Internuclear distance R-distribution of high-order harmonic generation from H₂⁺ and its isotopes

Highlights: • R-Distribution of molecular high-order harmonic spectra. • Numerically solving the Non-Bohn-Oppenheimer time-dependent Schrödinger equation. • The appearance of the even-order and the odd-order harmonics. • Harmonic emission from charge-resonance-enhanced-ionization. • Harmonic emission from dissociative ionization. - Abstract: Internuclear distance (R) distribution of high-order harmonic generation from H₂⁺ and its isotopes have been theoretically investigated. We found that in the presence of the few-cycle pulse, the harmonics mainly produce from R = 2.0 a.u. to R = 4.5 a.u. and the odd harmonics can be found in the harmonic spectra. With the increase of the pulse duration, the harmonics can be generated from R = 2.0 a.u. to the larger internuclear distance (e.g. R = 15.0 a.u.) and the even/odd harmonics can be obtained in the below/above-threshold harmonic regions. Moreover, the intensities of the harmonics can be enhanced in the presence of the multi-cycle pulse, but the intensities of the below-threshold harmonics can be reduced with the increase of the nuclear mass (e.g. D or T). The time-dependent nuclear motions, the ionization probabilities, the time-frequency analyses of the harmonic spectra and the R-dependent time-dependent wave functions have been shown to explain the R-distribution of the harmonic spectra from H₂⁺ and its isotopes.