Recoil-Induced Asymmetry of Nondipole Molecular Frame Photoelectron Angular Distributions in the Hard X-ray Regime

TL;DR

This study reveals how recoil effects influence nondipole photoelectron angular distributions in N$_2$ at 40 keV, showing unexpected asymmetries linked to molecular fragmentation and recoil dynamics, crucial for future XFEL experiments.

Contribution

It uncovers a novel recoil-induced asymmetry in molecular photoelectron emission, extending understanding of nondipole effects at high photon energies.

Findings

Strong forward-backward asymmetry due to nondipole effects

Unexpected asymmetry aligned with molecular fragmentation

Recoil effects cause preferential ion breakup along recoil axis

Abstract

We investigate angular emission distributions of the 1s-photoelectrons of N$_2$ ionized by linearly polarized synchrotron radiation at $h \nu=40$ keV. As expected, nondipole contributions cause a very strong forward-backward asymmetry in the measured emission distributions. In addition, we observe an unexpected asymmetry with respect to the polarization direction, which depends on the direction of the molecular fragmentation. In particular, photoelectrons are predominantly emitted in the direction of the forward nitrogen atom. This observation cannot be explained via asymmetries introduced by the initial bound and final continuum electronic states of the oriented molecule. The present simulations assign this asymmetry to a novel nontrivial effect of the recoil imposed to the nuclei by the fast photoelectrons and high-energy photons, which results in a propensity for the ions to break up along the axis of the recoil momentum. The results are of particular importance for the interpretation of future experiments at XFELs operating in the few tens of keV regime, where such nondipole and recoil effects will be essential.