A simple generalization of the energy gap law for nonradiative processes

TL;DR

This paper revisits and generalizes the energy gap law for nonradiative processes, incorporating temperature effects and low frequency modes, leading to improved predictive accuracy over the original law.

Contribution

It provides a new generalized form of the energy gap law that accounts for temperature and low frequency vibrational effects, enhancing its applicability.

Findings

The generalized law improves rate predictions compared to the original.

Comparison with stationary phase approximations suggests a universal interpolation formula.

Test calculations confirm the effectiveness of the generalized model.

Abstract

For more than 50 years, an elegant energy gap (EG) law developed by Englman and Jortner [Mol. Phys. {\bf 18}, 145 (1970)] has served as a key theory to understand and model nearly exponential dependence of nonradiative transition rates on the difference of energy between the initial and final states. This work revisits the theory, clarifies key assumptions involved in the rate expression, and provides a generalization for the cases where the effects of temperature dependence and low frequency modes cannot be ignored. For a specific example where the low frequency vibrational and/or solvation responses can be modeled as an Ohmic spectral density, a simple generalization of the EG law is provided. Test calculations demonstrate that this generalized EG law brings significant improvement over the original EG law. Both the original and generalized EG laws are also compared with stationary phase approximations developed for electron transfer theory, which suggests the possibility of a simple interpolation formula valid for any value of EG.