Laser Driven Chemical Reactions
883
co ( a 3 n ~+ COL-CO~ +c IC];,,,= 1.2 x IO-'' cm3 particles-' sec-'
L?.,,cQt -+
CQT
(where CO denotes vibrationally excited CO). Knowing from the quantum yield calculation that only 1.6% of (a3n) CO react, KI and K Z are determined to be respectively 1 9 x 10-12 and 1.2 x cm3 particles-1 sec-l. This calculation amumes that fluorescence emission from the long-lived a3H state of 60 to be negligible.
richment resulting from the preferential absorption of the The data ob2062.4-i! iodine line by 13C1e0 and 12C180. tained reveal some of the finer details of this isotope effect. One distinctive feature displayed in a plot of 13C relative abundance in CsOz us. 13C relative abundance in GO is explained in terms of energy transfer from (a3II) CO. Quantum yield determinations reveal that this energy transfer is probably the predominant process occurring for the disappearance of (a3II) CO and numerous photopbysical and photochemical processes occur after excitation.
Conclusion The isotope separation factors determined for 13C in ' in COz indicate a large isotope enC3O2 and 6 0 2 and 120
Acknowledgment. This research was sponsored by the National Science Foundation under Grant No, GP-14518.
riven Chemical Reactions of Dimitrogen Tetrafluoride with Hydrogen an Sulfur Hexafluoride with Hydrogen1 Juhri L. Lyman* and Reed J. Jensen 1 0 s Ai'amos Scientific Laboratory, University of California, Los Aiamos, New Mexico 87544 (Received August 30, 1972)
Reactions of NzF4 or SFe with HZ werr initiated with a high-power pulsed C02 laser. Explosion threshold data free from wall effects were obtained for butene-stabilized mixtures of NzF4 and H2. Ultraviolet, visible, and infrared chemiluminescence data were used to corroborate the occurrence of chemical explosions. HF laser gain was observed a t early times after laser irradiation of SFe-Hz mixtures. In the case of NzF4-Hz mixtures, lower HF laser gain was measured during the preexplosion induction time and was strongly quenched by the explosion. An energy-dependent, unexpectedly short induction time occurred between the laser irradiation and the onset of the explosion. This induction time and observation of early time 'HF laser gain are indicative of reaction acceleration due to vibrational excitation.
Introduction In this study we employ vibrational excitation from a pulsed CO2 laser to ignite the explosion of mixtures of NzF4 plus Hz and to induce the chemical reaction of SF6 with Hz.Infrared laser excitation has been shown to be useful in laser-induced fluorescence studies of energy transfer,2a and Odiome, et a1.,2b demonstrated a dramatic increase in the reaction rate of HCl with K atoms for HC1 molecules that are vibrationally excited as a result of irra, ~ the exdiation with an HG1 laser. Karlov, et ~ l . induced plosion of mixtures of BC13 and Hz with a continuous wave (cw) COP laser, and Basov, et aL,* used a cw COz laser to initiate reactions in several gas mixtures. These authors attributed tlhe observed reactions to vibrationally excited molecules. However, Buchwald, et a1.,5 observed the laser-driven pyrolysis of NHJ, and concluded that his data and similar work with cw COz laser radiation could be explained on the basis of thermal energy input. The data reported here indicate that (1) pulsed infrared laser radiolysis is a viable method for obtaining a fast nonequilibriu m increase in the energy of a system, and (2) that vibrational excitation results in chemical
reaction more rapidly than if the same amount of energy were added thermally. Experimental Section The COz TEA laser used in these experiments is described elsewhere.6 However, in this work, the beam was reduced to give a spot size of 0.35 cm2 at the reaction cell except where otherwise noted. The laser pulse was approximately 1 psec full width at half maximum and lasing occurred for about 8 &see(Figure lb, ref 6). U. S. Atomic Energy Commission. (a) C. B. Moore, Annu. Rev. Phvs. Chem., 22, 387 /1572), and references therein, including C. B. Moore, Advan. Chem. Phys., to be published; (b) T. J. Odiorne, P. R. Brooks, and J. V, V . Kasper, J. Chem. Phys., 5 5 , 1980 (1971). N. V. Karlov, N. A. Karpov, Yu. N. Petrov, A. M. Prokhorov, and 0. M. Stel'makh, JETPLett., 14, 140 (1971). N. G. Basov, E. P. Markin, A. N. Oraevskii, and A. V . Pankratov, Sov. Phvs. Doki., 16, 445 (1971); N. G. Basov, E, P. Markin, A. N. Oraevskii, A. V. Pandratov, and A. N. Skashkov, JETP Lett., 14, 165 (1971). M. I. Buchwald, R. McFarlane, and S. H. Bauer. Third Conference on Chemical and Molecular Lasers, St. Louis, Mo., May 1-3, 1972. J. L. Lyman and R. J. Jensen, Chem. Phys. Lett., 13, 421 (1972).
(1) Work performed under the auspices of the
(2)
(3) (4)
(5)
(6)
The Journal of Physical Chemistry, Vof. 77, No. 7. 7973
John L. Lyman and Reed J. Jensen
884
1 2 0 pscc / div
2 0 pscc /div
Figure 1 . Emission from laser initiated reactions: (a) 26.8% N2F4, 72.5% H T , 0.7% C4Hs, pressure = 36.8 Torr, spot size = 0.35 c:m2, and laser energy = 0.20 J (upper trace is the photo-
multiplier response, 50 mV/division; lower trace is the infrared detector response, 2 mvldivision); (b) 33.3% SFe, 66.7% Ha, pressure = 27.9 To'rr, spot size = 0.35 cm2, and laser energy = 0.36 J (upper trace is the photomultiplier response, 1 mV/ division; lower trace ici the infrared detector response, 0.5 mV/ division).
The reaction cell consisted of a 2.5 X 3.8 X 5.1 cm brass block with a 1.27-cm diameter hole centered in its small face and drilled thruugh it lengthwise. A 0.556-cm diameter hole centered 0.793 em from the front of the cell was drilled at right angles to the 1.27-cm hole and fitted with CaF2 windows, The gas mixtures were introduced through the rear of the cell and the C02 laser pulse entered through a BaFz window on the front of the cell. A 1P28 (S-5) photomultiplier was used to monitor the visible and ultraviolet emission through one of the CaF2 side windows. A modified Czerny-Turner monochromator was used with the photomultiplier for spectral analysis of the light emission in the visible and ultraviolet regions of the spectrum. The infrared emission in the region from 2 to 7 1was monitored by a Philco GPC-216 infrared detector. The frequency range was determined by the response of the GeAu detector and the transmittance of the CaF2 window. The HF optical gain and absorption measurements were made by splitting the output from a small HF probe laser into two beams and passing one through CaF2 side windows and routing the other (reference) outside the reaction cell. The reference beam was passed through a Jarrell-Ash 0.5-m monochromator. The two beams were directed by mirrors to a pair of infrared detectors. The detectors used were either InAs photovoltaic infrared detectors or GeAu infrared detectors from Santa Barbara Research Gorp. A Tektronk 556 dual-beam oscilloscope was used to display the response of the photomultiplier and infrared detectors used in these experiments. The Journal of Physical Chemistry, Vol. 77,
No. 7,
1973
The S F O - H ~probe laser consisted of a 30 cm long by 1 cm i.d. quartz discharge tube fitted with CaF2 Brewster angle windows and with electrodes for longitudinal discharge. The optical cavity was defined by a totally reflecting mirror and a diffraction grating for wavelength selection, The laser radiation was coupled out of the cavity by a CaF2 flat set a t approximately 45" from the optical axis. The time between C02 laser pulse and the HF probe laser pulse was controlled electronically. The probe laser output consisted of a submicrosecond pulse a t a single selectable HF vibrational-rotational transition frequency. The N2F4 used was determined mass spectrometrically to be 97% pure. The major impurities were N2 (I.@%), N20 (0.5%), and N2F2 (0.4%). The purities of the H2 and SFo used were 99.9 and 99.8%, respectively. Phillips Petroleum Co. 99.5% pure cis-2-butene was used to inhibit spontaneous explosions in NzF4-Ha mixtures. The gases were premixed in a stainless steel vessel and gas pressures were measured with a Texas Instruments Model 145 precision pressure gauge. The energy incident on the reaction cell was measured by splitting off a few per cent of the beam to a Hadron Model 100 thermopile. The estimated accuracy of the laser energy measurement was *5% with a precision of A2%. NzF4-Hz-Butene Explosion Experiments Explosive reactions of a range of mixtures of N2F4, H2, and cis-%butene were initiated with the pulsed C02 laser. The N2F4 absorbance of GO2 laser radiation in two pressure ranges (Figure 2, ref 6) is given approximately by the expressions log I,/I = 0.00391P (P < 18 Torr) log I J I = 0.0078ZP
-
0.071
(P
18 Torr)
where 1 is the path length and P is the N2F4 pressure. Both N2F4 and its dissociation product NF2 have infrared absorption bands in the 10.6-1 r e g i ~ n . The ~ , ~ vs-NFz symmetric stretch band of N2F4 is at 946 cm-l and the ~3 asymmetric stretch band of NF2 is in the region from 930 to 940 cm-l. Both species could contribute to observed absorption. Mixtures consisting of 2% or less of butene and 20 to 73% Ha were investigated. Figure l a shows a typical oscillogram of the photomultiplier and infrared detector response to the laser-initiated explosion. The intensity of the emitted radiation was strongly dependent on gas composition and pressure. The infrared emission intensity increased linearly with pressure for a given gas composition. The visible--ultraviolet emission was most intense for the stoichiometric ratio of H2/N2F4 = 2. If Hz/NzF4 was decreased from 2 to 1h while keeping the N2F4 and butene partial pressures constant, the visible-ultraviolet emission intensity was reduced by a factor of 100. For near stoichiometric and hydrogen-rich mixtures, the intensity of the radiation emitted from the exploding gases was nearly independent of laser energy. It was also observed that the reaction propagated well out of the irradiated region. The laser served only to initiate an explosive reaction, and the observed properties of the reaction were independent of the method of initiation after ignition had occurred. (7) J R Durig and R . C Lord, Spectrochm Acta, 19, 1877 (1963) (8) M D Harmony, R J Myers L J Schoen D R Lide Jr , and D E Mann,J Chem Phys , 35, 1129 (1961)
Laser Driven Chemical Reactions
885
* 4
P-74.4 Torr P.36.8Torr P*20.8Torr
100' I
L
g ' Y
n 50,
0
0.1
I
I
1
0.2
0.3
0.4
-
LASER ENERGY ( J ) Figure 2. Explosion thresholds for three different gas compiositions. P is the total initial pressure and the laser spot size 1s 0.35 cm2: (mixture 1 ) 78.0% N 2 F 4 , 20.0% H z , and 2.0% CdHs;
(mixture 2) 26.8% N 2 F 4 . 72.5% H2, and 0.7% 26.8% N 2 F 4 , 71.5% Ha, and 1.7% C4H8.
C4H8;
0 0
1
0.3
0.4
LASER ENERGY ( J 1 Figure 3. Reciprocal induction time vs. laser ehergy at three different pressures of mixture 2 in Figure 2; laser spot size = 0.35 cm2.
(mixture 3)
The spectral distribution of the radiation emitted d!uring the explosive reaction of a gas mixture containing 5!1.5 Torr of NzF4, 57.4 Torr of Hz, and 1.34 Torr of cis-2-butene was determined by passing the emitted light through a monochromator. The region scanned was from 260 to 600 nm a t 20-nm intervals with 20-nm slits. The regionis of strong emission were then rescanned a t 5-nm intervals with 5-nm slits. Four strong bands and one weak band were observed. The stronger bands were identified as (0,O) and (0,l) bands of the CN violet system a t 388 and ,422 nm and the (0,O) and (0,l) bands of the C Z Swan bands; a t 517 and 564 nm. The weak band was attributed to the (A311 --*X3,?-)transitionof'NHat 336nm. The infrared emission was not investigated spectroscspically; however, emission from vibrationally excited HF in the 2.6-3-p region was likely a major portion of the observed infrared emission. It was observed that for a given gas composition a.nd pressure, there was a laser energy below which an explosive reaction would .not occur. Figure 2 shows plots of that threshold for three different gas compositions. Explosions occur only to the right of the lines. Mixture 1 was hydrogen lean, mixture 2 vias slightly hydrogen rich, and the butene-NzF4 ratio was constant for these two mixtures. Note that mixture 2 was more easily ignited than 1 below 7 5 Torr even thoiigh the concentration of the laser absorbing species (NzF4) waB considerably less than in mixture 1. Mixture 3 differed from 2 only in mole fraction of cis2-butene. The ;added butene increased the explosion threshold apparently by consuming reactive species such as H or F atoms. Kuhn and Wellmans have investigat,ed and discussed the effect of this and other inhibitors on the thermal ignition of this reaction. The induction time, T , between the beginning of the COz laser pulse and the first detectable visible-ultraviolet emission was strongly dependent on the laser pulse energy, as well as pressure and gas composition. Induction times in the range from 4 to 300 psec were observed. The phenomena that determine the induction time appeared to be nearly independent of wall effects and diffusion out of the irradiated region on this short time scale. For EXample, the time f i x M ;%tomsto diffuse out of the center of the irradiated region is on the order of 300 psec for 50 Torr of gas mixture. 'I'hesle induction times are not closely re-
a2
0.1
"."
I
E * 0.30 A
--
J
E = 0.25 J E = 0.20 J
0.21
'0
4/
Y
7 0.1
I
0 0
I
-
I
t 20
A
I
1
40
60
2
80
PRESSURE (Torr) Figure 4. Reciprocal induction time vs. pressure for mixture 2 in
Figure 2; laser spot size = 0.35 cm2.
lated to the much longer thermal induction times such as those measured by Kuhn and Wellmans for this reaction. The latter are strongly dependent on diffusion, wall deactivation of active species, and wall temperature. Figure 3 shows a typical plot of 1 / against ~ laser energy for one gas composition a t several pressures. The induction times for all gas mixtures investigated obeyed the following equation l/t
= k(E
-
E,)
(1)
k is a function of pressure and gas composition, E is the laser energy, and EO is the threshold energy for laser ignition of the reaction. Figure 4 is a plot of 1 / ~ against total gas pressure for several laser energies. The reciprocal of 7 increases nearly as P . Laser Initiated Reaction of SF6 with HZ The COz laser radiation is strongly absorbed by the U S band of SF6 a t 946 cm-l. The absorption coefficient of sF6 a t 10.6 p is several times that of NzF4 a t the laser intensities used in these experiments and this effective coupling of the laser radiation to gas mixtures containing SFe made possible the laser initiation of its reaction with Hz. This reaction was less thoroughly studied than the NzF4-Hz-butene reaction; however, the following observa(9) L. P. Kuhn and C. Wellman, Inorg. Chem., 9, 602 (1970) The Journal of Physical Chemistry, Vol. 77, No. 7, 1973
John L. Lyman and Reed J. Jensen
886
C02 laser pulse, but the gain was greater for the s p 6 - H ~ system. It is noticeable in the SFS-H~system that the gain on the P1(4)transition lasted longer than on the Pz(6) transition, and that the absorption of the Pz(6) line did not become as intense as that of the P1(4) line. In the N2F4-Hz system, the gain occurred well before the explosion of the gas and no gain was observed during the explosion. The induction times for the NzF4-Hz system of Figure 5 were from 20 to 10 psec, as the laser energy varied from about 0.5 to 0.7 J.
I
r/Io C
TIME (psec)
Transmitted to incident intensity ratio of an HF probe laser pulse in the laser driven reaction of NzF4 and Hz. The CO:! laser energies range from 0.5 to 0.7 J, the spot size is 0.35 cm2,and the path length of the probe laser is 1 .O cm. Figure 5.
9
I 0
P,
I 14)
P2 (6)
I.o
N2 F4 = H2
I
I
A
P = 28.7 Torr
1 I
1
TIME (psrc)
Transmitled to incident intensity ratio of an HF probe laser pulse in the laser driven reaction of s F 6 and Hz. The COz laser energies range from 0 5 to 0 7 J, the spot size is 0.35 c m 2 and the path langth of the probe laser I S 1 0 cm. Figure 6 .
tions were naade. (1) The intensity of the light emitted from the reaction increased with increasing laser energy. (2) The amount of gas that reacted, as determined by the increase in pressure, was small and increased with increasing laser energy. (3) Although SF6 is a stronger absorbier of 10.6-p radiation than is N2F4, the threshold for laser initiation of the reaction is greater for the SE’6-Hz system. (4) As shown in Figure Ib, the time of reaction was less sharply defined than the explosive reaction of N2F4 and H2. The intensity of the emitted radiation was also much less than in the NzF4-Hz system. These observations indicate that the SF6-& reaction was not a laser ignition of an explosion, but was merely reaction of species activated by absorption of laser energy.
eveloped by the Reacting Media HF laser gain measurements were made on the C02 laser driven reactions of N2F4 with HZ and SF6 with €32, by probing the reacting mixtures at various times after the beginning of the 602 laser pulse. The ratio of transmitted to incident intensity of the H F probe laser signal was plotted against time for reactions of the two systems in Figures 5 and 6. The accuracy of the determination of I / & was approximately &6%. The path length of the probe beam through the COZ laser irradiated gas was approximately 1 cm. There was evidence of laser gain in the X#4-f& reaction during the first few microseconds of the The .Journal of Physical Chemistry, Vol. 77, No. 7, 1973
Discussion The observed short delays between the COZ laser irradiation of the reacting gas and the onset of laser gain and explosion indicate that rates of some dissociative reactions are increased by vibrational excitation. Figure 7 helps to illustrate some of the features of the absorption of laser radiation by NzF4. This figure shows the results of a calculation of the extent of NzF4 dissociation into the product NF2 as well as the temperature as functions of deposited laser energy a t 10 psec after the beginning of the laser pulse. The measured laser absorption6 was used in the calculation, and thermal equilibrium was assumed. The genera1 features would be unchanged if an enhanced N2F4 dissociation rate due to vibrational excitation were used. Note from Figure 7 that the dissociation of N2F4 tends to act as a heat sink. That is, the temperature is kept constant near 500 K if the laser energy absorbed is insufficient to dissociate all of the N2F4. High temperatures result if the laser energy is increased above that point. Figure 7 also shows that above some threshold energy, the concentration of NF2 increases linearly with energy. The linear dependence of NF2 Concentration on laser energy was experimentally observed. A mixture of 106 Torr of 20% NzF4 and 80% Ar was irradiated with laser energies ranging up to 0.5 J and with a laser spot size of 2.3 cm2. The measured concentration of NF2 10 psec after the beginning of the laser pulse is given by [NF’z] = 8.33 x - 0.1) mol/cc. The rate of C02 laser-induced dissociation of N2F4, previously studied by Lyman and Jensen,6 is much faster than the thermal dissociation rate that has been measured by shock tube methods.lOJ1 This enhanced rate of the laser-induced reaction was attributed to nonequilibrium population of vibrationally excited states of N2F4. A chain reaction mechanism is proposed for the Iaserdriven reaction of N2F4 with Hz with the following as chaininitiating reactions N2F4* C M -2NFz + M AH = 1- 20 kcal - Evlb NF2* f M
-
NF
+F
(2) fM
A H = C 66 kcal - Evlb (3) where the rates of both of these endothermic reactions are greatly accelerated by laser excitation of vibrational modes of the reactants. Reaction 2 is very rapid for the laser energies used in these experiments. Comparison of Figure 2 with Figure 7 shows that the concentration of NF2 is very high a t laser energies near the explosion threshold. This NF2 evidently reacts very rapidly by reaction 3 when the NFz is vibrationally excited as from laser absorption or as a result of (10) A. P. Modica and D. F Hornig, J. Chem fhys , 49,629 (1968) (11) L. M Brown and B. deB Darwent, J Chem Phys , 42, 2158 (1965)
Laser Driven Chemical Reactions
887
give H atoms. Thus, the H atom concentration during the induction period will be given approximately by [HI = CI(E - Eo). The rate of heat release i s limited by the slowest of' the propagating reactions. If reaction 6 is the slowest
4
b_
,2.0
d[HF]/dt = 2&[NFZ][H] = 2hB[NF,]C,(E - E,) The factor 2 arises because every chain cycle releases two
HF molecules. Assumption 2 of the model allows easy
- 500
integration
-
A[HF] = 2ks[NF,]C,(E
E,) At
which can be rearranged to A(HF)/C,2h,[NF,]
300 0.2
0.I
0
0.3
0.4
LASER ENERGY ( J ) Figure 7. Calculation of NFz concentration and temperature as functions of laser energy at 10 psec after the COz laser pulse begins for a laser spot size of 0 35 cm2 and N2F4 concentration mol/cc (-316 5Torr) of 8.8 X
vibrational-vibrational energy transfer from N2F4. By analogy with tlw laser-induced dissociation of NzF4, the amount of F produced during the laser pulse by reaction 3 should inrrease linearly with laser energy above some threshold. The observation of HF laser gain only during the C02 laser pulse indicates that reaction 3 does indeed occur a t that time and is followed by the very fast reaction
F' t-
iH2
--+
kF*
+H
14)
A third possible chain-initiating reaction proposed by Kuhn and Wellman9 as the chain-initiating reaction for thermal explosions of KzF4 and W2 is
NF,
4-
Hz -+FST\IF2 + H AH
= C 25 kcal
(5)
and is believed to play only a This reaction is minor role in the present experiment. Figures 2 and 7 show that evlm high concentrations of both reactants do not ensure that an explosion will occur. Since the laser-induced dissociation of N2F4 is very fast, the NF2 concentration will be high during most of the induction period. The propagating reactions likely to be important, thereforr, are those involving NF2, such as NF, 4- t4 -+HF 4- N F (6) N F I- NF2+ N2F, + F (7 ) as well as reaction 4. A simple model is proposed to explain the observed explosion thresholds and induction times. The model includes the cham reaction mechanism suggested above with acceleration of dissociation of both N2F4 and NF2 by vibrational excitation. The model assumes, for a given gas composition and pressure, that (1)thc number of chain-propagating species (F and possibly El) produced during the C02 laser pulse increases linead\ with laser energy above some threshold Eo; (2) the temperature and the concentrations of major species remain esseritially constant during the induction period (since the concentration of initial chain propagators is small compaied with the concentration of H2, NF2, and NzF4); and (3) the induction time is the time taken for the heat release from the propagating exothermic reactions to heat the medium to the ignition temperature. Reaction 4 is 30 fa& the F atoms produced during the laser pulse by roaction 3 react immediately with Hz to
(E
E,) At
The left-hand side of the- above equation is proportional to the temperature rise of the medium, and the temperature rise necessary for ignition for a given gas composition is constant. Therefore 1/~ = l/At = h(E
-
E,)
The same result is obtained if reaction 7 is the slowest. This simple model does give the proper dependence of the induction time on laser energy, and requires that vibrational excitation does indeed accelerate the dissociation rate of NF2. However, in order to acquire further insight into the processes involved, a more comprehensive model was employed. The model included ( I ) reaction of F with H2 (reaction 4) and the subsequent deactivation of the vibrationally excited HF by V-T and V-V energy transfer processes (the Appendix of the paper by Kerber, et U L . , ~ lists ~ these reactions and their rate constants); (2) dissociationl0*14 of NzF4 and NF2; (3) reaction15 of N with nitrogen fluorides; and (4) vibrational reiaxation of the nitrogen fluorides. In order to simulate the effect of vibrational excitation on the dissociation rate of N2F4 and NF2, a vibrational temperature was defined that included all of the vibrational modes of both N2F4 and NFz. This vibrational temperature was used in the rate coefficients of these two dissociation reactions. The Vibrational relaxation time was assumed to be lo-? sec atm. The rate equations of the reactions involved were solved numerically by computer and laser gain was calculated for several HF vibrational-rotational transitions. As expected, the calculations showed that there was a high degree of vibrational excita'tion during the laser pulse as is shown in Figure 8. The calculated vibrational and translational temperatures are plotted out to 40 psec where the explosion is just beginning to occur as indicated by the temperature rise. Computer simulations with 20 Torr of N2F4, 40 Torr of H2, and relaxation time of 10-7 sec atm gave the following for induction time l / t = 0.79(E
where
T
-
0.925)
is in microseconds and energy is in joules. This
(12) G. L. Schott, L. S. Blair, and J, D. Morgan, of this laboratory, demonstrated in a shock tube study of the NzFI-Hz reaction in an argon diluent that the reaction between N F 2 radical and H Z does not take place on the time scale of the experiment except at iemperatures above approximately 1700°K (to be published). (13) R . L. Kerber, G. Emanuel. and J. S. Whittier, Appi. Opt., 1 1 , 1112 (1972), (14) A. P. Modica and D. F. Hornig, J. Chem. Phys., 43, 2739 (1965). (15) S . W. Rabideau (the rate coefficients for reaction of H with all nitrogen fluorides were taken to be the same as the measured rate coefficient for reaction of H with NF3), J. Msgn. Resonance, to be published. The Journal of Physical Chemistry, VoJ. 77, No. 7, 1973
John L. Lyman and Reed J. Jensen
888 I. 6
1.4
1.2
* 1.0
v
aw
I
F:
0.8
u)
>
0
n ~
0.6
CI
0.4
0.2
- A
0
3
IO
IS
20
25
30
35
40
l l M E ( MICROSECONDS 1 Figure 8. Computer-generated plot of translational and vibrational temperature during t h e induction time of a laser-driven reaction of 15 Torr of hL2F:d with 30 Torr of H2. The relaxation time is 0.1 psec atm and t h e laser energy is 1 .O J.
compares with an experimental result of
1 1 ~ 0.80(E - 0.107) Note that the computer-generated equation has the proper form, but the computer threshold energy is much larger than the experimental value. Inclusion of reaction 7 in the code gave some improvement, as follows ljz =
0.77(E
-
0.700)
The threshold energy in this expression is still large by about a factor of 7 . Altering the vibrational relaxation time did not improve the agreement with the experimental expression. However, close agreement with the experimen t was achieved by increasing the rate coefficient for reaction 3 during the laser pulse well above that given by including the calculated vibrational temperature in the rate coefficient. Attempts t o simulate the experimentally measured H F laser gain (and absorption) met with the same difficulties. Gain was indeed predicted as shown in Figure 9, but it occurred later than the experimentally observed gain (Figure 5 ) . Here again the experimental results could be duplicated by assuming a very rapid rate of reaction 3 during the laser pulse. We must therefore conclude that the rate of formation of 1F atoms during the CQz laser irradiation of N2F4 is greatly enhanced by vibrational excitation of the reactant molecules when the laser pulse intensity is on the order of 1 MW/cm2. The initial reactions operative in the laser-driven reaction of SF6 with 132 are probably SF," 11- M -3SF, F + M (8)
+
The Journal of Physical Chemistry, Vol. 77, No. 7, 1973
0
1
2
3
4
1
6
7
8
9
1
0
TIME (MICROSECONDS)
Figure 9. Computer-generated plot of transmitted to incident intensity ratio of a Pl(4) H F laser transition, The reactants are 14.35 Torr each of N 2 F 4 and Hz, laser energy is 1.0 J, and the relaxation time is 0.5 psec atm.
F
+
H2 4 HF*
+ €3
(4)
where the rate of (8) is accelerated by vibrational excitation. Since reaction 4 is fast on the time scale of this experiment, the H F laser gain will reflect the rate of F atom production. If F atoms were produced thermally, the gain would occur near the end of the laser pulse. However, gain was actually observed during the first few microseconds (Figure 6) which indicates that reaction 8 is operative. This conclusion was further justified by a computer model of the laser-initiated reaction. The computer program discussed above was modified to include the dissociation16 of SF6 and measured vibrational relaxation rates1? of SF6. The peak calculated gain (using an SFs dissociation rate based on the vibrational temperature of sF6) on both P1(4) and P2(6) HF transitions occurred about 4 psec later than the peaks of the experimental gain curves. Much better agreement was obtained when an even faster rate of dissociation was assumed during the CQ2 laser pulse. We conclude then that vibrational excitation accelerated the rate of dissociation of SF6 when it was irradiated with intense CQz laser pulses. Acknowledgment. We wish to acknowledge the support from the NDEA Fellowship for John L. Lyman through Brigham Young University.
(16) J. F. Bott and T. A. Jacobs, J. Chem. Phys., 50, 3050 (1969). ( 1 7 ) T. B. Hodkinson and A. M. North, J . Chem. SOC.A , 885 (1968), J.
