Ionization of helium by an ultrashort extreme-ultraviolet laser pulse

Abstract

We theoretically study photoelectron angular distributions and related anisotropy parameters for the competition of one- and two-photon ionization of He in an ultrashort extreme-ultraviolet laser pulse, using numerical results from the time-dependent Schrödinger equation. We explore the transition between the two processes for variation of the pulse duration, peak intensity, and central frequency or photoelectron energy. In the results obtained for fixed pulse parameters the transition and interference between the processes can be observed in the even (β2, β4) and odd (β1, β3) parameters, respectively, while the onset of the impact of three-photon ionization is observed via β5 at high peak intensities. Finally, we show that the transition can be observed via the even anisotropy parameters in pulses without carrier-to-envelope stabilization as well as in free-electron laser pulses with fluctuating pulse shape.

Asymmetries in ionization of atomic superposition states by ultrashort laser pulses

“…the ground state, conventional anisotropy and asymmetry parameters fail to provide comprehensive tools for the analysis of photoionization from atomic superposition states. For example, the simplest case of a competition between one-and two-photon ionization processes can be analyzed using asymmetry parameters if the atom is prepared in the ground state [20][21][22][23][24][25][26][27] . In contrast, these analysis tools are either not applicable or do not provide a straightforward interpretation for the same processes if the atom is in the superposition of two states.…”

“…The present calculations are performed utilizing single active electron potentials for He atom and Ne atom 42 with the electron initially prepared in a superposition of the ground and an excited state. The TDSE has been solved by expanding the wavefunction in spherical harmonics (up to for all relevant m values), as described in 27 . The computations have been performed on a radial grid of 300 a.u.…”

“…Probing atoms and molecules in their ground or excited states with ultrashort laser pulses opens a new regime where several linear and nonlinear ionization pathways compete and interfere [20][21][22][23][24][25][26][27] . For example, it has been shown how the competition between resonant and nonresonant pathways depends on the pulse width 20 .…”

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

“…Over the last few decades, studies of nonlinear light-matter interaction have received much attention, both theoretically [1][2][3][4][5][6][7][8][9] as well as experimentally [10][11][12][13][14]. With the advancement of extreme ultraviolet (EUV) and X-ray light sources such as free-electron lasers (FELs) [15][16][17], energy restrictions of optical lasers have been removed and new opportunities to investigate ionization of inner-shell electrons of atoms and molecules arose [18].…”