Correlating Stokes Shift with Huang‐Rhys Parameter for Diatomic Molecules: Effects of Finite Temperature, Anharmonicity and Breakdown of Franck‐Condon Approximation

Abstract

In order to compute optical absorption and fluorescence spectra, the harmonic oscillator model for molecular vibrations under Franck‐Condon approximation for optical transition at absolute zero temperature is often considered for ease of numerical simulation, which never truly reproduce the experimental spectra. In this article, we show how systematic introduction of realistic situation, i. e., anharmonicity of vibrations, thermalization and non‐adiabatic transition leading to failure of Franck‐Condon approximation plays a colossal role in determining the spectral shape of a model diatomic molecule. In particular, we investigate how the Huang‐Rhys parameter, describing the relative positioning of the ground and excited electronic surfaces, is correlated with the Stokes shift, considering both right‐ and left‐shifted potential wells, which is crucial to estimate changes in molecular length‐scale from experimentally determined quantities. Further, the result for iodine is shown to have excellent agreement with experimental absorption spectrum. These findings are promised to bring insights into origin of spectral shapes in complex polyatomic molecules.