I. Three-center versus four-center HCl-elimination in photolysis of vinyl chloride at 193 nm: Bimodal rotational distribution of HCl (v <= 7) detected with time-resolved Fourier-transform spectroscopy

Following photodissociation of vinyl chloride at 193 nm, fully resolved vib ration-rotational emission spectra of HCl in the spectral region 2000-3310 cm(-1) are temporally resolved with a step-scan Fourier-transform spectrome ter. Under improved resolution and sensitivity, emission from HCl up to ups ilon = 7 is observed, with J > 32 (limited by overlap at the band head) for upsilon = 1-3. All vibrational levels show bimodal rotational distribution with one component corresponding to similar to 500 K and another correspon ding to similar to 9500 K for upsilon less than or equal to 4. Vibrational distributions of HCl for both components are determined; the low-J componen t exhibits inverted vibrational population of HCl. Statistical models are s uitable for three-center (alpha, alpha) elimination of HCl because of the l oose transition state and a small exit barrier for this channel; predicted internal energy distributions of HCl are consistent but slightly less than those observed for the high-J component. Impulse models considering geometr ies and displacement vectors of transition states during bond breaking pred ict substantial rotational excitation for three-center elimination of HCl b ut little rotational excitation for four-center (alpha, beta) elimination; observed internal energy of the low-J component is consistent with that pre dicted for the four-center elimination channel. Rate coefficients 33.8 and 4.9X10(11) s(-1) for unimolecular decomposition predicted for three-center and four-center elimination channels, respectively, based on Rice-Ramsberge r-Kassel-Marcus theory are consistent with the branching ratio of 0.81:0.19 determined by counting vibrational distribution of HCl to upsilon less tha n or equal to 6 for high-J and low-J components. Hence we conclude that obs erved high-J and low-J components correspond to HCl (upsilon, J) produced f rom three-center and four-center elimination channels, respectively. (C) 20 01 American Institute of Physics.