Influence of multiple ionization thresholds on harmonic generation from Ar +

We apply time-dependent $R$-matrix theory to investigate harmonic generation from ground-state Ar${}^{+}$ with $M=0$ at a wavelength of 390 nm. Contributions associated with the different $3{s}^{2}3{p}^{4}$ ionization thresholds are assessed, including the interference between these. The dominant contribution originates from the second ionization threshold, $3{s}^{2}3{p}^{4}\phantom{\rule{0.16em}{0ex}}{}^{1}\phantom{\rule{-0.16em}{0ex}}D$. Changes to the harmonic yields arising from the higher $3s3{p}^{5}$ thresholds are also assessed. We further confirm that Ar${}^{+}$ has a higher harmonic yield than He for the same laser pulse, despite having a higher ionization threshold.