Photoelectron Angular Distributions of Nonresonant Two-Photon Atomic Ionization Near Nonlinear Cooper Minima

Abstract

Photoelectron angular distributions of the two-photon ionization of neutral atoms are theoretically investigated. Numerical calculations of two-photon ionization cross sections and asymmetry parameters are carried out within the independent-particle approximation and relativistic second-order perturbation theory. The dependence of the asymmetry parameters on the polarization and energy of the incident light as well as on the angular momentum properties of the ionized electron are investigated. While dynamic variations of the angular distributions at photon energies near intermediate level resonances are expected, we demonstrate that equally strong variations occur near the nonlinear Cooper minimum. The described phenomena is demonstrated on the example of two-photon ionization of magnesium atom.

High-intensity laser experiments with highly charged ions in a Penning trap

“…Then the laser electric field is much smaller than the electric field of the nucleus at the position of the electron, the electron wave functions are only weakly perturbed and the interaction can be described by non-perturbative approaches. Theory predicts here on the one hand positive giant resonances at specific photon energies characteristic for each particle and also dips where the ionisation cross section drops by several orders of magnitudes-the so-called Cooper minima [17]. There is typically only a slight dependence on the photon energy expected for other photon energies.…”

“…In the region of 92 eV instead the ionisation cross section drops by several orders of magnitude. This is called the cooper minimum and can be addressed to a minimum in the matrix element between the s ground-state wave function and the wave function of the photo-ionised electron [17].…”

Special Issue “Interaction of Ionizing Photons with Atomic and Molecular Ions”

“…Hofbrucker et al [4] note that two-photon ionization is one of the most fundamental nonlinear processes in which two photons are simultaneously absorbed and an electron is emitted. They treat the problem of non-resonant two-photon ionization in magnesium and examine the energy dependence of the resulting photoelectron angular distributions and in particular the angular distribution near the nonlinear Cooper minimum.…”