Emission spectra and relaxation dynamics of excited 79Br2 in Ar and Kr matrices

The photochemical dynamics of Br2 trapped in Ar and Kr matrices have been studied. Following laser excitation into either the B 3Π(0+u) or 1Π(1u) states, emission occurs in the near infrared spectral region. Analysis of this emission indicates that it consists of three electronic transitions which have been assigned as B 3Π(0+u) v′ = 0→X 1Σ+g (previously reported,) A 3Π(1u) v′ = 0→X 1Σ+g and (tentatively) A′ 3Π(2u)→X 1Σ+g. The B→X emission lifetime was measured to be 8.0±0.5 μs in Ar matrices (5.3±0.5 μs in Kr) and is in good agreement with both earlier determinations in matrices and the gas phase B→X radiative lifetime. The observed A→X emission lifetime of 170±10 μs in Kr is approximately the same as the measured A→X gas phase radiative lifetime, but appears shortened in Ar matrices where the lifetime is 67±4 μs. The lifetime of the A′→X transition is 11±1 ms in Ar and 6±1 ms in Kr matrices. The spectral and temporal data are consistent with an overall energy transfer mechanism involving both vibration and electronic relaxation. The intramolecular electronic relaxation is somewhat restricted in that the highest lying B 3Π(0+u) state is populated only by direct absorption into its bound and continuum levels, whereas the two lower states A 3Π(1u) and A′ 3Π(2u) acquire population after either the 1Π(1u) or B 3Π(0+u) states are initially excited. Both the initial vibrational and electronic relaxation rapidly (< 20 ns) routes the excited population into the lowest v′ = 0 levels of the A and B states (and tentatively A′), where subsequent decay occurs on a 10−6–10−2 s timescale. All of the observed emission lifetimes are independent of temperature, laser power and Br2: matrix dilutions ranging from 1:500–1:10 000.