Coupling-strength incoherence and the theory of resonances for a system of several sparse states and quasicontinua

Abstract

The phenomena of unimolecular radiationless decay and vibronic dissipative broadening in large molecules are studied by developing a theoretical treatment of a system of several sparsely spaced zero-order states coupled to a dense set of states that have unlike physical character, thereby extending earlier work on the line shape for a single and a double resonance. Observable quantities, such as the absorption line shape, that involve averages over the dense states are shown to depend on diverse microscopic details of the quasicontinuum states only collectively through the values of a small number of matrix element correlation parameters and matrix element averages. The effects of interstate interactions on the values of the matrix-element correlation parameters and matrix-element averages are examined, and related sum rules are obtained. The physical consequences implied by uncorrelated matrix elements for nearby dense states are studied, and it is shown that relaxing the overly restrictive assumptions of earlier model treatments qualitatively changes the theoretical predictions obtained, generally in the direction of improved agreement with experiment.