Origin of the Low Energy Structure in Above Threshold Ionization

TL;DR

This paper develops an ab initio analytic theory explaining the low energy structures in above threshold ionization, attributing them to Coulomb scattering effects and rescattering, with results matching experimental observations.

Contribution

It introduces a regularization scheme for the S matrix revealing the Coulomb origin of VLES and LES, and explains their dependence on laser polarization.

Findings

VLES results from multiple Coulomb forward scattering without photon absorption.

LES involves Coulomb threshold effects and forward rescattering with photon absorption.

The theory predicts slow electrons near zero momentum, consistent with recent experiments.

Abstract

We present an ab initio analytic theory to account for both the very low energy structure (VLES) [C. Y. Wu et al., Phys. Rev. Lett. 109, 043001 (2012); W. Quan et al., Phys. Rev. Lett. 103, 093001 (2009)], and the low energy structure (LES) [W. Quan et al. Phys. Rev. Lett. 103, 093001 (2009); C.I. Blaga et al., Nat. Phys. 5, 335 2009)] of above threshold ionization. The origin of both VLES and LES lies in a forward scattering mechanism by the Coulomb potential. We parameterize the S matrix in terms of ?, which is the displacement of the the classical motion of an electron in the laser field. When ? = 0, the S matrix is singular, which we attribute to be forward Coulomb scattering without absorption of light quanta. By devising a regularization scheme, the resulting S matrix is non-singular when ? = 0, and the origins of VLES and LES are revealed. We attribute VLES to multiple forward scattering of near-threshold electrons by the Coulomb potential, with no absorption of light quanta, signifying the role of the Coulomb threshold effect. We attribute LES to be due to the combined role of the Coulomb threshold effect and rescattering in the forward direction by the Coulomb potential with the absorption of light quanta. A comparison of theory with experiment confirms these conclusions. Further more, recently Dura et al. [Sci. Rep. 3, 2675 (2013)] reported the detection of slow electrons at near zero momentum, at 1.3 meV, which is much below the VLES, almost at threshold. Our theoretical formulation gives rise to slow electrons at near zero momentum and at threshold. In addition, for circularly polarized fields, it conserves the angular momentum in the ionization process which necessitate the disappearance of the VLES, LES and the slow electrons near threshold.