Dynamics of linear and T-shaped Ar–I2 dissociation upon B—X optical excitation: A dispersed fluorescence study of the linear isomer

We report the dispersed fluorescence spectra of the linear and the previously well-studied T-shaped isomers of Ar–I2 following B←X optical excitation for vpump516– 26, below the I2 dissociation limit. The linear isomer has a continuum excitation spectrum. For excitation at the highest pumping energy (vpump526), the product vibrational state distribution is nearly identical to that observed for excitation above the I2(B) dissociation limit; it shows a broad, nearly Gaussian distribution of I2(B) vibrational states, with about 22% of the available excess energy deposited in translation of the Ar1I2. This gives direct evidence that the ‘‘one-atom cage’’ effect seen above the I2(B) dissociation limit is attributable to the linear Ar–I2 isomer. The product vibrational state distribution becomes increasingly Poisson for decreasing excitation energies, and only about 7% of the excess energy is deposited in translation for vpump516. The bond energy in the linear isomer is determined from the spectra, 170(61.5)<D09(linear Ar–I2(X))<174(61.5) cm . A bond energy of D09(T-shaped Ar–I2(X))5142615 cm 21 is estimated based on the linear to T-shaped population ratio observed in the beam, which is about 90 cm smaller than that determined from fluorescence spectra. We suggest that electronic quenching in the T-shaped isomer is nearly 100% for the highest vibrational level produced by vibrational predissociation. © 1999 American Institute of Physics. @S0021-9606~99!02041-3#