Floquet calculation of harmonic generation from hydrogen molecular ions in strong monochromatic laser fields

We present Floquet calculations of high harmonic generation (HHG) for the lowest two electronic states of the $\mbox{H}_2^+$ ion by strong continuous-wave laser fields. We solve the non-Hermitian matrix problem to get accurate solutions to the periodic time-dependent Schr\"odinger equation (TDSE) by applying a pseudospectral representation combined with a complex absorbing potential method. This represents an alternative approach to direct TDSE solutions to obtain the harmonic spectra for the ion. We compare our HHG rates for the lower and upper states of $\mbox{H}^{+}_{2}$, which correspond to the gerade and ungerade ground states in the field-free case, with previously obtained results in the literature. We show that the enhancement of the ionization rates at the critical internuclear separation $R_{c}\approx 8\,au$ plays some role in the appearance of very strong harmonic orders $n=5-11$ at $\lambda = 1064\,nm$ and $n=5-9$ at $\lambda = 800\,nm$ and intensity $I=10^{14}\,W/cm^{2}$.