ABSTBACTS , .

70-ns transverse electric discharge. It is assumed that the electric discharge dissocia- tes a fraction of the NF3 to give an initial concentration of F atoms. The dominant reaction is

F . + R H S H F f R with most the the H F vibrationally excited. After a time delay to build up gain, the laser operates saturated with the output power roughly proportional to

4HFIldt = h[FoI[RHoI

-exp (-h[RHdt) where Fo and RHo are initial densities.

Rate constants from chemical laser experiments are within 30 percent of published rates for HZ, CH4, CZHG, cyclo- C3HG, C3H8, n - C4H10, and neo-Cs-Hlz. It appears that this system can be used to obtain kinetic rates for fluorine-atom/ hydrogen-compound reactions. Using the preceding assumptions, reaction rates near room temperature are obtained from experi- mental time constants Dz(O.89 X lot3), i~o-C~H~(4 .4 x lO13), CYC~O-CGH~Z(~O X 1013), CeH4(CH3)~(6.9 X 1013), AsHs(3.4 X lO13), SiH4(10 X W 3 ) , n - C3HG(3.0 X

GeH4(10 X 1013) and SbH3(9 X 1013 cm3 1013); and with a larger experimental error

mole-1.s-1).

WB9. Vibrational Excitation of HC1 Formed in the Addition-Elimination Reaction Between NH and Vinyl Chloride, P. R. Poole and G. C. Pimentel, University of California, Berkeley, Calif. 9.4780.

The presence of even a small amount of hydrazoic acid prevents normal photo- elimination laser emission from vinyl chloride. For example, no laser emission results from 15 torr of a 1 : l O mixture of vinyl chloride and argon if as much as 0.3 torr of HN3 is added. We have attempted to discriminate between deactivation (by HNI or NH) of HClZ formed by photo- elimination and possible production of vibrationally cold HC1 from the addition- elimination reaction between NH and HzCCHC1,

NH + H2C = CHCl- HCl*

+ HBCC 3 N,

AH S -63 kcal.

A “tandem laser” was used, with vinyl chloride as the driver reaction and with a mixture of HN3, vinyl chloride, and argon as the slave reaction. These experiments and spectroscopic analyses of the final products suggest that the quenching of the vinyl chloride emission is due to the addition- elimination reaction (not predominantly to deactivation) and that the elimination reaction does produce vibrationally cold HCI. The amount of added argon signifi- cantly affects the behavior of the system; chain decomposition of HN3 may be involved. These results will be described and their probable significance will be discussed.

WBIO. Tandem Laser Studies of the Vibrational Energy Distribution in Hydrogen Fluoride Formed by the Reaction of Methylene with Methyl Fluoride, J. L. Roebber,‘ Natheastern University, Boston, Mass., and G. C.

Pimentel, University of California, Berkeley, Calif. 94Y20.

Measurements of H F population ratios (N./N,-t) produced in the decomposition of chemically activated ethyl fluoride are described. The vibrationally excited ethyl fluoride was generated by the insertion of a methylene radical into a carbon-hydrogen bond of methyl fluoride. Flash photolysis of ketene was the source of the methylene radicals. Despite the large exothermicity of this and other insertion-elimination reactions, little H F vibrational excitation is detected.2 Consequently, it is necessary to be able to measure population ratios that may not alone sustain laser action. The “tandem laser” technique3 can be so used and it has been applied to this insertion- elimination chemical laser. Various photo- elimination lasers can be used as driver (e.g., vinyl fluoride) and, with the methy- lene-methylfluoride system in the same cavity, distinct attenuation of the vinyl fluoride emission is observed. The types of independent measurements needed to deter- mine quantitatively the rate constant ratio will be considered and experiments to measure kl/ko for the CH2-CH3F reaction will be described.

1 On leave with the Universitv of California. Berkeley, Calif. 94720.

2 M. C. Lin, J . Phya. Chem.. vol. 75, p. 3642, 1971. SF. M. G. Tablas and G . C. Pimentel. IEEE J .

Quantum Electron.. vol. &E-6, p. 176, 1970.

WB11. Vibrational Excitation of HF Formed by Elimination from Chemically Activated Ethyl Fluorides, E. Cuellar- Ferreira and G. C. Pimentel, University of California, Berkeley, Calif, 94780.

The vibrational excitation of HF formed by elimination from chemically activated ethyl fluorides, with particular attention to pairs of reactions that give the same products from different precursors was investigated. For example, both 1, l-di- fluoroethane and 1, 2-difluoroethane form vinyl fluoride and HF. Different H F vibra- tional excitation in these reactions would have interesting stereospecific implications

‘201

concerning the reverse addition reaction.’ The first experiments were based upon

flash photolysis of 1, 3-difluoroacetone. The fluoromethyl radicals thus formed, on re- combination, give chemically activated 1, 2-difluoroethane. Elimination of H F re- sults,2 but the system alone does not produce laser emission. When the photolysis takes place in one cell of a tandem laser, with H F from vinyl fluoride acting as driver in the other cell, the vibrational excitation of H F due to elimination from 1, 2-difluoroethane can be measured. Attempts to determine quantitatively the rate constant ratios will be described.

vol. 53, p. 3453, 1950.

p. 1786, 1964.

1 M. J. Berry and G. C . Pimentel. J . Chem. Phys..

2 G. 0. Pritchard et al. , J . Phys. Chem.. vol. 68.

SESSION WC

Wednesday, May 3, 1972 2:OO-5:OO P.M. HF Chemical Lasers

Chairman: T. A. Jacobs

WC1. Enhanced Initiation and High Pres- sure Operation of the HF Chemical Laser, L. D. Hess, Hughes Research Laboratories, Malibu, Calif. 90165.

Reaction rates and laser performance of HrFzHe mixtures were found to be strongly influenced by the initial rate of fluorine atom formation. Varying F atom production rates were obtained by adjust- ment of flashlamp discharge energies and, more effectively, by incorporating a mole- cular fluoride (MoFG) having a high photo- dissociation cross section into the Hz-Ft mixtures. Some of our experimental ments of laser output energies and chemical efficiencies are shown in Table I. These results demonstrate that MoFG is quite effective for enhancement of laser perform- ance from HF produced by the Hz-F~ chain reaction and illustrate the importance of rapid reaction initiation for increased chemical efficiency. Pronounced reductions in laser pulse durations and increases in laser powers were also observed when MoFB was added to Ht-Fz-He mixtures. Computer calculations of chemical concentrations (including vibrationally excited HF) and rates as a function to time indicate that the observed effects of MoF6 on laser perform- ance can be qualitatively explained in terms of chemical kinetics.

Additionally, H F laser experiments were performed to study the relative deactivation

TABLE I HF LASER ENERGIES AND EFFICIENCIES

Partial Pressures (torr) output Chemical

MOFG He (mJ 1 (percent) Hz Fz Energy Efficiency

3.9 3 .9 0 155 4 3.9 3.9 1.3 155 17

0.18

7.6 7.6 0 0.77

7.6 7.6 303 25 0.58 8

1 .o 0.19

7.6 7.6 2.5 303 35 0.81 14.5 14.5 0 566 14.5

16 14.5 3.6 566 65 0.79

0.19

303