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

Abstract

Following photodissociation of vinyl chloride at 193 nm, fully resolved vibration-rotational emission spectra of HCl in the spectral region 2000–3310 cm−1 are temporally resolved with a step-scan Fourier-transform spectrometer. Under improved resolution and sensitivity, emission from HCl up to v=7 is observed, with J>32 (limited by overlap at the band head) for v=1–3. All vibrational levels show bimodal rotational distribution with one component corresponding to ∼500 K and another corresponding to ∼9500 K for v⩽4. Vibrational distributions of HCl for both components are determined; the low-J component exhibits inverted vibrational population of HCl. Statistical models are suitable for three-center (α, α) elimination of HCl because of the loose 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 geometries and displacement vectors of transition states during bond breaking predict substantial rotational excitation for three-center elimination of HCl but little rotational excitation for four-center (α, β) elimination; observed internal energy of the low-J component is consistent with that predicted for the four-center elimination channel. Rate coefficients 33.8 and 4.9×1011 s−1 for unimolecular decomposition predicted for three-center and four-center elimination channels, respectively, based on Rice-Ramsberger-Kassel-Marcus theory are consistent with the branching ratio of 0.81:0.19 determined by counting vibrational distribution of HCl to v⩽6 for high-J and low-J components. Hence we conclude that observed high-J and low-J components correspond to HCl (v, J) produced from three-center and four-center elimination channels, respectively.