Interaction of a model atom exposed to strong laser pulses: Role of the Coulomb potential (original) (raw)
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Physical Review A, 2006
Two-dimensional ͑2D͒ electron momentum distributions and energy spectra for multiphoton ionization of atoms by intense laser pulses, calculated by solving the time-dependent Schrödinger equation ͑TDSE͒ for different wavelengths and intensities, are compared to those predicted by the strong-field approximation ͑SFA͒. It is shown that the momentum spectra at low energies between the TDSE and SFA are quite different and the differences arise largely from the absence of a long-range Coulomb interaction in the SFA. We further found that the low-energy 2D momentum spectra from the TDSE exhibit ubiquitous fanlike features where the number of stripes is due to a single dominant angular momentum of the low-energy electron. The specific dominant angular momentum in turn has been found to be decided by the minimum number of photons needed to ionize the atom only. The electron momentum spectra predicted by models modified from the SFA are also examined and found to lack the fanlike features as in the SFA.
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.
On the role of the Coulomb potential in strong field atomic ionization dynamics
Journal of Electron Spectroscopy and Related Phenomena, 2007
In this paper, we present a model aimed at exploring the role of the Coulomb potential in the mechanism of ionization of atomic hydrogen exposed to a strong low frequency pulsed laser field. Our approach is based on the solution of the time-dependent Schrödinger equation in momentum space. Although we are in a frequency and intensity regime where tunnelling is expected to dominate, our results indicate that the atomic structure associated to the Coulomb potential plays a significant role for low energy ejected electrons.
In-depth analysis of Coulomb–Volkov approaches to ionization and excitation by laser pulses
Physica Scripta, 2007
In perturbation conditions, above-threshold ionization spectra produced in the interaction of atoms with femtosecond short-wavelength laser pulses are well predicted by a theoretical approach called CV2 − , which is based on Coulomb-Volkov-type states. However, when resonant intermediate states play a significant role in a multiphoton transition, the CV2 − transition amplitude does not take their influence into account. In a previous paper, this influence has been introduced separately as a series of additional sequential processes interfering with the direct process. To give more credit to this procedure, called modified CV2 − (MCV2 −), a perturbation expansion of the standard CV2 − transition amplitude is compared here to the standard time-dependent perturbation series and the strong field approximation. It is shown that the CV2 − transition amplitude consists merely in a simultaneous absorption of all photons involved in the transition, thus avoiding all intermediate resonant state influence. The present analysis supports the MCV2 − procedure that consists in introducing explicitly the other quantum paths, which contribute significantly to ionization, such as passing through intermediate resonances. Further, this analysis permits to show that multiphoton excitation may be addressed by a Coulomb-Volkov approach akin to MCV2 − .
Secondary peaks in the atomic ionization by a resonant laser pulse
Journal of Physics: Conference Series, 2012
The above-threshold ionization of atoms by XUV short laser pulses with frequencies resonant with a ground-excited state transition is investigated. A theory based on a variational expression with trial wave functions for the final and the initial states is presented. For the former we use a Coulomb-Volkov wave function, and for the latter a close-coupling solution of the time-dependent Schrödinger equation considering a few bound states and their depletion towards the continuum. We find that this discrete-continuum Coulomb-Volkov theory provides a very good description of the photoelectron spectrum. In particular it accounts for the splitting of the above-threshold ionization peaks by Rabi oscillations and the appearance of secondarypeaks observed in full numerical simulations.
Semi-analytical model of hydrogen ionization by strong laser pulse at low field frequencies
Journal of Physics: Conference Series, 2014
We consider the interaction of hydrogen atom with a very intense low frequency laser pulse. The Henneberger-Kramers representation of the time-dependent Schrödinger equation is the most appropriate one for this purpose. It is shown that in the case of very low frequencies, the quantum dispersion of the electron wave packet plays a dominant role in the dynamics of the atom.
Physical Review A, 2014
We studied the elementary processes of excitation and ionization of atomic hydrogen in an intense 800-nm pulse with intensity in the 1.0 to 2.5 × 10 14 W/cm 2 range. By analyzing excitation as a continuation of above-threshold ionization (ATI) into the below-threshold negative energy region, we show that modulation of excitation probability and the well-known shift of low-energy ATI peaks vs laser intensity share the same origin. Modulation of excitation probability is a general strong field phenomenon and is shown to be a consequence of channel closing in multiphoton ionization processes. Furthermore, the excited states populated in general have large orbital angular momentum and they are stable against ionization by the intense 800-nm laser-they are the underlying reason for population trapping of atoms and molecules in intense laser fields.
Two-electron atoms in short intense laser pulses
Physical Review A, 1998
We discuss a method of solving the time dependent Schrödinger equation for atoms with two active electrons in a strong laser field, which we used in a previous paper [ A. Scrinzi and B. Piraux, Phys. Rev. A 56, R13 (1997) ] to calculate ionization, double excitation and harmonic generation in Helium by short laser pulses. The method employs complex scaling and an expansion in an explicitly correlated basis. Convergence of the calculations is documented and error estimates are provided. The results for Helium at peak intensities up to 10 15 W/cm 2 and wave length 248 nm are accurate to at least 10 %. Similarly accurate calculations are presented for electron detachment and double excitation of the negative hydrogen ion.
Physical Review A, 2004
The interaction between an atom and a short laser pulse is studied in the case where both photon energies are smaller than the ionization potential and perturbation conditions prevail. Under these conditions, a full numerical solution of the time-dependent Schrödinger equation for a hydrogen atom initially in its ground state shows that secondary peaks show up in the above-threshold ionization (ATI) spectrum. We introduce here an easyto-implement approximation that sheds light on these features. This approach, which is based on Coulomb-Volkov-type states, is an extension of a previous theory that applies only when photon energies are greater than the ionization potential. We show that introducing a coupling to intermediate bound states into the approach permits to nicely reproduce the full numerical electron spectrum for smaller photon energies. Further, it allows us to trace back unambiguously the secondary peaks to a manifold process, i.e., excitation of transient bound states followed by multiphoton absorption, and to show that the main ATI peaks may be enhanced by intermediate state contributions. When bound states with 1 ഛ n ഛ 4 are included, the approach provides excellent predictions for photon energies as low as half the ionization potential.