Quantum dissipative systems beyond the standard harmonic model: features of linear absorption and dynamics

TL;DR

This paper investigates non-standard harmonic and anharmonic effects on the linear absorption and dynamics of dissipative quantum systems using a stochastic Schrödinger equation approach, revealing new spectral features and dynamics.

Contribution

It introduces a model with differing curvatures in ground and excited states and explores anharmonic effects using Morse potentials, advancing understanding beyond standard harmonic approximations.

Findings

Curvature differences cause additional sub-structure in absorption spectra.

Anharmonicity influences population dynamics and spectral features.

Model explains features of the stiff-stilbene photoswitch.

Abstract

Current simulations of ultraviolet-visible absorption lineshapes, and dynamics of condensed phase systems, largely adopt a harmonic description to model vibrations. Often, this involves a model of displaced harmonic oscillators that have the same curvature. Although convenient, for many realistic molecular systems this approximation no longer suffices. We elucidate non-standard harmonic, and anharmonic effects, on linear absorption and dynamics using a stochastic Schr\"{o}dinger equation approach to account for the environment. Firstly, a harmonic oscillator model with ground and excited potentials that differ in curvature is utilised. Using this model, it is shown that curvature difference gives rise to an additional sub-structure in the vibronic progression of absorption spectra. This effect is explained, and subsequently quantified, via a derived expression for the Franck-Condon coefficients. Subsequently, anharmonic features in dissipative systems are studied, using a Morse potential, and parameters that correspond to the diatomic molecule $H_{2}$ for differing displacements and environment interaction. Lastly using a model potential, the population dynamics and absorption spectra for the stiff-stilbene photoswitch is presented and features are explained by a combination of curvature difference and anharmonicity in the form of potential energy barriers on the excited potential.