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.