# Time delays from one-photon transitions in the continuum

## TL;DR

This paper experimentally measures how the time delay in attosecond photoionisation depends on the angular momentum of electrons, revealing a universal continuum-continuum transition delay of up to 12 attoseconds near ionisation threshold.

## Contribution

First experimental quantification of angular momentum dependence in continuum-continuum photoionisation delays using energy and angle-resolved spectra.

## Key findings

- Delay between s- and d-electrons can reach 12 as near threshold.
- Universal continuum-continuum delay confirmed in helium and hydrogen.
- Angular momentum significantly influences photoionisation time delays.

## Abstract

Attosecond photoionisation time delays reveal information about the potential energy landscape an outgoing electron wavepacket probes upon ionisation. In this study we experimentally quantify, for the first time, the dependence of the time delay on the angular momentum of the liberated photoelectrons. For this purpose, electron quantum-path interference spectra have been resolved in energy and angle using a two-color attosecond pump-probe photoionisation experiment in helium. A fitting procedure of the angle-dependent interference pattern allows us to disentangle the relative phase of all four quantum pathways that are known to contribute to the final photoelectron signal. In particular, we resolve the dependence on the angular momentum of the delay of one-photon transitions between continuum states, which is an essential and universal contribution to the total photoionization delay observed in attosecond pump-probe measurements. For such continuum-continuum transitions, we measure a delay between outgoing s- and d-electrons as large as 12 as close to the ionisation threshold in helium. Both single-active-electron and first-principles ab initio simulations confirm this observation for helium and hydrogen, demonstrating the universality of the observed delays.