Two-electron atoms in short intense laser pulses (original) (raw)
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Physical Review Letters, 2006
We present ab initio computations of the ionization of two-electron atoms by short pulses of intense linearly polarized Ti:sapphire laser radiation beyond the one-dimensional approximation. In the model the electron correlation is included in its full dimensionality, while the center-of-mass motion is restricted along the polarization axis. Our results exhibit a rich double ionization quantum dynamics in the direction transversal to the field polarization, which is neglected in the previous models based on the onedimensional approximation.
High-order harmonic generation (HHG), attosecond pulse train (APT), and non-sequential double ionization (NSDI) in the He atom under high intense femtosecond laser pulses are calculated by time-dependent Schrodinger equation (TDSE) in one dimension (1D). By considering the mutual electron-electron and electron-nuclei interactions along with calculating the He atom ground state wave function by imaginary time propagation (ITP) method, besides calculating probability density of electrons, dipole acceleration, HHG, and APT, we could generate the well-known "knee structure" in the probability of the He atom ionization against the intensity in an ionization boundary condition model. The results are in good agreement with the experimental data reported by Walker et al. [B. Walker et al. Phys. Rev. Lett. 73, 1227 (1994)].
Resonant and non-resonant ionization of helium by XUV ultrashort and intense laser pulses
We study the multiphoton ionization of He by a strong ultrashort laser pulse. We solve the time-dependent Schrödinger equation numerically by means of a spectral method of configuration-interaction type (CI). Our method is based on an expansion on B-spline functions, more precisely each two-electron configuration is built as an antisymmetrized B-spline product for the (two-dimensional) radial part and dipolar spherical harmonics for the (four-dimensional) angular part. We focus on both the atomic structure calculations and the laser–atom dynamics. We show that our approach allows one to properly treat the electron–electron correlations so as to provide an accurate description of many eigenstates simultaneously. The method is applied in two different situations: (a) the multiphoton excitation and ionization of helium with photon energies ranging from 5.4 to 32.6 eV and (b) the two-photon ionization of helium below the He + (2l) threshold and three-photon ionization above the He 2+ threshold with photon energies ranging from 25.8 to 34 eV. The results are compared with other CI calculations (where a comparison is possible) and, in case (a), to Hartree–Fock calculations. In case (b) we particularly focus on the dynamics of the production of doubly-excited structure by laser pulses with a duration which is shorter than the autoionization lifetime.
Precise Calculation of Single and Double Ionization of Hydrogen Molecule in Intense Laser Pulses
2010
A new simulation box setup is introduced for the precise description of the wavepacket evolution of two electronic systems in intense laser pulses. In this box, the regions of the hydrogen molecule H2, and singly and doubly ionized species, H2^+ and H2^{+2}, are well discernible and their time-dependent populations are calculated at different laser field intensities. In addition, some new regions are introduced and characterized as quasi-double ionization and their time-dependencies on the laser field intensity are calculated and analyzed. The adopted simulation box setup is special in that it assures proper evaluation of the second ionization. In this study, the dynamics of the electrons and nuclei of the hydrogen molecule are separated based on the adiabatic approximation. The time-dependent Schrödinger and Newton equations are solved simultaneously for the electrons and the nuclei, respectively. Laser pulses of 390 nm wavelength at four different intensities (i.e., 1 × 1014, 5 × 1014, 1 × 1015, and 5 × 1015 W cm-2) are used in these simulations. Details of the central H2 region are also presented and discussed. This region is divided into four sub-regions related to the ionic state H+H- and covalent (natural) state HH. The effect of the motion of nuclei on the enhanced ionization is discussed. Finally, some different time-dependent properties are calculated, their dependencies on the intensity of the laser pulse are studied, and their correlations with the populations of different regions are analyzed.
Time-dependent method in the laser–atom interactions
Computer Physics Communications, 2011
We introduce a recent developed time-dependent method used in the study of laser-atom interactions. The key ingredients of the method are that (1) we propagate the wave function in real space in a finite region. The region is split into two parts, an inner-region and an outer-region. Once the electron moved into the outer-region, the wave function is projected into momentum space and propagated analytically to avoid the reflection from the boundary.
Precise description of single and double ionization of hydrogen molecule in intense laser pulses
The Journal of Chemical Physics, 2012
In this paper, a new simulation box is introduced for two electronic systems in intense laser pulses. In this box, the region of hydrogen molecule, single ionization and second ionization are precisely recognized and time dependent of population of these regions are reported. In addition, a new regions is introduced and characterized as quasi-double ionization regions and the time dependent population of these regions are calculated and compared at different intensities. The special character of the simulation box is that it is designed in order that to assure the overall second ionization is taken to account. In this study, the dynamics of the electrons and the nuclei of hydrogen molecule are separated based on the adiabatic approximation. The time dependent Schrödinger and Newton equations are solved simultaneously for the electrons and the nuclei respectively. Four different intensities are used in the simulation; 1 × 10 14 , 5 × 10 14 , 1 × 10 15 and 5 × 10 15 W cm −2 with the 390 nm wavelength for all the intensities. The details of the H 2 region is represented and discussed. This region is divided to four sub-regions related to the ionic state H + H − and covalent (natural) state HH. The effect of the internuclear distance and motion of nuclei on the enhanced ionization is discussed. Finally, some different time dependent properties are calculated and their relations with the characteristics of the laser pulse and the population of different regions are studied.
Quantum model for double ionization of atoms in strong laser fields
Physical Review A, 2008
We discuss double ionization of atoms in strong laser pulses using a reduced dimensionality model. Following the insights obtained from an analysis of the classical mechanics of the process, we confine each electron to move along the lines that point towards the two-particle Stark saddle in the presence of a field. The resulting effective two dimensional model is similar to the aligned electron model, but it enables correlated escape of electrons with equal momenta, as observed experimentally. The timedependent solution of the Schrödinger equation allows us to discuss in detail the time dynamics of the ionization process, the formation of electronic wave packets and the development of the momentum distribution of the outgoing electrons. In particular, we are able to identify the rescattering process, simultaneous direct double ionization during the same field cycle, as well as other double ionization processes. We also use the model to study the phase dependence of the ionization process.
Ionization of a single hydrogen-like atom by laser pulse of near-atomic strength
2007
The dynamics of high-harmonic generation and atom ionization by a strong and superstrong laser field are studied. In contrast to many earlier works, the present theory does not impose limitations on the laser field's strength. We solve the nonrelativistic problem of a single hydrogen-like atom's ionization from the ground state by a short laser pulse of subatomic, atomic, and superatomic field strength. Within the framework of the proposed method, we investigated the matrix elements of the ionization transition and revealed its substantially nonlinear dependence on the laser field strength. Both ionization and recombination processes are taken into account. The proposed method enables us to take into account the arbitrary order multiphoton ionization processes.
Interaction of a model atom exposed to strong laser pulses: Role of the Coulomb potential
Physical Review A, 2013
With the help of the solution of the time-dependent Schrödinger equation in momentum space, we study the above-threshold ionization spectrum resulting from the interaction of atomic hydrogen with an infrared and XUV short laser pulses. Our calculations are based on a model where the kernel of the nonlocal Coulomb potential is replaced by a finite sum of N symmetric separable potentials, each of them supporting one bound state of atomic hydrogen. Here, we consider only the case of 1s, 2s, and 2p states. Thus, the theory fully accounting for the important 1s-2p transition, explains the photoelectron spectrum as well as the total ionization probability for the resonant case. We compared the results given by our theory with the numerical solutions of the time-dependent Schrödinger equation.
Two-photon double ionization of the helium atom by ultrashort pulses
Journal of Physics B: Atomic, Molecular and Optical Physics, 2010
Two-photon double ionization of the helium atom was the subject of early experiments at FLASH and will be the subject of future benchmark measurements of the associated electron angular and energy distributions. As the photon energy of a single femtosecond pulse is raised from the threshold for two-photon double ionization at 39.5 eV to beyond the sequential ionization threshold at 54.4 eV, the electron ejection dynamics change from the highly correlated motion associated with nonsequential absorption to the much less correlated sequential ionization process. The signatures of both processes have been predicted in accurate \textit{ab initio} calculations of the joint angular and energy distributions of the electrons, and those predictions contain some surprises. The dominant terms that contribute to sequential ionization make their presence apparent several eV below that threshold. In two-color pump probe experiments with short pulses whose central frequencies require that the sequential ionization process necessarily dominates, a two-electron interference pattern emerges that depends on the pulse delay and the spin state of the atom.