Atomic harmonic generation in time-dependent R-matrix theory

TL;DR

This paper advances the calculation of harmonic spectra in multielectron atoms using time-dependent R-matrix theory, comparing different dipole operator forms and validating results against the HELIUM code.

Contribution

Developed a method within TDRM theory to accurately compute harmonic spectra for multielectron atoms, including the effects of different basis sets and operator forms.

Findings

Harmonic spectra are consistent across dipole length, velocity, and acceleration forms.

Dipole acceleration calculations break down at lower harmonics.

TDRM results agree well with the HELIUM code, especially with pseudostate basis sets.

Abstract

We have developed the capability to determine accurate harmonic spectra for multielectron atoms within time-dependent R-matrix (TDRM) theory. Harmonic spectra can be calculated using the expectation value of the dipole length, velocity or acceleration operator. We assess the calculation of the harmonic spectrum from He irradiated by 390 nm laser light with intensities up to 4 x 10(14) W cm(-2) using each form, including the influence of the multielectron basis used in the TDRM code. The spectra are consistent between the different forms, although the dipole acceleration calculation breaks down at lower harmonics. The results obtained from TDRM theory are compared with results from the HELIUM code finding good quantitative agreement between the methods. We find that bases which include pseudostates give the best comparison with the HELIUM code, but models comprising only physical orbitals also produce accurate results.