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Journal of Physics B Atomic and Molecular Physics

The 6s-6d quadrupole resonances in two-photon ionisation of caesium. Measurement of ion asymmetry and spin polarisation

H Kaminski, Joachim Keßler, K J Kollath

Published Jul 14, 1979• 9 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/12/13/001

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For pt.I see ibid., vol.19, p.79 (1986). The authors study the two-photon ionisation of unpolarised target atoms of caesium in their ground state or in one of the two first excited states 6P 1/2 or 6P 3/2 in the non-resonant region. Circularly, linearly and unpolarised light are considered. For each case they give a systematic analysis of generalised transition probabilities, angular distributions and polarisation phenomena. In particular, it is shown that polarised electrons can be obtained even with linearly or unpolarised light.

Journal of Physics B Atomic and Molecular Physics•Jan 14, 1986•3 citations
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.
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.
The two-photon ionisation of atomic caesium from the ground state irradiated with monochromatic circularly polarised radiation is analyzed theoretically in the framework of spin-orbit coupling, which leads to the existence of a differential spin polarisation of the ejected electrons in the z direction. This may give important information about the intermediate resonances and also for the off-resonant region where multiphoton ionisation takes place through intermediate virtual transitions. The essential features of differential spin polarisation are studied and the analysis of the results leads to the suggestion that it should be possible to obtain highly polarised electrons according to the properties of the incident radiation used.
We report measurements of photoelectron angular distributions via electric quadrupole transitions in cesium atoms using multiphoton excitation techniques. Experimental results are presented for the cases of one-photon resonant, two-photon ionization via the 6d 2{\mathrm{}}^{2}2D3/2{D}_{3/2}D3/2​ and 6d 2{\mathrm{}}^{2}2D5/2{D}_{5/2}D5/2​ electric quadrupole transitions and for one-photon resonant, three-photon ionization via the 5d 2{\mathrm{}}^{2}2 D3/2{\mathrm{D}}_{3/2}D3/2​ and 5d 2{\mathrm{}}^{2}2D5/2{D}_{5/2}D5/2​ transitions. The ratios of the total ionization rates for linearly and circularly polarized light have also been measured for the 6d 2{\mathrm{}}^{2}2D3/2{\mathrm{D}}_{3/2}D3/2​ and 6d 2{\mathrm{}}^{2}2D5/2{D}_{5/2}D5/2​ states. A theoretical analysis and detailed comparison with the experimental results are given.
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.
For pt.I see ibid., vol.19, p.79 (1986). The authors study the two-photon ionisation of unpolarised target atoms of caesium in their ground state or in one of the two first excited states 6P 1/2 or 6P 3/2 in the non-resonant region. Circularly, linearly and unpolarised light are considered. For each case they give a systematic analysis of generalised transition probabilities, angular distributions and polarisation phenomena. In particular, it is shown that polarised electrons can be obtained even with linearly or unpolarised light.