Sølve Selstø

Exact nondipole Kramers-Henneberger form of the light-atom hamiltonian: an application to atomic stabilization and photoelectron energy spectra.

The exact nondipole minimal-coupling Hamiltonian for an atom interacting with an explicitly time- and space-dependent laser field is transformed into the rest frame of a classical free electron in the laser field, i.e., into the Kramers-Henneberger frame. The new form of the Hamiltonian is used to study nondipole effects in the high-intensity, high-frequency regime. Fully three-dimensional nondipole ab initio wave packet calculations show that the ionization probability may decrease for increasing field strength. We identify a unique signature for the onset of this dynamical stabilization effect in the photoelectron spectrum.

Strong orientation effects in ionization of H+2 by short, intense, high-frequency light pulses

We present three-dimensional time-dependent calculations of ionization of arbitrarily spatially oriented H+2 by attosecond, intense, high-frequency laser fields. The ionization probability shows a strong dependence on both the internuclear distance and the relative orientation between the laser field and the internuclear axis. The physical features are explained in terms of two-center interference effects.

Absorbing boundary conditions for dynamical many-body quantum systems

In numerical studies of the dynamics of unbound quantum mechanical systems, absorbing boundary conditions are frequently applied. Although this certainly provides a useful tool in facilitating the description of the system, its applications to systems consisting of more than one particle are problematic. This is due to the fact that all information about the system is lost upon the absorption of one particle; a formalism based solely on the Schrodinger equation is not able to describe the remainder of the system as particles are lost. Here we demonstrate how the dynamics of a quantum system with a given number of identical fermions may be described in a manner which allows for particle loss. A consistent formalism which incorporates the evolution of sub-systems with a reduced number of particles is constructed through the Lindblad equation. Specifically, the transition from an N-particle system to an (N − 1)-particle system due to a complex absorbing potential is achieved by relating the Lindblad operators to annihilation operators. The method allows for a straight forward interpretation of how many constituent particles have left the system after interaction. We illustrate the formalism using one-dimensional two-particle model problems.

A master equation approach to double ionization of helium

It is demonstrated how a numerical approach based on absorbing boundaries may be used to describe the process of non-sequential two-photon double ionization of helium. Contrary to any method based on solving the Schrodinger equation alone, this numerical scheme is able to reconstruct the remaining particles as one particle is absorbed. This may be used to distinguish between single and double ionization. A model of reduced dimensionality, which describes the process at a qualitative level, has been used. The results have been compared with a more conventional method in which the time-dependent Schrodinger equation is solved and the final wavefunction is analysed in terms of projection onto eigenstates of the uncorrelated Hamiltonian, i.e. with no electron–electron interaction included in the final states. It is found that the two methods indeed produce the same total cross sections for the process.

Nonsequential Two-Photon Double Ionization of Atoms: Identifying the Mechanism

We develop an approximate model for the process of direct (nonsequential) two-photon double ionization of atoms. Employing the model, we calculate (generalized) total cross sections as well as energy-resolved differential cross sections of helium for photon energies ranging from 39 to 54 eV. A comparison with results of ab initio calculations reveals that the agreement is at a quantitative level. We thus demonstrate that this complex ionization process can be described by the simple model, providing insight into the underlying physical mechanism. Finally, we use the model to calculate generalized cross sections for the two-photon double ionization of neon in the nonsequential regime.

Formulae for partial widths derived from the Lindblad equation

A method for calculating partial widths of auto-ionizing states is proposed. It combines either a complex absorbing potential or exterior complex scaling with the Lindblad equation. The corresponding classical rate equations are reproduced, and the trace conservation inherent in the Lindblad equation ensures that the partial widths sums up to the total width of the initial auto-ionizing state.

LASER EXCITATION OF ANGULAR RYDBERG WAVEPACKETS

We consider the dynamics of the formation of high angular momentum Rydberg states in a hydrogen by laser excitation. The Schrodinger equation is solved by expanding the wavefunction in spherical |n,l,m states. Results obtained by expansions over a large and a small number of basis functions are compared in order to identify the main excitation dynamics. The final population is studied by simpler models and the survival of only odd angular momentum in the long-time limit states can be explained within an analytical model. From this model it is shown that the survival of odd high-l states originates from symmetry properties of the time development operator and that the population of even l states may occur with ultra-short laser pulses.

Scattering in a quantum dot: the role of resonances

The dynamics of a system in which an electron is incident on a populated quantum dot is studied by resolving the time dependence of the system. Specifically, one electron is initially at rest in the ground state of the dot, whereas the incident electron has a narrow velocity distribution. In addition to the probabilities of reflection and transmission, the probability of excitation is found as a function of energy. Moreover, probabilities of both electrons being ejected or both being captured are obtained. The latter process involves spontaneous emission. It is found that the dynamics is strongly influenced by the presence of doubly excited states; reflection becomes considerably more probable in the energetic vicinity of such states. This, in turn, contributes to an increase in the probability of trapping also the second electron within the dot.

Dynamics of H(2p) ionization in ultrashort strong laser pulses

The ionization dynamics of an initially excited aligned H(2p, m = 0) atom exposed to short intense laser pulses is studied in the non-perturbative regime based on a three-dimensional numerical solution of the time-dependent Schrodinger equation on a spherical grid. The laser pulse is given a linear polarization vector which defines an angle θ with the symmetry axis of the initial 2p state. Strong orientation effects for ionization are found as a function of polarization direction for high laser frequencies. The angular distribution of the photo-electron spectrum shows two characteristic features related to ionization dynamics and interference of parallel versus perpendicular states with respect to the polarization direction of the field. For high enough field intensities, the ionization probability saturates below unity. In this limit, the angular electronic distribution is insensitive to the laser polarization direction. Another characteristic feature is a complete suppression of multiphoton peaks which results in kinetic emission spectra dominated by slow electrons.

Relativistic ionization dynamics for a hydrogen atom exposed to superintense XUV laser pulses

We present a theoretical study of the ionization dynamics of a hydrogen atom exposed to attosecond laser pulses in the extreme ultraviolet region at very high intensities. The pulses are such that ...