Moi Bon Leong.

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DOUBLE RESONANCE SPECTROSCOPY AS A DIAGNOSTIC TOOL
AND ANALYTICAL TECHNIQUE FOR ATOMIC SPECTROSCOPY



BY



MOI BON LEONG



A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN

PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY



UNIVERSITY OF FLORIDA
1988



Dedicated to my father, my brother, and my sister for t'nei"
undying support and trust in me. In memory of my mother who did not
live to see the growth of her son but will always be remembered by
him for the time she spent when she was here.



ACKNOWLEDGMENTS

I would like to sincerely thank Dr. James D. Winefordner for his
guidance, inspiration, and encouragement during my three and one half
years in the finest spectroscopy group anywhere. It has been both a
learning experience and a great joy to work with such a distinguished
and friendly individual. I would like to thank you for the
opportunity.

I would like to acknowledge Dr. Benjamin Smith and Dr. Nicolo
Omenetto for their invaluable support and help as well as their
wisdom and guidance.

Lastly, I would like to thank the friends that I have made
during my quest, because without them it would not have been nearly
as fun and enjoyable. Of this host of friends, I especially thank
the clan for allowing me to eat with them. Thanks for the good
times, and I hope we remain friends and not strangers. I hope to see
you down the road: Brad, Tony, Benny, Jorge, Tom, Mark, Mike M.,
Mike R., Wellington, Richard, Chris, Joe, Ben, Andres, Doug, Leigh
Ann, Tiing, the secretaries (Jeanne, Chris, Susan, and Robin), and
Dave Berber ich.



TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS i i i

LIST OF TABLES vi

LIST OF FIGURES vi i

ABSTRACT xii

CHAPTERS

1 INTRODUCTION 1

Basic Principles of Two-Photon Methods 1

Brief Review of Double Resonance Spectroscopy for

Atomic Species 4

Intent of Dissertation 5

2 FLUORESCENCE DIP SPECTROSCOPY FOR THE MEASUREMENT

OF ATOMIC PARAMETERS 8

Introduction to Fluorescence Dip Spectroscopy 8

Experimental Facilities and Considerations 17

Results and Discussion 25

Conclusions 55

3 ATOMIC FLUORESCENCE AND IONIZATION MECHANISM FOR

LEAD IN AIR-ACETYLENE FLAME 61

Basic Principles of Atomic Fluorescence and

Ionization Spectroscopies 61

Experimental Facilities and Considerations 64

Results and Discussion 69

Conclusions 79

4 MEASUREMENT OF ATOMIC FLUORESCENCE FOR LEAD IN A
GRAPHITE TUBE ATOMIZER 81

Introduction to Graphite Tube Atomizers 81

Brief Review of Atomic Fluorescence Using Graphite

Furnace Atomization 83



Experimental Facilities and Considerations 85

Results and Discussion 94

Conclusions 1 08

5 FINAL COMMENTS AND FUTURE WORK 111

APPENDICES

A GLOSSARY OF TERMS AND SYMBOLS 114

B LIMITING CASES FROM FIGURE 2-4 116

C SAHA EQUATION 117

REFERENCES 1 1 8

BIOGRAPHICAL SKETCH 125



LIST OF TABLES



Table Page

2-1 Experimental Components for Fluorescence Dip

Spectroscopy 19

2-2 Experimental Parameters for Fluorescence Dip

Spectroscopy 23

2-3 Comparison of Gate Widths with Respect to Slope,

Intercept, and r Correlation Coefficient 30

2-4 Comparison of Relative (Ratio) of Atomic Parameters 43

2-5 Comparison of Steady State Saturation Dip Parameter

with Respect to the Second Laser Excitation Wavelength. .. .54

3-1 Experimental Components for Laser Enhanced Ionization

and Laser Excited Atomic Fluorescence Spectroscopies 66

3-2 Experimental Parameters for Laser Enhanced Ionization

and Laser Excited Atomic Fluorescence Spectroscopies 70

4-1 Experimental Components for Double Resonance Laser
Excited Atomic Fluorescence in a Graphite Tube
Atomizer 87

4-2 Experimental Parameters for Double Resonance Laser
Excited Atomic Fluorescence in a Graphite Tube
Atomi zer 93

4-3 Peak Fluorescence Signals for 100 pg of Lead and

Related Noise Figures 1 03

4-4 Laser Excited Atomic Fluorescence of Lead in a

Graphite Tube Atomizer: Absolute Detection Limits

as Obtained by Single Step and Two-Step Excitation 105



LIST OF FIGURES



Figure Page

1-1 Diagrammatic Representations of (a-c) Two-Photon

and (d ) Double Resonance Excitation Schemes 2

1-2 Double Resonance Excitation Schemes Followed by

(a) Fluorescence or (b) Ionization Detection 6

2-1 Generic Energy Level Diagram for Fluorescence

Dip Spectroscopy 9

2-2 Cross-Sectional Area of the Atomizer Including
Excitation Beam and Fluorescence Geometry and
Prefilter and Postfilter Effects 11

2-3 Rate Expressions for a Three-Level System 13

2-4 Expression for the Steady State Population of the
First Excited State in Terms of the Radiative and
Collisional Rate Coefficients 1 4

2-5 Theoretical Expressions for Fluorescence Dip
Spectroscopy for the Limiting Case of Optical
Saturation of the First Transition 15

2-6 Block Diagram of Experimental Setup for Fluorescence

Dip Spectroscopy 18

2-7 Partial Energy Level Diagram for Sodium 21

2-8 Scan of the Second Excitation Laser Through the
Upper Level Transitions of Sodium at 0.10 mJ per
Pulse with the Fluorescence Wavelength at 589.6 nm 26

2-9 Scan of the Second Excitation Laser Through the Upper
Level Transitions of Sodium at 0.73 mJ per pulse with
the Fluorescence Wavelength at 589.6 nm 27

2-10 Scan of the Second Excitation Laser Through the Upper
Level Transitions of Sodium at 8.33 mJ per pulse with
the Fluorescence Wavelength at 589.6 nm..... 28



vii



2-11 Plot of the Relative Dip and the Relative Fluorescence
Intensity as a Function of Observation Height Above
the. Load Coil 31

2-12 Measurement of the Relative Fluorescence Intensity at
588.995 nm With and Without the Second Laser at
568.266 nm as a Function of the Second Laser Irradiance. . .33

2-13 Measurement of the Relative Fluorescence Intensity at
589.592 nm With and Without the Second Laser at
568.266 nm as a Function of the Second Laser Irradiance .. .31

2-14 Reciprocal Plot of Relative Dip vs Spectral Energy

Density with 2 ns Gate Width and 568.3 nm Transition 35

2-15 Reciprocal Plot of Relative Dip vs Spectral Energy

Density with 2 ns Gate Width and 568.8 nm Transition 36

2-1 6 Reciprocal Plot of Relative Dip vs Spectral Energy

Density with 4 ns Gate Width and 568.3 nm Transition 37

2-17 Reciprocal Plot of Relative Dip vs Spectral Energy

Density with 4 ns Gate Width and 568.8 nm Transition 38

2-18 Reciprocal Plot of Relative Dip vs Spectral Energy

Density with 600 ns Gate Width and 568.3 nm Transition. .. .39

2-19 Reciprocal Plot of Relative Dip vs Spectral Energy

Density with 600 ns Gate Width and 568.8 nm Transition. .. .40

2-20 Theoretical Reciprocal Plot of Relative Dip vs

Spectral Energy Density 41

2-21 Plot of Relative Dip vs Spectral Energy Density with
589.0 and 568.3 nm Excitation and Fluorescence
Detection at 589.0 nm 44

2-22 Plot of Relative Dip vs Spectral Energy Density with
589.0 and 568.3 nm Excitation and Fluorescence
Detection at 589.6 nm 45

2-23 Plot of Relative Dip vs Spectral Energy Density with
589.0 and 568.8 nm Excitation and Fluorescence
Detection at 589.0 nm 46

2-24 Plot of Relative Dip vs Spectral Energy Density with
589.0 and 568.8 nm Excitation and Fluorescence
Detection at 589.6 nm 47



2-25 Plot of Relative Dip vs Spectral Energy Density with
589.6 and 568.3 nm Excitation and Fluorescence
Detection at 589.0 nm 48

2-26 Plot of Relative Dip vs Spectral Energy Density with
589.6 and 568.3 nm Excitation and Fluorescence
Detection at 589.6 nm 49

2-27 Plot of Relative Dip vs Spectral Energy Density with
589.6 and 568.8 nm Excitation and Fluorescence
Detection at 589.0 nm 50

2-28 Plot of Relative Dip vs Spectral Energy Density with
589.6 and 568.8 nm Excitation and Fluorescence
Detection at 589.6 nm 51

2-29 Theoretical Plot of Relative Dip vs Spectral Energy

Density 53

2-30 Partial Energy Level Diagram for Palladium 57

2-31 Measurement of Relative Fluorescence Intensity at
3^.1 nm With and Without Laser Excitation at
565.5 nm as a Function of Laser Irradiance 58

2-32 Partial Energy Level Diagram for Calcium (II) Ion 60

3-1 The Three Basic Pathways of Atomic Fluorescence:
a) Resonance Fluorescence, b) Direct Line
Fluorescence, and c) Stepwise Line Fluorescence 62

3-2 Block Diagram of Experimental Setup for Laser
Enhanced Ionization and Laser Excited Atomic
Fluorescence Spectroscopies 65

3-3 Partial Energy Level Diagram for Connected Double

Resonance Excitation and Ionization of Lead 67

3-4 Partial Energy Level Diagram for Connected Double

Resonance Excitation of Lead 71

3-5 Scan of the Second Excitation Laser With the

Fluorescence Wavelength at 239.379 nm and the First
Excitation Laser at 283.306 nm for Lead 73

3-6 Scan of the Second Excitation Laser With the

Fluorescence Wavelength at 261.418 nm and the First
Excitation Laser at 283.306 nm for Lead 74



ix



3-7 Temporal Behavior of the Ionization Signal as
Recorded by the Oscilloscope With Full Laser
Irradiance in Both Beams (283.306 and 600.193 nm) 76

3-8 Temporal Behavior of the Ionization Signal as
Recorded by the Oscilloscope with the Laser
Irradiance at 600.193 nm Decreased by a 100-Fold
and the 283.306 nm Laser Irradiance Unchanged 78

4-1 Graphite Furnace Designs 82

4-2 Block Diagram of the Double Resonance Laser Excited

Atomic Fluorescence in a Graphite Tube Atomizer 86

4-3 Graphite Tube Atomizer Setup..... 90

4-4 Chart Recorder Tracings of the Furnace Emission
Noise at the Four Fluorescence Wavelengths
Investigated in this Work: a) 405.733 nm,
b) 261.413 nm, c) 239.379 nm, and d) 216.999 nm 97

4-5 Boxcar Output for Laser Induced Noise into the
Detector System for Single Resonance Excitation
at 283.306 nm and Fluorescence Wavelength at
405.783 nm: a) ^0% Transmission Neutral Density
Filter Placed Between the Laser and the Graphite
Furnace and b) \% Transmission Neutral Density
Filter Placed as in a 98



4-6 Boxcar Output for Laser Induced Noise into the

Detector System for Double Resonance Excitation at
283.306 and 600.193 nm and Fluorescence Wavelength
at 216.999 nm: a) Laser Operated at Full Power,

b) 10? Transmission Neutral Density Filter Placed
Between the First Laser (283.306 nm) and the Graphite
Furnace, c) \% Transmission Neutral Density Filter

Placed as in b 99

4-7 Boxcar Output for Fluorescence of 100 pg of Lead
With Excitation at 283.306 and 600.193 nm and the
Fluorescence Wavelength at 216.999 nm: a) Both
Laser Operated at Full Power, b) 1 0% Transmission
Neutral Density Filter Placed Between the First
Laser (283.306 nm) and the Graphite Furnace, and

c) \% Transmission Neutral Density Filter Placed

as in b 102

4-8 Partial Energy Level Diagram for Disconnected

Double Resonance Excitation of Lead 106



4-9 Comparison of Connected and Disconnected Double

Resonance Excitation of Lead 10(



xi



Abstract of Dissertation Presented to the Graduate School

of the University of Florida in Partial Fulfillment of the

Requirements for the Degree of Doctor of Philosophy

DOUBLE RESONANCE SPECTROSCOPY AS A DIAGNOSTIC TOOL
AND ANALYTICAL TECHNIQUE FOR ATOMIC SPECTROSCOPY

BY

MO I BON LEONG

April, 1988

Chairman: James D. Winefordner
Major Department: Chemistry

Double resonance spectroscopy is a technique whereby two lasers
(i.e., two-color) are used to excite two real and different atomic
transitions in a species of interest. The first step of the excita-
tion process is the absorption of photons by the atomic species to
the first excited state from the ground state. This is subsequently
followed by a secondary absorption of photons from the first excited
state to the second excited state. Once the atomic species have
reached the second excited state, there are several methods for
monitoring the events that occur, among these are ionization and
fluorescence measurements. In the present study, three different
measurement methods are used with three different atomization
cells. The measurements demonstrate the usefulness of double
resonance excitation processes as a diagnostic tool for flames,
plasmas, and possibly, graphite furnaces, as well as an analytical



xii



method for the measurement of lead at femtogram levels in a graphite
tube atomizer.



xiii



CHAPTER 1
INTRODUCTION



Basic Principles of Two-Photon Methods
The development of pump lasers such as the nitrogen, excimer
(i.e., XeCl), and Nd:YAG (Neodymi urn: Yttrium Aluminum Garnet) together
with the organic dye laser has opened up the realm of analytical
atomic spectroscopy. Because organic dye lasers are not stand-alone
instruments, they are always coupled with a pump laser such as the
ones mentioned above. A majority of the analytical work to date has
thus involved the use of a pump laser in conjunction with an organic
dye laser. The addition of a second dye laser to this system has
provided the opportunity to perform two-color experiments.

Two-photon excitation methods usually involve a minimum of three
levels. These levels, however, are not necessarily real and well-
defined (i.e., virtual levels and ionization continuum). Figure 1-1
illustrates several types of two-photon excitation schemes. Two-
photon excitation taken in its basic form involves the utilization of
the same wavelength twice. The energy from the two photons excites
the atom from the ground state to a real excited state via a virtual
level that is midway between the ground and real excited states. The
real excited state must have the same symmetry and multiplicity as
that of the ground state for it to be a two-photon allowed transition
as shown in Figure 1-1a. However, as shown in Figures 1-1b and 1-1c,



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the ionization continuum can be considered as a level. In both of
these cases, the second photon promotes the species of interest from
a real or virtual level to the continuum, but not a well-defined
level in terms of the specific state (e.g., symmetry and multi-
plicity) to which the species has been promoted. These cases have
been used to investigate atoms that are not easily accesible by one-
photon excitation schemes (1-24).

Double resonance excitation is a limiting case of two-photon
excitation (25). This excitation scheme incorporates three real,
distinct levels. The species of interest absorbs photons of a
discrete energy such that excitation occurs from the ground state to
the first excited state. Once the species are in the first excited
state, a second set of photons with a different and discrete energy
than that of the initial excitation is absorbed such that promotion
of the species occurs from the first excited state to the second
excited state as shown in Figure 1-1d. Thus, all further discussion
pertains to this specific excitation scheme.

To populate the first excited state, the wavelength of the first
laser is tuned to a resonance transition of the atomic species. This
first transition should be saturated; that is, there is an equal
population in the ground state and the first excited state due to the
laser irradiance. Once this has been attained, an increase in laser
irradiance should have no or little effect on the overall distribu-
tion between the two states, and hence, fluctuations in laser
irradiance have no effect. The second excited state is then
populated when the wavelength of the second laser is tuned to the



transition that couples the first and second excited states. Again,
saturation is desired but not always necessary because the laser
irradiance of the second laser can be varied to provide the optimal
performance characteristics of the experiment.

Brief Review of Double Resonance Spectroscopy
for Atomic Species

The technique of double resonance excitation by two lasers has
been growing primarily due to the development of more powerful
lasers. With this development, more work has gone into the investi-
gation of atoms in the ultraviolet (200 nm) to the near infrared
(-900 nm) regions (26-38). The more powerful pulsed lasers have
provided high pumping rates such that the rate of absorption exceeds
the radiative and/or radiationless deactivations (25) (e.g., colli-
sional de-excitation). The pumping rate of the first laser, that is,
the rate at which the first excited state is populated, must exceed
(i.e., beat) the deactivation processes from that first excited
state. If this does not occur, then there is not a sufficient number
of species in that first excited state to populate, to a great
extent, the second excited state.

A majority of the publications to date have been studies of the
high lying (Rydberg) states that are close to the ionization
continuum (26-28,32,35,36) as well as studies of the fine and
hyperfine structures of specific levels (29-31 , 33, 3^) • More
recently, fluorescence and ionization measurements have appeared in
the literature with promising and exciting results (39-48).



Fluorescence monitoring from the second excited state to states below
the first excited state or even to the ground state as well as those
transitions that lie between the first and second excited states
(Figure 1-2a) have provided several interesting advantages. These
advantages include high selectivity, the detected fluorescence can be
blue-shifted relative to both pumping wavelengths (reduction of laser
scatter effects) , and transition probabilities are generally greater
for those transitions originating from the second excited state than
the first excited state (39,40). Ionization measurements, e.g., two-
color laser enhanced ionization in flames, have provided some of the
best detection limits for several elements (41-48) in analytical
atomic spectroscopy. The primary advantage was that the atoms were
much closer to the ionization continuum than in a one-color situation
(Figure 1-2b), and hence, the collisional processes that promoted the
atoms into the continuum were significantly increased. In addition
to this, there was increased selectivity as well as increased
sensitivity by one to three orders of magnitude. Thus, double
resonance excitation schemes have opened up new avenues of
exploration in analytical spectroscopy.

Intent of Dissertation

In this dissertation, double resonance excitation schemes were

investigated in three different atomization sources [i.e., flames,

inductively coupled plasmas (ICPs), and graphite furnaces] to obtain

information about the atomic parameters of elements, mechanistic



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Figure 1-2 Double Resonance Excitation Schemes Followed
Fluorescence or (b) Ionization Detection



by (a)



studies in laser enhanced ionization (i.e., temporal separation of
photoionization and collisional ionization), and the measurement of
ultratrace levels of lead by atomic fluorescence. Three different
techniques were used to gather this information, and they are
discussed individually. These excitation schemes have demonstrated
the usefulness and the viability for analytical atomic spectroscopy.



CHAPTER 2

FLUORESCENCE DIP SPECTROSCOPY FOR THE

MEASUREMENT OF ATOMIC PARAMETERS



Introduction to Fluorescence Dip Spectroscopy
Principles of Fluorescence Dip

Fluorescence dip is, as the term implies, a dip in the
fluorescence. To better explain this phenomenon, it is best
illustrated with a generic energy level diagram (Figure 2-1). Atoms
are produced from the atomization of a liquid sample, converted to an
aerosol and then to a dry particle and finally to submicroscopic
species such that the species of interest are lying in their ground
state. The first excitation laser is fixed to a resonance transition
of the analyte such that the first excited state is populated. The
extent of this population should be in an optical saturation mode;
that is, both the ground and first excited states are equally
populated assuming g = g . The resulting fluorescence from the
first excited state, whether it be a resonant or nonresonant
situation, is monitored. The second excitation laser is allowed to
pass such that this laser beam arrives at the center of the
atomization cell temporally and spatially coincident with respect to
the first excitation laser beam. This second laser excites atoms
from the first to the second excited state.

A fluorescence dip occurs because the first excited state has
been depleted. This depletion is a function of the average laser




Figure 2-1 Generic Energy-
Spec troscopy



Level Diagram for Fluorescence Dip



10



irradiance of the second excitation laser. It is important to note
that the laser irradiance of the second excitation laser must exceed
the radiative and radiationless deactivation pathways from the second
excited state.
Theoretical Considerations

The theory, as proposed by Omenetto et al . (49), was the first
general treatment of the fluorescence dip as it pertained to atoms
and/or ions in atomic spectroscopy. Several assumptions were made:
1 ) the time behavior of the laser pulses was approximated by a step-
like function, i.e., with a rise time equal to zero; 2) the lasers
were spatially homogeneous in the fluorescence volume; 3) the atomic
vapor was dilute (no self-absorption, prefilter, and postfilter
effects); 4) the spectral bandwidths of the excitation lasers were
much greater than the absorption profiles; and 5) the rate equation
approach was considered valid; i.e., coherence effects were neglected
(49). More discussion of points 3 and 5 is presented below.

The atomic vapor is dilute if there are no self-absorption,
prefilter, and postfilter effects. Self-absorption occurs from the
reabsorption of fluorescence photons, within the excitation volume,
as they traverse the atom reservoir (50). Prefilter effects are
those regions in which the analyte of interest is present and is
illuminated by the excitation beam but the resulting fluorescence is
not viewed by the detector (Figure 2-2) (50). Postfilter effects are
those regions in which the analyte of interest is present but the
region is not illuminated by the excitation source (Figure 2-2) (50).



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The rate equation approach is considered valid if coherence
effects are neglected. Coherence effects are interactions between
the analyte of interest and the laser field. This usually occurs at
very high pumping rates (i.e., 10 10 s~ 1 or greater). The high
pumping rates result in short interaction times (i.e., less than
100 ps). At such short times, the rate equation approach fails
because the atoms are oscillating between the first and second
excited states. Thus, the atoms are not exclusively lying in the
first excited state. At longer times, 1 ns or greater, the coherence
effects are negligible because they have been damped out.

The rate equations for the system illustrated in Figure 2-1 (see
Appendix A) are presented in Figure 2-3 (49). The time dependent
solutions for the population of the levels can be obtained after many
mathematical manipulations but are not shown here. However, the
conclusions from these solutions indicate that when the radiatively
induced rates significantly exceed the collisional rates, the
attainment of the steady state population of the final level is also
reached, and that when the first excitation laser is capable of
optical saturation, the steady state population of the ground and
first excited states depends upon the values of the radiatively
induced rate of the second excitation laser (49).

From the time dependent solutions and further mathematical
manipulations, it is possible to write an expression for the steady
state population of the first excited state in terms of the radiative
and collisional rate coefficients as shown in Figure 2-4 (see



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Appendix A). Utilizing this equation, the limiting cases (i.e.,
absence of the second excitation laser; optical saturation of the
first transition; optical saturation of the second transition but not
the first transition; and optical saturation of both transitions) for
the steady state population of the first excited state can be derived


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Online LibraryMoi Bon LeongDouble resonance spectroscopy as a diagnostic tool and analytical technique for atomic spectroscopy → online text (page 1 of 6)