[en] The author obtained an analytical expression of exchange interaction between initial ionic channel and exit covalent channels for positive-negative ions collision when the ions are separated by a large distance using an asymptotic method with unperturbed atomic wavefunctions. The expression, in which two equivalent electrons in H^- ion are taken into account, is used to calculate total charge transfer (mutual neutralisation) cross sections for H⁺ + H^- and D⁺ + H^- systems at low collision energy based on multichannel Landau-Zener model. The results obtained are compared with existing experimental data

[en] We investgate the high-order harmonic generation(HHG) emission of helium atom exposed to the midinfrared fields (λ = 2128 nm) with polarization gating by the strong field approximation(SFA) method. It is shown that the cut-off of HHG from helium atom driven by the fields is extended, and the supercontinuum spectra on the platuea near the cut-off are obtained. As a result, an isolated 44.5 as pulse is generated by superposing the some range of the supercontinuum spectra. In order to explain the emission process of HHG, we also perform the time-frequency analysis. Our results indicate that the emission of attosecond pulses outside the gating are suppressed when the polarization gating is used, thus a single isolated ultrashort pulse can be generated. (authors)

[en] Solving time-dependent Schroedinger equation numerically, we investigate the high-order harmonic generation and ionization probability of one dimensional, two dimensional and three dimensional hydrogen atom exposed to intense laser field. In the tunneling ionization regime, our results show that the HHG plateau features and cutoff positions of model hydrogen atoms are well agreement with those of real hydrogen atom, and the trend of changing of the ionization probabilities with time is similar, but the values of ionization probabilities for model atoms are different from ones for three dimensional hydrogen atom. We explain the reason for the difference of ionization probabilities between model atoms and real hydrogen atom according to the semiclassical three-step model. (author)

[en] There exist notable discrepancies with regard to the resonance structures when compared R-matrix calculations from the Opacity Project and other sources with recent absolute experimental measurements by Bizau et al. (2005 Astronomy and Astrophysics. 439 387) for Be-like ions N³⁺ and O⁴⁺ and B-like ions N²⁺,O³⁺ and F⁴⁺. To remove the differences between theory and experiment, we performed close-coupling calculations based on the R-matrix formalism for the photoionizations of ions mentioned above both for the ground states and the first excited states in the near threshold regions. The present results are compared with experimental ones by Bizau et al. and earlier theoretical ones. Excellent agreement is obtained between our theoretical predictions and the experimental photoionization cross sections. The present calculations show a significant improvement in comparison with previous theoretical results. (author)

[en] We present a method to obtain a large elliptical attosecond pulse train of He atom driven by a trichromatic circular polarization laser pulse composed of the counterrotating circularly polarized laser pulse with frequencies ω, 3ω, and frequency 2ω with a weak intensity. We calculate the high-order harmonic spectra emitted by helium atoms driven by two-color and three-color circularly polarized laser pulses and the synthesized attosecond pulse train by using the strong field approximation method. It is found that the ellipticity of the attosecond pulse train emitted by He atom driven by the two-color laser pulse joined the third color laser pulse with a low intensity can be enhanced, compared with the case of two-color laser pulse. A big ellipticity for attosecond pulse train can be obtained for atom whose initial state is s state by adjusting the intensity ratio of ω, 3ω laser pulses with the appropriate intensity of the third color laser with frequency 2ω. (authors)

[en] The B-spline expansion technique and the method of model potential of atom have been applied to study the properties of Rydberg states of lithium. Our results for Stark maps, oscillator strength distributions for Li are in good agreement with other reported theoretical calculations and experimental measurement. It shows that the method we used can be applied to investigate the properties of alkali-metal atoms in external static electric field. Finally, we obtain the evolution of the oscillator strength spectrum and Shannon Entropy of Li with the different static electric fields. (author)

[en] By solving the time-dependent Schrodinger equation numerically, we study the high harmonics of one-dimensional He atom exposed to the combined field composed of two infrared laser pulses. It is shown that the harmonic spectrum is extended to I_p + 5.7U_p in the case of combined fields, thus we can obtain shorter trains of attosecond pulses. By adjusting the phase delay between the lasers, we can obtain an isolated 33.8 as pulse. Furthermore, analyzing the characteristic of generation of the attosecond pulse based on the wavelet transform, it is found that the contribution of the long electron trajectory on harmonic generation is suppressed, thus we can obtain the single attosecond pulse. (authors)

[en] By using the precise model potential of lithium atom with the single-active-electron approximation, the properties of low-order harmonics below ionization threshold emitted by lithium atoms in the laser field are studied by numerically solving the three-dimensional time-dependent Schroedinger equation. Our results show that for the low-order harmonics below the ionization threshold of lithium atom, in addition to the usual odd harmonics, there are also side-peak structures. The radiation characteristics of these low-order harmonics are analyzed by using the synchronous squeezing transform (SST) techniques. We found that these side-peaks are composed of the transition between the dressed states of atoms in the laser field and the transition from the excited state to the ground state in field-free case. If there is still a radiation for the side-peak after the laser pulse ends, the energy difference between the side-peak position and the main peak can be used to estimate the maximum Stark shift for the excited state of atom in laser field. (authors)

[en] Based on the classical theory, in this paper we optimize the intensity ratio of basic frequency field and low frequency field in the combined field. Then solving time-dependent Schrodinger equation of one-dimensional model helium atom driven by the optimized combined field, we find that high-order harmonic generation cut-off position of helium atom is extended. In addition, taking into account the initial phase of the basic frequency field, we obtain an isolated sub-37 attosecond pulse from the high-order harmonic generation of Helium atom driven by the combined field. (authors)

[en] The cross sections of charge transfer in He⁺ + H^- and Li⁺ + D^-collisions at low energies have been calculated by using the multichannel Landau-Zener model. An asymptotic method is used to obtain the coupling matrix elements between initial ionic channel and exit covalent channels for the colliding systems. Our results are compared with recent experimental data