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 rota tional 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 sl it nozzle expansion. These fluorescence quantum yields provide vibrati onal predissociation efficiencies for B state I2-Ar as a function of v ibrational 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 pr edissociation. Fluorescence intensities of combination bands with exci tation in the van der Waals modes were also found to have oscillating v' dependencies with patterns nearly identical to that for the bands w ithout van der Waals mode excitations. Thus, these oscillations must a rise from the electronic predissociation channel, rather than the vibr ational channel. Deconvolution of the lifetime of B state I2-Ar into v ibrational and electronic lifetimes indicates that the similar overall lifetimes at v' of 18 and 21 result from a twofold increase in the el ectronic lifetime at v' = 21, which compensates for a decrease in the vibrational lifetime. Assumption of a smooth v' dependence for the vib rational lifetime leads to oscillatory predicted overall lifetimes of 35, 77, 82, 51, and 30 ps over the v' range of 20-24, respectively. Ba sed on symmetry arguments, as well as the observed vibrational prediss ociation efficiencies, the electronic predissociation of I2-Ar must ar ise 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.