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Journal of Physics B Atomic and Molecular Physics

Photoelectron polarisation in two-photon ionisation with two different light beams

R. Parzyński

Published Feb 14, 1980• 10 citations
Publisher

Abstract

A new formula is proposed for the spin-polarisation degree of photoelectrons, emitted from the ground state of alkali atoms in two-photon ionisation caused by two different light beams. The formula is derived within the fine structure scheme and can be applied to polarised as well as unpolarised atoms, all conceivable experimental geometries, all possible combinations of polarisation states of the exciting and ionising light beams, and for near-resonance with a P doublet intermediate state.

DOI
10.1088/0022-3700/13/3/009

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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.

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A I Andrjushin, Alexey E. Kazakov

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.

Journal of Physics B Atomic and Molecular Physics•Oct 28, 1980•2 citations
A new formula is proposed for the spin-polarisation degree of photoelectrons, emitted from the ground state of alkali atoms in two-photon ionisation caused by two different light beams. The formula is derived within the fine structure scheme and can be applied to polarised as well as unpolarised atoms, all conceivable experimental geometries, all possible combinations of polarisation states of the exciting and ionising light beams, and for near-resonance with a P doublet intermediate state.
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.
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.
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.
A density-matrix formulation is given of two-photon ionisation of alkali atoms, for two incoming light pulses of arbitrary direction, polarisation, pulse duration and time difference. The frequency of one of the pulses coincides with a resonant transition. The emphasis is on separating out angular momentum factors from the basic physics contained in the radial matrix elements of the atomic electric dipole. The bandwidth of the exciting light is assumed large compared to natural and induced linewidths. The polarisation of the photoelectron is calculated with or without saturation of the excitation for various values of the nuclear spin.
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.