Two-Color Strong-Field Photoelectron Spectroscopy and the Phase of the Phase

TL;DR

This paper introduces a two-color phase-of-the-phase spectroscopy method to analyze how weak second-harmonic fields influence electron yields in intense-laser ionization, revealing target-sensitive structural information.

Contribution

It develops and demonstrates a novel spectroscopy technique that quantifies phase effects on electron momentum distributions, enabling high-precision structural analysis.

Findings

Universal RPC and PP spectral structures observed across targets

Spectra sensitive to target structure and dynamics

Simple model explains electron trajectory effects

Abstract

The presence of a weak second-harmonic field in an intense-laser ionization experiment affects the momentum-resolved electron yield, depending on the relative phase between the $\omega$ and the $2\omega$ component. The proposed two-color 'phase-of-the-phase spectroscopy' quantifies for each final electron momentum a relative-phase contrast (RPC) and a 'phase of the phase' (PP), describing how much and with which phase lag, respectively, the yield changes as function of relative phase. Experimental results for RPC and PP spectra for rare gas atoms and CO$_2$ are presented. The spectra demonstrate a rather universal structure that is analyzed with the help of a simple model based on electron trajectories, wave-packet spreading, and (multiple) rescattering. Details in the PP and RPC spectra are target sensitive and thus may be used to extract structural (or even dynamical) information with high accuracy.