The role of dipole-forbidden autoionizing resonances in non-resonant one-color two-photon single ionization of N$_2$

TL;DR

This study combines experimental and theoretical methods to investigate how dipole-forbidden autoionizing resonances influence the angular distributions in non-resonant one-color two-photon ionization of N₂ molecules, revealing their role in the ionization dynamics.

Contribution

It provides the first detailed analysis of dipole-forbidden autoionizing resonances in two-photon ionization of N₂, highlighting their impact on photoelectron angular distributions.

Findings

Autoionizing resonances cause rapid changes in electron angular distributions.

Dipole-forbidden resonances belong to Hopfield series and compete with direct ionization.

Experimental and theoretical results agree on the role of these resonances.

Abstract

We present an experimental and theoretical energy- and angle-resolved study on the photoionization dynamics of non-resonant one-color two-photon single valence ionization of neutral N$_2$ molecules. Using 9.3 eV photons produced via high harmonic generation and a 3-D momentum imaging spectrometer, we detect the photoelectrons and ions produced from one-color two-photon ionization in coincidence. Photoionization of N$_2$ populates the X $^2\Sigma^+_g$, A $^2\Pi_u$, and B $^2\Sigma^+_u$ ionic states of N$_2^+$, where the photoelectron angular distributions associated with the X $^2\Sigma^+_g$ and A $^2\Pi_u$ states both vary with changes in photoelectron kinetic energy of only a few hundred meV. We attribute the rapid evolution in the photoelectron angular distributions to the excitation and decay of dipole-forbidden autoionizing resonances that belong to series of different symmetries, all of which are members of the Hopfield series, and compete with the direct two-photon single ionization.