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Soviet Journal of Quantum Electronics

Polarization of electrons by resonant two-photon ionization of unpolarized atoms

A. I. Andryushin, Alexey E. Kazakov

Published Nov 30, 1980• 0 citations
Publisher

Abstract

General expressions are derived and investigated for the degree of polarization of electrons as a result of resonant two-photon ionization of unpolarized atoms having the S state as the ground state. The exciting and ionizing electromagnetic fields have arbitrary degrees of polarization and directions of propagation. It is shown that in cases where the matrix elements of P → S transitions from the resonant state to the continuum predominante over the matrix elements of P → D transitions, completely polarized photoelectrons may be obtained. These cases are found for sodium and potassium atoms.

DOI
10.1070/qe1980v010n11abeh010320

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General expressions are derived and investigated for the degree of polarization of electrons as a result of resonant two-photon ionization of unpolarized atoms having the S state as the ground state. The exciting and ionizing electromagnetic fields have arbitrary degrees of polarization and directions of propagation. It is shown that in cases where the matrix elements of P → S transitions from the resonant state to the continuum predominante over the matrix elements of P → D transitions, completely polarized photoelectrons may be obtained. These cases are found for sodium and potassium atoms.
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.
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The physical effects which result in the polarization of the electrons produced in the ionization of unpolarized atoms by an electromagnetic field in the optical frequency range are discussed. Attention is focused on the Fano effect and resonance many-photon ionization. Basic equations for the angular and polarization properties of the photoelectrons are derived. A corresponding experiment is described, and the results are compared with the theoretical predictions. The outlook for the use of these processes to develop sources of polarized electrons is discussed.
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.
Sodium atoms are excited to either the 32P323^{2}P_{\frac{3}{2}}32P23​​ or 32P123^{2}P_{\frac{1}{2}}32P21​​ state using a linearly polarized dye-laser beam and are subsequently ionized using a linearly polarized nitrogen-laser beam. Angular distributions of the ejected electron have been both measured and calculated for several relative orientations of the photon polarization vectors. The difference between the sss- and ddd-continuum partial-wave phase shifts ${\ensuremath{\delta}}_{s}\ensuremath{-}{\ensuremath{\delta}}_{d}$ and the ratio of the radial dipole matrix elements (dsdd\frac{{d}_{s}}{{d}_{d}}dd​ds​​) are obtained.
A beam of unpolarized electrons is crossed at right angles with a beam of partially polarized sodium or potassium atoms (degree of polarization P A approximately=21%) and resonance lines from the excited 2 P 3/2, 1/2 states are observed in a direction perpendicular to both the atom and electron beams for electron energies from threshold to 200 eV. The polarization of the emitted photons is measured in the form of the Stokes parameters P 1 , P 2 and P 3 . The present measurements for sodium atoms are in better agreement with the theoretical values than the previously published experimental results. No other experimental or theoretical results are available for potassium atoms.