Studies of BrCl by laser-induced fluorescence. Part 2.—State-selected kinetics in the excited B3Π(0+) state of BrCl

Abstract
Resolved rotational levels in the vibrational states 7 v′ 3 of electronically-excited B3Π(0+) BrCl molecules have been excited, using pulses from a narrow-band tunable dye laser. Fluorescence lifetimes((τ= 1/Γ) have been measured as a function of J′, v′ and pressure. All rotational levels of the v′= 7 state of 79Br35Cl(B) were predissociated. Their lifetimes were found to shorten systematically with increase in rotational energy, from 0.91 µs for J′= 7 down to 0.24 µs for J′= 16. In order to suppress disproportionation of BrCl, all studies were carried out using BrCl mixed with typically a seven-fold excess of Cl2. Both upward and downward vibrational energy (V ↔ V) transfer was found to be significant above 30 mTorr total pressure, within the relatively long lifetimes (25 µs) of the lower vibrational states (v′ 6) of BrCl(B). Lifetime measurements were made in two separate total pressure ranges: at low pressure, from 30 to 200 mTorr; and at high pressures, from 3 to 11 Torr. At low pressures, a strong pressure-dependence of the fluorescence decay rate constant Γ was found. This pressure-dependent rate is attributed almost entirely to rapid vibrational transfer, which depletes the population of emitters by upward V ↔ V transfer into the unstable part of the energy level manifold above v′= 6. Values for the lifetime, extrapolated to zero pressure, for v′= 4, 3, were determined to be (τ0= 35+11 –9)µs. After the first few µs, fluorescence decay at high pressures was exponential; Γ for this regime showed a much less marked dependence on total pressure than at low pressures. Vibrational relaxation is essentially complete under these conditions, and the pressure-dependence of Γ is attributed to quenching of a Boltzmann distribution of BrCl(B) molecules consisting of 75 % in v′= 0 and 25 % in v′= 1. However, electronic quenching is relatively slow, and for BrCl + Cl2(90 %) mixtures, has collision efficiency around 2 × 10–3.

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