Enhancement and suppression of nonsequential double ionization by spatially inhomogeneous fields.

Using the three-dimensional classical ensemble approach, we theoretically investigate the nonsequential double ionization of argon atoms in an intense laser field enhanced by bowtie-nanotip. We observe an anomalous decrease in the double ionization yield as the laser intensity increases, along with a significant gap in the low momentum of photoelectrons. According to our theoretical analysis, the finite range of the induced field by the nanostructure is the fundamental cause of the decline in double ionization yield. Driven by the enhanced inhomogeneous field, energetic electrons can escape from the finite range of nanotips without returning. This reduces the possibility of re-scattering on the nucleus and imprints the finite size effect into the double ionization yield and momentum distribution of photoelectrons in the form of yield decline and a gap in the photoelectron-momentum distribution.

Control of the spider-like interference structure in photoelectron momentum distributions of a helium atom in a few-cycle nonlinear chirped laser pulse.

We investigate theoretically the photoelectron momentum distributions (PMDs) of the helium atom in the few-cycle nonlinear chirped laser pulse. The numerical results show that the direction of the spider-like interference structure in PMDs exhibits periodic variations with the increase of the chirp parameter. It is illustrated that the direction of the spider-like interference structure is related to the direction of the electron motion by tracking the trajectories of the electrons. We also demonstrate that the carrier-envelope phase can precisely control the opening of the ionization channel. In addition, we investigate the PMDs when a chirp-free second harmonic (SH) laser pulse is added to the chirped laser field, the numerical results show that the interference patterns can change from only spider-like interference structure to both spider-like and ring-like interference structures.

Photoelectron interference of He atoms in the attosecond ionization gating.

We theoretically investigate the photoelectron momentum distribution of He atoms by numerically solving the time-dependent Schro¨dinger equation (TDSE) in few-cycle ionization gating, which is synthesized by two linearly polarized laser pulses. When applying the TDSE, we can clearly see the spider-like structures in the photoelectron momentum spectra. We also find that the spider-like structures can be isolated by changing the relative phase. The directionality of the spider-like structure is changed from right-side to left-side and the ring-like interference structure gradually appears in the photoelectron momentum spectra when increasing the relative phase. The interference patterns observed in TDSE are recaptured well by the quantum-trajectory Monte Carlo (QTMC) model. We separate the ionization time window of the tunneling electron by analyzing the ionization rate. With the help of QTMC simulation, we illustrate the change of the interference structure and its directionality in the photoelectron momentum spectra. By changing the relative phase, the forward-backward asymmetry of the momentum distribution of the emitted electrons can also be controlled. Moreover, we find that the relative contribution of the nonrescattering and the rescattering trajectories can be controlled. These properties are beneficial for the application of photoelecron holography in probing atomic and molecular structures and dynamics.

Spectroscopic properties and spin-orbit coupling of electronic excited states of the germanium dimer.

The electronic structure and spectroscopic properties of the germanium dimer have been investigated by high-level ab initio calculations with consideration of spin-orbit coupling (SOC). The potential energy curves (PECs) of 19 Λ-S states, as well as those of 52 Ω states generated from the Λ-S states, are calculated by the multireference configuration interaction plus Davidson correction (MRCI + Q) method. The properties of the F3Σ+u1-X3Σ-g1 and H3Σ-u1-X3Σ-g1 transitions as well as the interactions of the F3Σ+u and H3Σ-u states with other excited states induced by SOC are investigated based on the calculated SO matrix and the PECs of the Ω states. Our results indicate that the previously observed spectra of Ge2 in the range 20 500-22 000 cm-1 should be assigned as the transition between the Ω = 1g component of the X3Σ-g state and Ω = 1u of the F3Σ+u state. Moreover, owing to the strong SOC with the repulsive 25Πu state, the H3Σ-u state is predissociative, leading to the Ge(3P2) + Ge(3P1) channel at vibrational levels higher than v' = 6. Our theoretical study would provide comprehensive information and shed light on understanding the spectroscopy and dynamics of the electronic excited states of Ge2.

Investigation of electron vortices in time-delayed circularly polarized laser pulses with a semiclassical perspective.

We theoretically investigate strong-filed electron vortices in time-delayed circularly polarized laser pulses by a generalized quantum-trajectory Monte Carlo (GQTMC) model. Vortex interference patterns in photoelectron momentum distributions (PMDs) with various laser parameters can be well reproduced by the semiclassical simulation. The phase difference responsible for the interference structures is analytically identified through trajectory-based analysis and simple-man theory, which reveal the underlying mechanism of electron vortex phenomena for both co-rotating and counter-rotating component. This semiclassical analysis can also demonstrate the influences of laser intensity and wavelength on the number of arms of vortices. Furthermore, we show the influence of the Coulomb effect on the PMDs. Finally, the controlling of the ionization time intervals in the tens to hundreds of attosecond magnitude is qualitatively discussed.

Theoretical study on predissociation of B3Σu- of sulfur dimer.

A high level ab initio study on electronic states of sulfur dimer S2 is carried out by utilizing multi-reference configuration interaction plus Davidson correction (MRCI+Q) method. The modification to the electronic structures caused by spin-orbit coupling (SOC) effect is taken into consideration by the state-interacting method with the full Breit-Pauli Hamiltonian. The potential energy curves (PECs) of 19 Λ-S states and 52 Ω states generated from the Λ-S states are calculated. With the aid of calculated SOC matrix elements and the PECs of the Ω states, we discuss the predissociaiton of the B3Σu- state. The variation of line width of B3Σu-- X3Σg- transitions with vibrational quantum number v' of the B3Σu- are determined from Fermi golden rule calculations. Our study indicate that the predissociation of the B3Σu- state is induced by the strong spin-orbit coupling with different electronic excited states, resulting in the abnormal dependence of the dissociation rate on vibrational states.

Multiple recollisions in nonsequential double ionization by counter-rotating two-color circularly polarized laser fields.

With the three-dimensional (3D) classical ensemble method, we theoretically investigate the recollision dynamics in strong-field nonsequential double ionization (NSDI) of Ar by counter-rotating two-color circularly polarized laser fields. With the analysis of the NSDI trajectories, we find that not only multiple-recollision but also single-recollision processes occur in the double ionization events. Furthermore, the multiple-recollision and single-recollision processes both undergo the recollision-induced excitation with subsequent ionization (RESI) and recollision-induced ionization (RII). The angle between the momentum and the force of the laser field at the recollision moment can affect the times of the recollision.

Nonsequential double ionization of Mg from a doubly excited complex driven by circularly polarized laser field.

With the classical ensemble method, the correlated-electron dynamics of Mg atom from a doubly excited, transition Coulomb complex in few-cycle circularly polarized (CP) laser field at low laser intensity is theoretically investigated. The low energy transfer during the recollision process indicates that the two electrons cannot release directly, but it can pass through a doubly excited state, and then escape with the ionization time difference. The numerical results show that the feature of the sequential double ionization (SDI) can be observed in the nonsequential double ionization (NSDI) process. The SDI-like results demonstrate that the intermediate state has lost any memory of its formation dynamics. The distribution of the angle between the two release directions of the two electrons also depends on the ionization time difference. Finally, the influence of e-e Coulomb repulsion is discussed.

Nonsequential double ionization channels control of Ar with few-cycle elliptically polarized laser pulse by carrier-envelope-phase.

The carrier-envelop-phase (CEP) dependence of nonsequential double ionization (NSDI) of atomic Ar with few-cycle elliptically polarized laser pulse is investigated using 2D classical ensemble method. We distinguish two particular recollision channels in NSDI, which are recollision-impact ionization (RII) and recollision-induced excitation with subsequent ionization (RESI). We separate the RII and RESI channels according to the delay time between recollision and final double ionization. By tracing the history of the trajectories, we find the electron correlation spectra as well as the competition between the two channels are sensitively dependent on the laser field CEP. Finally, control can be achieved between the two channels by varying the CEP.

Quantum control of electron wave packet during high harmonic process of H2(+) in a combination of a circularly polarized laser field and a Terahertz field.

By solving a two-dimensional time-dependent Schrödinger equation we investigate high harmonic generation (HHG) and isolated attosecond pulse generation for the H2+ molecular ion in a circularly polarized laser pulse combined with a Terahertz (THz) field. The harmonic intensity can be greatly enhanced and a continuum spectrum can be obtained when a THz field is added. The HHG process is studied by the semi-classical three-step model and the time-frequency analysis. Our studies show that only short trajectories contribute to HHG. Furthermore, we present the temporal evolution of the probability density of electron wave packet, which perfectly shows a clear picture of the electron's two-time recombination when a THz field is added, and it is the main mechanism of HHG. By superposing the harmonics in the range of 216-249 eV, an isolated attosecond pulse with a duration of about 69 attoseconds can be generated.