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.

Photodissociation Imaging of Diatomic Sulfur (S2)

“…2. The lifetimes of these B-state levels 15 are typically much shorter than the few-nanosecond timescale for the current measurements of S atom production. The onset of photodissociation to yield exclusively 3 P 2 + 3 P 2 S atoms at the lowest energy asymptote is at 35636.9 cm -1 above the lowest rotational level of the ground state; a revision of the value of the bond energy is thus indicated, and supported by the TKER measurements for the photofragments.…”

“…Photodissociation of S 2 has been examined here from the S( 15 The 1 1 Π u state correlates exclusively to the (2,1) atomic limit (Fig. 10).…”

“…The later study by Wheeler et al 15 investigated the levels of the B state above the first dissociation limit, measuring linewidths for v' = 10−19 in the range 1 to >20 cm -1 by cavity ring down spectroscopy (CRDS). The experimental linewidths were used to derive excited state lifetimes, and interpretation of the data was supplemented by modelling of the predissociation rates, aided by ab initio calculation of potential energy curves of u symmetry correlating to the S( 3 P) + S( 3 P) dissociation limit.…”

“…Potential energy curves derived from electronic structure calculations by Wheeler et al 15 for the lower lying electronic states of S 2 are shown in Figure 1. At large interatomic distances, the B 3 Σ u -state correlates diabatically to the second dissociation channel (S( In previous studies 15 of S 2 photoexcitation, line-broadening in the absorption spectrum, which is a signature of reduction in excited state lifetime caused by the onset of dissociation pathways, was used to deduce which PE curve(s) cause the predissociation of the B 3 Σ u -state.…”

“…At large interatomic distances, the B 3 Σ u -state correlates diabatically to the second dissociation channel (S( In previous studies 15 of S 2 photoexcitation, line-broadening in the absorption spectrum, which is a signature of reduction in excited state lifetime caused by the onset of dissociation pathways, was used to deduce which PE curve(s) cause the predissociation of the B 3 Σ u -state. Such studies did not, however, provide information on the subsequent dissociation dynamics, and any predictions about the production of different fine-structure (J) states of the resultant S( 3 P J ) atoms could only be based on limiting case models of adiabatic or sudden dissociation.…”