Angular momentum dependence in multiphoton ionization and attosecond time delays

TL;DR

This paper extends the asymptotic approximation for attosecond ionization time delays by incorporating angular momentum dependence, validated through experiments and ab initio calculations for atomic and molecular targets.

Contribution

It introduces a rigorous method to include angular momentum effects in the asymptotic theory, improving accuracy for higher-order and threshold ionization delays.

Findings

Asymptotic theory validity is extended to threshold energies.

Angular momentum dependence can be incorporated via partial-wave-specific correction factors.

Method is validated against ab initio calculations across a wide energy range.

Abstract

Attosecond ionization time-delays at photoelectron energies above typically 10 eV are usually interpreted using the so called asymptotic approximation as a sum of the atomic or molecular delays with a universal laser-induced contribution. Here, we employ a two-harmonic RABITT (Reconstruction of Attosecond Beating by Interference of Two-photon Transitions) configuration to isolate the multiphoton pathways and measure the ionization time delays as a function of the dressing field intensity. We show that the validity of the asymptotic theory can be extended to the threshold or to higher-order contributions by rigorously treating the angular-momentum dependence of the continuum-continuum transitions into universal and easily computable partial-wave-specific correction factors. Our asymptotic treatment is also valid for higher-order interfering amplitudes while significantly simplifying their evaluation and providing a transparent physical interpretation. The validity of the method for atomic and molecular targets in the vicinity of resonances, ionization thresholds, and for both the emission-integrated and angularly resolved signal is confirmed by comparison to ab initio calculations over a wide energy range.