Study On High-order Harmonic Generation From Vibrating Diatomic Molecules

High-order harmonic generation (HHG) from molecules is one of the nonlinear and non perturbative phenomena during the interaction between molecules and intense, ultrashort laser pulses. The hydrogen molecular ion as the simplest molecule has been studied extensively. By solving the time-dependent Schrodinger equation, the harmonic spectra are studied, including the interference effect and the dependence of the harmonic intensity on the internuclear distance. The harmonic spectra are also used to probe the frequency information of vibrational levels.We present the harmonic spectra obtained by Born-Oppenheimer approximation from H2+ . By calculating the harmonic spectrum for fixed nuclei, we find that the minima positions causing by the interference effect disagree with the position predicted by the simple double-slit interference model. The recombination process is investigated showing that the failure of the interference model is the plane-wave approximation which results in the problem of the conversion from the kinetic energy of electron to the energy of emitted photon. The interference effect form vibrating molecule is studied and it is found that there are several extreme in the harmonic spectrum arising from the nuclear movement therefore the molecular structure is complicated to image for vibrating molecules. Applying the time-frequency analysis, we can see the interference effect more clearly.We present a fully quantum mechanical calculation of the harmonic spectra from vibrating molecules H2+ and D+2 . The laser-induced dynamics is studied and it is found that the emission efficiency of the harmonics is correlated with the time varying internuclear separation. The high-order harmonic generation emitted from different initial and final vibrational states may cause the oscillation of the harmonic intensity following the nuclear oscillation. By considering the molecules prepared in vibrationally coupled states, the frequency information of the coupled vibrational levels can be extracted from the harmonic yields. We therefore demonstrate the vibrational dynamics initiated by the pump or by the probe pulse can be investigated simultaneously by the HHG processes.