Vibrational coherence in nonadiabatic dynamics

In this paper we explore temporal vibrational coherence effects in nonadiabatic radiationless transitions between two electronic states in a large molecule or in the condensed phase, accounting explicitly for the role of the (intramolecular and/or medium) vibrational quasicontinuum of the final slates. Our treatment of the time evolution of the wave packet of states and of coherence effects in the nonradiative population probabilities of the reactants and the products rests on the diagonalization of the Hamiltonian of the entire multimode system, with supplementary information being inferred from the effective Hamiltonian formalism. New features of the vibrational Franck-Condon quasicontinuum, which originate from weak, but finite, correlations between off-diagonal coupling terms, were established. The state dependence of the off-diagonal couplings V-s alpha between the doorway states manifold {\s]} and the quasicontinuum {\alpha]} was quantified by the correlation parameters eta(ss') = [Vs alphaValpha s']/[(V-s alpha(2)][V-s'alpha(2)]](1/2), where [] denotes the average over the relevant energy range. Calculations were conducted for a Franck-ConcIon four-mode system consisting of n(s) = 100 doorway states and n(alpha) = 3000 quasicontinuum states. The correlation parameters for all pairs of doorway states are considerably lower than unity (\eta(ss')\ less than or similar to 0.4), obeying propensity rules with the highest values of \eta(ss')\ corresponding to a single vibrational quantum difference, while for multimode changes between \s] and \s'] very low values of \eta(ss')\ are established. Quantum beats in the population probabilities of products and reactants in nonadiabatic dynamics are characterized by an upper limit for their modulation amplitudes xi congruent to (Gamma/Delta E)eta (for Delta E/2 pi Gamma greater than or equal to 1), where Gamma is the decay width of the doorway states and Delta E is their energetic spacing. These low xi values originate from a small (similar to Gamma/Delta E) contribution to the off-diagonal matrix elements of the nonradiative decay matrix in conjunction with low correlation parameters, The amplitudes of the quantum beats in nonradiative temporal dynamics provide dynamic information on the larger correlation parameters eta(ss') Our theoretical and numerical analysis was applied for temporal coherence effects in nonadiabatic electron transfer dynamics in a Franck-Condon quasicontinuum of Mulliken charge transfer complexes [K. Wynne, C. Reid, and R. M. Hochstrasser, J. Chem. Phys. 105, 2287 (1996)]. This accounts for the ''preparation'' (signature of coherent excitation), for thr low amplitudes of coherent temporal modulation of reactants and products (xi congruent to 0.05-0.06 determined by the eta(ss') parameters) and for the dominating contributions' to temporal coherence (subjected to propensity rules). (C) 1997 American Institute of Physics.