Origin of the side peaks appearing in the above-threshold ionization spectra of Mg

Complete test of publication follows. Above-threshold ionization (ATI), a well studied process by now, is a process in which atoms absorb more than the minimum number of photons required to be ionized. The ATI spectrum consists of a series of peaks equally separated by the photon energy. We have theoretically investigated above-threshold ionization and ionization yield of Mg, a two-valence electron atom, in linearly and circularly polarized laser pulses. Since the first excited state of Mg is located only at a few eV from the ground state while those of rare gas atoms are at least several eV from the ground state, one can imagine that the low energy states may play important roles in the ATI spectra of Mg compared with rare gas atoms. The laser frequencies we have chosen correspond to the second and third harmonic of a Ti-sapphire laser. Numerical ab-initio data have been obtained by the solving the time-dependent Schrodinger equation on a discretized atomic basis. Interestingly we have found that the ATI peaks are accompanied by side peaks, which, after the detailed theoretical study to be presented in this paper, are attributed to the real excitation of some off-resonant low-lying bound state, such as 3skp (k=3,4,5, ... etc.), by the spectral wing of the laser pulse. To our knowledge, the appearance of the side peaks in the ATI spectra has not been yet reported for other atoms such as H, He, and rare-gas atoms, since, according to our interpretation, these side peaks are associated with the real excitation of a certain off-resonant low-lying bound state by the spectral wing of the pulse if the photon energy is in the visible-UV region, which naturally requires a strong single-photon coupling to appear as side peaks in the ATI spectra. That is, the origin of the ATI side peaks of Mg is directly connected to its richer electronic structure; as mentioned above, the first excited state of Mg is located just a few eV above the ground state compared to 19.8 eV for the first excited state of He. Thus, the fact that the multiphoton absorption is required to reach the excited states of He and other rare gas atoms does not allow those states to contribute to the appearance of the side peaks in the ATI spectra. In this paper, we theoretically demonstrate how we could pinpoint the physical origin of the side peaks we have obtained in the numerical ATI spectra.