Accurate photoionisation cross section for He at non-resonant photon energies

TL;DR

This paper presents highly accurate calculations of the helium photoionisation cross section across a broad energy range, confirming previous theoretical results and highlighting deviations from experimental data outside estimated errors.

Contribution

The study introduces a full configuration-interaction approach with complex scaling for precise cross section calculations and extends results with a universal hydrogenic tail model.

Findings

Results agree with previous calculations within 0.01% error

Confirms deviations from experimental data outside error estimates

Introduces a universal tail model for high-energy extrapolation

Abstract

The total single-photon ionisation cross section was calculated for helium atoms in their ground state. Using a full configuration-interaction approach the photoionisation cross section was extracted from the complex-scaled resolvent. In the energy range from ionisation threshold to 59\,eV our results agree with an earlier $B$-spline based calculation in which the continuum is box discretised within a relative error of $0.01\%$ in the non-resonant part of the spectrum. Above the $\He^{++}$ threshold our results agree on the other hand very well to a recent Floquet calculation. Thus our calculation confirms the previously reported deviations from the experimental reference data outside the claimed error estimate. In order to extend the calculated spectrum to very high energies, an analytical hydrogenic-type model tail is introduced that should become asymptotically exact for infinite photon energies. Its universality is investigated considering also H$^-$, Li$^+$, and HeH$^+$. With the aid of the tail corrections to the dipole approximation are estimated.