Effects of laser parameters on orbital competition in high-order harmonic generation from a hydrogen cyanide molecule

Multiorbital involvement in high-order harmonic generations (HHG) emitted from molecules exposed to an intense ultrashort laser pulse is essential for probing the structures and dynamics of molecules. A comprehensive understanding of how lower-lying orbitals contribute to HHG under varying laser parameters is needed; however, current research has primarily focused on variations in laser intensity for specific molecules, while investigations into other laser parameters remain desirable. In this study, we systematically investigate the effects of laser parameters, including pulse duration, intensity, and wavelength, on the lower-lying orbitals’ fingerprints in HHG emitted from hydrogen cyanide (HCN) molecules. By solving the time-dependent Schrödinger equation when HCN is aligned with the laser field, we find that the relative contribution of HOMO and HOMO-1 to HHG is not fixed but rather depends sensitively on the laser conditions. In particular, the competition between HOMO and HOMO-1 varies with laser intensity and pulse duration but remains relatively stable across different wavelengths. Notably, as pulse duration and intensity increase, HOMO-1 rapidly suppresses HOMO, controlling total HHG in both spectral shape and HHG yield. By analyzing the harmonic time-frequency profiles, we attribute these dependencies to differences in the growth rates of the ionization probability and the recombination transition dipole as we vary the laser parameters. Interestingly, the competition in HHG differs from that in ionization probability: while HOMO dominates for short pulses, HOMO-1 becomes more favorable for longer pulses. Our findings highlight the critical importance of selecting appropriate laser conditions to investigate the ultrafast dynamics of inner-shell electrons through HHG, particularly concerning multiorbital coupling.