Power-law decay in the nonadiabatic photodissociation dynamics of alkali halides due to quantum wavepacket interference

TL;DR

This paper demonstrates that nonadiabatic photodissociation of alkali halides exhibits a power-law decay due to quantum interference, contrasting with the commonly assumed exponential decay, and introduces a diagrammatic method to analyze this phenomenon.

Contribution

It develops a diagrammatic approach and analytical formula to explain power-law decay in nonadiabatic dynamics, highlighting quantum interference effects.

Findings

Power-law decay with exponent -1/2 observed in simulations.

Quantum interference from wavepacket bifurcation causes decay.

Analytical formula matches numerical wavepacket results.

Abstract

The nonadiabatic photodissociation dynamics of alkali halide molecules excited by a femtosecond laser pulse in the gas phase are investigated theoretically, and it is shown that the population of the photoexcited molecules exhibits power-law decay with exponent -1/2, in contrast to exponential decay, which is often assumed in femtosecond spectroscopy and unimolecular reaction theory. To elucidate the mechanism of the power-law decay, a diagrammatic method that visualizes the structure of the nonadiabatic reaction dynamics as a pattern of occurrence of dynamical events, such as wavepacket bifurcation, turning, and dissociation, is developed. Using this diagrammatic method, an analytical formula for the power-law decay is derived, and the theoretical decay curve is compared with the corresponding numerical decay curve computed by a wavepacket dynamics simulation in the case of lithium fluoride. This study reveals that the cause of the power-law decay is the quantum interference arising from the wavepacket bifurcation and merging due to nonadiabatic transitions.