Numerical simulations of molecular multiphoton excitation models

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Abstract
In this paper we report the results of numerical simulations of the intramolecular dynamics of a model system for multiphoton excitation of large molecules, where the low energy range is represented by a single discrete state, while the quasicontinuum is mimicked by two or three manifolds of molecular eigenstates. The random coupling model (RCM), where the radiative coupling matrix elements are assumed to be random functions of the level indices, yields conventional rate equations describing consecutive–reversible transitions for the populations with golden rule rates. In addition, numerical simulations were conducted for a constant coupling model (CCM) and for a separable random coupling model (SRCM), confirming the counterintuitive analytical results for these model systems. The time evolution of a RCM system is determined by the distribution function of the coupling elements and not by individual coupling terms, and the intramolecular dynamics is essentially determined by the lower moments (average and variance) of the distribution function. On the basis of numerical simulations we have shown that a radiative RCM, based on the molecular eigenstates, is equivalent to an intramolecular RCM founded on a zero‐order molecular basis with a small number of optically active modes, random anharmonic coupling, and constant selective radiative interaction terms. Our computer experiments provide evidence for the validity of a strong coupling kinetic master equation for the RCM and suggest that random coupling is essential for the erosion of phase coherence effects in the multiphoton excitation of a molecular quasicontinuum.