Zhao Zeng-Xiu

Effects of Bounding Potential on High-Order Harmonic Generation with H+2

We present harmonic spectra from hydrogen molecular ions with different ionization energy Ip. By comparing the recombination matrix element from helium ions and hydrogen molecular ions, we verify that the interference effect in high-order harmonic generation originates from the recombination interference. A numerical study on recombination matrix elements in 1D shows that ω = Ek + Ip holds only when an exact continuum wavefunction is used, and the positions of extrema in harmonic spectra can be predicted accurately. We demonstrate that the positions of extrema in harmonic spectra are mainly affected by bounding potential dependence of the recombination interference.

Alignment-Dependent Ionization of CO2 in the Intense Laser Fields: Single-Active-Electron Approach

We perform Hartree-Fock calculations to obtain occupied orbitals for triatomic molecule CO2, based on one-center method and B-splines to deal with cusps and speed up convergence. Both the orbital energies and charge distribution are in good accordance with the reference data. The valence orbital is propagated with single-active-electron approximation, and the alignment-dependent ionization yield peaks about 40 degrees. However, there are discrepancies between our results and the experimental data, and many-electron effects may need to be exploited further to describe them.

Elliptical High-Order Harmonic Generation from H+2 in Linearly Polarized Laser Fields

We investigate the elliptical high-order harmonic generation (HHG) from the ground state of H+2 subjected to linearly polarized laser fields, by numerically solving the three-dimensional (3D) time-dependent Schrodinger equation (TDSE) and using the strong-field approximation (SFA) models. Highly elliptical HHG is yielded at intermedial alignment angles from the TDSE, while the standard SFA model fails to predict this. By including the coulomb potential and the stark shift corrections, we yield qualitative agreement results with the TDSE. The comparisons show that in the description of elliptical HHG, both the coulomb potential and the stark-shift are necessary.

Theory of Tunnel Ionization in Complex Systems

We present a detailed derivation of the Ammosov–Delone–Krainov theory of tunnel ionization in complex systems (CS-ADK). A few mistakes in the previous works have been found and corrected. The theory is then applied to CO2, showing that CS-ADK yields better agreement with experiment than the molecular ADK theory.

Radiative Transition Line Shape in Warm and Dense Argon Plasma: a Two-Centre Model Study

A two-centre model is employed to study the electronic structure of argon plasma at a density of 1022 cm−3 and a temperature of 5eV. The model takes into account the influence of the nearest neighbour on the electronic structure of the radiator and gives a proper description of the transient molecular behaviour in dense plasmas where the mean interatomic distance can be the order of the orbital spatial extent. The orbital energies of quasi-molecular Ar2 against the interatomic distance are calculated in the framework of density functional theory. It is shown that the orbital energies are not monotonic functions of the distance because of two competitive effects: the repulsive potential between electrons and the attractive potential between electrons and nuclei. The characteristics of this two-centre system can be categorized by three regimes depending on the ratio between the electron wavelength and the interatomic distance: atomic regime, screened atomic regime, and quasi-molecular regime. This classification agrees qualitatively with the conclusion of full quantum-mechanical approach [Yonger S Met al 1988 Phys. Rev. Lett. 61 962].

Electron Interaction with Lithium Atoms and Electron Impact Broadening of the Lithium Resonance Line

Electron collision with lithium atoms is calculated by using R-matrix method. The scattering parameters are applied in the determination of the electron impact broadening and shift of the lithium resonance line 2s2S - 2p2P. The width and shift are obtained based on the impact approximation. A largest discrepancy of 30 percent for the broadening width is found from a semiclassical calculation.

Local-microfield enhancement of high-order harmonic generation and the associated electron dynamics

The high-order harmonic generation (HHG) by local-microfield enhancement is investigated using the simple diatomic model H2+, which is initially prepared in coherent superposition of the ground state and the first excited state. The high-order harmonic spectroscopy is calculated by solving time-dependent schrodinger equation (TDSE) using the split-operator technique in a large internuclear distance. Numerically we use asymmetric-spatial mask functions, multi-trajectories contributions are revealed in the harmonic spectra, which contribute to different frequencies of harmonic spectroscopy. Also we numerically solve the Newton equation to get the classical analysis, the maximum return energies show great agreement with the cut-off energies of high-order harmonics from different trajectrories. Due to the tunneling ionization has a sensitive response to the instantaneous electric field, all the trajectories can be controlled by the carrier-envelop phase of intense few-cycle laser field. It is shown that electrons migrating directly from one nucleus to the neighboring one without typical tunneling ionization, contribute dominantly to the lower order harmonics, and the intensities are dramatically enhanced along with the enhancement of the local-microfield. The migration of electrons between nuclei in multi-nuclei model will mimic the dynamics of electrons in solids, which will shed light on HHG from solids especially with local impurity.

THz wave emission from argon in two-color laser field

Terahertz (THz) wave emission from argon atom in a two-color laser pulses is studied numerically by solving the one-dimensional (1D) time-dependent Schr?dinger equation. The THz spectra we obtained include both discontinuous and continuum ones. By using the special basis functions that we previously proposed, our analysis points out that the discontinuous and continuum parts are contributed by bound?bound and continuum?continuum transition of atomic energy levels. Although the atomic wave function is strongly dressed during the interaction with laser fields, our identification for the discontinuous part of the THz wave shows that the transition between highly excited bound states can still be well described by the field-free basis function in the tunneling ionization regime.

Strong-field approximation for the ionization of N{sub 2} and CO{sub 2}

We investigate the ionization of N{sub 2} and CO{sub 2} using the strong-field approximation (SFA) in both the length gauge (LG) and the velocity gauge (VG). Comparisons have been made among the different versions of SFA, the molecular Ammosov-Delone-Krainov (ADK), and recent experiments. The study also shows that the VG SFA displays a larger laser parameter (intensity and wavelength) dependence than the LG SFA, which is not observed in experiments and first principal calculations. The LG SFA is preferable for description of the strong-field ionization of N{sub 2} and CO{sub 2}.We investigate the ionization of N{sub 2} and CO{sub 2} using the strong-field approximation (SFA) in both the length gauge (LG) and the velocity gauge (VG). Comparisons have been made among the different versions of SFA, the molecular Ammosov-Delone-Krainov (ADK), and recent experiments. The study also shows that the VG SFA displays a larger laser parameter (intensity and wavelength) dependence than the LG SFA, which is not observed in experiments and first principal calculations. The LG SFA is preferable for description of the strong-field ionization of N{sub 2} and CO{sub 2}.