Nonadiabatic decay of metastable states on coupled linear potentials

TL;DR

This paper investigates the nonadiabatic decay of metastable states at avoided crossings of coupled potential energy curves, providing non-perturbative lifetime calculations validated across multiple methods and analyzing their dependence on system parameters.

Contribution

It introduces a comprehensive non-perturbative approach to calculate lifetimes of metastable states on coupled potentials, comparing multiple methods and analyzing parameter dependence.

Findings

Lifetimes increase by about ten orders of magnitude with increased adiabaticity parameter V.

Non-perturbative methods agree well, validating their use for MSAC lifetime calculations.

Semi-classical Landau-Zener predictions can be significantly inaccurate, up to twenty times off.

Abstract

Avoided crossings of level pairs with opposite slopes can form potential energy curves for the external degree of freedom of quantum particles. We investigate nonadiabatic decay of metastable states on such avoided crossings (MSACs) using diabatic and adiabatic representations. The system is described by a single scaled adiabaticity parameter, $V$. The time-independent two-component Schr\"odinger equation is solved in both representations, and the nonadiabatic lifetimes of MSACs are determined from a wave-function flux calculation and from the Breit-Wigner formula, leading to four lifetime values for each MSAC. We also solve the time-dependent Schr\"odinger equation in both pictures and derive the MSAC lifetimes from wave-function decay. The sets of six non-perturbative values for the MSAC lifetimes agree well, validating the approaches. As the adiabaticity parameter $V$ is increased by about a factor of ten, the MSAC character transitions from marginally to highly stable, with the lifetimes increasing by about ten orders of magnitude. The $\nu$-dependence of the lifetimes in several regimes is discussed. Time-dependent perturbation theory is found to yield approximate lifetimes that deviate by $\lesssim 30\%$ from the non-perturbative results, while predictions based on the semi-classical Landau-Zener tunneling equation are found to be up to a factor of twenty off, over the ranges of $V$ and $\nu$ studied. The results are relevant to numerous atomic and molecular systems with quantum states on intersecting, coupled potential energy curves.