Absolute HHG yields in argon from R-matrix with Time Dependence calculations

We present ab initio calculations of the high-order harmonic generation (HHG) spectrum in argon using the R-matrix with Time Dependence (RMT) method [1] for strong-field, few-cycle optical pulses. The calculations reproduce the harmonic structure, cutoff energy, and Cooper minimum observed experimentally [2]. The all-electron RMT method complements previous work, which has typically employed single-active-electron methods combined with pseudopotentials. We present the spectrum in absolute units, noting that the integrated yield (energy fluence) is, in principle, measurable and provides a stringent benchmark for various HHG calculations. We investigate the strong carrier-envelope-phase (CEP) dependence of the HHG spectra. A central focus is the time window used in the Fourier transform to calculate the HHG spectrum. Experimentally, for observation times shorter than the relevant spontaneous decay and decoherence times, but longer than the pulse length, residual bound-state coherence, associated with excited Rydberg populations, appears in the spectrum. Likewise, TDSE simulations exhibit time-window-dependent features in the HHG spectrum. We also report the influence of common windowing procedures on these effects. 

[1] A.C. Brown et al., Comp. Phys. Commun. 250 (2020) 107062.

[2] C. Guo et al., J. Phys. B 51 (2018) 034006.