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Europhysics Letters (EPL)

An Optical Sector of Electron-Atom Resonances: Its Possible Detection and Relevance for Resonant-Process Studies

F. Giusiano, G. Mambriani

Published Sep 10, 1993• 1 citation

Abstract

The analysis of a theoretical equation for the double differential cross-section of electron-atom collisions, in which "bremsstrahlung" (BS) photons are produced, and in particular of its dependence on both photon energy and electron scattering angle, shows that there is an optical sector of electron-atom resonances, that is, the appearance of resonance dips in BS cross-section. When considering, for instance, the resonance that the rare gases exhibit at nearly 0.5 eV below excitation threshold, this optical sector, or in other words the BS photon energy distribution, presents one or two resonancelike structures whose positions depend on both electron energy and scattering angle. By suitably choosing these parameters, one of these optical-resonance images can be made to occur at easily detectable photon energies. A successful experiment could be performed by utilizing an electron-photon coincidence method such as those used for studying coherence and correlation in electron-atom collisions. Particular conditions for such an experiment are considered, and its possible relevance for better insight into electron-atom resonances is outlined.

DOI: 10.1209/0295-5075/23/8/003

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The analysis of a theoretical equation for the double differential cross-section of electron-atom collisions, in which "bremsstrahlung" (BS) photons are produced, and in particular of its dependence on both photon energy and electron scattering angle, shows that there is an optical sector of electron-atom resonances, that is, the appearance of resonance dips in BS cross-section. When considering, for instance, the resonance that the rare gases exhibit at nearly 0.5 eV below excitation threshold, this optical sector, or in other words the BS photon energy distribution, presents one or two resonancelike structures whose positions depend on both electron energy and scattering angle. By suitably choosing these parameters, one of these optical-resonance images can be made to occur at easily detectable photon energies. A successful experiment could be performed by utilizing an electron-photon coincidence method such as those used for studying coherence and correlation in electron-atom collisions. Particular conditions for such an experiment are considered, and its possible relevance for better insight into electron-atom resonances is outlined.

High-energy cross sections for the excitation of the 3s state of the hydrogen atom by simultaneous collision with electrons and photons are calculated under the second-order Bethe-Born approximation, both as functions of electron energy and of laser intensity. Resonant structures are shown to arise as a function of laser-photon frequency. The behaviour of these resonances is described and their origin explained. It is concluded that these resonances are characteristic of the simultaneous electron-photon excitation process itself and can occur for any atomic target, under suitable conditions.

Abstract We considered the influence of light-dressed-atom effect on two-photon laser-assisted electron-atom scattering in the Born approximation. We performed calculations for the ground state of a Hydrogen atom. We demonstrated that for small scattering angles, the main contribution to the cross section is made by the combined process, when one of the photons is absorbed by the atomic electron and the second one by the scattered electron. However, in the UV region, when two-photon resonances with highly excited atomic states arise, the main contribution to the cross section is determined by the inverse polarization Bremsstrahlung absorption.

Synopsis We considered the influence of light-dressed-atom effect on two-photon laser-assisted electron-atom scattering in the Born approximation. We performed calculations for the ground state of a Hydrogen atom. For small scattering angles, the main contribution to the cross section is made by the combined process, when one of the photons is absorbed by the atomic electron and the second one by the scattered electron. However, in the UV region, when two-photon resonances with highly excited atomic states arise, the main contribution is determined by the inverse polarization Bremsstrahlung absorption.

A brief review is given on development of electron-photon coincidence studies from electron impact of atoms. The theory of electron-photon correlations is outlined in detail in terms of cross sections of excited atoms and excited state charge clouds. The state of a target atom is determined by the various orientation and alignment parameters and the multipole moments of the excited atom. General considerations, such as coherence effects in impact excitation, the influence of fine and hyperfine structures in correlations are taken into account. The relationship between scattering amplitudes (including their phases) and target parameters is worked out for singlet and doublet transitions.

Electrons are scattered elastically by caesium atoms in a crossed-beam experiment. The dependence of the cross section on collision energy and scattering angle has been measured; the collision energies studied are in the range from 0.8 to 20 eV. Resonant structures are observed in the elastic cross section at an energy of 1.49 eV and around the 6 2 P 1/2 threshold at 1.39 eV. It is believed that the 1.49 eV resonance is due to an autoionising F state of Cs - which is consistent with theoretical predictions. The angular distributions of the elastic cross section agree satisfactorily with theory for collision energies below 2 eV. At higher energies larger deviations from theory are observed.

A theoretical description is presented for the process in which photons emitted from atoms which have been stepwise excited firstly by electron impact and then by resonant interaction with coherent radiation are detected in coincidence with the inelastically scattered electrons. The optical excitation is modelled using a full quantum electrodynamic theory. Calculations of the Stokes parameters are performed for the Hg (I=0) scheme; electron excitation from the 6 1 S 0 ground state to the 6 1 P 1 state followed by resonant laser excitation to the 6 1 D 2 state, with analysis being performed on the fluorescence from the 6 1 D 2 -6 3 P 1 relaxation channel. It is found that for this case, even though manifestations of optical pumping are evident, the Stokes parameters do not vary by more than +or-4% from the weak intensity limit.