The Kramers Problem in the Energy-Diffusion Limited Regime

TL;DR

This paper develops an analytical method to calculate the time-dependent transmission coefficient in the energy-diffusion limited regime of the Kramers problem, extending previous high-friction approaches to low friction and low temperature conditions.

Contribution

It introduces a parallel analytical approach for the low-friction regime of the Kramers problem, providing explicit formulas for the rate coefficient's full time and temperature dependence.

Findings

Analytic results match numerical simulations across regimes.

Long-time behavior aligns with Kramers' low-friction limit.

Energy loss rate is key to the transmission coefficient evolution.

Abstract

The Kramers problem in the energy-diffusion limited regime of very low friction is difficult to deal with analytically becasue of the repeated recrossings of the barrier that typically occur before an asymptotic rate constant is achieved. Thus, the transmission coefficient of particles over the potential barrier undergoes oscillatory behavior in time before settling into a steady state. Recently Kohen and Tannor (JCP Vol. 103, Pg. 6013, 1995) developed a method based on the phase space distribution function to calculate the transmission coefficient as a function of time in the high-friction regime. Here we formulate a parallel method for the low-friction regime. We find analytic results for the full time and temperature dependence of the rate coefficient in this regime. Our low-friction result at long times reproduces the equilibrium result of Kramers at very low friction and extends it to higher friction and lower temperatures below the turn-over region. Our results indicate that the single most important quantity in determining the entire time evolution of the transmission coefficient is the rate of energy loss of a particle that starts above the barrier. We test our resutls, as well as those of Kohen and Tannor for the Kramers problem, against detailed numerical simulations.