Quantum dynamics of the intramolecular vibrational energy redistribution in OCS: From localization to quasi-thermalization

TL;DR

This study uses quantum dynamics to explore how vibrational energy redistributes within OCS molecules, revealing a transition from localized to thermalized states and highlighting the slow energy flow leading to dissociation.

Contribution

It provides a detailed quantum-mechanical analysis of IVR in OCS, connecting localization, chaos, and thermalization, and demonstrates the slow energy redistribution prior to dissociation.

Findings

Identifies a localization to delocalization transition linked to quantum chaos.

Shows near-thermalization occurs at high energy and short times (~1 ps).

Reveals IVR is slow, with dissociation happening before full thermalization.

Abstract

We report a fully quantum-dynamical study of the intramolecular vibrational energy redistribution (IVR) in the electronic ground state of carbonyl sulfide (OCS), which is a prototype of an isolated many-body quantum system with strong internal couplings and non-Rice-Ramsperger-Kassel-Marcus (RRKM) behavior. We pay particular attention to the role of many-body localization and the approach to thermalization, which currently are topics of considerable interest, as they pertain to the very foundations of statistical mechanics and thermodynamics. We employ local-mode (valence) coordinates and consider initial excitations localized in one local mode, with energies ranging from low to near the dissociation threshold, where the classical dynamics have been shown to be chaotic. We propagate the nuclear wavepacket on the potential energy surface by means of the numerically exact multiconfiguration time-dependent Hartree method and employ mean local energies, time-dependent and time-averaged populations in quantum number space, energy distributions, entanglement entropies, local population distributions, microcanonical averages, and dissociation probabilities, as diagnostic tools. This allows us to identify a continuous localization $\rightarrow$ delocalization transition in the energy flow, associated with the onset of quantum chaos, as the excitation energy increases up to near the dissociation threshold. Moreover, we find that at this energy and $\sim$1 ps the molecule nearly thermalizes. Furthermore, we observe that IVR is so slow that the molecule begins to dissociate well before such quasi-thermalization is complete, in accordance with earlier classical-mechanical predictions of non-RRKM behavior.