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Journal of Physics B Atomic Molecular and Optical Physics

Generation of unpolarized Auger electrons from linearly polarized radiation

U. Kleiman, Birgit Lohmann, Karl Blum

Published Jan 1, 1999• 6 citations
Publisher

Abstract

Unpolarized Auger electrons with an isotropic angular distribution are predicted for a primary excitation or ionization of initially unpolarized atoms using completely linearly polarized photons. This phenomenon depends neither on the alignment of the intermediate state nor on the dynamics of the Auger decay and can be observed within a reaction plane that is oriented perpendicular to the direction of the classical electric field vector. A full qualitative and quantitative analysis of this behaviour is given. The effect can be utilized experimentally to generate unpolarized electrons of a fixed energy.

DOI
10.1088/0953-4075/32/9/105

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Unpolarized Auger electrons with an isotropic angular distribution are predicted for a primary excitation or ionization of initially unpolarized atoms using completely linearly polarized photons. This phenomenon depends neither on the alignment of the intermediate state nor on the dynamics of the Auger decay and can be observed within a reaction plane that is oriented perpendicular to the direction of the classical electric field vector. A full qualitative and quantitative analysis of this behaviour is given. The effect can be utilized experimentally to generate unpolarized electrons of a fixed energy.
It is shown that new effects in light scattering by oriented atoms and in orientation of an unpolarized atomic target due to the light scattering arise when channels of dissipation for the light energy are open. Thus, in scattering of linearly-polarized or unpolarized light, the degree of linear polarization and the angular distribution of scattered light depend on the initial atomic orientation. On the other hand, the orientation of unpolarized atoms appears in the scattering of unpolarized photons without registration of the scattered photon polarization. The orientation vector is perpendicular to the scattering plane, and the effect can be observed in a coincidence experiment.
Abstract The dipole radiation emitted by an atom excited by a unidirectional electron beam has a nonuniform angular distribution which is simply related to the percentage polarization P of the radiation emitted perpendicular to the beam. P was first calculated using the OppenheimerPenney (O.-P.) theory. In this theory the probability of excitation of an upper quantum state and the probability of subsequent emission of a polarized photon from such a state are considered independently. P is finally expressed in terms of the cross-sections QM for excitation of states of definite component of angular momentum along the direction of the electron beam. In general, P is dependent on detailed numerical calculations of QM j, but the selection rule A = 0 removes this dependence at threshold. In the O.-P. theory allowance may be made for fine structure and hyperfine structure, but the theory is ambiguous when the f.s. or h.f.s. separations are comparable with the line width. A theory is therefore developed which is based on the calculation of the probability of a polarized photon being emitted by the complete system of atom + electron. The ambiguity of the O.-P. theory is removed by integration over line profiles, but the expressions reduce to O.-P. expressions when the f.s. or h.f.s. separations are much smaller or much larger than the line width. The Lyot line of hydrogen is an intermediate case for which the line widths and the h.f.s. separations are comparable. Assuming the validity of the Born approximation, a simple expression is obtained which allows the QM to be calculated from the angular distribution of the scattered electrons. Theoretical predictions are compared with experimental results. For the Na£) lines the predicted polarization is small enough to escape experimental detection. Polarizations observed by - Skinner & Appleyard in 1927 for various Hg lines rise to maxima with decreasing electron energy, and then tend to values close to zero at threshold. These experimental results at low energies appear to be inexplicable in terms of the reactions considered, but if the polarization curve above the maximum is extrapolated to threshold, the theory and experiment are found to be in reasonable agreement. Further experimental work is thought to be desirable
A density-matrix approach has been developed for the angular distribution and polarization of radiative emission during single-photon atomic transitions for a general set of steady-state excitation processes in an arbitrary arrangement of static (or quasistatic) electric and magnetic fields. Particular attention has been directed at spectroscopic observations in the intense fields of the high-power ion diodes on the Particle Beam Fusion Accelerator II (PBFA II) and SABRE devices at Sandia National Laboratories and at magnetic-field measurements in tokamak plasmas. The field-dependent atomic eigenstates are represented as expansions in a complete basis set of field-free bound and continuum eigenstates. Particular emphasis has been given to directed-electron collisional excitations, which may be produced by an anisotropic incident-electron velocity distribution. We have allowed for the possibility of the coherent excitation of the nearly degenerate field-dependent atomic substates, which can give rise to a complex spectral pattern of overlapping Stark-Zeeman components. Coherent excitations may be produced by a beam of electrons that are spin-polarized at an angle with respect to the propagation direction or by nonparallel electric and magnetic fields. Our main result is a general expression for the matrix elements of the photon-polarization density operator representing the total intensity, angular distribution, and polarization of the atomic radiative emission. For the observation of radiative emission in the direction of the magnetic field, the detection of linearly polarized emission, in addition to the usual circularly polarized radiation, can reveal the presence of a perpendicular electric field or a coherent excitation mechanism.
Polarized radiation following electron impact.---If a unidirectional beam of electrons, of velocity slightly in excess of the resonance potential, is projected through a vapor at a very low pressure so that effects due to atomic collision and resonance radiation are negligible, and in the absence of an applied field and resulting Larmor precession, there is reason to suspect that the radiation emitted perpendicular to the electron beam should be partially polarized, especially for lines corresponding to $\ensuremath{\Delta}j=1$.Polarization in mercury and sodium vapor.---Experiment showed a polarization of about 30 percent for $\ensuremath{\lambda}2537$ with the greatest intensity of the electrical vibration having a direction perpendicular to the electron beam. No polarization was detected for the unresolved sodium D-lines, confirming the recent observation of Kossel and Gerthsen.Polarization in mercury with field of 3 gauss.---A field parallel to the electron beam produced no effect. With the field perpendicular to the beam the radiation along the field was unpolarized while that perpendicular to the field was reduced in polarization to 13 percent. These results suggest that the atom radiates with the same distribution of polarization and intensity as in zero field but with the atom rotated through an angle determined by the time of excitation and the precessional velocity.
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.
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.