Kinetics and Dynamics on the Formation of S₂(X³Σ_g^–,a¹Δ_g) in the S(¹D) + OCS Reaction

Abstract

The reaction of electronically excited sulfur S((1)D) with OCS has exothermic channels generating S(2) in two electronic states X(3)Σ(g)– and a(1)Δ(g). The a(1)Δ(g) state is correlated directly to the reactants via the spin-allowed singlet surface; the X(3)Σ(g)– state, on the other hand, is a product of the spin-forbidden channel. There has been no report on kinetic evidence for the simultaneous generation of the two electronic states, although the two electronic states have been detected so far. The previous studies showed that little energy was released into rotation or vibration of the S(2) products despite large heats of reactions (228 and 175 kJ mol(-1) for generation of X(3)Σ(g)– and a(1)Δ(g), respectively). In the present study, S((1)D) was generated by the photolysis of OCS at 248 nm in a buffer He at 298 K, and the resulting two electronic states of S(2) (X(3)Σ(g)– and a(1)Δ(g)) were detected with dispersed laser-induced fluorescence (LIF) via the B(3)Σ(u)(-)–X(3)Σ(g)– and f(1)Δ(u)-a(1)Δ(g) transitions, respectively. Not only excitation but also dispersed fluorescence spectra made it possible to find a single rotational line of the vibrational level of interest. The time-resolved LIF intensities of the initial growth of the X(3)Σ(g)– and a(1)Δ(g) states showed identical OCS pressure dependences, giving the overall rate coefficient of the S(1D) + OCS reaction to be [3.2 ± 0.2(2σ)] × 10(-10) cm3 molecule(-1) s(-1). The simultaneous generation of the two electronic states indicates that the intersystem crossing plays a role in opening the spin-forbidden channel. As for the reaction dynamics, vibrational levels up to v = 19 of X(3)Σ(g)– and 11 of a(1)Δ(g) have been detected, which is distinctly different from the previous studies. The reaction mechanism has been discussed on the basis of the potential energies reported so far.