High-order harmonic generation of a hydrogen molecular ion in few-cycle chirped laser fields.

Using the solution of the time-dependent Schrödinger equation beyond the Born-Oppenheimer approximation, we investigate high-order harmonic generation of the hydrogen molecular ion (H2+) in a few-cycle chirped laser pulse. Our results demonstrate that a double-plateau structure in the high-order harmonic spectra shows up for H2+ subjected to a few-cycle chirped laser field when the chirp parameter changes from negative to positive. Theoretical analysis indicates that the dependence of the double-plateau structure on the chirp of the laser pulse is primarily attributed to the influence of quantum diffusion effects. Moreover, it is found that enhanced harmonic yields in the low-energy region and suppressed harmonic yields in the high-energy region appear, including the nuclear vibration, which can be attributed to competition between effects of ionization probability and squared nuclear autocorrelation function modulus. Further analysis reveals that the effects of the carrier-envelope phase (CEP) on high-order harmonic generation are strongly dependent on the chirp parameter. The above finding highlights the critical role of nuclear dynamics in modulating both spectral features and the fundamental mechanisms governing high-order harmonic generation.