Phase-Dependence of Resonant and Antiresonant Two-Photon Excitations

TL;DR

This study investigates the phase behavior of two-photon ionization processes in helium near resonances and antiresonances using high-resolution spectroscopy with tunable XUV and NIR lasers, revealing rapid phase changes and validating models.

Contribution

It provides the first detailed phase measurements of two-photon matrix elements around resonances and antiresonances, combining experimental data with theoretical models.

Findings

Rapid phase changes observed near resonances and antiresonances.

Agreement between experimental results and theoretical models.

Extension of phase measurement techniques to three-photon ionization.

Abstract

Measurements of the phase of two-photon matrix elements are presented for resonant and antiresonant two-color ionization of helium. A tunable, narrow-bandwidth, near-infrared (NIR) laser source is used for extreme ultra-violet (XUV) high-harmonic generation (HHG). The 15th harmonic of the laser is used within (1+1') XUV+NIR two-photon ionization, and tuned in and out of resonance with members of the 1s$n$p $^1$P$_1$ ($n=3,4,5$) Rydberg series, covering a broad spectral range with high spectral resolution. The technique allows to observe characteristic rapid changes in the phase of the two-photon matrix elements around the resonances and, previously unobserved, at the antiresonances between the resonances. Similar effects are observed for (1+2') XUV+NIR three-photon ionization. The experimental results are compared to a perturbative model and to numerical solutions of the time-dependent Schr\"odinger equation (TDSE) in the single active electron (SAE) approximation, elucidating the origin and dependences of the observed phenomena.