Dynamics of Tunneling Ionization using Bohmian Mechanics

Recent attoclock experiments and theoretical studies regarding the strong-field ionization of atoms by few-cycle infrared pulses revealed new features that have attracted much attention. Here we investigate tunneling ionization and the dynamics of the electron probability using Bohmian Mechanics. We consider a one-dimensional problem to illustrate the underlying mechanisms of the ionization process. It is revealed that in the major part of the below-the-barrier ionization regime, in an intense and short infrared pulse, the electron does not tunnel \through" the entire barrier, but rather already starts from the classically forbidden region. Moreover, we highlight the correspondence between the probability of locating the electron at a particular initial position and its asymptotic momentum. Bohmian Mechanics also provides a natural definition of mean tunneling time and exit position, taking account of the time dependence of the barrier. Finally, we find that the electron can exit the barrier with significant kinetic energy, thereby corroborating the results of a recent study [Camus et al., Phys. Rev. Lett. 119 (2017) 023201]

Two-photon Double Ionization of H2_2 in Intense Femtosecond Laser Pulses

Triple-differential cross sections for two-photon double ionization of molecular hydrogen are presented for a central photon energy of 30 eV. The calculations are based on a fully {\it ab initio}, nonperturbative, approach to the time-dependent Schroedinger equation in prolate spheroidal coordinates, discretized by a finite-element discrete-variable-representation. The wave function is propagated in time for a few femtoseconds using the short, iterative Lanczos method to study the correlated response of the two photoelectrons to short, intense laser radiation. The current results often lie in between those of Colgan {\it et al} [J. Phys. B {\bf 41} (2008) 121002] and Morales {\it et al} [J. Phys. B {\bf 41} (2009) 134013]. However, we argue that these individual predictions should not be compared directly to each other, but preferably to experimental data generated under well-defined conditions.Comment: 4 pages, 4 figure

Connection between Superelastic and Inelastic Electron-Atom Collisions Involving Polarized Collision Partners

It is shown how the results of a recent experiment by Jiang, Zuo, Vuković, and Bederson [Phys. Rev. Lett. 68, 915 (1992)], who investigated low-energy electron scattering from laser-excited polarized sodium atoms in the initial (3p) 2P°3/2 (F=3, MF=3) state, can be related to the inelastic 3S→3P transition involving initially unpolarized electron and atom beams. Hence, this method can provide an independent check of the traditional electron-scattering experiment with unpolarized beams

"Close-coupling and distorted-wave calculations for electron-impact excitation of the (5p56p) states of xenon"

Klaus Bartschat is a professor of physics at Drake University, Des Moines, Iowa.We report on a series of calculations for electron impact-excitation of the (5p^5)6p states in xenon from the ground state (5p^6)^1S_0. As in previous calculations for other noble-gas targets, we find strong evidence of channel coupling for all incident energies considered (between threshold and 200\,eV). Although qualitative agreement with the experimental results of Fons and Lin (Phys. Rev. A 58 (1998) 4603) is achieved, severe quantitative discrepancies of sometimes more than a factor of two remain

Photoelectron angular distribution in two-pathway ionization of neon with femtosecond XUV pulses

We analyze the photoelectron angular distribution in two-pathway interference between non\-resonant one-photon and resonant two-photon ionization of neon. We consider a bichromatic femtosecond XUV pulse whose fundamental frequency is tuned near the $2p^5 3s$ atomic states of neon. The time-dependent Schr\"odinger equation is solved and the results are employed to compute the angular distribution and the associated anisotropy parameters at the main photoelectron line. We also employ a time-dependent perturbative approach, which allows obtaining information on the process for a large range of pulse parameters, including the steady-state case of continuous radiation, i.e., an infinitely long pulse. The results from the two methods are in relatively good agreement over the domain of applicability of perturbation theory

Complete break-up of the helium atom by proton and antiproton impact

We present a fully {\it ab initio}, non-perturbative, time-dependent approach to describe single and double ionization of helium by proton and antiproton impact. A flexible and accurate finite-element discrete-variable-representation is applied to discretize the problem on the radial grid in spherical coordinates. Good agreement with the most recent experimental data for absolute angle-integrated cross sections is obtained over a wide range of incident projectile energies between 3 keV and 6 MeV. Furthermore, angle-differential cross sections for two-electron ejection are predicted for a proton impact energy of 6 MeV. Finally, the time evaluation of the ionization process is portrayed by displaying the electron density as a function of the projectile location.Comment: 4 pages, 4 figure

Excitation of Ar 3p⁵4s-3p⁵4p Transitions by Electron Impact

Electron-impact excitation of argon from the 3p54s (J=0,2) metastable states to the 3p54p (J=0,1,2,3) manifold has been investigated in the semirelativistic first-order distorted-wave and plane-wave Born approximations. The results are compared with recent experimental data of Boffard et al. [Phys. Rev. A 59, 2749 (1999)] and R-matrix predictions by Bartschat and Zeman [Phys. Rev. A 59, R2552 (1999)]. In cases for which perturbative approaches are expected to be valid, the plane-wave Born approximation is found to be sufficiently accurate and thus allows for an efficient calculation of results over a wide range of collision energies