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HOT BAND ROTATIONAL RELAXATION TIME IN CARBONYL SULFIDE BY TRANSIENT INFRARED-MICROWAVE DOUBLE RESONANCE

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300 N ZEEB RD , ANN ARBOR, Ml 48106
8218505
Leap, John William
HOT BAND ROTATIONAL RELAXATION TIME IN CARBONYL SULFIDE
BY TRANSIENT INFRARED-MICROWAVE DOUBLE RESONANCE
University ofIllinois at Urbana-Champaign
University
Microfilms
I n t e r n a t i o n a l 300N.ZeebRoad,AnnArbor.MI48106
PH.D. 1982
HOT BAND ROTATIONAL RELAXATION TIME IN CARBONYL SULFIDE
BY TRANSIENT INFRARED-MICROWAVE DOUBLE RESONANCE
BY
JOHN WILLIAM LEAP
B.S., Purdue University, 1969
M.S., Purdue University, 1970
THESIS
Submitted in partial fulfillment of the requirements
for the degree of Doctor of Philosophy in Electrical Engineering
in the Graduate College of the
University of Illinois at Urbana-Champaign, 1982
Urbana, Illinois
UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN
THE GRADUATE COLLEGE
MARCH 1 9 8 2
WE HEREBY RECOMMEND T H A T T H E THESIS BY
JOHN WILLIAM LEAP
F.NTTTT.F.n
HOT BAND ROTATIONAL RELAXATION TIME IN CARBONYL
SULFIDE BY TRANSIENT INFRARED-MICROWAVE DOUBLE RESONANCE
BE ACCEPTED IN PARTIAL FULFILLMENT OF T H E REQUIREMENTS FOR
T H E DEGREE O F
DOCTOR OF P
Director of Thesis Research
Committee on
Chairman
^-j^f^rt^. t?. ^ ^ L ^ ^ ^ ^ u ^
t Required for doctor's degree but not for master's
iii
DEDICATION
This thesis is dedicated to my wife, Jane, who cheerfully
endured much inconvenience during the course of this thesis.
iv
ACKNOWLEDGEMENTS
Professor P. D. Coleman has provided invaluable guidance
and educational, challenging research opportunities.
Others
from the Electro-Physics Lab, especially Professor T. A.
DeTemple, Dr. M. K. Gurnick, Dr. E. G. Malk, and Dr. A. T.
Rosenberger, also gave particularly important advice and assistance.
Mrs. J. H. Smith proficiently typed the text and pre-
pared the figures of this thesis.
The University of Illinois
staff of the various offices, libraries, storerooms, and technical shops have given me excellent service.
I would also like to thank:
Dr. A. Maki (National Bureau
of Standards) for communicating spectroscopic data prior to
publication; Mr. J. Ruso (Microwave Associates, Inc.) for providing the microwave detector; Mr. W. Asbill, Dr. W. Kennedy,
and Dr. W. Kunz (Watkins-Johnson Company) for loaning the
microwave oscillator and amplifier.
Funding was provided by the Energy Research and Development
Administration, the Air Force Office of Scientific Research, the
Joint Services Electronics Program, and the National Science
Foundation.
Equipment support was provided by the University
of Illinois Physical Electronics Affiliates Program.
V
PREFACE
Most infrared laser systems have molecular gases as essential components:
Frequency sensitive absorbers for filters,
saturable absorbers for isolators and pulse shapers, and gain
media for amplifiers and oscillators which are often optically
5
pumped by another infrared laser. The high Q (^ 10 ) associated
with low pressure gases usually restricts their use to narrow
frequency bands centered about the fixed frequencies of the many
naturally occurring molecular resonances.
Of the multitudinous
number of resonances in a particular molecule, only a small
fraction have been used in applications. This is because the
molecules tend to be in states with relatively little energy.
Laser operation at many additional frequencies is possible if
transitions between hotter states are considered.
Mathematical treatments of quantum electronic systems often
involve the assumption that all population differences and all
polarizations relax at the same rate.
There is very little ex-
perimental evidence to support this assumption, especially for
highly excited molecules.
The aim of this thesis is to determine how the relaxation
rates compare for internally hot and cold molecules.
Carbonyl Sulfide (OCS) was chosen because a pulsed C0_ laser
can be used to populate a specific state with three vibrational
quanta and the induced changes in microwave transmission can be
VI
conveniently used to determine the relaxation time of this
highly vibrationally excited state.
When broadly tunable in-
frared sources become available, the transient infrared-microwave (IR-MW) double resonance technique used here will be generally applicable for determining relaxation rates for many
more states of many more molecules.
vii
TABLE OF CONTENTS
Page
I.
II.
III.
IV.
V.
INTRODUCTION
1.1. Overview
1.2.
Carbonyl S u l f i d e i n Quantum
Electronics
1.3. Previous Relaxation Experiments
with OCS
1.4. Relation of This Thesis to Previous
Work
1.5. Selection of Experimental Technique
1
1
4
5
IR-MW TRANSIENT MODEL
2.1. Assumptions
7
7
1
2
EXPERIMENT
3.1. Concept
3.2. Excitation Laser
3.3. Diagnostic Microwave Bridge
3.4. Signal Processing
19
19
19
27
29
RESULTS
36
DISCUSSION
39
APPENDIX:
MICROWAVE BRIDGE DETAILS
41
LIST OF REFERENCES
45
VITA
50
viii
LIST OF TABLES
Table
1.
Page
Relaxation experiments in Carbonyl
Sulfide. Key: IR, infrared, J, rotational
quantum number; LIF, IR laser induced IR
fluorescence; MW, microwave; N3-S, excited
nitrogen; pT, product of pressure and decay
time; r, rotation to rotation; T, decay
time; Ti, population decay time; T2, polarization decay time; UV, ultraviolet, vx, the
number of vibreitionai quanta in the i t h mode
(vx) ; W , vibration to vibration; VT,
vibration to translation; Av, line half
width = (1/27TT)
3
ix
LIST OF FIGURES
Figure
1.
Page
Normalized expected microwave gain pulse
shapes, |m | proportional to infrared impulse
excitation strength, m =-1000 (top curve) and
m 0 =-30 (lower c u r v e ) . If T,=T 2 =T, t h e n T =T;
if T1#T2, then T slnfTg/T-^/Cr^1-^1)
13
Lumped c i r c u i t analog of IR-MW system.
R 1 C 1 = T 1 ; V 1 ^ AN; R 2 C 2 =T 2 ; V2 «->• y
16
3.
T r a n s f e r f u n c t i o n model of IR-MW system
17
4.
T r a n s i e n t IR-MW e x p e r i m e n t a l a p p a r a t u s . SA,
s e l e c t i v e a b s o r b e r ; PS, plasma s h u t t e r , CW MW,
c o n t i n u o u s microwave; H and E, magic t e e a r m s ;
L, matched l o a d ; TR MW, t r a n s i e n t microwave
b r i d g e imbalance
20
C0~ l a s e r p u l s e , not t r u n c a t e d , 480 Pa NH-,
mtravavity.
Photon d r a g d e t e c t o r o u t p u t
a m p l i f i e d by a 5 kHz t o 500 MHz a m p l i f i e r
and d i s p l a y e d on a 350 MHz o s c i l l o s c o p e
24
C0 2 TEA l a s e r p u l s e s (20 n s / d i v . ) l i k e F i g ure 5 except plasma s h u t t e r u s e d t o t r u n c a t e
p u l s e s t o l e s s than 100 ns d u r a t i o n
26
Step r e s p o n s e of v i d e o p r o c e s s i n g s y s t e m .
Droop o r sag = 10% i n 600 ns and r i s e t i m e =
75 n s , a l t e r n a t e l y T , = 5700 ns and T. =
45 ns
7
7
31
2.
5.
6.
7.
8.
Transfer function model of experiment with
exponentially decaying population difference
density AN as the forcing function. D is a
transition strength factor defined in section
2.1. G3 is a detection and amplification factor.
T3 = 5.7 ps and T. = 45 ns determine the
video sag and risetime
32
X
Figure
9.
Numerically evaluated expected pulse width
at half maximum, T,=T =T, T,=5.7 ys, T. =
45 ns
Page
35
10.
Transient IR-MW waveforms, all 200 ns per
hor. aiv. Top Three: enlarged, inverted
tracing of oscilloscope display, 0.13 Torr (17 Pa)
Bottom: expected waveform, T,=T2=T=210 ns,
impulse excitation 0.52 div. to right of
vertical (gain) axis, circles are vertically
offset sampled points from top waveform
37
11.
Relaxation rate (T~ ) vs. OCS pressure (p) .
I : + one std. dev., six pulse width
values
© : three nearly equal data points
0 : single data point
>^: double data point
(g): triple data point
38
Microwave bridge. Dashed rectangle indicates
portion containing OCS, key: D, detector; DR,
dc return; E, difference port of magic tee;
EB, E plane bend; H, sum port of magic tee;
I, isolator; IA, input attenuator; IMT, input
magic tee; M, modulator, MA, microwave amplifier; OMT, output magic tee, PA probe attenuator; S, waveguide switch; VA video amplifier,
YO, YIG-tuned oscillator; Z_, matched load
42
12.
1
I.
INTRODUCTION
1.1.
Overview
Rotational relaxation time (T) for a highly vibrationally
and rotationally excited state (03
0, J=24) of low pressure (p)
Carbonyl Sulfide (OCS) has been measured by observing transient
gain at 7.4 GHz after termination of C0 2 laser excitation (P„(24)
line).
The observed pT=32 ns«Torr = 4.3 ps«Pa value agrees very
well with current literature values for states with fewer vibrational and fewer rotational quanta.
Specific state relaxation
times are generally unknown for molecular states with several
vibrational quanta because of measurement difficulty.
Observation of infrared-microwave (IR-MW) double resonance
allows identification of the OCS state pumped by the CO- Pg(24)
line.
This identification is in contrast to explanations in re1 2
cent articles ' concerning collisionless multiphoton dissociation of OCS.
A Bloch equation model (Chapter II) is used to describe the
IR-MW experimental procedures (Chapter III), form of results
(Chapter IV), and generality of conclusions (Chapter V ) .
1.2.
Carbonyl Sulfide in Quantum Electronics
i /• T O
The most common (95%) isotope ( 0
"5 0
C
S) of Carbonyl
Sulfide is the molecule studied in this thesis. It has been
used to generate 3 '4'5 electromagnetic radiation (2.3 THz to 88
6 7
THz) by electric discharges, to convert ' CO, laser radiation
(30 THz) to lower frequency (790 GHz to 16 THz) r a d i a t i o n by
o p t i c a l pumping, and to Q-switch
a CO- l a s e r .
These quantum e l e c -
t r o n i c s a p p l i c a t i o n s used molecules in t h e ground e l e c t r o n i c
s t a t e , as i s t h e case for a l l following d i s c u s s i o n s .
Vibrational
(v-, , v 2 , v^) and r o t a t i o n a l (J) quantum numbers adequately specify
the molecular s t a t e .
The o r i e n t a t i o n a l (M) quantum s t a t e s can
usually be grouped and t r e a t e d on an average b a s i s .
In OCS t h e
frequencies of allowed infrared and microwave t r a n s i t i o n s a r e
separated s u f f i c i e n t l y t h a t such single-frequency r a d i a t i o n
t e r a c t s with a t most two v i b r a t i o n - r o t a t i o n s t a t e s .
in-
For mosc
quantum e l e c t r o n i c a p p l i c a t i o n s , the f a s t e s t r e l a x a t i o n from a
s p e c i f i c s t a t e i s the most important.
1.3.
Previous Relaxation Experiments with OCS
The a v a i l a b l e experimental OCS r e l a x a t i o n information can
be categorized e i t h e r as r e l a x a t i o n time and linewidth data
(Table 1) or a s c o l l i s i o n a l preference r u l e s .
There appears t o
be no previous r e l a x a t i o n time o r linewidth measurement for
s p e c i f i c OCS s t a t e s with more t h a n two v i b r a t i o n a l quanta.
The temperature dependence 15 of T, implies that r e l a x a t i o n i s
more l i k e l y for smaller c o l l i s i o n v e l o c i t i e s .
I t has been claimed 17 that t h e only r e l i a b l e linewidth d a t a
for large J i s Av = (l/2irT 2 ) = 7 . 1 MHz/Torr = 53 kHz/Pa, J = 8-<-7
18
18
of
OCS. This i s in agreement with a t h e o r e t i c a l model
which
p r e d i c t s a very s l i g h t increase (20%) in linewidth a s J i n c r e a s e s
3
Table 1.
Relaxation experiments in Carbonyl Sulfide. Key: IR,
infrared; J, rotational quantum number; LIF, IR laser
induced IR fluorescence; MW, microwave; N2, excited
nitrogen; pT, product of pressure and decay time; r,
rotation to rotation; T, decay time; T]_, population
decay time; T2> polarization decay time; UV, ultraviolet; V£, the number of vibrational quanta in the
ith mode (\>j_) ; W , vibration to vibration; VT, vibration to translation; Av, line half width = (l/2irT) .
Technique
LIF
Specific State
Excited Probed
/
N2-MW
(excite-probe)
IR-UV
(excite-probe)
IR
non-linear
absorption
Transient
MW
MW
Linewidth
IR-MW
(excite-probe)
/
/
Ref.
Results
9
(pT)vy=(pT)VT=l ms«Torr=0.13 s«Pa
2v2-n>, v-,-»-T
10
(pT) w = lras-Torr=0 .13 s -Pa
N*-v3
11
(pT)VT=1 ms«Torr=0.13 s-Pa
Vague
v
/
11
/
12
13
14
/
/
/
v=0, J<2
(pT1) = (pT2)=26 ns«Torr=3.5 ys«Pa
Av= 6 MHz/Torr=45 kHz/Pa
15
v=0, J<3
pT1(200 K)=0.7pT1(300 K)
11 varies < 10% with |M|
16
J=2-«-l Av within a few %
for v=0, v3=l, v2=2° and
for v=0 of several isotopes
17
j=8<-7, v=0 of 18 OCS
Av=7.1 MHz/Torr=53 kHz/Pa
/
/
2 J(pT) w = l us«Torr=0.13 ms-Pa
1 (pT) r=29 ns -Torr=3 . 9 \xs -Pa
Mode
This v 3 =3, J=24 ^-doublet
Work (pT) = 32 ns«Torr=4.3 ys«Pa
to about 20 but relatively constant from J=15 to J=25. An
19
earlier experiment
had found linewidths increasing significantly with J, (1/2TTT) = 17 MHz/Torr - 130 kHz/Pa for J=16-KL5,
but the reliability of these results has been questioned. 17'18
18
The theoretical model
for predicting linewidth of ground vibra-
tional states has the OCS molecular quadrupole moment as an adjustable parameter which was then determined by fitting predictions to available J=l->-2 linewidth data.
Using this fitted
quadrupole moment, contributions to linewidth were then computed
for dipole-dipole, dipole-quadrupole, quadrupole-dipole, quadrupole-quadrupole, and dispersion interactions.
The calculated
contribution due to dipole-dipole interaction is between 80%
and 90% of the total calculated linweidth.
Dipole-dipole inter-
action appears to have the strongest J dependence which is a 20%
increase as J increases from 0 to 20.
Collisional preference rules 20 have been observed using
Stark-modulated microwave double resonance. For v=0, J<4, OCSOCS collisions tend 21 to induce AJ=+1 changes (dipole allowed).
For v-=l, J^3, the preference 2 2 is for AJ=+1 parity conserving
changes (vibrational angular momentum I also changes).
1.4.
Relation of This Thesis to Previous Work
23
Neither reliable linewidth data
laxation time measurements
states with either:
'
'
nor rotational state re-
appear to be available for
(a) more than two vibrational quanta, or
5
(b) more than eight rotational quanta.
The rotational relaxa-
tion time for such a state is presented in this thesis.
Credible interpretation of the previously mentioned experiments1 '2 dealing with "collisionless" multiphoton dissociation
of OCS, depends upon measured relaxation time for the identified
state first populated by the pump laser.
The first populated
state pumped by P.(24) CO- laser radiation is shown to be (03
J=24).
measured
0,
Note 0310 is Fermi resonant24 with 01 1 ! and 11 1 0. The
pT value (32 ns-Torr = 4.3 ys'Pa) was determined by ob-
serving the 7.4 GHz pulsed g a m (Z doublet transition) as a
function of time for a range of pressures of pure OCS.
1.5.
Selection of Experimental Technique
Infrared-microwave (IR-MW) transient double resonance ex-
periments are attractive because the excitation and diagnosis
g
can both be state specific. Infrared spectroscopic data
can
be used to identify the specific OCS state (03
0, J=24), £
doublet notation as in Ref.25, populated by a C0„ P_(24) laser.
This data is consistent with a still more reliable value
'
the line center frequency of the OCS (01lf0, J=25) •+ (03lf0,
J=24) absorption which is 330 MHz greater than the line center
frequency of the CO- P-(24) laser.28 The rotational distribut
peaks at J=22 so there is a usable fraction (0.0016) of the
molecules normally in the lower state (01
0, J=25) . The
pumped transition is,however, an overtone (weak) and is off-
for
resonance with respect to the C0 2 laser line center.
Excitation
will be dramatically enhanced by using radiation at a frequency
about 330 MHz higher than C0 2 line center.
Because of its gain
bandwidth (^ 2 GHz) and high pulse power, a grating-tuned
transversely-excited atmospheric pressure (TEA) laser was chosen
as the infrared excitation source.
The strongly absorbing rotational transitions (03 0, J=24) -*•
If
03 0, J=23 or 25) could be probed with a 30 0 GHz microwave
source.
Diagnosis by probing the much weaker absorbing I doub-
let transition (03lf0, J=24) •* (03
0, J=24) , was chosen instead
since more microwave circuit components were available at its
frequency27 (7.4252 GHz).
7
II.
IR-MW TRANSIENT MODEL
2.1.
Assumptions
To interpret transient IR-MW signals it is necessary to
29 30
model
'
the molecular system.
It is convenient to model
excitation as the effect a strong, short duration (d<<T) infrared laser "impulse" has on the populations of two states A
and B whose transition frequency is resonant with that of the
infrared laser.
After the laser pulse, the change in microwave
transmission is then attributed to the change in population
difference between states B and C.
It is assumed that the microwave transition is pressure
broadened. Since the Doppler half width 31 .
in OCS is AvD =
-7
8x10 v, pressure broadening is dominant when
T <<
(1/2TTAVD)
= 27 ys
v = 7.4 GHz
There is a velocity dependence
32
of the relaxation rates which
causes a slight time variation of T but this is ignored as a
small effect and T is considered a constant for a given pressure.
If << is interpreted to be a factor of three or more and if
laser pulse duration d = 100 ns
for 300 nsec - T - 9 ys .
this model is appropriate
By truncating the laser pulse and
8
by correcting for non-zero pulse width, a T as short as 100
ns may be measured.
Molecular relaxation times determine the time evolution of
the infrared-induced microwave gain.
Conversely one should be
able to extract information about molecular relaxation times
from the observed microwave gain pulse shape.
The steady state equilibrium microwave absorption is extremely small since the Boltzmann factors are nearly equal for
states B and C.
This is true even before vibration-vibration
and vibration-translation relaxation return excess population
to lower states. The infrared laser pulse can be considered
to suddenly create a microwave inversion by adding a large population to state B. A relaxation matrix treatment 33 and phenomenological equations
lead to very similar descriptions of
the system which can be pictured as a Feynman-Vernon-Hellwarth
(FVH) vector pointing exactly along the population difference
axis immediately after laser impulse.
Although there is a
population inversion, initially there is no polarization, hence
34
no initial gain or dispersion.
Rephrased for emphasis:
in-
stantaneous microwave power absorbed is not proportional to
instantaneous population difference except under steady state
conditions.
In addition to direct relaxation from B to C there
will be relaxation from B and C to other states, reducing the
vector magnitude with time. Two levels m
lation reservoir
is equivalent
contact with a popu-
to a three-level system.
Note this "third" level is not the original laser-absorbing
molecular state, but rather represents a sink for population
lost from the two-level system after termination of laser pulse.
In this experiment, the on-resonance microwave gain is observed.
The meter-kilogram-second (mks) International System
of Units (SI) expression for absortion y(t)
-(t/T1)
-(t/T2)
- e
(m0-l) e
coy A N Q T 2
Y(t) =
o
cTl
1 +
1 - (T2/T1)
where
y(t) < 0 means g a m = \y{t) |
(meter)~
co = angular microwave frequency
(second)~
y = transition matrix element
(coulomb«meter)
e
is
= vacuum permittivity
= 8.85xl0~12 (farad/meter)
c = vacuum speed of light
g
= 3x10 (meter/second)
n = Planck constant/2TT
.-34
(joule-second)
= 1.055x10
10
AN = population difference density, AN < 0 implies gain.
-.3
(meter)
AN- = equilibrium population difference density, by definition ANQ > 0
(meter)"3
_ AN(t=0)
m
m
0
AN^
(dimensionless)
T1 = population difference decay time
(second)
T 2 = polarization decay time
(second)
The above expression was obtained from Equations (28) , (40), and
(241) of Ref.30 by dividing by 4-rre to convert from centimetergram-second (cgs)-Gaussian units, and setting the off-resonance
parameter Aw=0.
It is assumed that laser pumping is an impulse excitation
at t=0.
If microwave probing is weak, AN(t) is assumed to ex-
ponentially decay to ANQ with time constant T-, . ANQ is the
rotational equilibrium value before slower vibration-rotation
relaxation, and |m-| > 1000 for the strong excitation used in
this experiment.
For T, £ T- expressions for the slope, peak amplitude and
its time T
are
11
D^Q- 1 )
dy(t) .
dt
W2/T:1)
l -
where
f "t/T2
coy2AN
D = e c"h
o
fT2l
37
(second*meter)~
(m0-l)
Y(t)
-t/Tl^
Y(T p )| -
DT, 1 +
max
e
-TP/T,
1.
e
"T„/T-^
P 2
1 - (T2/Tj_)
An(T2/T1)
T =
P
T"1
- T"1
For a given m- and T-, larger T. implies that the gain increases longer (T larger) and reaches a larger peak value. To
compare shapes for various ratios (T2/T,) ^ 1, it is necessary
to normalize waveforms both with respect to amplitude and time
to reach peak values.
If 0.5 £ (T-/T,) £ 2, the difference be-
tween any two of these doubly-normalized response curves is less
than 5% of the peak value and the curves are practically indistinguishable.
This lack of pulse shape dependence upon (T-/T..)
is a result of the assumed strong infrared excitation and resonant microwave probing.
Unless T, or T- is known by other
means, only a characteristic time constant T = &n(T2/T,)/
(T7 - Ti ) can be determined.
(T2/T1) a 1, one can treat T
Since it is very likely that
= T, = T_.
12
For T, = T 2 = T
= T,
Y(t) = DT 1 + (m 0 -l)(t/T)e" { t / T )
The pulse shape of this is shown in Figure 1 and expressions
for slope, peak amplitude, and its time T , are given below.
-D
dy(t)
dt
T1=T2=T
i.
-
Y(t)
max
T
P
(m0-D f e -t/T _
- (t/T)e-t/T
T
e
—
DT 1 +
*•
(m--l)e"1
'
= T
At an OCS pressure of 40 Pa (T ~ 100 n s ) , there are 1.7x10 +19
.3
molecules per m in the correct state to absorb infrared radiation but they are distributed over a Doppler profile.
If 1/17
of these are pumped to the upper microwave transition state,
AN 0 (m Q -l) = -10 + l 8 m~ 3 .
The l doublet transition y 2 = 4*10~ 63
2
2
coulomb meter and a microwave probe intensity less than
2
50 kW/m
will affect the I doublet population decay rate less
than 1% for pressures above 3 Pa. Typically a probe power of a few
milliwatts through WR-90 waveguide (cross-sectional area = 2.3
2
-7
cm , effective length = 6 m) would yield a 10
watt increase in
i i
—5 -1
the milliwatt probe, Y _ a v = 2x10
m . At lower pressures
UlclX
13
GAIN
-
1.0
0.8
•
0.6
0.4
0.2
0
(t/Tp)
Figure 1,
Normalized expected microwave gain pulse shapes,
|m | proportional to infrared impulse excitation
strength, mQ=-1000 (top curve) and m-=-30 (lower
curve). If Ti=T-=T, then T =T; if T.^T0, then T =
r *
P
i. z.
p
ln(T2/T1)/(T-1-T2l) .
14
there will be a smaller fraction of the infrared Doppler profile pumped by the laser and the resulting microwave gain will
be lower.
The impulse response of the OCS gas has now been described.
Waveforms actually observed in the laboratory are altered (at
least slightly) by finite bandwidth amplifiers.
This will be
considered later (Section 3.4) by multiplying the gas transfer
function by the instrument transfer function.
citation is assumed, |m 0 -l| >>> 1 and
Y(t) =
_, G _,
T, - T ,
X
e
2
- e
= Gte"^
Y(t)
TX=T2=T
where
Q
=
OV2M0 ™0
eo c TI
(second-meter)
The Laplace transform of Ylt) is
r(a)
^
^
(S + T1"L) (S + T 2 X )
Since strong ex-
15
To see the effect of non-zero pulse duration, it is helpful to look at a lumped circuit system (Figure 2) which has the
same transfer function (Figure 3) and impulse response as the
OCS gas. For the lumped circuit, G = T 7 T I . After the input
pulse, the system in Figure 2 is completely specified by two
variables V,(t) and V 2 (t).
If there is no excitation after t=0,
the system can be described (t>0) in terms of V,(0) and V 2 (0).
VL(t) = V ^ O l e * 1 ^ )
T X
2
V-(t) = V (0) -3T-*
T -T
-(t/T,)
-(t/T-)
x
e
- e
* | if T1?*T2
T
V2(t) = V1(0) (t/T)e-(t/T)
if T ^ T ^ T '
A l l e x c i t a t i o n s l i m i t e d t o t<0 t h a t y i e l d t h e same V,(0)
V 2 ( 0 ) , y i e l d t h e same V 2 ( t ) , t > 0 .
and
These v a l u e s of V, (0) and
V„(0) c o u l d r e s u l t from an impulse of a r e a A o c c u r r i n g a t t = - r ,
+ (T/TL)
A = 11 V ^ O J e
T
=
¥2
T
2"T1
V 2 (0)
In
(Tj1-^1)^
1 - vTToT "
T
-1
i f Tl5*T2
V 2 (0)
T = T
vTToT
i f T 1 =T 2 =T
16
V\A
^/V^
o
6•
+
•
Figure 2. Lumped circuit analog of IR-MW system.
Vx ^ AN; R 2 C 2 =T 2 ; V 2 ++ y.
R 1 C 1 =T 1 ,
17
« * » 3. Transfer f u n o t i o n
m o d e l o £ is_m
syt ^_
18
If V (t) is observed after excitation has ceased, then V_(t)
is just the response to an impulse at t=-T.
Excitation of OCS gas by a laser pulse of non-zero duration
d, yields the same results as true impulse excitation provided
the observation of microwave gain is made after excitation has
ceased.
From a practical standpoint, the pulse duration should
be less than T, and T 2 .
Excessively long pulses lead to (1)
smaller values of AN(0) since relaxation to other levels occurs
during the pulse and (2) significant charge on actual capacitors
in the video signal processing chain (Section 3.4) prior to t =
-T.
The latter complicates waveform interpretation.
Since
population difference density AN is a physical quantity and
the laser impulse is an artifice, the former will be considered
as the system driving function m
section 3.4.
19
III.
EXPERIMENT
3.1.
Concept
The basic idea of the IR-MW experiment is that the micro-
wave transmission through a sample of OCS gas, is changed as the
gas responds to infrared excitation.
Measurement of the mole-
cular response time is the experimental goal and microwave probing
is the diagnostic technique. After excitation has been accomplished, a microwave bridge or interferometer
(Figure 4) can be
used to detect very slight changes in either the amplitude or
phase of the microwave signal passing through the excited gas.
The IR-MW experiment has two functional parts, IR excitation
and MW diagnostics, which will now be discussed in more detail.
3.2.
Excitation Laser
A CO- transversely-excited atmospheric
pressure (TEA) laser
provides the infrared radiation used to pulse excite the OCS gas.
The 3.3 m laser cavity is formed by a 65% reflecting flat mirror and a grating with a 10m radius of curvature.
Inside the
cavity is a 50 cm long TEA gain section and a 190 cm long, low
pressure tube which can be filled with a frequency-selective
absorbing gas. An intracavity iris is used to suppress transverse mode oscillation.
Four KC1 Brewster windows in the cavity
cause the laser output to be linearly polarized.
Perpendicular
orientation of the infrared and microwave electric fields was
£----"-:%
•f
4+
Jm
\l
PS
A
r
;
CW MN
A
- \
ft
ft
» I
O
c
s
SA
C 0 2 TEA
^
Figure 4.
TR MW
Transient IR-MW experimental apparatus. SA, selective absorber; PS, plasma shutter; CW MW, continuous microwave; H and E, magic tee arms; L,
matched load; TR MW, transient microwave bridge
imbalance.
20
21
chosen since then molecules oriented for the strongest infrared absorption are also oriented for the strongest interaction
with the microwave field.
The TEA gain section is pulse excited by triggering a hydrogen thyratron (ITT 7666/Ku73) which discharges a 0.1 yF capacitor charged to 20 kV. Sidewall spark plugs preionize the discharge mixture containing a trace amount of Ferrocene vapor.
The main discharge is across the 2 cm gap between 3.5 cm wide
electrodes shaped to give a uniform discharge. A nitrogen-lean
mixture of CO-, N- and He (6:2:11) is used to obtain a tailless
pulse envelope.
The frequency-selective intracavity cell is used to slightly
alter the frequency content of the laser pulse in such a way
that a larger percentage of the OCS molecules are excited.
As
was previously mentioned, effective excitation is hindered by
a 330 MHz mismatch between the line center frequencies of laser
emission and OCS absorption which is inhomogeneously (Doppler)
broadened with full width at half maximum of 50 MHz. Both of
these spectral characteristics are especially troublesome for
excitation of OCS at low pressures. Low pressures would otherwise be desirable for measuring relaxation times.
A single frequency laser tuned to the correct frequency can
fully excite a homogeneously broadened absorption.
However,
reproducible fine frequency tuning of a CO_ single frequency
22
38
laser is not trivial.
Without tuning, an arbitrarily power-
ful C0 2 laser could fully excite even the inhomogeneously broadened absorption.
Another alternative to either a tunable single
frequency laser or arbitrarily powerful laser is a laser with a
broad spectral output which matches or overlaps the inhomogeneously broadened absorption.
A narrow band (50 MHz) noise
source, centered at the OCS absorption frequency would be ideal.
This could be conceptually realized by using an RF noise signal
to control an infrared modulator of the CO- laser beam.
One simple excitation source which was used successfully is
a multimode C0 2 TEA laser with an estimated emission bandwidth
of 600 MHz.
The longitudinal mode interval was 45 MHz so at
least one longtiudinal mode frequency would be within the OCS
Doppler profile.
The energy in the many other modes did not
excite the OCS and the one or two effective laser modes only
excited portions of the OCS Doppler profile.
Excitation of
the OCS gas was improved by adding an intracavity, narrow band,
saturable absorber which tends to suppress laser oscillation of
modes with frequencies which are too low to match that of the
OCS absorption. This is a selective gain discrimination tech39
40 41
nique.
Ammonia (NH3) gas has an absorption '
line
1
v2s(4,3) •*• 2v2a(5,3) at 1043.16 cm" which is ^300 MHz lower
28
in frequency than C0 2 line center
and was used in this experiment as an intracavity absorber. During laser oscillation
23
build-up, modes with frequencies above line center experience
higher round trip gain than that of lower frequency modes, therefore the peak of laser spectral brightness is shifted to more
closely match the OCS absorption.
As used in the IR-MW experiment, the intracavity saturable
absorber also modulates the laser output to obtain the desired
spectral broadness. As the laser intensity increases, the NHabsorption becomes saturated or bleaches. This lowers the
cavity loss and alters the NH, index of refraction.
The reduced
cavity loss makes possible higher instantaneous power.
of refraction change, sweeps the mode frequencies.
The index
The net re-
sult is a better frequency spectrum overlap between the CO- laser
and the OCS absorption.
It is possible to obtain a train of
short duration (< 5 ns) pulses with 1.3 kPa of NH 3 m
the in-
tracavity cell, however, larger IR-MW signals and a larger energy
deposited in the OCS were obtained with 480 Pa of NH 3 in the intracavity cell.
Typical laser pulses are shown in Figure 5.
The relative energy deposited in OCS was roughly determined by observing the size of the signal from a microphone in
a side arm of a 2.5 cm diameter, 30 cm long Pyrex sample cell
with KC1 Brewster windows.
This spectrophone cell, placed out-
side the CO- laser cavity and filled to a static OCS pressure
of 5 to 40 Pa, was used only to verify excitation of the
OCS gas and was not used during the IR-MW experiment.
24
100 n s / d i v .
mm minM
1 1 »f
50
ns/div.
mam
20 n s / d i v .
Figure 5. C0 2 laser pulse, not truncated, 480 Pa NH3 intracavity. Photon drag detector output amplified by a
5 kHz to 500 MHz amplifier and displayed on a 350 MHz
oscilloscope.
25
A Plasma shutter 42 '43 located outside the laser cavity, was
used to truncate the laser pulse.
The shutter was a confocal
arrangement of two f/2.5 antireflection-coated germanium lenses.
Clean nitrogen (N2) gas flowed through the focal region and
breakdown intensity could be adjusted by varying N- pressure.
During IR-MW experiments the shutter was operated without external ultraviolet ionization.
The resulting multimode pulse
envelope, shown in Figure 6, had a fall time less than a few
ns
and pulse energy of 25 to 50 millijoules.
During plasma
formation in the plasma shutter, in addition to amplitude modulation of the beam there is frequency or phase modulation 42 due
to the rapidly changing index of refraction in the plasma.
This
resulting spectral broadening further enhances the effectiveness
of the excitation of OCS, but the enhancement is small compared
to that due to the intracavity NH3 absorber.
The plasma shutter fully blocks the C0 2 laser pulse only
while laser intensity is high enough to maintain the plasma and
for a short time
('v 1 ys) thereafter.
If a nitrogen-rich mix-
ture is used in the TEA laser, a low intensity pulse tail persists for longer than the shutter blocking time.
This trans-
mitted tail appears in the form of a low intensity secondary
pulse which causes a secondary IR-MW signal during the decay
of the first IR-MW signal.
While carefully controlled pulse se-
quencies are sometimes intentionally used,
the secondary IR
26
ESSilUilliillUC
I • • • • 4
HMIfMViWUI
gj
Figure 6.
a H
i
a H I M H V V H
i
a H H
ll
H H I i a H
l
C0 2 TEA laser pulses (20 ns/div.) like Figure 5
except plasma shutter used to truncate pulses to less
than 100 nsec duration.
27
pulse was not desired in this experiment, so the nitrogen-lean
mixture previously mentioned was used.
3.3.
Diagnostic Microwave Bridge
As has been discussed, the expected size of the 7.43 GHz
absorption is very small relative to the microwave probing power.
A microwave bridge31 '44 '45 can be adjusted to separate the small
transient imbalance signal from the large probe power.
Signals
due to absorption or phase shift can be obtained independently
by operating with a large amplitude or phase imbalance respec44
tively.
Provisions for such adjustments and other construction details are explained in the appendix.
To keep the infrared excitation as strong as possible, it
is desirable to use waveguide with a small cross section.
Since
only one dimension of a rectangular cross section need be larger
than one-half of a free space wavelength, in principle the other
dimension could be very small provided that relaxation by collisions with walls is negligible.
For the convenience of the
experimenter, available WR90 waveguide was used. Its relatively
2
small cross-section (2.3 cm ) keeps the infrared intensity
reasonably high for all gas molecules in the waveguide.
The
laser pulse and singly-distilled OCS (Dry Ice-acetone trap)
were admitted into the three meter length of waveguide via a
pierced E plane bend.
Through another such bend, the transmitted
laser pulse could be monitored and the waveguide evacuated.
28
Static OCS pressure was measured with a capacitance manometer.
Although WR-90 is usually used with frequencies above 8.2 GHz,
less than 5% attenuation per meter is expected at 7.43 GHz with
ideal copper walls.
The WR-90 cut-off frequency is 6.56 GHz.
The 7.43 GHz guide wavelength A
= 8.6 cm which is 2.12 times
the free space wavelength A- = 4.04 cm.
This means that a micro-
wave probe perturbation propagates at slightly less than half
the velocity of the laser pulse. The slow microwave propagation
can be considered as a superposition of two repeatedly crossing
plane waves reflecting from sidewall to sidewall. The effective microwave path length is (^CTMn) times the length of the
waveguide section or the gas effective attenuation constant is
(Ay /A-)
times larger. 44 The difference in infrared and microu
wave propagation times becomes significant for long interaction
lengths.
In a three-meter length of WR-90 waveguide, an ideal
infrared impulse copropagating with a 7.43 GHz wave
11 ns
aneous.
causes an
microwave perturbation even if gas response is instantThis 11 ns
storage time is negligible compared to
the gas relaxation times measured with this configuration (Figure 4).
If the laser pulse propagation direction were reversed,
the storage time would almost triple to 31 ns. Longer storage times are possible for other IR-MW configurations having
longer effective microwave absorption pathlengths.
Two of
these configurations were used to observe IR-MW double resonance
29
signals although relaxation times were not measured.
First, a
slotted line detector was used to observe laser induced perturbation of a microwave standing wave minimum.
Second, the tran-
sient imbalance of an equal arm reflection bridge was observed.
Compared to the transmission bridge,the other two arrangements
had twice the IR-MW interaction length and four times the storage time.
Construction details of the microwave transmission
bridge which was used to measure relaxation times are in the
appendix.
3.4.
Signal Processing
Signal processing bandwidth must be adequate to avoid loss
of information but not excessive since unnecessary noise is then
included.
For reasonable reproduction of the microwave transient,
it is necessary for the video processing risetime to be less
than about 1/2 of the observed transient risetime.
For the
video processing system used in this experiment, measurements
were limited to pressures £ 40 Pa.
The microwave bridge was adjusted for a slight excess of
power through the unperturbed arm.
The transient part of the
imbalance microwave envelope, was then proportional to the microwave transient absorption (- gam) . The microwave imbalance
(constant level plus transient part) was amplified (^ 30 dB)
by a Watkins Johnson WJ 5310-514, 6 dB noise figure amplifier.
The amplified microwave imbalance was then envelope-detected
30
with a Microwave Associates MA400 54 diode in a Hewlett Packard
440A coaxial detector mount.
The transient video signal was
amplified (^ 33 dB) by a Texas Instruments TIEF 152 amplifier
with 16 KHz to 18 MHz passband and a 125 £2 input impedance.
The
overall video high frequency cutoff was set by the prefilter
and preamp of a PAR 160 boxcar integrator not otherwise used.
The data reported here was taken with a prefilter setting of 10
MHz.
The output of the 160 preamp via a 30 m long RG58 delay
line, was displayed on a Tektronix 485 oscilloscope and recorded
photographically.
The measured risetime (Figure 7) of the video
signal processing system (TIEF 152, dc return, 160 prefilter
and preamp, delay line, and oscilloscope) was 75 ns.
The
risetime of the detector, in detector mount loaded with TIEF 152,
was ^ 15 ns .
The
microwave bridge storage time previously
discussed, was 11 ns.
Since the squares of cascaded stage
risetimes are added to obtain the square of the overall risetime, 46 the detector and storage time contributed negligibly
(~ 2 ns ) to
the overall measurement system risetime which is
therefore assumed to be the same as the measured video system.
The low frequency cutoff, determined by TIEF 152 and dc
46
current return, leads to a sag
step response.
the 75 ns
or droop (Figure 7) m
the
The effect of this sag (10% in 0.6 ys) and
risetime
has been included (Figure 8) as a modifi-
cation of the expected pulse shape.
sponse is expected to be
The displayed impulse re-
31
500 ns/div.
50 ns/div.
Figure 7.
Step response of video processing system. Droop or
sag = 10% in 600 ns and risetime = 75 ns, alternately T
T-3 = 5700 ns and T 4 = 45 ns.
32
OCS
ENSEMBLE
PHASING
VIDEO
FREQUENCY
LIMITS
LOW
HIGH
AN
m,
Figure 8. Transfer function model of experiment with exponentially decaying population difference density AN as
the forcing function. D is a transition strength
factor defined in section 2.1. G3 is a detection
and amplification factor. T3 = 5.7 ys and T4 =
45 ns determine the video sag and risetime.
33
f (t) = m 0 D G 3 T 4 1
-j£
-t^l
-1 mT-L /m-lT m -lTv/m-l(Tm -l, Te
<V - l > <3 " l > 4 " l>
- T"1
-t/T,
e
(TT^-T"1) ( T ^ - T " 1 ) (TT1-^1)
- Ti 1
"t/T3
(T'1-!"1) (T^-T"1) (T"1-^1)
^
(T
1 1 " T 4 1 ) < T 2 1 " T 4 1 ) (Ta 1 -^ 1 )
"t/T4l
e
where T, and T. have been determined to be 5.7 ys and 4 5ns,
respectively.
If either T-^ or T 2 were significantly different from the
other, the ratio of falltime to risetime would be noticeably
larger than if T, z T 2 .
If T, and T 2 are within a factor of
two of each other, the difference in pulse shape from T.=T_
is slight.
The approach taken here is to assume T.. z T_ z T
and compare the expected waveform with that actually displayed.
If a T can be found to give a reasonable fit to a displayed
experimental waveform then T, * T 2 « T is a reasonable conelusion. The expected display waveform 47 for T,=T2=T is:
34
f (t) = m Q D G 3 T-1
4
-1
-t/T
- T
t e
1
-1
[(T^-T" ) (T^-T )
(T^-T -1 ) 2 (T^-T -1 )2
-1
- T
-t/T;
(T'1-T31)2(TT1-T31)
-1
- T4
+
(T'1-T41)2(T31-T~1)
-t/T,
where T- = 5.7 ys and T. = 45 ns.
The displayed waveforms were recorded photographically
and have noise (±5% to ±10% of peak signal).
Values of T were
obtained from the displayed waveform by relating (Figure 9) T
to the pulse width at half maximum.
For a given percentage of
uncertainty in pulse width, the percentage uncertainty in T
is smallest for values along maximum slope portion of Figure 9
curve.
This and the earlier mentioned assumption that T £ laser
pulse duration, mean that pressures should be low enough that
T 2 100 ns.
35
PULSE
WIDTH
(ns)
2000
1000 500 300 200 100
X
50
100
200 300 500 1000 T
(ns)
Figure 9.
Numerically evaluated expected pulse width at
half maximum, T,=T2=T, T3=5.7 ys, T.=45 ns.
36
IV.
RESULTS
Transient infrared microwave double resonance signals
were observed over a range of pressures to determine (pT) =
32 ns»Torr = 43 ys«Pa.
Three example waveforms are shown in
Figure 10. The top one (with less than typical noise) compares
favorably with an expected waveform assuming a T,=T2=T model.
The sum of the squares of the differences between expected and
obtained values (greater than 1 division) is less than the sum
of the squares of the uncertainties of the obtained values.
In other words, the experimental waveforms can be adequately
modeled assuming T,=T2=T.
The pulse width at half maximum was
determined from several (2 to 6) waveforms for each of 8 pure
OCS pressures over the range of 5 to 40 Pa (40 to 300 mTorr).
These pulse widths were then converted (Figure 9) to values of
T.
The dependence of relaxation time (T) upon OCS pressure is
shown in Figure 11. The straight line is a least squares fit
to the values of T~
pressure.
slope (pT)
ys*Pa.
obtained for a pulse width average at each
The straight line has intercept T-
=0.62 MHz and
= 0.0309 MHz/mTorr, or (pT) = 32 ns«Torr = 4.3
37
T
Figure 10.
1
1
1
1
1
I
1""1 '"I
1
Transient IR-MW waveforms, all 200 ns per hor. div.
Top Three: enlarged, inverted tracing of oscilloscope display, 0.13 Torr (17 Pa) . Bottom: expected
waveform, T..=T2=T=210 ns, impulse excitation 0.52
div. to right of vertical (gain)axis, circles are
vertically offset sampled points from top waveform.
38
10
0
Figure 11,
20
30
0.3
(Torr)
-J
40
(Pa)
Relaxation rate (T~ ) vs. OCS pressure (p).
± one std. dev., six pulse width values
I
three nearly equal data points
©
single data point
double data point
x
triple data point.
o
®
39
V.
DISCUSSION
It appears (pT ) = (pTj = 32 ns-Torr =4.3 ys-Pa ade-
quately describe the relaxation of OCS molecules excited to
(03
0,J=24).
These "hot" molecules behave very much like
"cold" v=0, J=0,l,2 molecules.
Above some level of excitation
or internal energy, the relaxation behavior may change, but
this demarcation is above v-=3, J=24, for OCS. Often the analysis of quantum electronic systems (e.g., multiphoton-multilevel) is so complicated that only a single relaxation parameter T is used for all levels.
It is reassuring to find this
is indeed a reasonable procedure for one reasonabaly highly excited molecule.
Since the relaxation times appear to be the
same for both hot and cold OCS molecules, if emission is obtained between two such states, the tunability and spectral
purity will be approximately the same as between cold states.
Since there are many more hot states, the number of possible frequencies is much larger.
The observed collisional relaxation time for the v=3, J=24
state was slightly larger (20%) than typical literature 12 '13'14
values for v=0, J=0,l,2 states.
However since this experiment
is measuring a composite parameter T rather than T-^ or T 2 individually and since the experimental apparatus could not be
calibrated using v=0, J=0,l,2 states, the 20% difference could
be attributed to differences in the measurement techniques or
40
experimental error.
(03
The collisional relaxation rate for the
0, J=24) state is therefore considered not significantly
different from that of v=0, J=0,l,2 states. This conclusion
16
is in agreement with the claim
that several vibrational quanta
do not affect linewidth, although this claim was based on a
study of only states (v3=l and v2=2°) with E vibrational sym24
metry,
which is the same as that of nearly all of the collision partners. The 03 0 state has II vibrational symmetry. The
results of this thesis conflict with early linewidth measure19
17 18
ments
for J=15-*-16 but support the conclusion
'
that the
presence of rotational excitation does not significantly enhance rotational relaxation.
41
APPENDIX:
MICROWAVE BRIDGE DETAILS
The overall experimental configuration has been shown
in Figure 4.
This appendix contains additional details which
may be helpful to persons duplicating or extending the measurements presented in this thesis.
The bridge (Figure 12) was used
to increase the ratio of transient signal to continuous microwave power at the detector, D.
The microwave probing radiation was provided by a Yittrium
Iron Garnet (YIG) tuned Gallium Arsenide Bulk-Effect (Gunn)
Diode.
This Watkins-Johnson oscillator WJ-5008XX,provided a
maximum of 8 milliwatts of 7.4 GHz power to the input magic tee
of the bridge via:
a 3 dB attenuator (Narda micro-pad 4778-3),
a dc block (Sage 5504), 8 cm of RG55 cable, a UC 397/U coax to
waveguide adapter, a WR112 to WR90 taper, an isolator (Ferrotec
I-155-L) , a rotary waveguide switch (FXR X641A) , and a 0 ->• 100
dB attenuator (Dymec DY5029) .
Typically the 0 > 20 dB probe attenuator, PA, (FXR X155A)
was set to 1 dB and the probe power through the OCS gas was
about 3 milliwatts.
The 0 H- 50 dB reference attenuator, RA,
(HP X382A) and reference phase shifter, $, (Lectronic Research
Labs DBG-915) were adjusted for minimum power out of the difference port (E arm) of the output magic tee, OMT.
The bridge
was constructed such that the reference and probe electrical
path lengths were equal.
Each magic tee, impedance matched
(3H3~®:
YO
42
x<
1A
PA v
r ^4—^C
EB
EB
Figure 12.
J
M
H
IMT
H OMT
Microwave b r i d g e . Dashed r e c t a n g l e i n d i c a t e s portion containing OCS, key: D, d e t e c t o r ; DR, dc r e t u r n ; E, difference port of magic t e e ; EB, E plane
bend; H, sum p o r t of magic t e e ; I , i s o l a t o r ; IA,
input a t t e n u a t o r ; IMT, input magic t e e ; M, modulat o r ; MA, microwave amplifier; OMT, output magic
t e e ; PA probe a t t e n u a t o r ; $ reference phase
s h i f t e r ; RA, reference a t t e n u a t o r ; S, waveguide
switch; VA, video a m p l i f i e r ; YO, YIG-tuned o s c i l l a t o r ; Z-, matched load.
43
at 7.4 GHz, was really a Technicraft TR 641230 E-H hybrid tee
with no matching elements at the junction, but with a fourscrew tuner on the E arm and a slide screw tuner on the H arm.
The output magic tee, OMT, included a 50 ym thick Mylar
gasket between the TR 641230 and the four-screw tuner.
This
ground loop breaking device, shunted by 1 Mil, was installed
as a precaution against electrical interference. The 7.4 GHz
isolation between the side arms of these custom matched tees
was greater than 30 dB.
RA and <f> could be conveniently ad-
justed such that the power out of the E arm of OMT, was 70 dB'
below the 3 milliwatt power passing through the OCS gas. This
balancing was done after the YIG-tuned oscillator frequency
was selected and the probe arm was filled to the desired static
OCS pressure.
To simplify waveform interpretation it is desirable for the
transient part (video) of the detector output to be linearly
proportional to the microwave transient gam.
This was ac-
complished by reducing RA until the detector rectified current
increased to 600 yA.
The dc return, DR, was a 1 mH choke
(14ftresistance) plus a 5 Q current sensing resistor which was
shorted during pulse observation.
The system noise figure is
relatively insensitive to the amount of rectified detector current, provided the microwave amplifier, MA, is not overloaded.
MA, via a coax to waveguide adapter with a sliding short, presented a matched load to the E arm of OMT.
44
The entire signal processing chain could be tested by
using modulator, M, to generate a transient bridge imbalance.
The modulator was a 30 dB cross guide coupler with a sliding
short and broadband detector mount on opposite ends of the
cross.
A test modulation signal was applied to a 1N415C diode
in the detector mount.
This slightly altered the transmission
through the main coupler arm.
The OCS gas was confined to the pierced E plane bends (EB)
and the three-meter length of waveguide, by 50 ym thick
Mylar epoxied to EB flanges adjacent to PA and M.
The bend
piercing holes (6.4 mm diameter) were fitted with tubes
whose axes matched that of the straight waveguide section.
45
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50
VITA
John William Leap was born November 3, 1947 in Louisville,
Kentucky.
In 1969 Purdue University awarded him the degree
of Bachelor of Science in Electrical Engineering with distinction and awarded him a certificate of completion of the Research and Development Option in Electrical Engineering.
In
1970 Purdue University also awarded him the degree of Master
of Science in Electrical Engineering.
While attending Purdue
University, he was an undergraduate research assistant and
graduate teaching assistant m
the School of Electrical
Engineering and a graduate research assistant m
the School
of Material Science and Metallurgical Engineering.
He served
in the United States Navy Civil Engineer Corps, specializing
in public works management. While employed by Intel Corporation, he worked to improve integrated circuit reliability.
While employed by Varian Associates, he worked on development
of nuclear magnetic resonance spectrometer equipment.
At the
University of Illinois, he has been a graduate research
assistant in the Electro-Physics Laboratory of the Department
of Electrical Engineering.
He has presented research results
at the 1978 IEEE MTT-S International Microwave Symposium:
Paper C6.1, "Two-Photon Pumping of a Four-Level System in
Ammonia to Obtain 12.16 ym Radiation for Isotope Separation",
features of which were also reported as part of "Hot Band Lasing
in NH 3 ", IEEE J. Quan. Elec. QE-15, 838 (1979).
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