Nonperturbative resonant strong field ionization of atomic hydrogen

TL;DR

This paper studies nonperturbative resonant strong field ionization of atomic hydrogen, revealing how ionization and AC Stark shifts affect bound state dynamics and photoelectron spectra, with implications for future high-energy light sources.

Contribution

It introduces an analytical model and ab initio solutions to predict and analyze nonperturbative effects in strong field ionization, including spectral shifts and broadening.

Findings

Pronounced shift and broadening of the Autler-Townes peaks with increasing field strength

The right Autler-Townes peak remains narrow and shifts less compared to the left peak

Autler-Townes duplet can be resolved with 20 fs pulses, enabling experimental observation

Abstract

We investigate resonant strong field ionization of atomic hydrogen with respect to the 1s-2p-transition. By "strong" we understand that Rabi-periods are executed on a femtosecond time scale. Ionization and AC Stark shifts modify the bound state dynamics severely, leading to nonperturbative signatures in the photoelectron spectra. We introduce an analytical model, capable of predicting qualitative features in the photoelectron spectra such as the positions of the Autler-Townes peaks for modest field strengths. Ab initio solutions of the time-dependent Schroedinger equation show a pronounced shift and broadening of the left Autler-Townes peak as the field strength is increased. The right peak remains rather narrow and shifts less. This result is analyzed and explained with the help of exact AC Stark shifts and ionization rates obtained from Floquet theory. Finally, it is demonstrated that in the case of finite pulses as short as 20fs the Autler-Townes duplet can still be resolved. The fourth generation light sources under construction worldwide will provide bright, coherent radiation with photon energies ranging from a tenth of a meV up to tens of keV, hence covering the regime studied in the paper so that measurements of nonperturbative, relative AC Stark shifts should become feasible with these new light sources.