Competitive channels in the photodissociation of thionyl chloride

Abstract

The photodissociation dynamics of thionyl chloride are investigated by a one-dimensional resonance enhanced multi-photon/time-of-flight technique (REMPI/TOF) at a dissociation wavelength around 235 nm. Nascent sulfur monoxide molecules and chlorine atoms were detected state-specifically under collision-free conditions for parallel and perpendicular polarisation geometries. Dissociation and detection were performed using the same laser. Polarisation-dependent and state-specific TOF profiles were converted into kinetic energy distributions using a least squares fitting method taking into account velocity-dependent spatial fragment anisotropies. Kinetic energy distributions are narrow and structureless for SO and bimodal for both spin-orbit states of Cl. The bimodality reflects the competition between two-and three-body decay: SOCl 2 + hn ! SOCl + Cl and SOCl 2 + hn ! SO + 2 Cl. Chlorine atoms are preferentially released along the polarisation vector of the dissociation laser. The spatial distribution of Cl is characterised by an anisotropy parameter of 0.8 AE 0.2 for the two-body decay and 0.2 AE 0.2 for the three-body decay. SO molecules are isotropically released with low kinetic energy together with two ground state chlorine atoms. These products are most likely to be generated in a sequential three-body decay on a potential energy surface of A 0 symmetry. Another surface of A 00 symmetry instantaneously produces fast chlorine atoms in a two-body decay process to equal amounts in the ground and the excited spin-orbit states. The corresponding SOCl fragment carries about 50% of the available energy as internal energy.

Molecular halogen elimination from halogen-containing compounds in the atmosphere

“…The photochemistry of SOCl 2 has been well investigated, producing three dissociation channels: (1) SOCl + Cl, (2) SO + Cl 2 , and (3) SO + 2Cl. The insight into photolysiswavelength dependence of dissociation channels with the related branch ratios, 12,[100][101][102] anisotropy parameter for each channel, 12,13,103 dissociation characterization for the threebody production, 12,104 and nascent vibrational and spin-state distribution of SO 103,105 was mainly gained. The prior studies are essentially focused on probing of the Cl (Cl*) and SO fragments for understanding the dynamical complexity especially for the channels (1) and ( 3).…”

“…A concerted three-body dissociation (SO + Cl + Cl) was reported to be the main channel accounting for 480% of the overall process at 193 nm, whereas no contribution of such a three-body dissociation was observed when the wavelength was changed to 248 nm. 103 The atomic Cl recombination from this dissociation channel is thus neglected. Further, the reaction of eliminated Cl with SOCl 2 requires two SOCl 2 molecules that are opposed to the pressure dependence measurement.…”

Three-Body photodissociation of thionyl chloride

“…Since the time resolved parent ion intensity reflected the excitation and decay processes of the excited states of SOCl 2 , it was acceptable to assign a lifetime constant of 166 fs to the decay dynamics of the initially excited states. Roth et al [33] observed that the photodissociation of SOCl 2 at 235 nm proceeded through two potential-energy surfaces A ′′ and A ′ . According to their previous study, the femtosecond pump laser at 235 nm launched a nuclear wave packet on the 3A ′′ and 3A ′ surfaces.…”

“…The photodissociation of SOCl 2 has previously been studied experimentally for the photolysis wavelengths of 248, 235, and 193 nm. At 248 nm, PTS study [24] suggested that the two-body dissociation channel (SOCl 2 +hν→SOCl+Cl) becomes the main channel and the molecular elimination channel (SOCl 2 +hν→SO+Cl 2 ) contributes only insignificantly to the dissociation process [25]; however, no contribution of the three-body dissociation channel (SOCl 2 +hν→SO+2Cl) has been observed at this wavelength. These results agree well with the conclusions obtained by using microwave spectroscopy and laser-induced fluorescence (LIF) at the same wavelength [26].…”

“…This result hinted a competition between the two-body dissociation channel and the three-body one [28]. A one-dimensional resonance enhanced multi-photon/time-of-flight technique (REMPI/TOF) has also been used to study the photodissociation dynamics of thionyl chloride at 235 nm [25]. This study also observed the competition between the three-body and two-body dissociation channels, and indicated that the three-body decay was the most probably sequential.…”