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

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Spectroscopy

Background:

• Current simulations of condensed-phase systems often use simplified harmonic oscillator models for molecular vibrations.
• These models, assuming identical potential curvatures for ground and excited states, are insufficient for many realistic molecular systems.
• Accurate modeling requires accounting for nonstandard harmonic and anharmonic vibrational effects.

Purpose of the Study:

• To elucidate the impact of nonstandard harmonic and anharmonic vibrational effects on linear absorption and dynamics in condensed-phase systems.
• To investigate the role of differing potential energy curvatures between electronic states.
• To analyze anharmonic features in dissipative systems and their influence on molecular dynamics.

Main Methods:

• Utilized a stochastic Schrödinger equation approach to incorporate environmental interactions.
• Employed a harmonic oscillator model with differing ground and excited state potential curvatures.
• Studied anharmonicity using a Morse potential for H2 and a model potential for stiff-stilbene.

Main Results:

• Differing potential curvatures introduce additional substructure in vibronic progressions of absorption spectra, quantified via Franck-Condon coefficients.
• Anharmonic features, including potential energy barriers, significantly influence population dynamics and absorption spectra.
• The stiff-stilbene photoswitch's spectral features are explained by a combination of curvature differences and anharmonicity.

Conclusions:

• Standard harmonic oscillator approximations are inadequate for describing complex molecular vibrations in condensed phases.
• Nonstandard harmonic effects (curvature differences) and anharmonicity are crucial for accurately simulating absorption spectra and dynamics.
• The developed approach provides a more realistic framework for understanding molecular behavior in condensed-phase environments.