title: Dissociation of energy-selected c-C2H4S+ in a region 10.6-11.8 eV: threshold photoelectron-photoion coincidence experiments and quantum-chemical calculations.
abstract: Dissociation of energy-selected c-C2H4S+ was investigated in a region of 10.6-11.8 eV with a threshold photoelectron-photoion coincidence technique and a synchrotron as a source of vacuum ultraviolet radiation. Branching ratios and average releases of kinetic energy in channels of formation of c-C2H4S+, CH3CS+, and HCS+ were obtained from well-resolved time-of-flight peaks in coincidence mass spectra. Measured average releases of kinetic energy for channel CH3CS+ + H of least energy are substantial and much greater than calculated with quasiequilibrium theory; in contrast, small releases of kinetic energy near the appearance onset for channel HCS+ + CH3 agree satisfactorily with statistical calculations. Calculations of molecular electronic structures and energetics of c-C2H4S+ and C2H3S+ isomers and various fragments and transition states were also performed with Gaussian 3 method to establish dissociation mechanisms. A predicted dissociation energy of 11.05 eV for c-C2H4S --> HCS+ + CH3 agrees with a linearly extrapolated threshold at 10.99+/-0.04 eV and a predicted dissociation mechanism that c-C2H4S+ isomerizes to CH3CHS+ before dissociating to HCS+ + CH3 supports the experimental results. The large releases of kinetic energy for channel CH3CS+ + H might result from a dissociation mechanism according to which c-C2H4S+ isomerizes to a local minimum CH3CSH+ and then dissociates through a transition state to form CH3CS+ + H.
authors.affiliation: Department of Applied Chemistry, National Chiao Tung University, 1001, Ta Hsueh Road, Hsinchu 30010, Taiwan.
authors.forename: Yung-Sheng
authors.initials: YS
authors.lastname: Fang
authors.forename: I-Feng
authors.initials: IF
authors.lastname: Lin
authors.forename: Yao-Chang
authors.initials: YC
authors.lastname: Lee
authors.forename: Su-Yu
authors.initials: SY
authors.lastname: Chiang