A single short pulse produced by the excited state of Na atom exposed to an infrared laser

Abstract

The time-dependent Schrodinger equation of alkali metal Na atom in an infrared laser field is solved numerically by using the pseudo-spectral method. In the calculation, an accurate model potential of Na atom is used. The bound state energy levels, which are consistent with experimental data, are obtained with the potential, so that we can study the characteristics of high-order harmonic generation for emission of the exited stated of Na atom. Our results show that the high-order generation spectrum of emission of 4s, and 5s excited states of Na atom is super-continuum in the over-barrier ionization regime. By superposed certain orders harmonics below threshold, a single pulse can be obtained with the central frequency from high frequency of visible light to the ultraviolet band. Through the calculated ionization probability of Na atom and the time-frequency analysis by wavelet transform of the superposed harmonics, it reveals that the emission process of low-order harmonic generation in over-barrier ionization regime is different from in the tunnel ionization regime.