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

• Main Authors: Smith, Luke D, Dijkstra, Arend G
• Format: Journal Article
• Language: English
• Published: 27-08-2019
• Subjects: Physics - Chemical Physics; Physics - Quantum Physics
• DOI: 10.48550/arxiv.1908.10130

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.