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Journal of Physics B Atomic and Molecular Physics

Two-photon ionization of trivalent atoms as a source of polarized electrons

Péter Faragó, D W Walker

Published Oct 1, 1973• 16 citations
Publisher

Abstract

It is argued that photo-electrons emitted from trivalent atoms in a two-photon ionization process are totally polarized if the incident light is of total circular polarization and its wavelength matches the resonance condition for the transition between ground state (n 2 P 1 2/) and the first excited state (n+1) 2 S 1 2/. The estimated photoionization yield compares favourably with that of the analogous process in alkali atoms. Some of the most important conditions for a successful experiment are considered.

DOI
10.1088/0022-3700/6/10/002

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It is argued that photo-electrons emitted from trivalent atoms in a two-photon ionization process are totally polarized if the incident light is of total circular polarization and its wavelength matches the resonance condition for the transition between ground state (n 2 P 1 2/) and the first excited state (n+1) 2 S 1 2/. The estimated photoionization yield compares favourably with that of the analogous process in alkali atoms. Some of the most important conditions for a successful experiment are considered.
The theory of multiphoton ionization for an atomic system of arbitrary complexity is developed using a density matrix formalism. An expression is obtained which determines the differential N-photon ionization cross section as a function of the polarization states of the target atom and the incident radiation. The parameters which characterize the photo-electron angular distribution are related to the general reduced matrix elements for the N-photon transition. Two-photon ionization of unpolarized atoms is treated as an illustration of the use of the theory. The dependence of the multiphoton ionization cross section on the polarization state of the incident radiation, which has been observed in two- and three-photon ionization of Cs, is accounted for by the theory. Finally, the photoelectron spin polarization produced by the multiphoton ionization of unpolarized atoms, like the analogous polarization resulting from single-photon ionization, is found to depend on the circular polarization of the incident radiation.
An experiment is reported on two-step photoionization of Cs via the 6P 3/2 intermediate state using circularly polarized radiation from a GaAs laser for the first step (8521 AA) and from an Ar-ion laser for the second step. Values for the ratio of ion production rates for right- and left-handed circular polarization in the second step and for the spin polarization of the photoelectrons are presented. The measurements are interpreted on the basis of a calculation which takes into account the hyperfine splitting of the levels involved in the first transition. A description in terms of fine-structure splitting of the 6P state is shown to be totally inadequate, even when allowing for possibly large spin-orbit effects in the continuum. The process is evaluated as a possible source of a spin-polarized electron beam.
General equations are proposed for the integrated cross section and degree of spin polarization of photoelectrons emitted by ground-state alkali metal atoms in two-photon ionization by two different optical beams. The equations, which are derived in the fine-structure approximation, are valid for polarized and unpolarized atoms, parallel experimental geometries, completely polarized (elliptically, circularly, and linearly polarized) and completely unpolarized optical beams, and also near resonance with an nP intermediate doublet level.
Abstract An all-optical, near-resonant laser atom trap is used to prepare an electronically excited and polarized gas target at mK-temperature for complete photo-ionization studies. As a proof-of-principal experiment, lithium atoms in the 2 2 P 3/2 ( m l = +1) state are ionized by a 266 nm laser source, and emitted electrons and Li + ions are momentum analyzed in a COLTRIMS spectrometer. The excellent resolution achieved in the present experiment allows not only to extract the relative phase and amplitude of all partial waves contributing to the final state, it also enables to characterize the experiment regarding target and spectrometer properties. Photo-electron angular distributions are measured for five different laser polarizations and described in a one-electron approximation with excellent agreement.