Harmonic generation by noble-gas atoms in the near-IR regime using ab initio time-dependent $R$-matrix theory

We demonstrate the capability of ab initio time-dependent $R$-matrix theory to obtain accurate harmonic generation spectra of noble-gas atoms at near-IR wavelengths between 1200 and 1800 nm and peak intensities up to $1.8 \times 10{}^{14} \mathrm{W}/\mathrm{cm}{}^2$. To accommodate the excursion length of the ejected electron, we use an angular-momentum expansion up to $L_{\max }=279$. The harmonic spectra show evidence of atomic structure through the presence of a Cooper minimum in harmonic generation for Kr, and of multielectron interaction through the giant resonance for Xe. The theoretical spectra agree well with those obtained experimentally.

Figure 1

Calculated harmonic spectra of Kr at a wavelength of 1800 nm for different peak laser intensities.

Figure 2

Calculated harmonic spectrum of Kr at a peak intensity of $1.8\times 10{}^{14} \mathrm{W}/\mathrm{cm}{}^2$ for different laser wavelengths.

Figure 3

HHG spectrum for Kr calculated using RMT (dotted, red line), smoothed spectrum (solid, red line), and the experimental spectrum (dashed, blue line, [6]) at a wavelength of 1800 nm and a peak intensity of $1.8 \times 10{}^{14} \mathrm{W}/\mathrm{cm}{}^2$. Although the theoretical spectrum extends to 190 eV, the experimental spectrum was only reported for photon energies up to 150 eV. The experimental data have been renormalized for comparison with the theoretical spectrum.

Figure 4

The HHG spectrum of Kr at a wavelength of 1800 nm and a peak intensity of $1.8 \times 10{}^{14} \mathrm{W}/\mathrm{cm}{}^2$ for a 10-cycle pulse with a 4-2-4 profile (a) and a four-cycle pulse with a 2-0-2 profile (b).

Figure 5

The harmonic spectrum for Xe irradiated by an 11 fs, 1800-nm laser pulse with a peak intensity of $1.9 \times 10{}^{14}$ W/$\mathrm{cm}{}^2$ as obtained in experiment (dashed, black line, [6]) and using the present approach, either including emission of a 4d electron (solid, red line) or excluding this emission (dotted, blue line). The thick lines indicate the smoothed theoretical results. The experimental data have been renormalized for comparison with the theoretical spectra.