High-order harmonic generation from CH4 and CD4 molecules in the presence of a static electric field.

In this paper, an ab initio approach is used to study high-order harmonic generation (HHG) from methane and its deuteriated isotopomer within the framework of time-dependent density functional theory. The time-dependent Kohn-Sham equations are numerically solved in real-time and real-space evolution scheme. We showed that by adding a static electric field to the two-color chirped laser field, not only is the harmonic cutoff frequency extended remarkably, but also the quantum paths of the HHG are modified significantly. Furthermore, our results showed that due to the faster Jahn-Teller distortion of CH 4 cation relative to CD 4 cation, the high-order harmonic signals are more intense in heavier isotopomer. Additionally, it is found that by adding a static electric field, the difference in the cutoff frequency of the high-order harmonics increases. Finally, in order to obtain a deeper insight into the physical origin of the HHG enhancement, the semiclassical calculations and quantum time–frequency analysis are carried out. [ABSTRACT FROM AUTHOR]