Intense blue-green fluorescence from a structured band centered at lambda approximately 484 nm was observed from Ar, CF3I and NF3 gas mixtures excited by an electron beam. This emission was tentatively assigned to the E yields A transition of the iodine monofluoride (IF) molecule. The fluorescence efficiency of the IF(E yields A) band and the IF (E) state radiative lifetime were estimated to be approximately 6% and 15 ns, respectively. The emission band structure, the short IF(E) radiative lifetime and the Franck-Condon shift between the E and A states suggest that IF is an attractive candidate for a blue-green laser

Eden, J. G.

Verdeyen, J. T.

English

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https://ntrs.nasa.gov/search.jsp?R=19800011200 2020-03-21T19:44:04+00:00Z
(NASA-CR-162618) LASING CHARACTERISTICS Of	 N80-19480
GAS MIXTURES INVOLVING UFG: APPLICATION TO
NUCLEAR PUMPING OF LASERS final Report
(Illinois Univ.) 19 F HC A02/MF A01
	 Unclas
CSCL 20E G3/36 47512
Final Report
NASA Grant NSG 1609
Lasing Characteristics of Gas :fixtures Involving UF6:
Application to Nuclear Pumping of Lasers
Prepared fur
National Aeronautics and Space Administration
Langley Research Center, Hampton, VA 23665
26 ;larch 1980
r
1
i
by
J.T. Verdeyen
	 'V
J.G. Eden
Q
Gaseous Electronics Laboratory
Department of Electrical Engineering
University of Illinois at Urbana-Champaign
Urbana, Illinois 61801
Our research efforts during this past year of NASA
sponsorship have concentrated on identifying atoms or molecules
that are attractive as potential nuclear-pumped lasers. These studies
*
have led to the examination of the iodine-monofluoride (IF ) molecule.
Specifically, as a direct result of NASA funding, the following
accomplishments have been realized in the past year:
*
1) The blue-green emission spectrum of IF in a-beam
excited Ar, NF  and CF 3 I mixtures has been observed and
assigned to the E -; A3 II 1 transition.
2) From fluorescence measurements, the radiative lifetime of
the excited molecule has been estimated to be ti 15 ns.
*
3) The IF kinetic formation chain has been determined
to be: Ar -► Art + I -> IF , where * indicates an
electronically excited state of the atom or molecule
Indicated
and
*	 *
4) The rate constants for quenching of I and IC'.
by various atoms and molecules (including UF 6 ) have
been measured.
Simply stated, IF appears to be a very attractive choice
for a laser to be pumped by the gaseous core reactor. For instance,
the E -► A3 IT1
 band of IF lies in the blue-green and,therefore, UF6
*
ground state absorption will be negligible. Also, quenching of IF
by UF6
 is sufficiently small that a potential IF laser would be
compatible with ti 1-10 Torr of UF 6 . Also, since IF contains one
fluorine atom, then UF6 can be used to form the laser's upper state.
1
Finally, the highly structured nature of the IF blue-green
emission ')and, the short (ti 15 ns) IF* radiative lifetime and the
Franck-Condon shift between the IF E and A J II I states all indicate that
the small signal optical gain from 	 IF should be large.
In the last two months, a research group at Garching
has succeeded in obtaining stimulated emission from IF. Lasing occurs
at a - 491 nm. It is our intention of continuing the study of IF
and, in particular: 1) to measure the pump power threshold for
the laser and 2) determine the efficiency of an IF oscillator
or the extraction efficiency of a dye-laser saturated IF amplifier.
The experimental results obtained during this past year
are presently being written up for publication. However, the highlights
of this experimental effort are summarized in the attached paper
which has been submitted for publication.
2
gh
IODINE-E10NOFLUORIDE (IF) EMISSION SPECTRUM AND
FORMATION KINETICS IN ELECTRON BEAM PRODUCED PLASMAS*
S. B. Hutchison, J. G. Eden and J. T. Verdeyen
Department of Electrical Engineering
University of Illinois
Urbana, Illinois 61801
ABSTRACT
Intense blue-green fluorescence from a structured band cat
at A % 484 nm has been observed from Ar, CF 3I and NF3 gas mixtc
(T - 300'1) excited by a ,%, 3 no FM electron beam. This emission is
tentatively assigned to the E -• A3w l transition of the iodine-
monofluoride (IF) molecule. From the temporal behavior of the IF * and
I2 (342 nm) emission, it has been determined that the IF(E) level is
formed primarily by excitation transfer from I* ( 4P) states. The
fluorescence efficiency of the IF(E ♦ A) band and-the IF*
 (E) state
radiative lifetime have been estimated to be ti 6% (in 99.83% Ar/.13%
luCF3 I /.047. NF3
 mixtures, PTOTAL ti 2300 Torr) and ti 15 ns, respec-
tively. The emission band structure, the short IF(E) radiative life-
time and the Franck-Condon shift between the E and A states suggest
that IF* is an attractive candidate for a blue-green laser.
*Work supported in part by the NASA Langley Research Center under
Contract No. NSG 1609.
The spectroscopic properties of the diatomic halogen molecules
have been studied extensively for several decades and since 1975, sti-
mulated emission has been obtained from each member of this molecular
family (I2 , Br2, CE2 and F2).l
In contrast, although numerous studies of the low-lying A-1 rl and
B3 r0 states of the interhalogen molecules (BrF, ICE, etc.) have been
conducted, 2-5
 for many of these molecules little detailed information
regarding higher excited states is available. Consequently, only
recently have the interhalogens received attention as W and visible
laser candidates.
Huestis and co-workers 6
 have observed strong fluorescence bands
(from electron beam (a-beam) excited Ar and ICI or IBr gas mixtures)
at 385 and 430 nm which were attributed to ionic to covalent transi-
tions of IBr and ICI, respectively. Optical gain was observed on the
blue ICI band but, due to ground state ICI absorption, 6 , 7
 oscillation
was not obtained. Diegelmann at a1 8 subsequently reported lasing at
285 nm from the CEF molecule when formed in a-beam excited Ne/F2/Cl2
gas mixtures. However, the identity of the states responsible for
these a-beam pumped ICE, IBr or CIF emission bands remains unclear.
The observation of intense blue-green spontaneous Emission from
e-beam excited Ar/CF3 I/NF3
 gas mixtures is described here. To our
- knowledge, this emission band, which peaks near A ti 484 nm, has not
been reported previougly9-11 and is tentatively assigned to the E - ►
AN 1
 transition of the iodine-monofluoride (IF) molecule. Both CF3I
and CH3I have been utilized as iodine donors to minimize absorption in
the 450-500 nm spectral region.12-14
The experimental apparatus used in these studies consisted simply
2
of a Febetron 706 a -beam generator, an optical call and gas handling
equipment and a detection system. After traversing &-%, 1 cm thick
atmospheric drift region, an intense (ti 1 kA-cm-2),,%, 3 no FMI beam
of 600 keV electrons irradiated a mixture of Ar, CF3I and NF3 con-
tained in a stainless-steel cell. The emission from the excited gases
was viewed transverse to the a-beam axis by a 0.6 m spectrograph (in
first order) and Polaroid Type 57 (or Kodak Tri-X) film and by a
biplanar photodiods with an S-20 surface. Bandpass or calibrated
neutral density filers were placed in the optical path to prevent
saturation of the photodiode and to isolate the desired spectral
region. Research grade rare gases and technical grade CF 3I, NF3 and
F2 (as supplied by the manufacturer) were used in these experiments.
Passivation of the a-beam cell and gas handling system was
accomplished by allowing"- 50 Torr of F2 to stand in the cell for ti
0.5 h. Finally, calibration of the observed spectra was afforded by a
Hg pen lamp.
Upon exciting 2000-4000 Torr Ar, 3 Torr CF3I and 1 Torr NF3 gas
mixtures with the a-beam, the intense blue-green emission band shown
In Fig. 1 was observed. Vibrational structure in the band is evident
and maximum emission occurs for X 1%, 484 nm but the fluorescence
extends to wavelengths < 430 nm. When either the NF3 or CF3 I was
removed from the gas mixture, the band of Fig. 1 vanished. Also, the
same spectrum was recorded (although with diminished intensity) if NF3
was replaced by F2
 or if He or Ne was used as the mixture diluent.
Therefore, this blue-green band is attributed to the diatomic iodine-
monofluoride molecule (IF) and is the longest wavelength diatomic
halogen emission yet observed for relativistic electron beam excita-
3
tion. Table I shows that the ti 484 nm peak wavelength of the IF
spectrum is consistent with the known wavelensths M of the other
iodine-monohalide emission bands (e-beam produced).
In order to identify the electronic states responsible for the
484 nn emission, an analysis of the Ia and IBr spectra (from a-beam
irradiated Ar, CF 3I and CA 2 or IBr gas mixtures) was undertaken. This
approach was adopted simply because the excited states of ICA and IBr
are better known than those of IF. For ICA, a close coincidence was
found to exist between the measured emission spectrum and
V 1 - 0,1 - v" - 3-6 transitions of the ICI (E-0 Pff 1) band, as cal-
culated from the spectroscopic constants and vibrational transition
assignments given by Rosen. 3
 The peak IC1 emission at 433 nm was
identified as the (0,4) transition of the E - A 3rr 1 band. A similar
analysis of the a-beam excited IBr band led to an analogous
conclusion: that the band peak at a 'L 356 nm corresponds to the (0,4)
transition of the D -* A 37► 1 band. 15 Neither the ICA nor IBr spectra
could be explained by assuming Bn 0 to be the lower energy state,8
a result which may bear on attempts to identify the origin of the 342
nm band of I2.16,17
Preliminary analysis 18 of the wavelength separation between the
local maxima of the IF* emission spectrum (Fig. 1) suggests that we
(upper level) ti 250 cm-1 . Hence if the emission band peak at A ", 484
nm is assigned to the E i A (0,4) transition (consistent with the ICA
and IBr spectra), then Te(E) ti 37967 cm 1 . Table I shows that these
rough estimates of Te
 and we are in agreement with the trends
established by 12, IBr and Ia . Moreover, if we and T. of the E state
are taken to be those estimated here, then the vibrational lines in
4
the IF* spectrum apparently originate from the v' - 0,1 levels of the
E state and terminate on v" - 3-6 of the A3% level.
In addition to the IF band, the only emission detected (from the
Ar/CF3 I/NF3
 plasmas) between 250 and 650 nm was due to I2* (342 um).
So, in order to investigate the formation kinetics of the IF*
molecule, the temporal behavior of both the I2* and IF* fluorescence
was recorded using bandpass filters (I2"max - 18% at A O - 352 nm,
NA - 12 nm FWWI; IF* :Tm&x - 70% at A 0 - 495 nm, AA - 38 nm FWWO .
Over a wide range of gas mixture compositions, the time decay of
IF*
 and I2*
 waveforms (for an example, see Fig. 2) was found to be
identical to within experimental error. This suggests that these
excited molecules are fed by a common precursor. A similar conclusion ►
was reached in ref. 6 from experiments involving Ar/IQ plasmas.
Partial energy level diagrams for Ar, I, I 2 and IF are illustrated in
Fig. 3. The I 2
 states were redrawn from refs. 16 and 19 and the IF (X
and A) levels were taken from the paper by Birks, Gabelnick and
Johnston. 11
 The E state of IF is included simply to indicate its
position relative to the I 2*
 (1432, -3w 2g) state but its equilibrium
radius is probably slightly larger than shown in the Figure. Clearly,
at least one of the I * ( 4P) levels has sufficient energy to simulta-
neously break either the CF3-I or NF2-F bonds (2.32 and 2.52 eV,
respectively) and populate low vibrational levels of I 2*
 or IF
Consequently, the kinetic scheme for IF *
 production is expected to be
analogous to that proposed by Huestis at al for IC1. 6
 That is:
5
k1Ar*
 (or Ar *2) + CF3I ♦ I* ( 4P) + CF3	(1)
and
k2• NF3 ♦ IF* ( E) + NF2	(2)
I* ( 4P) +
k3
• CF3I ♦ I2* (1432, 3 a29 ) + CF3,	 (3)
where Ar*
 represents one of the low-lying Ar ( 3P) states and Art* is
the metastable 3 E u excimer state.
Figure 4 shows the dependence of the time and wavelength inte-
grated IF*
 fluorescence on the argon partial pressure for Ar, 2 Torr
CF3I and 1 Torr NF3 gas mixtures. Initially, the integrated spontane-
ous emission increases as ti ^^=^ which suggests that Ar t*
 may be
primarily responsible for I *
 formation in Eqn. (1). Above ti 2300
Torr, the IF*
 fluorescence levels off due to increased Ar quenching of
the molecule. Here, it should be noted that the internal energy of
the Ar t ( 3I) molecule is ti 1000 cm-1 in excess of that required to
break the CF3-I bond and subsequently excite I to the 6s4P1/ 2 or 3/2
levels. However, Ar* ( 3P) atoms are capable of populating I* states
ti 3
. 104 cm-1 above the 4p levels. Also, the Art* excimer formation
process, Ar* ( 3P) + 2 Ar + Ar2* ( 3I) + Ar, competes with Eqn. (1) for
the available Ar* ( 3P) atoms. Taking the Ar 2* formation rate 20 to be
1.7 - 10-32 cm6
-s
-1 
- [Ar1 2 and letting kl - 4.7 - 10-10 cm3_8-121
then for 3 Torr CF3 1 and PAr > 1700 Torr, Ar 2* formation is the domi-
nant Ar * ( 3p) loss mechanises. The apparent conclusion, then, is that
for the high Ar pressure gas mixtures characteristic of these experi-
ments, the Ar 2*
 molecule, rather than an Ar * ( 3P) atom, is the imme-
6
_j
diate precursor to the I * ( 4p) state.
From the single exponential decay of the IF *
 waveforms from low
pressure (PTOTAL ti 10 Torr) CF3 I/NF3 gas mixtures, the IF * (E) state
radiative lifetime has been estimated to be a 15 no. 22 This result is
consistent with the % 7 no lifetime for the upper state of the I 2 342
nm band. 19 , 23
 Therefore, the waveforms shown in Fig. 3 (for Ar, CF3I
and NF3 mixtures) reflect the quenching of I *
 by CF3I or NF3.
Measurements of the rate constants for co11iaiona1 quenching of I *
 by
various atoms and molecules and the IF* radiative lifetime will be
described elsewhere.22
Finally, by comparing the time-integrated IF *
 blue-green fluores-
cence to the 112 (C- B) emision from Ar/5r 1:2 gas mixtures, 19
 the
fluorescence efficiency of the IF* (E + A 3n l ) band in a-beam produced
2300 Torr Ar/3 Torr CF3 I/l Torr NF3 mixtures was found to be ti 6%.
Also, measurements of the branching ratio for IF* formation by
quenching of I *
 (BR = f IF* ( a ,t)d a dt/ f(IF * + I2* )dXdt) in 2300 Torr
Ar, PCF3 1, 1 Torr NF3 gas mixtures revealed that 25» ' BR < 444' for
4 > PCF3I > 1 Torr.
In summary, strong blue-green fluorescence has been observed from
electron beam excited Ar, CF 3I and NF3 gas mixtures. This emission
has been tentatively assigne? to the E - A3n l transition of the IF
molecule and is likely the same transition that is responsible for the
385 and 430 nm bands of IBr and IC1, respectively. The emission
spectrum and short radiative lifetime of IF* and the Franck-Condon
shift between the E and A states make the molecule attractive as a
potential laser. Also, none of the ground or excited state species
that are known to be present in Ar/CF 3 1/NF3
 plasmas absorb strongly in
7
the 480-500 nm spectral region. Since it would likely operate at room
temperature, an IF blue-green oscillator would represent an improve-
ment over the HgBr laser which requires operating temperatures in
excess of 100•C. Consequently, experiments to obtain lasing from this
molecule using both a-beam and discharge pumping are in progress.
The authors thank R. Dixon and Dr. R. W. Waynant of NU for the
use of a microdensitometer. Also, discussions with Prof. J.
Tellinghuisen of Vanderbilt University regarding the IF spectrum are
gratefully acknowledged.
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REFERENCES
I. For a set of references concerning the a-beam and discharge
pumped X2 (X a I, Dr, Ct and F) lasers, sees A. K. Hays, Opt.
G1001. 28, 209 (1979); also see ref. 8.
2. For a review, see J. A. Coxon, Chem. Soc. Report on 21olecular
Spectroscopy, London (1972) Vol. 1, Chap. 4.
3. D. Rosen, ad., Spectroscopic Data Relative to Diatomic molecules,
Pergamon Press, New York, N. Y. (1970).
4. M.A.A. Clyne and I. S. ticDerold, J.C.S. Farad4y II 74, 807 (1978).
S. N.A.A. Clyne and J. A. Coxon, Proc. Roy. Soc. A 298 9 424 (1967).
6. D. L. Huestis, R. M. Hill, D. J. Eckstrom, 2i. V. MCCusker, D. C.
Lorents, H. H. Nakano, Be Z. Perry, J. A. Margavicius and N. E.
Schlotter, SRI International Report No. MP78-07, (May, 1978)
pp. 53-61.
7. D. J. Seery and D. Britton, J. Phys. Chem. 68 9 2263 (1964).
Be M. Diagelmann, K. Kohl& and K. L. Kamp&, Opt. Comm. 29, 334 (1979).
9. R. A. Durie, Proc, Roy. Sac. A 207, 388 (1951).
10. R. A. Durie, Can. J. Phys. 44, 337 (1966).
11. J. W. Birks, S. D. Gabalnick and H. S. Johnston, J. Hole Spectry.
57, 23 (1975).
12. H. Okabe, Photochemistry of Small Molecules, J. Wiley and Sons,
New York, N. Y. 1978) p. 302.
13. K. Hohla and K. L. Kamp&, Z. Naturforsch. 27a, 938 (1972).
14. D. Porret and C. F. Goodeve, Proc. Roy. Soc. A 165, 31 (1938).
15. The upper energy levels for the IBr, ICE and IF emission bands
are likely the same state. However, the decision of whether to
label this state D or E must await more detailed (and higher
resolution) studies of the absorption and emission spectra of
these molecules.
16. R. S. Mulliken, J. Chem. Phys. 55, 288 (1971).
17. K. Wieland, J. 8. Tellinghuisen and A. !Sobs, J. Hole Spectry. 41,
69 (1972); J. Tellinghuisen, Chem. Phys. Lett. 49, 485 (1977)
18. J. Tellinghuisen, Vanderbilt Univ. private communication).
10
19. It. V. McCusker, D. C. Lorents, D. L. Huestis, R. It. Hill, H. H.
Nakano and J. A. Margevicius, Stanford Research Institute Report
No. IIP76-46 (May, 1976).
20. E. Ellis and N. D. Twiddy, J. Phys. B 12, 1366 (1969).
21. J. E. Velazco, J. H. Kolts and D. W. Setser, J. Chem. Phys. 69,
4357 (1978).
22. S. B. Hutchison, J. G. Eden and J. T. Verdeyen, (to be
published).
23. M. C. Sauer, Jr., W. A. Mulac, R. Cooper and F. Grieser, J. Chem.
Phys. 64, 4587 (1976).
3
11
FIGURE CAPTIONS
Fig. 1. Densitometer tracing of the IF* emission spectrum observed
from a-beam excited 99.83% Ar, 0.13% CF 3I, 0.04% NF3
(pTOTAL : 2300 Torr) gas mixtures. The spectrograph resolu-
tion was 0.16 nm and the vertical scale is linear in optical
density. The same spectral profile was observed regardless
of the iodine/fluorine donor molecules or rare gas diluent
used in the mixture.
Fig. 2. Temporal decay of the I 2* (342 nm) and IF * (484 nm) fluores-
cence waveforms for a 2300 Torr Ar/3 Torr CF 3 I/1 Torr NF gas
mixture. The I2*
 and IF* emissions were spectrally isolated
by bandpass filters. For clarity, the IF* waveform has been
intentionally displaced horizontally with respect to the
I2* trace.
Fig. 3. Partial energy level diagrams for the atomic and molecular
species involved in IF*
 formation in a-beam produced plasmas.
The 12 and IF (X and A) energy levels were redrawn from refs.
16 and 19 and ref. 11, respectively. Only the I* ( 4P1/ 2 and
4P5/2) states are shown for clarity. At the bottom of the
graph are also shown the NF 2-F and CF3-I bond energies.
Fig. 4. Variation of the time and wavelength integrated IF *
 fluores-
cence with the Ar partial pressure for pAr/3 Torr CF3 I/1 Torr
NF3 gas mixtures.
12
440
	 460	 480
	 500
A,nm
Fig. 1 Hutchison et al.
13
• w
Fig. Z Hutchison et al.
I
N
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yC
C
0.1
V IV
	 lu 30 40 50 60
t, ns
0	 1
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Q Q
I it .: : I I	 I
-
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^
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ch
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N
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15
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1000 30002000
PA. , Torr
j
Fig. 4 Hutchison et al.
16


Lasing characteristics of gas mixtures involving UFG: Application to nuclear pumping of lasers

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