Origin of Unexpected Low Energy Structure in Photoelectron Spectra Induced by Mid-Infrared Strong Laser Fields

TL;DR

This paper explains the origin of the low-energy structure in photoelectron spectra caused by mid-infrared strong laser fields, attributing it to multiple forward scattering and Coulomb effects, supported by a semiclassical model.

Contribution

It introduces a semiclassical model that combines tunneling and Coulomb effects to identify the cause of LES in above-threshold ionization spectra.

Findings

LES results from multiple forward scattering of ionized electrons.

Coulomb field disturbance influences the position of LES peaks.

Scaling laws relate LES parameters to laser intensity and wavelength.

Abstract

Using a semiclassical model which incorporates tunneling and Coulomb field effects, the origin of the unexpected low-energy structure (LES) in above-threshold ionization spectrum observed in recent experiments [C. I. Blaga et al., Nature Phys. {\bf 5}, 335 (2009) and W. Quanet al., Phys. Rev. Lett. {\bf 103}, 093001 (2009)] is identified. We show that the LES arises due to an interplay between multiple forward scattering of an ionized electron and the electron momentum disturbance by the Coulomb field immediately after the ionization. The multiple forward scattering is mainly responsible for the appearance of LES, while the initial disturbance mainly determines the position of the LES peaks. The scaling laws for the LES parameters, such as the contrast ratio and the maximal energy, versus the laser intensity and wavelength are deduced.