Femtosecond Transition-State Dynamics of Dissociating OCS on the Excited 1Σ+ Potential Energy Surface

Abstract

Ultrafast photodissociation dynamics of OCS on the dissociative potential energy surface (PES) of the electronically excited 1Σ+ state was investigated by photofragment excitation (PHOFEX) spectroscopy and time-dependent wavepacket calculations. The high-resolution PHOFEX spectrum of the entire 1Σ+−1Σ+ transition (63 300−69 350 cm-1) in the vacuum ultraviolet (vacuum UV) region was measured under jet-cooled conditions by using a tunable vacuum UV laser as an excitation light source and by monitoring the fragment S(1S) atom. Due to sufficient vibrational and rotational cooling in a supersonic jet, a simple and distinct vibrational progression of the 1Σ+−1Σ+ band was recorded free from vibrational hot bands and the broadening by rotational structure. The autocorrelation function obtained from a Fourier transform of the PHOFEX spectra clearly exhibited recurrences with a period of 42 fs, corresponding to a period of the vibrational motion at the transition state along the direction perpendicular to the dissociation coordinate. This interpretation of the ultrafast motion in the transition-state region was supported by the wavepacket calculation on the ab initio PES obtained in the present study, which afforded a period of 48 fs for vibrational motion along the in-phase CO and CS stretching mode at the transition state on the ab initio PES. The distinct six peaks broadened due to the fast dissociation process were assigned to the transitions at the vTS = 0−5 levels, where vTS represents a vibrational quantum number for the Feshbach resonances in the in-phase stretching mode at the transition state. For the vTS = 0 peak, the narrowest width corresponding to the lifetime, τ(vTS), of τ(0) = 133 fs was obtained. It was found that the dissociation lifetime first becomes shorter for a larger vTS, i.e., τ(1) = 44 fs and τ(2) = 27 fs, and then, the peak width becomes narrower for the higher vibrational states above vTS = 2, with corresponding lifetimes of τ(3) = 47 fs and τ(4) = 44 fs. This deceleration of the dissociation rate for the larger vTS states was interpreted as a result of trapping of a wave function in the in-phase stretching or the CO stretching vibrational motion suppressing the motion along the dissociation coordinate. All six main peaks in the PHOFEX spectra exhibited a characteristic asymmetric profile, originating from the interference between the zero-order discrete states for the in-phase stretching vibration at the transition state and the zero-order continuum states corresponding with the motion along the dissociation coordinate. In the main progression of the PHOFEX spectra, the reversal of the asymmetry direction was also observed. This q-reversal phenomenon was ascribed to the characteristic shape of the excited 1Σ+ PES near the transition region which causes a phase shift in the wave functions in a narrow energy range.