Vibrational Energy Relaxation (VER) is modelled to provide a rigorous investigation on the accuracy of the statistical regime interpretation of VER kinetics and to characterize the decoherence of quantum vibrational states. Results clearly demonstrate that when considering a typical chromophore in solution (the aqueous azide ion) the thermal equilibrium condition of the atomic-molecular classical coordinates determines the statistical behavior of the quantum-trajectories, with the corresponding density operator (always clearly showing an ergodic behavior) either being incoherent within the whole relaxation process or rapidly loosing its initial coherence.

Abstract

A theoretical-computational procedure, recently proposed for modelling Vibrational Energy Relaxation (VER) processes of a molecule (Quantum Center, QC) embedded in a complex atomic-molecular system, is extended and applied for analyzing in detail the features of the QC density matrix (DM) temporal evolution. The results, obtained using aqueous azide ion as a case study, show the total lack of coherence in the DM, when the system is prepared to be initially in a pure vibrational eigenstate. This finding is fully in line with the statistical interpretation of the process typically adopted also in the experimental studies where the relaxation processes are all described within the typical schemes of chemical kinetics. Consistently, when the initial vibrational state corresponds to an eigenstate mixture, although initially coherent, the DM relaxes to a fully incoherent condition with a mean lifetime related to the one of the diagonal elements relaxation. These specific DM features turn out to be essentially governed by the thermal equilibrium condition of the atomic-molecular classical coordinates which drive the ensemble of the quantum-trajectories toward the observed statistical regime. Finally, from the analysis of a single long timescale quantum vibrational trajectory it also clearly emerges its ergodic behaviour.