Predissociation of the B 3Σu− state of S2

Abstract

Predissociation of the B 3Σu− state of S2 has been investigated by a combination of cavity ring-down spectroscopy and model calculations. The experimental spectra of the B 3Σu−−X 3Σg−(v′,0) bands for 10⩽v′⩽22 span the wavenumber range 35 480–39 860 cm−1. Extensive variation is observed in the degree of rotational structure within the vibrational bands because of lifetime broadening caused by predissociation. Fits to the band contours give homogeneous linewidths for transitions to the B-state vibrational levels for 10⩽v′⩽17 that vary from ⩽1 cm−1 for the (10,0) band to 7±1 cm−1 for the (17,0) band with a maximum linewidth of 14±1 cm−1 for the (13,0) band. For v′⩾18, all bands are completely diffuse, indicating linewidths in excess of 15 cm−1. The experimental results are compared with the results of a theoretical model that uses a Rydberg–Klein–Rees (RKR) potential for the B 3Σu− state, ab initio calculations of the repulsive potentials that cross the B state, and Fermi golden rule calculations of the predissociation rates for the different repulsive potentials. Minor adjustments to the ab initio potentials, and an estimate of the spin-orbit coupling between the bound and repulsive states enable us to calculate predissociation rates in excellent agreement with the experimental observations. We deduce that the predissociation for v′⩽16 is predominantly via a 1Πu state, whereas for v′⩾17, coupling to a second repulsive state, suggested to be either a 5Σu− or 5Πu state, provides the primary mechanism for predissociation.