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Journal of Physics B Atomic and Molecular Physics

On the two-photon ionisation of atomic caesium via quadrupole 6D3/2-6D5/2resonances: fine-structure effects and electron spin polarisation

M. Jaouen, A. Declémy, A. Rachman, G Laplanche

Published Dec 14, 1980• 4 citations
Publisher

Abstract

The two-photon ionisation of caesium atoms in the spin-orbit coupling scheme is analysed when one uses the electric dipole plus quadrupole approximations. The electron spin polarisation is obtained on the allowed quadrupole resonances 6D 3/2 -6D 5/2 . Comparison with experimental results is made and a final discussion is presented.
DOI
10.1088/0022-3700/13/23/005

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Three-photon ionisation probability rates have been calculated for circularly and linearly polarised incident radiation applying time-dependent perturbation theory to the lowest non-vanishing order in the framework of the quantum defect method generalised to the spin-orbit coupling scheme. The infinite summations over the unperturbed atomic states have been performed on the basis of the Green's function formalism. Electron spin polarisation has been evaluated and the possibility of obtaining ejected polarised electrons in the direction of the incident radiation wavevector through three-photon ionisation by circularly polarised radiation is shown.

Journal of Physics B Atomic and Molecular Physics•Sep 28, 1981•2 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 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.
For pt.I see ibid., vol. 17, p.4643 (1984). The authors apply the theoretical results obtained in a previous paper in order to study the two-photon resonant ionisation of atomic caesium in the neighbourhood of the fine-structure doublet 7P 1/2 -7P 3/2 when the incident light is right-hand circularly polarised. This formalism allows one to consider these two resonant states on an equal footing and it includes the effects of the infinite summations involved in higher-order perturbation theory. The numerical results show that the two-photon resonant ionisation of caesium takes place through an anticrossing of levels, at least in the intensity range studied. The temporal behaviour of the probability exhibits two distinct temporal regimes: linear for long interaction times and cubic for the shorter ones. The intermediate regime is characterised by quantum beats. At high intensity values, it is found that the induced light shift of the resonances is time dependent. It is also shown that the contribution of the higher-order effects may explain the slight departures between the theoretical and experimental results in the off-resonant two-photon ionisation of atomic caesium.
Three-photon ionisation probability rates have been calculated for circularly and linearly polarised incident radiation applying time-dependent perturbation theory to the lowest non-vanishing order in the framework of the quantum defect method generalised to the spin-orbit coupling scheme. The infinite summations over the unperturbed atomic states have been performed on the basis of the Green's function formalism. Electron spin polarisation has been evaluated and the possibility of obtaining ejected polarised electrons in the direction of the incident radiation wavevector through three-photon ionisation by circularly polarised radiation is shown.