Quantum-resolved dynamics of excited states. Part 3.—Collision-free lifetimes of BrF(B)

Abstract

The lifetimes τ₀ of resolved ro-vibrational levels (v′, J′) in the exicted B³Π(0⁺) states of ⁷⁹BrF and ⁸¹BrF have been directly measured under collision-free conditions. Laser-induced flourescence with excitation from a pulsed, narrow-band dye laser has been used to probe all stable vibrational states 8 v′ 0 of the energy manifolds of both isotopic species.

The results for v′⩽ 4 and v′= 8 confirm our previous data for τ₀ based on Stern-Volmer extrapolations of measurements at higher pressures. For v′= 5, 6, 7, longer τ₀ values are now reported. For all rotational states with 5 v′ 0, τ₀ values are regular, with only small variations in τ₀ in the range 42–56 µs. The lower rotational states of v′= 6 and 7 show similar τ₀ values. The results are used to derive transition moment values for the B–X transition of BrF; a small variation of |R_e|² with v′ was found.

In the v′= 6, 7 levels, abrupt predissociation commences at the states (6,49) and (7,30); all higher states (6,J′) and (7,J′) have τ₀⩽ 100 ns. The predissociating state belongs to Herzberg′s case I(b) with formation of a potential maximum at r≃ 5.3 Å. The dissociation energy is thus established within narrow bounds : D⁰₀(⁷⁹BrF)= 20 622 ± 20cm^–1 or 246.64 ± 0.24 kJ mol^–1; D_e(BrF)= 20 953 ± 20 cm^–1.

The lifetime behaviour of the (8,J′) state is dissimillar to that of the (6,J′) and (7,J′) states, in that τ₀ for the v′= 8 state varies regularly and strongly with J′. This variation, for rotational energies ⩽ 100 cm_–1, gave a good fit to eqn (I) for predissociation, 1/τ₀= 1/τ₀₀+k_v′J′(J+ 1).