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Journal of Physics B Atomic and Molecular Physics

Photoelectron polarisation in resonant two-photon ionisation with superstrong continuous excitation

R. Parzyński

Published Dec 28, 1983• 2 citations

Abstract

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.

DOI: 10.1088/0022-3700/16/24/014

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Polarisation of photoelectrons produced from unpolarised alkali atoms is studied analytically for a wide range of the strengths and durations of the light pulse. In the particular case of long pulse durations, where the rate equation for the ionisation probability is approached, the photoelectron polarisation is shown to be a monotonically decreasing function of the pulse strength. In the case of weak pulses, usually dealt with by perturbation theory, the photoelectron polarisation is found to decrease continuously when the pulse duration is shortened. It is, however, found also that if neither the long pulse duration or the weak-field condition is fulfilled then an increase in pulse strength and a shortening of the pulse duration need not always affect the photoelectron polarisation destructively.

Journal of Physics B Atomic and Molecular Physics•Dec 28, 1984•1 citation

Angular distributions and polarisation phenomena in non-resonant multiphoton ionisation of alkali-metal atoms. I. Formal theory

G Laplanche, M. Jaouen, A. Rachman

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.

Journal of Physics B Atomic and Molecular Physics•Jan 14, 1986•8 citations

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

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

Use of the Green's function formalism in resonant two-photon ionisation of alkali-metal atoms. II. Two-photon ionisation of Cs near the 7P1/2-7P3/2resonances

M. Jaouen, G Laplanche, A. Rachman

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.

Journal of Physics B Atomic and Molecular Physics•Dec 14, 1984•4 citations

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.

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.

Polarisation of photoelectrons produced from unpolarised alkali atoms is studied analytically for a wide range of the strengths and durations of the light pulse. In the particular case of long pulse durations, where the rate equation for the ionisation probability is approached, the photoelectron polarisation is shown to be a monotonically decreasing function of the pulse strength. In the case of weak pulses, usually dealt with by perturbation theory, the photoelectron polarisation is found to decrease continuously when the pulse duration is shortened. It is, however, found also that if neither the long pulse duration or the weak-field condition is fulfilled then an increase in pulse strength and a shortening of the pulse duration need not always affect the photoelectron polarisation destructively.

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.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 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.