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Journal of Physics B Atomic and Molecular Physics

Two-photon ionization of caesium via the 62P3/2intermediate state: analysis of photoelectron spin polarization and saturation effects

Ernst H.A. Granneman, M Klewer, Marnix J. van der Wiel

Published Nov 11, 1976• 20 citations
Publisher

Abstract

The authors have investigated the two-step process 6 2 S 1/2 to 6 2 P 3/2 to (epsilon 2 S 1/2 , epsilon 2 D 3/2 , epsilon 2 D 5/2 ) in Cs for circular polarization of the radiation in both steps. Theoretical expressions are presented for the photoelectron spin polarization P z and the ion count-rate 'asymmetry' A i (ratio of ion rates for right or left circular polarization in the second step), based on the hyperfine-coupling scheme for the first step. These expressions contain ratios of the three radial matrix elements involved in the ionizing step. The experimental determination of P z and A i , for different groups of hyperfine transitions, permits an evaluation of these ratios. The experiment was performed on a Cs atomic beam using a pulsed GaAs laser at 8521 AA for the first step and a pulsed Ar-ion laser at 5017-4545 AA for the second step. Effects of saturation in the first transition are studied and optimal conditions for use of the process in a spin-polarized electron source are discussed.

DOI
10.1088/0022-3700/9/16/017

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The authors have investigated the two-step process 6 2 S 1/2 to 6 2 P 3/2 to (epsilon 2 S 1/2 , epsilon 2 D 3/2 , epsilon 2 D 5/2 ) in Cs for circular polarization of the radiation in both steps. Theoretical expressions are presented for the photoelectron spin polarization P z and the ion count-rate 'asymmetry' A i (ratio of ion rates for right or left circular polarization in the second step), based on the hyperfine-coupling scheme for the first step. These expressions contain ratios of the three radial matrix elements involved in the ionizing step. The experimental determination of P z and A i , for different groups of hyperfine transitions, permits an evaluation of these ratios. The experiment was performed on a Cs atomic beam using a pulsed GaAs laser at 8521 AA for the first step and a pulsed Ar-ion laser at 5017-4545 AA for the second step. Effects of saturation in the first transition are studied and optimal conditions for use of the process in a spin-polarized electron source are discussed.
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.
Measurements are reported on the photoelectron spin polarization and ratio of the ion count rates for linearly and circularly polarized light obtained in two-photon ionization of caesium atoms via the 7 2 P 1/2 and 7 2 P 3/2 intermediate resonance states. Depending on the pulse length of the radiation used the transitions have to be described either in the fine-structure or in the hyperfine-structure coupling scheme. A simple model is used to describe intermediate cases. From these measurements ratios are obtained for the various bound-free matrix elements.
In the two-photon ionisation of Cs atoms resonances may be expected which correspond to a quadrupole 6s-nd transition associated with the first photon. At the 6D 5/2 and 6D 3/2 resonances the ratio of photoionisation cross sections for circularly polarised and for linearly polarised light (ion asymmetry) and the spin polarisation of the photoelectrons ejected by circularly polarised light have been measured. The ratios of the radial transition matrix elements from the 6D to the epsilon P and epsilon F continuum states were derived from the experimental results.
We present a systematic theoretical analysis of two-photon ionization of cesium in the perturbation-theory regime. Matrix elements from diverse sources, such as quantum-defect theory and model potentials, as well as from experimental oscillator strengths, have been used in the calculation. The results are discussed in the context of recent measurements. In addition to total generalized cross sections (gcs) for linearly polarized light, ratios of gcs for linear to circular polarization, as well as photoelectron spin polarization, have been calculated. The various sets of matrix elements give results which are consistent, given variations that should be expected from the diversity of the sources. The disagreement with experiment, however, persists.
Measurements are reported of the ratio of the ion count rates obtained in two-photon ionization of caesium atoms for linearly and circularly polarized light at the nine wavelengths of the argon-ion laser (514.5-454.5 nm). Comparison of this data with the recent theoretical values of Teague et al. (1976) and Flank et al. (1976) shows discrepancies at all photon energies except near the 7P resonance. In addition, upper limits are presented for the absolute two-photon rates in the rhodamine-6G wavelength region (563-635 nm).
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.