Detecting Mercury Fluorescence Signature through Two-Photon
Todd B. Johnson
B.S., University of Colorado Denver, 2000
This thesis submitted to the
University of Colorado Denver
in partial fulfillment
of the requirements for the degree of
Master of Science
This thesis for the Masters of Science
Todd B. Johnson
has been approved
Johnson, Todd B. (M.S., Electrical Engineering)
Detecting Mercury Fluorescence Signature through Two-Photon Excitation.
Thesis directed by Dr. Tim Lei, Professor of Electrical Engineering,
University of Colorado Denver
This research focuses on the detection of elemental mercury and mercuric
compounds. For low surface levels of elemental mercury or mercuric
compounds a two photon (2P) light emitting diode (LED) excitation can be
used in order to detect and ultimately determine surface content. This is
accomplished by stimulated emission by a catalyst photon of energy
equivalent to the energy gap between the initial ground state and the first
excited state, with a second simultaneous photon excitation to the final energy
state. A sequential excitation by means of photonic radiation of two distinct
atomic transitions in order to detect a distinct emission, which is specific to
the nature and composition signature of elemental mercury, Hg, is the goal of
this research. The detection stage of the experiment will focus on the signature
properties of the substance around 546 nm when the mercury atoms relax to a
lower state by means of fluorescence.
The criteria of the process for the detection will be a repeatable
experiment on Hg to establish the detection of the first relaxation state of the
element at 546 nm. Further work will be made in order to detect mercuric
compounds by use of a linear method to analyze the areas of specific artifacts
for study of the surface content and/or contamination on these surfaces by
mercury or mercuric compounds.
This abstract accurately represents the content of the candidates thesis. I
recommend its publication.
Director of Thesis
To my brothers and sisters for their unconditional love and support through
all my endeavors, to my wife, Lisa, for putting up with me and giving me all the
love I could ask for. To my friends for keeping me going and all the support
they have given over the years. And to my Father who taught me value and
honor and to my Mother who taught me how to honor value.
I had to learn the math on my own.
I would first and foremost like to thank Dr. Tim Lei for all of his
guidance, support, and interest in my work, immense patience throughout this
thesis, and always having his door open for questions. This would never have
been possible without his assistance and knowledge. I would like to thank Dr.
Timberley Roane for being on my defense committee and for her time and
influence on the subject of the thesis, as well as the use of the laboratory for
the preparation of the mercury slides. I would like to thank Dr. Albert Rosa
for taking the time to discuss my thesis and for being on my defense
committee. I would like to thank Larry Scherrer for his trust in the CAPT
laboratory and for all the assistance in the setup and preparation of the optics
and equipment I needed. To my employer, Raytheon and everyone I work
with, for allowing me the opportunity to make this a possibility.
TABLE OF CONTENTS
LIST OF FIGURES.............................................viii
LIST OF TABLES...............................................x
2.1 Characteristics of Mercury.............................4
2.2 Mercury Hazards........................................5
2.3 Laboratory Mercury Samples.............................7
3. MERCURY DETECTION METHODS..................................9
3.2 Common Techniques.....................................10
3.2.1 Intrusive Methods................................10
3.2.2 Non-Intrusive Methods............................11
4. ULTRAVIOLET LIGHT OPTICS..................................15
4.2 UV LED Development and Structure......................18
5. PHOTONIC INTERACTION WITH ATOMS...........................21
5.2 Energy States.........................................25
5.3 Quantum States........................................29
6. RECENT EXPERIMENTATION....................................32
6.1 SFG Laser Diode Sensor at 253.7 nm....................32
6.2 SHG Laser Diode Sensor at 365 nm......................33
6.3 Two-Photon Laser-Induced-Fluorescence Detection.......34
7.2 Experiment Method......................................39
7.3 Data Collection........................................44
8.1 Research Conclusions...................................47
8.2 Research Revisions.....................................48
8.3 Future Study...........................................49
A.l Physical Constants.....................................51
A.2 Physical Constant Values...............................51
A.3 Useful Conversions.....................................51
LIST OF FIGURES
3.1 Thermo Scientific Niton XL3t handheld XRF...........................12
3.2 XRF excitation model................................................13
4.1 Electromagnetic spectrum chart......................................16
4.2 405 nm LED loss by coupling method in fiber.........................17
4.3 Typical deep UV LED structure.......................................19
5.1 Schematic of absorption of a photon-molecule interaction............23
5.2 Schematic of stimulated emission of a photon-molecule interaction..23
5.3 Energy level diagram for mercury (path A)...........................26
5.4 Grotrian energy level diagram for mercury...........................27
5.5 Jablonski diagram for possible excitation states....................28
5.6 Emission decay of given number of excited atoms.....................30
7.1 Excitation of mercury with two sources..............................36
7.2 Visual representation of photon excitation..........................37
7.3 Energy level diagram for mercury (path B)...........................38
7.4 Setup block diagram.................................................39
7.5 Detection setup (Same-Side Opposing)................................40
7.6 Vertical laboratory setup...........................................41
7.7 Vertical setup detail...............................................42
7.8 Relative optical distances..........................................43
7.9 Initial conditions..................................................44
7.10 405 nm LED with no 250 nm LED......................................45
7.11 250 nm LED with no 405 nm LED......................................45
7.12 250 and 405 nm with small 546 nm excitation........................46
LIST OF FIGURES
7.13 546 nm excitation detail.............................................46
8.1 Proposed mercury detection system.....................................49
LIST OF TABLES
2.1 General mercury properties.........................................5
2.2 Mercury slide material construction................................8
Mercury has a wide occurrence in the environment. It can be found in air,
water and soil. It exists in several forms: mercury in an elemental or natural
state, within inorganic compounds, and also organic compounds. The
toxicological effects of mercury and its compounds on biological organisms and
the environment have been notably recognized. Means for detection of mercury
are widely available and has several techniques.
Fluorescence detection is a spectroscopic process based on the absorption of
radiation of a certain wavelength that excites the electrons in a molecule and
causes them to emit light by the radiation deactivation of the excited atoms
toward the detection device. Both the absorption and the subsequent atomic
emission processes occur at wavelengths which are characteristic of the specific
signature of the atomic species present. Fluorescence spectroscopy (FS) is a
method for determining the presence of a number of environmental and
biomedical important elements such as mercury. This technique is an important
analytical tool for trace element analysis. Several books and review articles
have been published dealing with the theory and instrumentation of
fluorescence, most of which are mainly concerned with atomic fluorescence
spectroscopy (AFS) which is the analysis of a species when observed in
discrete, gaseous atoms, l18^20^
The current means of detecting mercury in trace amounts and in compounds
varies depending on the application, area of detection, type of detection, and the
knowledge of the boundary conditions of the detection apparatus. Mercury
alone, in its elemental state, is hazardous and care needs to be taken when
determining quantities in a restrictive environment. A brief description of the
analytical methods that are available for detecting, measuring, and monitoring
mercury, its metabolites, and other biomarkers will be covered in order to
understand the subject of the detecting method more readily. The intent is not to
provide an exhaustive list of analytical methods, rather a comparison of the
well-established standards for analysis. The method that will be used in this
project will focus on the excitation of elemental mercury and/or mercuric
compounds and the detection of the surface content levels by means of a dual
stage LED excitation through the fluorescence of the atom. The resonance
fluorescence line of 253.7 nm (6 Pi 6 So transition term symbols) is well
documented and utilized primarily for atomic fluorescence studies on mercury
but the theory behind the fluorescence process can be proven on a atom of
mercury in a non-gaseous state.
The detection stage of the project will show the characteristic signature of
the species by means of a dual spectrometer system, one observing from 195
nm to 525 nm, and the other from 500 nm to 1100 nm. The sample to be used
initially will be stable elemental mercury, Hg. The mercury content will then
be decreased in order to fulfill the goal of determining low levels in a linear
fashion. Mercuric compounds, such as mercuric chloride (HgCL), may also be
investigated to determine if the technique can be used for complex molecules
pertaining to an inert object. The concentration of the mercury in the compound
will be high enough at first to determine a good baseline in order to retrieve
good data and then reduced in order to give a linear determination of
The main portion of this research involves a bench-top experimentation to
develop a low cost mercury detector. The next phase of this research will be a
prototype hand-held or portable scanner. This will be discussed in order to
show possible avenues to either build on the research for a present application
or to evolve the process to acquire better data in a more efficient manner.
This research was an off-shoot of a project Dr. Timberley Roane,
Professor of Biology, University of Colorado Denver, was working on in
order to remove mercury, in the form of mercuric chloride, from Native
American artifacts located in museums across the United States and Canada.
These artifacts were preserved with the mercuric chloride and when given
back to their respective tribes under the Native American Graves Protection
and Repatriation Act (NAGPRA),1341 which requires any federal agency or
institution that receive federal funding to return any Native American cultural
items and/or human remains to their respective tribes or peoples, resulted in
mercury-toxicity related illness among tribal members handling the artifacts.
The cultural items are comprised of sacred objects and objects of cultural
patrimony. The primary function of the research in this paper is to provide a
basic mercury detector in order to be able to analyze sample areas of
homogenous or heterogeneous origin.
2.1 Characteristics of Mercury
To analyze any sort of metal, either in an environmental inert material, or in
biological material, the task is complicated not only by the method to be used
but also by the differing forms of inorganic and/or organic compounds being
analyzed. For mercury, this complication was overcome by starting an analysis
of a sample in its elemental state. By analyzing the mercury in a high
concentration the characteristics of the element could be understood and a
method developed for detection.
Mercury presents a very cumbersome problem being extremely volatile
and, therefore, easily lost when working with it in its elemental state or when
preparing a sample for analysis. Mercury is also very toxic and caution should
be exercised when dealing with the samples. It has a zero oxidation state [Hg]
where it exists as vapor or in a liquid phase, its mercurous state [Hg+] exists as
the form of inorganic salts, and its mercuric state [Hg2+] may form either
inorganic salts or organic mercury compounds.
There are seven isotopes of mercury which are quite stable. One of the
more common isotopes is Hg, which is stable and one of the most abundant
forms. This is the isotope that will be studied as the form of elemental mercury
and will be referred to as Hg. Mercury is in liquid phase at standard pressure
and standard temperature. The general properties of mercury are found in
Table 2.1. Mercury will dissolve to form amalgams with many metals with the
exception of iron. When mercury is heated it can also react with the oxygen in
the air and form mercuric oxide, which can be hazardous. Care should be taken
when there is a possible exposure to air.
TABLE 2.1 General mercury properties.|n|
Name, Symbol, Number Mercury, Hg, 80
Element Category Transition Metal
Group, Period, Block 12, 6, d
Standard Atomic Weight 200.59 g-mol'1
Electron configuration 1 s2 2s2 2pb 3s2 3p6 3dlu 4s2 4p6 4dl0 4f*4 5d,u 6s2
Density (near room temp.) 13.534 g-cm (liquid phase)
Melting Point, Boiling Point (-38.83C, -37.89 F), (356.73 C, 674.11 F)
Crystal structure rhombohedral
Ionization energies 1st: 1007.1, 2nd: 1810, 3rd: 3300 (kJ/mol)
2.2 Mercury Hazards
Most biological organisms require trace amounts of metal for their
nutritional purposes, such as iron, copper, manganese and zinc, but excessive
levels can be damaging to the organism. Mercury is not a vital nutrient nor is
it beneficial to any organism. The accumulation of mercury over time can be
detrimental to biological organisms. Mercury is a hazardous poison where all
the compounds that it is related to are poisons as well. It is not absorbed
readily through ingestion or skin contact; its main hazard is the result of the
potential to release mercury vapor. This is the main exposure process in which
mercury is transferred throughout the body is by means of the respiratory and
circulatory system. Toxic effects of mercury include brain damage, kidney
damage, and lung damage. Mercury poisoning results in several diseases,
including acrodynia, Hunter-Russell syndrome, and Minamata disease.1241 The
main symptoms of these diseases include sensory impairment, vision and
hearing, lack of coordination, memory loss, speech impediments, and mental
illness. The degree to of exposure, the chemical form of the mercury, and the
duration of exposure are all factors in determining the toxicity.
Mercuric chloride, HgCL, is also a corrosive sublimate, and highly
soluble. It forms a covalent compound and has little ionization in an aqueous
solution. Mercury is not toxic to plants and has, in the past, been used as an
insecticide, usually applied on the plant surface, where it is low in toxicity but
is easily absorbed by contact with the plant. This practice has been
discontinued due to the general toxicity of exposure.
Low-level mercury poisoning is characterized by fatigue, headaches, lack of
concentration, and hair loss. These tire symptoms are not at all distinctive from
characterizations like those caused by the fatigues of work or that of a falling
stock market. The levels of mercury over which devoted crusaders become
vocal are less than those causing low-level poisoning. These levels have no
indicators and they cause no clinical symptoms.
This research was always performed under strict safety precautions to
mitigate any risk of exposure to any person in the laboratory where the
experiment was conducted. The experimentation on the samples was
performed under a laboratory fume hood. This was also located in a laboratory
space where a high efficiency particle air (HEPA) filter was in use for the
environmental clean room. The researcheds) also utilized personal protective
equipment (PPE) in the form of nitrite gloves, lab coats, hair nets, and safety
goggles. The UC Denver Environmental Health and Safety Program  and
the Laboratory Health and Safety Plan '35' were used as a guideline for the
safety practices while in the laboratory environment.
2.3 Laboratory Mercury Samples
The mercury samples used in this research were prepared by means of
laboratory slide configurations where the mercury drops were locked down by a
thin cover slide then bonded to the microscope slide by means of super-glue or
a two-part epoxy. The microscope slides and the cover slides were laboratory
grade glass slides where the microscope slide had a concave indentation for
sample placement. Given mercurys surface tension, several slides were coated
with mineral oil in order to ease the flattening of the mercury droplet onto the
microscope slide. Mercury (Hg) was extracted by means of a syringe and
dropped onto the center of the prospective slide. Six different slides were
prepared with a differing combination of the various elements used for
The first prepared slide was used as a test slide for alignment and also to be
used as a control slide to distinguish background noise from the spectrometer.
The second slide was used to identify whether the mineral oil held any spectral
emissions in the range of the expected mercury signature. The four remaining
slides were a combination of the mercury, with or without mineral oil, and
either the epoxy or superglue to hold the cover slide in place, as summarized in
TABLE 2.2 Mercury slide material construction.
Mercury Mineral Oil Super Glue Two Part Epoxy
SLIDE 1 None None X None
SLIDE 2 None X X None
SLIDE 3 X X X None
SLIDE 4 X None X None
SLIDE 5 X X None X
SLIDE 6 X None None X
3. MERCURY DETECTION METHODS
Mercury is the leading cause for three-quarters of all heavy-metal
contamination or related warnings and advisories for threats to human health.
[24, Because of the increase in the variety of detection techniques these
warnings are a result of an increase in these methods and not a result of an
increase in the amount of contamination. Most of these techniques are currently
used for environmental analysis and are approved by federal agencies and
organizations, such as the Environmental Protection Agency (EPA), the
National Institute for Occupational Safety and Health (NIOSH), and the
American Public Health Association (APHA).
The current methods for mercury analysis often rely on capturing the
oxidized and elemental mercury in various solutions, and then analyzing the
solutions for mercury content. This method requires laboratory settings and
must destroy a portion of the sample or the whole sample to get the required
data. Quantification of mercury is usually performed by atomic absorption,
atomic emission or atomic fluorescence, mass spectrometry (MS), x-ray
fluorescence, or gold-film resistance measurements. Cold-vapor atomic
fluorescence spectroscopy (CVAFS) and cold-vapor atomic absorption
spectroscopy (CVAAS) are the most popular approaches for mercury analysis.
Most of these detection techniques are well developed as final steps in
determining total mercury in solid, liquid or gaseous states. ^
The appropriate testing method to use for the detection of a specific heavy
metal, such as mercury, will depend on the results that need to be quantified.
One must know what is driveled from a method and why. The sample material
or sample element can be tested under laboratory conditions or in a non-
destructive manner. For our purposes the sample will be in a laboratory setting
in order to create a method to use in a non-destructive means of analysis. This
might yield a method in order to obtain test results on homogeneous samples
which would usually indicate the consistent presence in the entire object.
However, that might also be used on heterogeneous samples (such as organic
substrates) where the specific test site/sample would be a universal method to
obtain quantitative results that are relevant to either a homogenous or a
3.2 Common Techniques
3.2.1 Intrusive Methods
These methods are not desired for the research involved but are covered
briefly to demonstrate available techniques which utilize similar analysis in the
field of spectroscopy.
EPA Method TO-15 |28' is used for the analysis of volatile organic
compounds (VOCs) for the range of air samples associated with vapor intrusion
investigations. It utilizes gas chromatography (GC) to accomplish a sample
separation and then a mass spectrometer (MS) for substance identification and
trace amounts. This method has the ability to look at a wide range of
compounds but it does not target any specifics in these compounds. The
compound lists it might use vary among differing laboratories and yield
inconclusive results. This method could not be utilized in a field or off-site
Another method that the EPA uses is Method 1631.1291 This method differs
is that is targets low-level mercury measurements by means of Cold Vapor
Absorption Fluorescence Spectrometry (CVAFS). It requires the sample be
immersed in an aqueous solution of bromine mono-chloride. The mercury is
thermally desorbed from a gold trap into a cold vapor atomic fluorescence
spectrometer. This method is highly intrusive and could not be used outside a
3.2.2 Non-Intrusive Methods
One of the main methods used for non-intrusive analysis is x-ray
fluorescence (XRF). All of the elements comprising the sample being tested
produce a unique set of characteristic x-rays that are an identifiable signature
for that specific element. Energy-dispersive x-ray fluorescence (EDXRF)
analyzers determine the chemistry of a sample by measuring the spectrum of
the characteristic x-rays emitted by the different elements in the sample when it
is illuminated by x-rays. These x-rays are emitted either from a miniaturized x-
ray tube, or from a small, sealed capsule of radioactive material.[31'
A fluorescent x-ray is created when the x-ray strikes an atom in the sample,
dislodging an electron from one of the atom's inner orbital shells. These shells
are usually the inner most part of the atom. For the atom to regain its stability
the vacancy left by the dislodged electron is filled with an electron from one of
the atom's higher energy orbital shells. The electron drops to the lower energy
state by releasing a fluorescent x-ray, as seen by FIGURE 3.2. The energy of
emission of the resultant x-ray is equal to the specific difference in energy
between two quantum states of the electron.
An XRF operates by extracting a small sample from a gas stream and
moving this sample through a very small pipe to the main sampling and analysis
compartment. The extracted sample is drawn through a resin-impregnated filter
tape. The filter tape collects both the particulate and vapor-phase metals. The
resulting deposit is then moved into analysis position where the metals are
quantified by EDXRF analysis while another sample is being collected. The
stack gas elemental concentrations are calculated from the EDXRF results,
mass flow meter data, and oxygen concentration.[3IJ
Incident Radiation from
The XRF method is widely available and extremely standardized by
means of the method of use and stable linearity. It is a well suited method for
the study of bulk analysis of trace elements. In theory the hand-held device
has the ability to detect an x-ray emission from virtually all elements. The
overall process does experience inefficiency due to the radiation of the
secondary beam being weaker than the primary beam. If the element being
radiated has a low energy, or a long wavelength, the beam, when passing
through any length of air, will result in attenuation. Non-portable XRF
analyzers maintain a vacuum with the sample where this does not become a
The method of choice for this project will be very similar to the way in
which the XRF analyzer operates. The fluorescence excitation of an LED will
yield less power than an x-ray and maintain lower intensities for analysis. This
type of analysis would have a smaller footprint than an XRF analyzer and
potentially utilize several distinct wavelength specified LEDs to determine
chemical characteristics. Further research is necessary for the specitation of a
new instrument which would become a new low-cost standard alternative to
material analysis through fluorescence spectroscopy.
4. ULTRAVIOLET LIGHT OPTICS
Ultraviolet (UV) light is defined as light with a wavelength shorter than
400 nm. Within this region there are specific bands: the near-UV spectral
region ranges from 400Dnm down to 300Dnm, the middle-UV region ranges
from 300 to 200Dnm, and shorter wavelengths from 200Dnm down to 10Dnm
belong to the far-UV region. Still shorter wavelengths belong to the extreme
UV (EUV). The term vacuum UV (below 200Dnm) refers to the wavelength
range where a vacuum is often used to mitigate the absorption of the light in
the air. The vacuum UV includes the far and extreme UV. The term UVA
stands for the range from 320 to 400Dnm, UVB for 280-320Dnm, and UVC
for 200-280Dnm. When comparing visible light and ultraviolet light, they
differ in essentially two distinct respects: one, UV has a short wavelength and
it can be focused very well; second, the photon energy is higher than the band-
gap energy of many compounds. This focused power and ability to overwhelm
the band-gap of most material, particularly biological tissue, results in UV light
being strongly absorbed by many substances. This absorption and the induced
excitation can lead to changes in the chemical structure or excitation of the
The optics packages needed to deal with UV are very specialized in the
respect that in order to get the full potential of the UV light the optical window
must be as transparent as possible to the UV source. There are important
material parameters for all UV applications. A low bubble content, or material
free from imperfections and inclusion content, a good homogeneity of the
refractive index, a small birefringence of the material used, and that the surfaces
are smooth and with very small roughness makes for a good UV opticly
transparent device. UV optics are often made from highly purified calcium
fluoride (CaF2) or fused silica, both of which have a very low UV absorption,
high homogeneity, low birefringence, and relatively high hardness. The CaF2
crystals also have a high physical stability and a high optical damage threshold
when compared to other fluoride materials. Both types of optics can be used
down to 200Unm. However, the calcium fluoride is brittle, naturally
anisotropic, and hygroscopic.1371 Another possible material choice is diamond,
which is transparent down to 230Dn/w and very strong and robust, but also very
FIGURE 4.1 Electromagnetic spectrum chart.
From the electromagnetic spectrum, FIGURE 4.1, it can be seen that the
region of UV radiation has a very high frequency and very short wavelength.
When coupling the UV into fiber optics is necessary, some general optical
fibers cannot be used because of their attenuation of the signal. Specialized
optical fiber can be used in the near-ultraviolet spectral region, although with
relatively high propagation losses. Fiber delivery of UV light is usually not
feasible for shorter wavelengths and/or high optical powers. For the 405 nm
LED in use for the experiment there was significant loss in the initial coupling
of the optics.
FIGURE 4.2 405 nm LED loss by coupling method into fiber.
This can be seen in Figure 4.2 where the initial measurement was made
via an Ocean Optics 2000+ spectrometer through 2 m length, 400 nm
diameter, of fused silica fiber optic cable 4.5 cm from the end of the LED ball
lens into the fiber at low power. The 405 nm LED is capable of 300 mA
continuous wave (CW) operation at 3.5 volts direct current (Vdc). Second, the
lower intensity waveform is the attenuated signal when coupled into fiber
optics. This coupling technique will be discussed further in the experiment
description of the research. In the far-UV region, or extreme ultraviolet light
(EUV) region, all solid materials are strongly absorbing the UV light, and air
causes strong attenuation below 200Unm. Most of the analysis done below
200 nm must be performed in vacuum environment to be able to detect the
photons in the florescence reaction.
4.2 UV LED Development and Structure
Mercury lamps have been widely used in the past for generation of UV
characteristic light. These bulbs are toxic due their construction where the
primary means of generating the UV signature is achieved by mercury. This
type of source is toxic, considered hazardous waste, produces ozone, and has a
short lifetime of use (8000 h).1171 These sources also require a high voltage, or
ballast, in order to excite the mercury. UV LEDs have no toxic elements, are
not hazardous, require low voltage, produce no ozone, and have a long lifetime
(100,000 h). This type of source is proficient for detection of hazardous
materials by means of fluorescence analysis techniques.
UV LEDs operate with several different wavelengths. Most of these
wavelengths are tuned, by means of the fabrication of the diode, to emit at a
single, centered, wavelength of 10 nm. The size of these sources is very small
and can be placed on a number of LED package types which make it versatile
with a small footprint and adjustable form factor. The LED transparent
materials on the package can be a flat window, or a ball or hemispheric lens.
These have an emission pattern of 60, 10, and 12, respectively with a spot
size from the ball lens of 2 mm and a focal length of 15 20 mm. Currently
their uses include purification and sterilization, bio-agent detection, biomedical
applications, and polymer and plastic curing.
The fabrication of these types of LEDs is similar to other, more common
LEDs. They are grown on a sapphire substrate by the use of metal-organic
chemical vapor deposition (MOCVD). It can be seen from Figure 4.3 that the
size of these devices have a very small footprint. The method of fabrication for
these wavelengths is quite complex and time consuming. These factors along
with their specialized use make any type of UV LED below 265 nm quite
FIGURE 4.3 Typical deep UV LED structure.1,81
These types of LEDs have a CW power output of 1 1.5 mW. Under pulse
mode operation, where the duty cycle can range from 1 2% at a rate of 1 kHz,
the power can increase in excess of 150 m W in optical power. These devices are
fairly stable but will show power degradation under stressed conditions where
the optical power might be decreased by 30%. They are also very susceptible to
the effects of electro static discharge (ESD) damage.
5. PHOTONIC INTERACTION WITH ATOMS
Atoms absorb and emit radiation as light of fixed, characteristic
wavelengths when excited. These upward or downward transitions occur
between distinct energy levels, conserving energy in the process. f5' When this
process takes place the absorption and emission of light corresponds to the
electrons within the atom moving away from the nucleus when the energy is
absorbed, or closer to the nucleus when the energy is emitted. There are four
forms of interaction that are possible for the atom to absorb energy or release it
in the form of a photon: absorption, spontaneous emission, stimulated emission,
and Raman scattering.  For the purposes of this research the process of
absorption and spontaneous emission will be discussed to enhance
understanding of the fluorescence of the mercury.
The interactions between electromagnetic radiation and matter cause
changes in the energy states of the electrons in matter. Electrons can be
transferred from one energy level to another by means of absorbing radiative
energy and emitting a photon at a characteristic frequency. If the atom is in a
lower energy level, or ground state, and is radiated with a photon, where the
photon is absorbed, this will raise the atom to a higher energy level. When this
energy is absorbed or emitted as a form of electromagnetic radiation, the energy
difference between these two energy levels (E2-E1) is a defining characteristic
of the frequency of the electromagnetic radiation. This is an example for an
excitation of two different energy states by one wavelength.
By Planks Law:
AÂ£ = Ei E2 = hv = = ho) (5.1)
The absorption characteristics presented in Einsteins model of absorption and
emission are as follows:
WAbsorption = B12 Nt p (5.2)
where W is the rate of transition ^12 is the Einstein coefficient. N is
number of molecules present at a given state, and p is the density of the
where 'll is the transition dipole moment which describes the strength of the
transition between the excited electronic states and the ground electronic state
excited by an incoming photon. This interaction is molecule specific.161
FIGURE 5.1 Schematic of absorption of a photon-molecule interaction.
For Spontaneous Emission:
WSP. Emi = ^
FIGURE 5.2 Schematic of spontaneous emission of a photon-molecule
The spontaneous emission rate is proportional to the number of molecules at
the excited state, N2, illustrated by Einsteins coefficient of spontaneous
emission. This coefficient is also proportional to the absorption coefficient.
According to Bohrs 2-dimensional theory for surface area interaction, if an
electron were to move from an outer orbit to an inner orbit a photon of light
should be emitted, having energy given by:
The wavelength (A.) of this photon would be demonstrated by the Plancks,
equation (5.1), where:
Where the Rydberg energy, Ry, is found by:
This equation is dependent on the susceptibility of the atom where there is a
dipole moment and it is not in a vacuum. Bohrs simple atomic theory of an
electron moving between fixed orbits can be used to explain the observed
spectra in experimentation and will aid in the detail needed for complex atoms
with more than one electron.
for shell orbitals fn = 1,2,3,...)
5.2 Energy States
When a photon of low energy, from a UV source, is used to radiate a
mercury atom at a ground state then only one of the two outermost electrons
in the 6s shell of the atom will absorb this energy and be promoted to an
excited state referred to as a triplet 63P' state as illustrated in FIGURE 5.3.
This energy can be calculated from equation (5.1):
c Vl = l7 (5.9)
E1 = hv (5.10)
From Planks constant and the speed of light (c) the first excitation to this
triplet state requires a 4.86 eV energy transfer to the bound electron of the
mercury atom. The probability of excitation to higher levels other than the 6s
shell electrons or the probability of excitations of any inner shell electron is
very low and will not be possible with the method used to excite the mercury
An excited atom will only occupy a higher energy level for a short time.
These are on the order of pico- seconds, 109 seconds. After this short time
they will return to a lower energy level by spontaneous emission. This is the
time it will take when emitting a photon and is on the order of micro- seconds,
10"6 seconds. The energy level has a characteristic average lifetime. The
different states can be seen in FIGURE 5.3, where the wavelength and required
energy for the transition is given.
I P. 10.43 *v
SERIES LIMIT 84.104.1 cr*
lOp 10d 9d QdkOK llll lOp 10d 9d 10 f 9f 8f 7f
9p ft 8d 6f 10s 9p 8d 6f
8p If r 7d -751 9* 8p 7 d -y- 5f
Vacuum wavelengths below 2000 A
Air wavelengths above 2000 A
FIGURE 5.4 Grotrian energy level diagram for mercury.
The Grotrian diagram, or term diagram, in FIGURE 5.4, shows the
specific and allowed transitions between the energy levels of atoms by means
of photon excitation. They can be used for one electron and multi electron
atoms. The diagrams take into account the specific selection rules related to
changes in angular momentum of the electron but it does not show all
relaxation states in the form of energy loss due to vibrational relaxation, loss
by means of heat, internal conversion (IC) or intersystem crossing (ISC).161
For the possible starts of relaxation, through a non-radiative process, the
Jablonski diagram in FIGURE 5.5 shows an example of possible energy
release from radiative decay by means of fluorescence or phosphorescence.
The ground state of the diagram, lowest level, shows that when an amount of
energy, in terms of photons, is absorbed into the molecule, it will excite. The
next stage is the method of releasing the energy. When a molecule has a
commonality of a singlet ground state (SO) such as mercury, the excited state
can produce either an internal conversion or move into a triplet state. From
this triplet state the molecule can then be instantaneously excited to another
level by means of another absorption state, not shown in the general diagram.
When the atom is in this high singlet stage of energy the electrons are paired,
and as the energy is dispersed, in transition to a triplet state, by means of
intersystem crossing, the molecule undergoes a non-radiative ISC.
5.3 Quantum States
In the area of quantum theory, the transition from one energy level to
another is described by statistical probability. The probability of transition
from higher energy level to a lower energy level is inversely proportional to
the lifetime of the higher energy level, given by the absorption and emission
probability density function:101
This can be seen in FIGURE 5.6 where the spontaneous emission of a
number of excited atoms, N, at a given time will decrease exponentially given
its cavity of volume V, transitional cross section a, spontaneous lifetime t, and
line shape function g.
FIGURE 5.6 Emission decay of a given number of excited atoms.181
The probability for different transitions is a characteristic of each
transition. When the transition probability is low for a specific transition, the
lifetime of this energy level is longer on the order of ^////-seconds, 10'
seconds, and this level becomes a quasi-stable or meta-stable level. This will
bring about a condition where a higher energy level is denser in population
than a lower level. This phenomenon is called population inversion and is the
key characteristic of the lasing process.|8'
If a population inversion exists between two energy levels, the probability
is high that an incoming photon will stimulate an excited atom to return to a
lower state, while emitting another photon of light. The probability for this
process depends on the match between the energy of the incoming photon and
the energy difference between these two levels.
6. RECENT EXPERIMENTATION
6.1 SFG Laser Diode Sensor at 253.7 nm
Purdue University was given a grant in 2007 by the Department of Energy
(DOE), National Energy Technology Laboratory (NETL), for the development
of a diode-laser-based sensor for atomic mercury. 1261 The atomic mercury
sensor uses UV radiation at 253.7 nm, sum frequency generated (SFG) by
mixing the output of a 375-nm external cavity diode laser (ECDL) with the
output of a 784-nm distributed feedback (DFB) diode laser into non-linear
optical crystal for the deep-UV range needed.
The UV beam is split into a reference beam and an absorption beam to
make absorption spectroscopy measurements of atomic mercury. It is then
filtered into a photo-multiplier tube (PMT). The absorption beam is directed
through a sample of atomic mercury and through an interference filter and
onto a PMT. The DFB laser is modulated via the injection current to tune the
wavelength of the UV radiation back and forth across the absorption lines of
atomic Hg at 253.7 nm. The signals from both PMTs and an etalon (to
measure frequency changes) are recorded on a personal computer with an
analog input/output card.
Theoretical atomic Hg absorption spectra are generated with a modified
version of the computer program described in that reference. Concentration of
atomic mercury is determined from a least-squares fit of the theoretical
absorption spectrum to the experimental spectra using an evolutionary fitting
Techniques are also being employed to resolve interference and improve
resolution to measure elemental and oxidized forms of mercury in low part-
per-billion (ppb) concentrations. For total mercury measurement using laser-
induced breakdown spectroscopy (LIBS), new approaches will be researched
to accurately measure mercury in the low ppb ranges in the presence of
6.2 SHG Laser Diode Sensor at 365 nm
The process of sum frequency generation (SFG) was used to generate 365
nm laser light for mercury detection. The radiation of a continuous wave
(CW) single mode laser diode used for this application due to its in analytical
spectrochemistry properties.  When this experiment was performed the
commercially available laser diodes at the time operated outside the range
needed and could only be found in the near-infrared and red wavelength
range. The UV range is attainable by second harmonic generation (SHG) of
the fundamental laser radiation in a non-linear crystal.
Power for the beam was achieved in a SHG crystal with a fundamental
power of about 30 mW. The low efficiency of the non-linear process limited
the detection power needed for the diode laser atomic absorption spectroscopy
(DLAAS). This experiment showed the way in which a SFG can overcome
the wavelength gap of SHG. Currently, there are laser diodes that are more
robust but the technique is useful if this situation arises. The experiment did
have a 50% loss in radiation due to absorption that could be avoided but
results yielded a high signal-to-noise (SNR) ratio which could be
characterized. This system at the time of practice proved to be attractive for
sensitive analytical applications.
6.3 Two-Photon Laser-Induced-Fluorescence Detection
Two-photon laser-induced-fluorescence (TP-LIF) detection of mercury
uses the sequential absorption of two photons, involving real atomic levels, to
produce atomic fluorescence at a wavelength shorter than either the pump or
the idler (seeding) wavelength. [221 For this research the pumping and idling
occurred at 254 and 408 nm, where the detection phase concentrated on
sampling at 185 nm. The preliminary results of the data indicated a sensitivity
of 103 atoms/cm3. The theoretic limits were given as low as a few atoms per
cubic centimeter. The driver for this research was the detection of gas-phase
mercury that can be used for the detection of mercury pollution and
environmental threats. The technique reported here involves the sequential
two-photon laser pumping of ground-state mercury atoms. The detection of
the photons was by a PMT, and at the time of the experiment, these devices
were not as sensitive as they are currently.
The results demonstrated that the integration over time for the detection
made the process for data retrieval very slow. The experiment could have
been corrected by increasing the optical collection efficiency of each PMT
assembly, increasing laser energy, and increasing the overall optical collection
efficiency of the system.[221
This research focuses on the detection of elemental mercury and mercuric
compounds. For low surface levels of elemental mercury or mercuric
compounds a dual stage light emitting diode (LED) will be used in order to
detect and ultimately determine surface content. This will be accomplished by
stimulated emission by a catalyst photon of energy equivalent to the energy
gap between the ground state and an initial state, which will be a virtual state.
Between this ground state and virtual state there will be a second emission
of a photon to create a spontaneous emission for detection. The experiment
provided sequential excitation by means of photonic radiation of two distinct
atomic transitions in order to detect a distinct emission that is specific to the
nature and composition signature of elemental mercury, Hg. FIGURE 7.1 is
a pictorial description of the process.
The detection stage of the experiment will focus on the signature
properties of the substance around 546 nm when the mercury atoms relax to a
lower state by means of fluorescence. This stage will be the resultant
spontaneous emission from the absorption of the photon energy in the
mercury. The criteria of the process for the detection will be a repeatable
experiment on Hg to establish the detection of the first relaxation state of the
element at 546 nm. Further work will be made in order to detect mercuric
compounds by use of a linear method to analyze the areas of specific artifacts
for study of the surface content and/or contamination on these surfaces by
mercury or mercuric compounds.
FIGURE 7.1 Excitation of mercury with two sources.
Through radiation by LED excitation, the elemental mercury can be brought
from a lower energy state by first applying a pulsed 250 nm LED to the initial
energy state, and then a concurrent 405 nm radiation of LED or Laser Diode, to
bring the mercury to a final energy state. From the diagram of FIGURE 7.1, the
arrowed line coming up from the ground state 6So, at 253.7 nm, depicts the
stimulated energy into the mercury to its first energy state at 63P]. The arrowed
line at 407.8 nm is the second excitation to the 7*So state.
The arrowed line, at 546 nm, is the wavelength that will be detected. This
dual excitation with spontaneous emission also has a wavelength at 184.9 nm
that can be detected. These are very well known and very documented
characteristic wavelengths.  However, the detection of this wavelength at
185 nm requires a photo-multiplier tube detector. The use of a PMT requires a
great deal of calibration and verification of the signal, and is also very costly.
An atom of mercury contains 80 paired electrons. When a radiative
photon, 253.7 nm, is absorbed by the outermost shells electron pair it will
cause a member of the pair to change energy states through absorption criteria
described by equation (5.12). When a subsequent photon of a different
wavelength, 405 nm, is absorbed at the same instance, the energy state will
change again almost instantaneously. As can be seen from FIGURE 7.2, the
546.1 nm wavelength will fluoresce, by means of spontaneous emission, as the
electron pair reestablishes stability.
FIGURE 7.2 Visual representation of photon excitation.
These events are all a part of the signature of mercury and establish
presence when observed or measured. There are numerous possibilities of
absorption that the element can maintain. From FIGURE 5.4 one can see
multiple paths for a certain energy state that the atom might assume under
certain conditions. Another possibility, given the LED source having a wide
spectrum, can be seen in FIGURE 7.3.
FIGURE 73 Energy level diagram for mercury (path B).
FIGURE 73 is representative of an alternate route the photon reaction
might have on the mercury. It is unclear which of the energy routes is being
taken because the source has a 5 nm difference from the center wavelength.
One of the notable differences is the quenching effect from one energy state to
another. This is found when there is presence of oxygen around the sample. ^
In FIGURE 53 (energy path A) the energy state goes from a singlet to a
triplet state by means of ISC. As seen in FIGURE 73 (energy path B) the
energy state stays in a triplet excitation state and decays to a lower triplet state.
Both of these means are possible and both can happen with the two LEDs
currently being used. One atom can function on pathway A and another can
proceed on pathway B. Given the use of a mass amount of mercury in the slide,
0.25 grams, there could be a path A and path B event occurring simultaneously.
7.2 Experiment Method
FIGURE 7.4 Setup block diagram.
The samples were placed in a horizontal configuration on a stable bread-
board. From Figure 7.4 the block diagram shows the general method of
excitation. Originally, the optics were placed at one side of the sample and the
detection fibers were placed on the adjacent side. This did not prove to be an
adequate method due to the difficulty in aligning the two LED to occupy the
same focal point. The selected method would be to have the two LED
converging, from either side, and illuminating the sample from two directions.
FIGURE 7.5 Detection setup (Same-Side and Opposing).
The approximate weight of the mercury is about 0.25 gram; this varies
from sample to sample because the mercury was placed in the glass slide with
a syringe. The samples were measured at relative room temperature and under
dark conditions. There was ambient light from windows but the lab space
was located in a UV protected area and the laboratory lights were turned off at
the time of measurement. Slide 1 was measured initially to give an idea if
there was any fluorescence from the glass, or adhesive, which was being used.
No fluorescence was detected.
Slide 4 and Slide 6 were used to take the initial measurements because
they were void of mineral oil that was present in Slide 3 and Slide 5. Mineral
oil was unnecessary for the construction of the slides. The experiment was
modified from the single-side approach into the horizontal opposing side
excitation. This gave an initial ease in aligning the two LED focal points into
one concise area. This method did yield one detection of a 546 nm peak
amidst a high amount of noise, but it was never found again. This might have
been resultant from the 250 nm LED power level. Initially, this was the only
LED on hand and great care was taken because of this fact, its cost and lack of
FIGURE 7.6 Vertical laboratory setup.
A second 250 nm LED was eventually purchased from the manufacturer
and the setup was modified for a vertical stance, where there was movement
and adjustment in all three axis for the sample as well as a rotational yaw.
This is illustrated in FIGURE 7.7 where the bread board is supporting the
vertical platform of the slide and the optics are converging on the sample from
two opposing angles approximately 30 degrees off the z-axis. The new 250
nm LED provided a greater power output, 100 m W to the older 47 mW LED,
at a distance of 12 mm from the Melles Griot power meter. This is also evident
from the initial spectrometer plots taken in Ocean Optics Spectra Suite
FIGURE 7.7 Vertical setup detail.
From this new vertical setup the sample was now hanging inside the slide
chamber but it did not move from gravitational forces due to the high surface
tension inside the glass. It was pressed against the cover slip and had a surface
area that could be viewed easier than the horizontal position. The fiber
detectors also had better access to the sample in this manner. For the readings
and measurements recorded, the fiber detectors were held by hand and not
locked into place. This presented two problems: first, signals from the upper
and lower end of the spectrum had to be saved individually and, second, the
measured signal was difficult to acquire and the relation from detector to
sample cannot be quantified at this time.
f- 25 f- 40
Lens 2 Lens 1
15 mm 18 mm 20 mm 70 mm
FIGURE 7.8 Relative optical distances.
The relative distances, as seen from FIGURE 7.8, is composed of two
lenses on the 405 nm LED and since the 250 nm LED had such a short focal
length with the ball lens it did not require additional optics, and this
wavelength could not be attenuated by any loss that might be incurred from a
lens. Options for focusing this wavelength would involve expensive lens and
might prove to be unbeneficial due to the optical power loss that might be
73 Data Collection
Data was collected by means of two Ocean Optics spectrometers. The lower
end spectrometer could acquire from 195 nm to 525 nm. The upper end
spectrometer could collect from 500 nm to 1100 nm. The spectrometers fed
data into a laptop where Spectra Suite was being run via USB cables. The LEDs
were powered by individual DC power sources through an AC outlet.
The first measurement made was of background noise in the system and are
the initial conditions of the set up as seen by FIGURE 7.9. Two graphs were
placed side by side but the scans were running simultaneously. The noise floor
on both detectors was normalized and free running when pointing at the set up
apparatus without a mercury slide. This did not change when a slide of glass,
Slide 1, was placed in the slide holder.
500 550 60C 650 700 750
850 900 950 WOO 1050 1100
FIGURE 7.9 Initial conditions.
intensity (counts) Intensity (counts)
The next two figures, 7.10 and 7.11, show the individual LEDs activated
when a mercury sample is inserted into the vertical stand. The strength of the
250 nm LED is notable because the mercury and/or glass absorbs much of the
300 350 400 450 5
FIGURE 7.10 405 nm LED with no 250 nm LED.
200 250 300 350 400 450 500
FIGURE 7.11 250 nm LED with no 405 nm LED.
The final two figures, 7.12 and 7.13, for the data collection present the small
signal from the excitation of the mercury. This signal is almost in the noise and
it is possible that it had been present before when measurements were made but
did not get noticed. A larger signal was expected from the experiment. Ideally
the signal should be strong enough to distinguish it from the noise.
700 800 900 1000 1100
FIGURE 7.12 250 and 405 nm with small 546 nm excitation.
FIGURE 7.13 405 546 nm excitation detail.
8.1 Research Conclusions
Mercury has a wide occurrence in the environment. The effects of mercury
and its compounds have been notably recognized. Means for detection of
mercury is widely available and has several techniques. Refinement of
mercury detection methods can play an important part in environmental
protection and substance analysis. Fluorescence spectroscopy is a method for
determining the presence of a number of environmental and biological
elements such as mercury. This is an important analytical tool for analysis.
The current means of detecting mercury in trace amounts and in
compounds varies depending on the application, area of detection, type of
detection, and the knowledge of the boundary conditions. These techniques
are efficient and proven but are generally intrusive in nature. Non-intrusive
techniques are anticipated to be developed in order to mimic the sensitivity of
The purpose of this research, and experimentation, was to be able to detect
mercury through a two-photon excitation. The method for the detection
through fluorescence spectroscopy that will be utilized was conducted through
the use of a dual stage LED excitation of the atoms. It was shown and proven
that the detection of the mercury, in a gaseous state, was possible through this
technique. The mercuric compounds were not, as of yet, analyzed by this
The sensitivity of this experimentation was very poor. Quantification of
the mercury could not be established from the current data. The mercury
signal at 546 nm is very much within range for detection, however, it is very
close to the noise. The signal to noise ration needs to be improved by alternate
means or through an evolution of the process for excitation and collection.
When this is accomplished the next step towards a system for linear
quantification will be possible. The experimentation in this research process
showed the possibility of all the applied aspects to the purpose of the project.
8.2 Research Revisions
To understand the process better the excitation scheme needs to be
constrained to allow one path of the excitation state, either path A or path B. It
is unclear due to the broad nature of the LEDs as to which path is is creating the
fluorescence signature. If a method for detection and analysis for quantification
is to be determined then the path that the atomic state assumes is vital. The two
separate paths do yield the same resultant wavelength, however, the aspect of
determining compounds must have a way in which a distinct, and repeatable,
energy transfer occurs.
To accomplish this there needs to be a means of feedback in order to
compensate or constrain the process to make a determination of energy paths.
This implies that the process will be specialized to enact one specific reaction
alone. Filtering on both sources will allow the sample to be excited with a
known wavelength and intensity. This filtering method, by means of
upper/lower band, or notched, filters could then be minimized or reduced in
order to achieve a desired result.
8.3 Future Study
Laser feedback probe
UV source feedback probe
FIGURE 8.1 Proposed mercury detection system.
The setup seen in FIGURE 8.1 is a viable example of a bench top detection
system that would incorporate a dual source and feedback structure. If the 400
nm source were to be fine-tuned through a filter and imaging optics, while being
probed through a feedback loop, it could be monitored and controlled for
differing modes and wavelength intensity. This would allow for a concise
analysis. A variable laser source is a way in which this could be accomplished,
albeit an expensive one. This laser source could be tuned from 400 410 nm
and have a peak power of 40 -50 mW, however, the power that might be used
for an experiment of this size would need a power output of 15 20 m W.
The UV source, where the range could be from 245 255 nm through
filtering techniques, would be ideal if it could be compared to an electrode-less
UV lamp. This type of source achieves emission lines of an extremely narrow
bandwidth which are congruent with the absorption lines of the specified
mercury sample. This would keep the cross-sensitivities minimized. Electrod-
less lamps are required to have a radiated radio frequency source with
temperature stabilization. The 250 nm LED did yield fair intensity and a bank
of these with a focused point might prove to be an excellent source, with
filtering it could prove more robust and useful in a field environment.
A.l Physical Constants
c = Speed of light in a vacuum (m/s)
e = Elementary charge (C)
80= Absolute dielectric constant (As / V m)
X = Optical wavelength (nm)
v = Optical frequency (Hz)
co = Angular optical velocity (2% v)
h = Planck constant
h = hi (2n)
k = Boltzmann constant (J / K)
p<,= Absolute magnetic constant
A.2 Physical Constant Values
So= 8.8542x10-12^4 5 / (Vm)
= 1.6022x10-19 C
c = 2.9979x108 m/ s
h= 6.6261 x 10-34 J s
h= 4.1356 xl0-i5eF5'
h= 1.0546 x IO-34 J 5
h= 6.5821 x 10-16 eVs
k= 1.3807 x IO-23 J/K
k= 8.6175x10-5 eV/K
Po= 1.2566 x 10-6 Vs/(A m)
A.3 Useful Conversions
eV = 1.6022 x IO-19 CV
1.6022 x IO-19 J
E = 1239.8 eV / (XI nm)
kT = 25.86 meV (at T = 300 K)
kT = 25.25 meV (atr = 20 C = 293.15 K) room temperature
Anisotropic is the property of being directionally dependent, as opposed to
isotropy, which means homogeneity in all directions. It can be defined as a
difference in a physical property (absorbance, refractive index, density, etc.)
for some material when measured along different axes.
Anthropogenic derived from human activities, as opposed to those occurring
in natural environments without human influence.
Biomarkers substance used as an indicator of a biologic state. It is a
characteristic that is objectively measured and evaluated as an indicator of
normal biologic processes, pathogenic processes, or pharmacologic
responses to a therapeutic intervention.
Birefringence is the decomposition of a ray of light into two rays when it
passes through certain types of material, such as crystals that are
directionally dependant, depending on the polarization of the light.
Elemental Mercury is the naturally occurring or ground state mercury; pure
Fluorescence a luminescence that is mostly found as an optical phenomenon
in which the molecular absorption of a photon triggers the emission of a
photon with a longer (less energetic) wavelength.
Heterogeneous an object or system consisting of multiple items having a
large number of structural variations.
Homogenous a single-phase system where the entire substance is similar
1 Definitions are from Wikipedia Internet Encyclopedia and SPIE.org.
Hygroscopic is the ability of a substance to attract water molecules from the
surrounding environment through either absorption or adsorption.
Isotope consisting of the different types of atoms (nuclides) of the same
chemical element, each having a different atomic mass.
Metabolites the intermediates and products of metabolism.
Phosphorescence the specific type of photoluminescence related to
fluorescence where the material does not immediately re-emit the radiation
it absorbs and but at a lower intensity for a long duration of time.
Specitation is the evolutionary process by which a new species arises.
APHA American Public Health Association
CVAAF cold-vapor atomic absorption fluorescence
CVAAS cold-vapor atomic absorption spectroscopy
CVAFS cold-vapor atomic fluorescence spectroscopy
CW continuous wave
DFB distributive feedback
DLAAS diode laser atomic absorption spectrometry
DOE Department of Energy
EDCL external cavity diode laser
EDXRF energy dispersive X-ray fluorescence
EPA U.S. Environmental Protection Agency
ESD electro-static discharge
GC Gas chromatography
HEPA high efficiency particulate air filter
LIBS laser-induced breakdown spectroscopy
MOCVD metal-organic chemical vapor deposition
MS mass spectrometry
NAGPRA Native American Graves Protection and Repatriation Act
NETL National Energy Technology Laboratory
NIOSH National Institute for Occupational Safety and Health
PMT photo-multiplier tube
PPE personal protection equipment
SFG sum frequency generated
SHG second harmonic generation
SNR signal to noise ratio
TP-LIF two-photon laser-induced fluorescence
VOC volatile organic compounds
XRF X-ray fluorescence
 Stark, H., and J. W. Woods. Probability and Random Processes with
Applications to Signal Processing, 3rd Edition. New Jersey: Prentice-
 Leon-Garcia, Alberto. Probability and Random Processes for Electical
Engineering, 2nd Edition. New York: Addison-Wesley Publishing
 Fowles, Grant. Introduction to Modem Optics, 2nd Edition. New York:
Dover Publications, 1989.
 Hecht, Eugene. Optics. 4th Edition. New York: Pearson Addison-
Wesley Publishing Company, 2002.
 Saleh, B. E. A., and M. C. Teich. Fundamentals of Photonics. New
York: John Wiley & Sons, Inc., 1991.
 Prasad, Paras. Introduction to Biophotonics. New York: John Wiley &
Sons, Inc., 2003.
 Blanchard, S., Bronzino, J., and J. Enderle. Introduction to Biomedical
Engineering. 2nd Edition. San Diego: Elsevier Academic Press,
 Gupta, Mool. Handbook of Photonics. New York: CRC Press, 1997.
 Hitz, Breck. Understanding Laser Technology, 2nd Edition. Tulsa:
PennWell Publishing Company, 1991.
 Chuang, Shun. Physics of Optoelectronic Devices. New York: John
Wiley & Sons, Inc., 1995.
 Silberberg, Martin. Chemistry: The Molecular Nature of Matter and
Change, 5th Edition. New York: McGraw-Hill, 2008
 Atkins, P., Physical Chemistry 6th Edition. Oxford: Oxford University
 Bhattacharya, Pallab. Semiconductor Optoelectronic Devices. New
Jersey: Prentice-Hall, Inc., 1994.
 Paschotta, Rudiger. Encyclopedia of Laser Physics and Technology.
New York: Wiley-VCH, 2008.
 Franzke, John. Sum frequency generation at 365 nm by two diode
lasers applied to the detection of mercury. Applied Spectroscopy
Reviews Vol. 41. 2006: 259-303.
 Anderon, Thomas. Development of New Optical Sensors for
Measurements of Mercury Concentrations, Specition, and
Chemistry. Department of Energy: DE-FC26-04NT42311. 2004.
 Curtis, L. Semiempirical Studies of Atomic Structure: Progress
Report. Department of Energy, Division of Chemical Sciences:
DE-F G0294ER14461. 1996.
 Bilenko, Y., Zhang, J., Hu, X., Lunev, A., Deng, J., Katona, T., Shur,
M., Gaska, R., and M. Khan. AlGaN Deep-Ultraviolet Light-
Emitting Diodes. Japanese Journal of Applied Physics Vol. 44.
 Gaska, M., and M. Shur. Ill-Nitride Based Deep Ultraviolet Light
Sources. SPIE Vol. 6894. 2008: 6894191-6894198.
 Morita, H., Tanaka, H., and Shigeru Shimomura. Atomic
Fluorescence Spectrometry of Mercury: Principles and
Developments. Spectrochimica Acta Vol. 50B. 1995: 69-84.
 Bauer, D., Campuzano-Jost, P., and A. Hynes. Rapid, Ultra-Sensitive
Detection of Gas Phase Elemental Mercury Under Atmospheric
Conditions Using Sequential Two-Photon Laser Induced
Fluorescence. The Royal Society of Chemistry Vol. 10.1039.4.
 Rodgers, M., Bradshaw, J., Liu, K., and D. Davis. Sequential Two-
Photon Laser-Induced Fluorescence Detection of Mercury.
Optical Society of America Vol. 0146-9592. 1982: 359-361.
 Cai, Yong. Atomic Fluorescence in Environmental Analysis.
Encyclopedia of Analytical Chemistry 2002: 2270-2292.
 Environmental Protection Agency. Mercury. Washington, D.C.: United
States Environmental Protection Agency, 2009. 10 February 2009
 Van Dyck, J. Stoltenberg, and D. Pengra. The Hanle Effect. Unknown:
2006. 25 February 2009
 Schauer, J., Armstrong, D., Gross, Deborah S., Hurley, J., Krabbenhoft,
David. Speciated Atmospheric Mercury: Gas/Particle Partitioning.
Transformations, and Source Characterization. Madison, WI: 2005.
10 January 2009
 Columbia Analytical Services. Measurement of Trace Level Mercury by
EPA Method 163. Kelso, WA: 2004. 8 March 2009
 Columbia Analytical Services. Analytical Considerations for Air
Samples for Vapor Intrusion Investigations. Kelso, WA: 2004. 8
 Columbia Analytical Services. Mercury in Water by Oxidation, Purge
and Trap, and Cold Vapor Atomic Fluorescence Spectrometry.
Kelso, WA: 2002. 8 March 2009
Babko, S., Montgomery, J., Battleson, D., Whitworth, C., Sears, J.,
Sunner, J., and Dean Gingery. Mercury Speciation Analysis by
Gas Chromatography/Electron Impact/Mass Spectrometry.
Bozeman, MT: 2001. 5 March 2009
[31 ] Thermo Scientific. Thermo Scientific NITON XL3p Analyzer.
Waltham, MA: 2009. 28 November 2008
 Occupational Safety and Health Administration. Safety and Health
Topics: Mercury. Washington, D.C.: 2009 10 January 2009
 Xiaomei, T., Barat, R., and Arthur T. Poulos. Real-time trace detection
of vapor-phase elemental mercury and its compounds. Bellingham,
WA: 1999. 2 April 2009
 U.S. Department of the Interior. National NAGPRA. Washington
D.C.:2009. 12 March 2009
 University of Colorado Denver. Laboratory Safety. Denver, CO.: 2007.
15 December 2008
 Lizotte, T., and O. Ohar. Vacuum isostatic micro/macro molding of
PTFE materials for laser beam shaping in environmental
applications: large scale UV laser water purification. Society of
Photographic Instrumentation Engineers, Los Angles CA: 2009 <
&conference_id=883721 &event_id=883681 &jsenabled= 1 >