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Physical Review Letters

Experimental Verification of the Fano Effect

Joachim Keßler, J. Lorenz

Published Jan 19, 1970• 44 citations
Publisher

Abstract

An absolute measurement of the electron polarization obtained by photoionization of cesium by circularly polarized light is reported. The measured polarization of 65 \ifmmode\pm\else\textpm\fi{} 15% confirms the prediction made by Fano and the assumptions of Seaton on spin-orbit interaction in photoionization of alkali atoms. They also show that it is possible to construct a photoionization source of polarized electrons without using a polarized atom beam.

DOI
10.1103/physrevlett.24.87

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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Jun 1, 2017•0 citations
An absolute measurement of the electron polarization obtained by photoionization of cesium by circularly polarized light is reported. The measured polarization of 65 \ifmmode\pm\else\textpm\fi{} 15% confirms the prediction made by Fano and the assumptions of Seaton on spin-orbit interaction in photoionization of alkali atoms. They also show that it is possible to construct a photoionization source of polarized electrons without using a polarized atom beam.
A spin-exchange optical pumping experiment to study collisions between cesium atoms and quasifree electrons is reported. In this experiment, electrons in a weak magnetic field were polarized through spin-exchange collisions with optically pumped cesium atoms. The cesium-electron collisions were the principal source of the electron-resonance linewidth, and they also gave rise to a shift in the center frequency of the electron resonance. The magnitudes of the linewidth and frequency shift depend upon the two-body scattering amplitude for elastic collisions, the cesium polarization, and the cesium density. From measurements of the electron linewidth and the frequency shift, a value is derived for the electron-cesium spin-flip cross section. The spin-flip cross section at 20\ifmmode^\circ\else\textdegree\fi{}C is found to be approximately 3.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$ cm2{\mathrm{cm}}^{2}cm2. The expressions for the linewidth and frequency shift due to spin-exchange collisions of electrons with alkali-metal atoms are generalized to cover the case where there is a spin-orbit interaction between the electron-alkali-metal-atom systems.
An experiment is reported on two-step photoionization of Cs via the 6P 3/2 intermediate state using circularly polarized radiation from a GaAs laser for the first step (8521 AA) and from an Ar-ion laser for the second step. Values for the ratio of ion production rates for right- and left-handed circular polarization in the second step and for the spin polarization of the photoelectrons are presented. The measurements are interpreted on the basis of a calculation which takes into account the hyperfine splitting of the levels involved in the first transition. A description in terms of fine-structure splitting of the 6P state is shown to be totally inadequate, even when allowing for possibly large spin-orbit effects in the continuum. The process is evaluated as a possible source of a spin-polarized electron beam.
It is argued that photo-electrons emitted from trivalent atoms in a two-photon ionization process are totally polarized if the incident light is of total circular polarization and its wavelength matches the resonance condition for the transition between ground state (n 2 P 1 2/) and the first excited state (n+1) 2 S 1 2/. The estimated photoionization yield compares favourably with that of the analogous process in alkali atoms. Some of the most important conditions for a successful experiment are considered.
Summary form only given. Spin plays a fundamental role in the electronic structure of matter, from single atoms and molecules through to condensed matter. However, spin dynamics and the spin response of electrons to ultra-short, strong laser pulses has remained largely unexplored until today. In 2011 and 2013, pioneering theoretical work [1, 2] extended the fundamental PPT theory [3] to treat ionization of p+ and p” electrons by circular fields. This work predicted the sensitivity of ionization probabilities in strong field ionization using circular fields to the magnetic quantum number, showing that counter rotating electrons can tunnel ionize easier. It also predicted the possibility to obtain spin polarized electrons from strong, circularly polarized fields with a high degree of efficiency. The preference of ionization was later confirmed experimentally by [4], however, the experiment in [4] did not provide a confirmation of the spin polarization predictions.