Laser-induced fluorescence studies of a supersonic molecular beam of bromine : Vibrational and rotational relaxation of bromine and collision-free lifetimes for Br2(B3Π(0u+))

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Abstract
Laser-induced fluorescence has been employed to measure collision-free lifetimes of various vibrational levels of Br2(B 3 Π(0u +)) using a supersonic molecular beam of bromine. The lifetimes lie in the range 3·2–5·8 μs and are in accord with previous studies. Using time-of-flight and laser-induced fluorescence methods, the efficiencies of rotational and vibrational relaxation of ground state Br2 have been studied by measuring the translational, rotational and vibrational temperatures of the Br2 in the expansion. For pure Br2, collision numbers Z R ∼ 1 and Z V ∼ 350 have been obtained. From measurements on seeded beams of Br2, vibrational relaxation cross sections have been obtained for Br2 with various gases. These cross sections lie in the range 0·18–2·41 Å2 and indicate collision efficiencies between 0·004–0·05. These values are less than corresponding values for I2 indicating a reduced relaxation efficiency. However, the values indicate very efficient vibrational relaxation of Br2 at the low temperatures of the beams, which is inconsistent with the usual Landau-Teller model. This is attributed to additional relaxation being achieved by the formation and subsequent dissociation of metastable or van der Waals complexes. Efficient V→R energy transfer is observed in collisions of Br2(ν = 1) with H2.