Periodic orbit bifurcations as an ionization mechanism: The bichromatically driven hydrogen atom

TL;DR

This paper explores how periodic orbit bifurcations influence ionization in a bichromatically driven hydrogen atom, combining classical and quantum analyses to predict ionization rates and mechanisms.

Contribution

It introduces a periodic orbit analysis method that accurately predicts ionization behavior and provides an empirical formula matching quantum simulations.

Findings

Classical and quantum simulations agree on ionization mechanisms.

Periodic orbit stability correlates with ionization rates.

Empirical formula reproduces quantum results with high accuracy.

Abstract

We investigate the multiphoton ionization of hydrogen driven by a strong bichromatic microwave field. In a regime where classical and quantum simulations agree, periodic orbit analysis captures the mechanism: Through the linear stability of periodic orbits we match qualitatively the variation of experimental ionization rates with control parameters such as the amplitudes of the two modes of the field or their relative phases. Moreover, we discuss an empirical formula which reproduces quantum simulations to a high degree of accuracy. This quantitative agreement shows the mechanism by which short periodic orbits organize the dynamics in multiphoton ionization. We also analyze the effect of longer pulse durations. Finally we compare our results with those based on the peak amplitude rule. Both qualitative and quantitative analyses are implemented for different mode locked fields. In parameter space, the localization of the period doubling and halving allows one to predict the set of parameters (amplitudes and phase lag) where ionization occurs.