Energy partitioning in photodissociation and photosensitization

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Abstract
The molecular dynamics of photodissociation and of electronic vibrational/translational energy transfer have been investigated in terms of an impulsive ‘half-collision’ model, modified to accommodate the possibility of changes in the potential energy functions (force constants) of the separating fragments. The model, which is suitable for any system which is suddenly prepared on a repulsive potential energy surface (e.g. following electronic excitation or radiationless transition), has been applied to the quenching of Hg(63P1) by CO and NO and to the photodissociation of ICN. Experimental data and alternative theoretical treatments are available for each of these systems. The new model is able to achieve a remarkable agreement with observation in the Hg/CO and NO systems by assuming the intermediate formation of a Hg(63P0) - CO or NO complex in which the C-O or N-O bond order increases by ∼ 0·6. Model calculations for the quenching of Cd and Zn(3P0) suggest that the distributions over final states would be very similar to that for Hg(3P0). A re-appraisal of the near U.V. photo-dissociation of ICN indicates that CN radicals should be formed preferentially in the Ã2π electronic state; those observed in the ground state should be vibrationally excited whether they appear as primary photo-products or not. In general, changes in the equilibrium bond length of the molecular fragment as the system transfers onto the repulsive potential surface, are crucial in determining both the level of vibrational excitation and the detailed distribution over final states. When the molecular fragment has a large force constant the ‘steepness’ of the potential surface is relatively unimportant.