A time-independent Hermitian Floquet approach for high harmonic generation in H2+ and HD+: Effect of nonadiabatic interaction in HD+

Abstract

We have theoretically investigated the high harmonic generation (HHG) in H(2)(+) and HD(+) using a time-independent Hermitian nonperturbative three-dimensional Floquet approach for continuous wave monochromatic lasers of intensities of 2.59x10(13), 4.0x10(13), and 5.6x10(13) W/cm(2), and wavelengths of 1064, 532, and 355 nm. For the moderate intensities and the wavelengths used, the Keldysh parameter gamma > 1 and no tunnel ionization occurs. We have endeavored to explain the dynamics of HHG in H(2)(+) and HD(+) within the framework of transitions due to electronic as well as intrinsic (for HD(+)) dipole moments and the nuclear motion on the field coupled ground and the first excited electronic states of these single-electron molecular ions, without considering any ionization. To evaluate the HHG spectra, the resonance Floquet quasienergy and the Fourier components of the Floquet state corresponding to the initial vibrational-rotational level v = 0, J = 0 have been calculated by solving the time-independent close-coupled Schrödinger equation following the Floquet ansatz. For HD(+), we have taken into account the symmetry breaking effect of the nonadiabatic interaction or coupling [beyond the Born-Oppenheimer (BO) dynamics] in our calculations of the HHG spectra and have compared the results with the HHG spectra of HD(+) obtained with the BO approximation.