EFFICIENCY AND MECHANISM OF ELECTRONIC PREDISSOCIATION OF B-STATE I-2-AR

The isomer of the I2-Ar complex which yields discrete bands in the B<--X spectrum is shown, as expected, to be T shaped on the basis of rotational structure observed in the vibronic bands. Precise fluorescence quantum yields for I2-Ar relative to I2 were measured via simultaneous acquisition of absorption and fluorescence excitation spectra in a slit nozzle expansion. These fluorescence quantum yields provide vibrational predissociation efficiencies for B state I2-Ar as a function of vibrational state from v' of 15 to 26. This is an oscillating function with local maxima at v' of 16, 22, and 26. For v' = 22 and 26, 7 3 % +/- 3 % of the complexes undergo vibrational, rather than electronic predissociation. Fluorescence intensities of combination bands with excitation in the van der Waals modes were also found to have oscillating v' dependencies with patterns nearly identical to that for the bands without van der Waals mode excitations. Thus, these oscillations must arise from the electronic predissociation channel, rather than the vibrational channel. Deconvolution of the lifetime of B state I2-Ar into vibrational and electronic lifetimes indicates that the similar overall lifetimes at v' of 18 and 21 result from a twofold increase in the electronic lifetime at v' = 21, which compensates for a decrease in the vibrational lifetime. Assumption of a smooth v' dependence for the vibrational lifetime leads to oscillatory predicted overall lifetimes of 35, 77, 82, 51, and 30 ps over the v' range of 20-24, respectively. Based on symmetry arguments, as well as the observed vibrational predissociation efficiencies, the electronic predissociation of I2-Ar must arise from coupling of the B state to the PI(g) state. This coupling may also be the dominant channel for collisional quenching of B state I2.