The underlying physics behind the molecular harmonic emission in relatively long $\sin {}^2$-like laser pulses is investigated. We numerically solved the full-dimensional electronic time-dependent Schrödinger equation beyond the Born-Oppenheimer approximation for simple molecular ion ${\mathrm{H}}_2{}^{+}$. The occurrence and the effect of electron localization, nonadiabatic redshift, and spatially asymmetric emission are evaluated to understand better complex patterns appearing in the high-order harmonic generation (HHG) spectrum. Results show that the complex patterns in the HHG spectrum originate mainly from a nonadiabatic response of the molecule to the rapidly changing laser field and also from a spatially asymmetric emission along the polarization direction. The effect of electron localization on the HHG spectrum was not observed as opposed to what is reported in the literature.

• Received 15 March 2016

DOI:https://doi.org/10.1103/PhysRevA.94.033415