High harmonic generation in H$_2^+$ and HD$^+$ by two-colour femtosecond laser pulses

[摘要] We have theoretically investigated the high harmonic generation (HHG) spectra of H$_2^+$ and HD$^+$ using a time-dependent wave packet approach for the nuclear motion with combined twocolour (1$𝜔_L$–3$𝜔_L$) pulsed lasers for ωL corresponding to wavelengths 1064 nm and 800 nm. The 1$𝜔_L$ and 3$𝜔_L$ lasers have peak intensities of $I_1^0 = 5.0 × 10^{13}$ W/cm$^2$ and $I_2^0 = 2.0 × 10^{14}$ W/cm$^2$, respectively. We have taken the pulse duration of $T = 50$ fs for both the fields, and the molecular initial vibrational level 𝑣0 = 0. We have argued that for these combinations, the harmonic generation due to transitions in the electronic continuum by tunnelling or multiphoton ionization may be neglected and only the electronic transitions within the two lowest electronic states would be important. Thus, the characteristic features of HHG spectra in the two-colour field are determined, in our model, by the nuclear motions on the two lowest field-coupled electronic states between which interelectronic and intraelectronic (due to the intrinsic dipole moments in case of HD+) radiative transitions can take place. We have studied the role of relative phase ($varphi$) of the two fields on the HHG spectra of the molecular ions. In case of HD+, the effect of nonadiabatic (NA) nonradiative interaction between the two lowest Born–Oppenheimer (BO) electronic states (1$s𝜎_g$, 2$p𝜎_u$) has been taken into account. Our calculations give realistic HHG spectra which are reasonably efficient and extended for both H$_2^+$ and HD$^+$ in the mixed two-colour field without involving the electronic continuum. The use of two-colour (1$𝜔_L$–3$𝜔_L$) field enables us to generate high harmonics beyond that achievable with a single 1$𝜔_L$ or 3$𝜔_L$ field of the corresponding intensity, frequency and pulse time.

[学科分类] 物理（综合）

[关键词] High harmonic generation;H$_2^+$;HD$^+$;two-colour laser pulses;wave packet;nonadiabatic interaction.