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 |Re|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 A. The dissociation energy is thus established within narrow bounds : D00(79BrF)= 20 622 ± 20cm–1 or 246.64 ± 0.24 kJ mol–1; De(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+kv′J′(J+ 1).