Reaction dynamics of Cl+H2S: Rotational and vibrational distribution of HCl probed with time-resolved Fourier-transform spectroscopy

Abstract

Following laser irradiation of a flowing mixture of S2Cl2 and H2S at 308 nm to initiate the reaction of Cl+H2S, vibration–rotation resolved emission spectra of HCl(v=1,2) in the spectral region 2436–3310 cm−1 are detected with a step-scan time-resolved Fourier-transform spectrometer. The Boltzmann-type rotational distributions of HCl(v=1) and HCl(v=2) yield rotational temperatures that decrease with reaction time; extrapolation to time zero based on data in the range 0.5–4.0 μs yields nascent rotational temperatures of 1250±70 K and 1270±120 K, respectively; an average rotational energy of 8.3±1.5 kJ mol−1 is determined for HCl(v=1,2), much greater than a previous report. Observed temporal profiles of the vibrational population of HCl(v=1,2) are fitted with a kinetic model that includes formation and quenching of HCl(v=1,2) to yield a branching ratio of 0.14±0.01 for formation of HCl(v=2)/HCl(v=1) and a thermal rate coefficient of k1=(3.7±1.5)×10−11 cm3 molecule−1 s−1. Combining an estimate of the vibrational population of HCl(v=0) based on a surprisal analysis of previous investigations on the reaction Cl+D2S, we report a ratio of vibrational distributions of HCl(v=0):(v=1):(v=2)=0.41:0.52:0.07, which gives an average vibrational energy of 23±4 kJ mol−1 for HCl. Internal energies, especially rotational energy, of HCl derived with this method is more reliable than with previous techniques; the fractions of available energy going into rotation and vibration of HCl are fr=0.12±0.02 and fv=0.33±0.06, respectively.