Time delay anisotropy in photoelectron emission from the isotropic ground state of helium

TL;DR

This study reveals that time delays in photoelectron emission from helium's ground state are anisotropic and depend on emission direction, challenging the assumption of angle-independence in isotropic states, with implications for attosecond measurements.

Contribution

The paper demonstrates the existence of angular dependence in photoelectron time delays from helium's isotropic ground state, supported by experimental and theoretical analysis, highlighting a previously unrecognized effect in photoionization.

Findings

Measured time delay anisotropy up to 60 attoseconds.

Time delay depends on emission angle relative to polarization axis.

Effect arises from interference of quantum states with different symmetries.

Abstract

Time delays of electrons emitted from an isotropic initial state and leaving behind an isotropic ion are assumed to be angle-independent. Using an interferometric method involving XUV attosecond pulse trains and an IR probe field in combination with a detection scheme, which allows for full 3D momentum resolution, we show that measured time delays between electrons liberated from the $1s^2$ spherically symmetric ground state of helium depend on the emission direction of the electrons relative to the linear polarization axis of the ionizing XUV light. Such time-delay anisotropy, for which we measure values as large as 60 attoseconds, is caused by the interplay between final quantum states with different symmetry and arises naturally whenever the photoionization process involves the exchange of more than one photon in the field of the parent-ion. With the support of accurate theoretical models, the angular dependence of the time delay is attributed to small phase differences that are induced in the laser-driven continuum transitions to the final states. Since most measurement techniques tracing attosecond electron dynamics involve the exchange of at least two photons, this is a general, significant, and initially unexpected effect that must be taken into account in all such photoionization measurements.