Origin of high energy enhancement of photoelectron spectra in tunneling ionization

TL;DR

This paper investigates the high energy enhancement in photoelectron spectra during tunneling ionization, revealing it as a classical effect caused by Coulomb momentum transfer leading to electron bunching.

Contribution

It demonstrates that the high energy enhancement is a classical phenomenon resulting from Coulomb momentum transfer, supported by Monte Carlo and quantum simulations.

Findings

High energy enhancement is classical in origin.

Coulomb momentum transfer causes electron bunching.

Quantum effects slightly amplify the spectrum.

Abstract

Recently, in a strong Coulomb field regime of tunneling ionization an unexpected large enhancement of photoelectron spectra due to the Coulomb field of the atomic core has been identified by numerical solution of time-dependent Schr\"odinger equation [Phys. Rev. Lett. \textbf{117}, 243003 (2016)] in the upper energy range of the tunnel-ionized direct electrons. We investigate the origin of the enhancement employing a classical theory with Monte Carlo simulations of trajectories, and a quantum theory of Coulomb-corrected strong field approximation based on the generalized eikonal approximation for the continuum electron. Although the quantum effects at recollisions with a small impact parameter yield an overall enhancement of the spectrum relative to the classical prediction, the high energy enhancement itself is shown to have a classical nature and is due to momentum space bunching of photoelectrons released not far from the peak of the laser field. The bunching is caused by a large and nonuniform, with respect to the ionization time, Coulomb momentum transfer at the ionization tunnel exit.