On the Cooper Minimum in Singly ionized and Neutral Argon

TL;DR

This study investigates the Cooper Minimum in singly ionized and neutral argon, analyzing how atomic structure details influence photoionization and high-harmonic generation spectra, revealing consistent minima positions and effects of magnetic alignment.

Contribution

It provides a detailed comparison of the Cooper Minimum in Ar+ and neutral Ar, highlighting the impact of atomic structure and magnetic alignment on spectral features using R-matrix calculations.

Findings

The Cooper Minimum appears at 55 eV in PICS and 60 eV in HHG spectrum.

HHG minimum shifts to higher energies compared to PICS minimum.

Magnetic quantum number M_L influences harmonic yield but not minimum position significantly.

Abstract

We present an analysis of the appearance of the Cooper Minimum in singly ionized argon in both the photoionization cross-section (PICS) and high-harmonic generation (HHG) spectrum. We employ two computational approaches based on the same R-matrix technique to provide a coherent description of the atomic structure of the Ar+ system, finding that the PICS and HHG spectrum are affected differently by the inclusion of additional residual ion states and the improved description of correlation effects. Both the PICS and HHG spectrum possess a clear minimum for all atomic structure models used, with the centre of the minimum at 55 eV in the PICS and 60 eV in the HHG spectrum for the most complete description employed. The HHG minimum is systematically shifted to higher energies with respect to the PICS minimum. We also find that the initial magnetic alignment (magnetic quantum number) of the Ar+ system does not affect substantially the position and shape of the HHG minimum (given a sufficiently detailed atomic structure description), but the harmonic yield is enhanced by two-orders of magnitude for M_L = 1 over M_L = 0. We also perform similar calculations for neutral argon, finding that this system is more sensitive to enhancements in the atomic structure description.