Valence Transitions of Br-2 in Ar Matrices: Interaction With the Lattice And Predissociation

Fluorescence spectra from v{prime}=0 of the B, A and A{prime} states of Br{sub 2}/Ar are presented for excitation wavelengths from 630 to 540 nm with high resolution, to evaluate isotopic splittings in emission and absorption. The observed progression of sharp zero phonon lines (ZPLs) from v{prime}=2 to v{prime}=19 in B excitation is used to derive spectroscopic constants. The ZPL broadening and the growing phonon sideband (PSB) contributions indicate an increase of matrix influence on the X-B transition with rising v{prime}. Contributions of the PSB are parameterized with the Huang-Rhys coupling constant S, where S=1 near the potential minimum reflects the electron-phonon coupling and S=4 close to Franck-Condon maximum originates from vibrational coupling. The PSB spectral composition correlates with the matrix phonon density of states, and the ZPL broadens and shifts with temperature. Two crossings with repulsive states (between v{prime}=4-5 and v{prime}=7-9) leading to matrix induced predissociation and a third tentative one between v{prime}=14 and 15 are indicated by ZPL broadening, population flow, and spectral shifts. The crossing energies are close to gas phase and matrix calculations. The stepwise flow of intensity from B via repulsive states to A{prime} and, similarly, from the A continuum to A{prime} is discussed. Emission quantum efficiency of the B state decreases from near unity at v{prime}=0 to less than 10{sup -3} at v{prime}=19. Broadening of ZPL near crossings yields predissociation times of 5 and 2.5 ps corresponding to probabilities of 5% and 10% per round-trip for the two lowest crossings, respectively.