Rigid cage effect on ICl photodissociation and B O+ fluorescence in rare gas matrices

The relaxation dynamics of matrix isolated ICl have been studied via time and wavelength resolved emission. Vibrationally relaxed B O+ fluorescence occurs following irradiation both above and below the gas phase dissociation barrier. The fluorescence quantum yield is near unity independent of excitation wavelength, and I35Cl and I37Cl progressions clearly separate in the emission spectrum. The excitation spectrum shows discrete thresholds for v′ = 0−3 and only smooth structure above the dissociation barrier, indicating that constrictive cage forces cause no significant vibrational quantization in the B O+ repulsive region. Irradiation below the v′ = 0 threshold also yields B O+ fluorescence by a weak sequential two photon process. Vibrational line shapes show that excitation below the barrier produces relaxed phonon emission, while irradiation above the barrier produces unrelaxed phonon structure. It is proposed that unrelaxed emission occurs from metastable orientational (librational) local minima in the v′ = 0 B O+ state. The photodissociation−recombination sequence appears to be a general method for populating phonon states not Franck−Condon accessible in bound−bound absorption.