Energy-conservation conditions in saddle-point approximation for the strong-field-ionization of atoms

TL;DR

This paper investigates how applying energy-conservation conditions in saddle-point approximations improves the accuracy and symmetry of strong-field ionization momentum distributions, aligning them better with numerical results.

Contribution

It introduces a method to enforce energy conservation in saddle-point calculations, enhancing the physical accuracy of strong-field ionization models.

Findings

Energy-conservation enforcement yields symmetric momentum distributions.

The method aligns saddle-point results with numerical integrations.

Improves predictive power of semi-classical ionization models.

Abstract

Orbit-based methods are widespread in strong-field laser-matter interaction. They provide a framework in which photoelectron momentum distributions can be interpreted as the quantum interference between different semi-classical pathways the electron can take on its way to the detector, which brings with it great predictive power. The transition amplitude of an electron going from a bound state to a final continuum state is often written as multiple integrals, which can be computed either numerically, or by employing the saddle-point method. If one computes the momentum distribution via a saddle-point method, the obtained distribution is highly dependent on the time window from which the saddle points are selected for inclusion in the "sum over paths". In many cases, this leads to the distributions not even satisfying the basic symmetry requirements and often containing many more oscillations and interference fringes than their numerically integrated counterparts. Using the strong-field approximation, we find that the manual enforcement of the energy-conservation condition on the momentum distribution calculated via the saddle-point method provides a unique momentum distribution which satisfies the symmetry requirements of the system and which is in a good agreement with the numerical results. We illustrate our findings using the example of the Ar atom ionized by a selection of monochromatic and bichromatic linearly polarized fields.