Photochemical reaction dynamics in SO2-acetylene complexes

Abstract

The dynamics of photoinduced reactions between electronically excited SO(2) molecule (A (1)A(2)<--X (1)A(1)) and acetylene molecule (X (1)Sigma(g) (+)) in the SO(2)-acetylene van der Waals (vdW) complexes (clusters) was studied. The SO(2) molecule was excited by frequency-doubled radiation of a tunable dye laser, and resonance enhancement multiphoton photoionization of the produced photofragments was induced by ArF (193 nm) laser radiation or by frequency-doubled radiation of a second tunable dye laser to observe the C(2)H radical. The HOSO radical was detected by its IR emission. We found that the main photodecomposition channel of the vdW complexes (clusters) involves the SO(2) ( *)+C(2)H(2)-->HOSO+C(2)H reaction. Indeed, the analysis of the action spectra of the excitation laser radiation showed that the photofragments emerging in our experimental conditions (SO(2), 5%; C(2)H(2), 5%; and Xe; P(0)=2 atm) originate from the SO(2)...C(2)H(2) vdW complex (cluster). We analyzed the structure of this vdW complex theoretically, obtaining C(s) symmetry, with the acetylene molecule located above the OSO plane. The resonance-enhanced multiphoton photoionization action spectra of the C(2)H (A<--X) photofragmentation and the IR emission spectra of the HOSO radical allowed the authors to probe the energy distribution between the photofragments formed. The reaction that involves transition of the acetylene H atom to the SO(2) oxygen should be the primary step of the process considered, followed by nonstatistic dissociation of the vdW complex (cluster), with the C(2)H radical formed in its vibrationless state and excited both rotationally and translationally, and the HOSO radical excited vibrationally, rotationally and translationally. The proposed reaction mechanism was discussed, employing transition-state and Rice-Ramsperger-Kassel-Marcus (RRKM) approaches. The kinetics of photofragment formation was investigated, yielding characteristic radical build-up time of 0.64 micros.