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Journal of Physics B Atomic and Molecular Physics

Angular distributions and polarisation phenomena in non-resonant multiphoton ionisation of alkali-metal atoms. II. Two-photon ionisation of atomic caesium

G Laplanche, M. Jaouen, A. Rachman

Published Jan 14, 1986• 3 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/19/1/015

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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.
For the last ten years, the production of polarised electrons has been of increasing interest in different fields of physics. One of the methods used to obtain polarised electron beams is based on the multiphoton ionisation of heavy alkali-metal atoms. The authors give a systematic analysis of the theoretical tools necessary for the study of the experimental quantities which can be measured in multiphoton ionisation experiments for moderate intensities in the non-resonant region. They use a description of the problem based on the density operator. The expansion of this density operator over the complete set of irreducible spherical tensor operators enables them to describe the different parameters attainable by experiments and so to construct the density matrix. This formalism allows the authors to study the total transition probabilities, the angular distributions and the differential polarisation degrees along any direction. They use the quantum defect theory as an atomic model. The atomic core polarisation is fully taken into account. The infinite summations running over the whole unperturbed atomic spectrum (discrete plus continuum) related to time dependent perturbation theory are performed using the Green's function formalism.
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.
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.
General expressions are presented and investigated for the degree of polarisation of electrons produced in the resonant two-photon ionisation of unpolarised atoms from the ground S state. The exciting and ionising electromagnetic fields are assumed to have arbitrary directions of propagation and arbitrary degrees of polarisation. It is shown that completely polarised electrons can be obtained when the matrix element of the P to S transition from the resonant state to the continuum is larger than the matrix element of the P to D transition. Such cases are realised in sodium and potassium atoms.
Non-perturbative completely analytic calculations of the spin polarisation of photoelectrons produced in resonant two-photon ionisation of alkali atoms with exciting and ionising light beams are presented. A well justified alkali atom model composed of four discrete states and the continuum is applied. Contrary to simpler models it enabled one to obtain results valid when the effective resonance Rabi frequency exceeds the frequency separation between the two components forming a given fine-structure energy doublet at resonance. It is shown that in this limit the photoelectron polarisation depends critically on whether a light or heavy alkali atom is ionised. In the former case the photoelectron polarisation is proved to be a copy of the electronic polarisation of the atom, whereas in the case of heavier alkalis it departs appreciably from that of the ionised atom due entirely to spin-orbit perturbation of radial wavefunctions of P discrete states. It is also shown how the author's present results go over into those previously obtained for weaker exciting fields.