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Physical Review A

Electron scattering from atoms in the presence of a laser field. IV

Marvin H. Mittleman

Published Aug 1, 1978• 18 citations
Publisher

Abstract

A recent experiment in which an electron scattering off an atom at a resonance energy is excited to a higher resonance energy by absorption of a laser photon is analyzed. There is more than one mechanism for the reaction, but the cross section, as a function of the laser frequency, is shown to have only one peak for each pair of electron-atom resonance states which can be connected by absorption of a photon. The width of the peak is the width of the upper resonance state.

DOI
10.1103/physreva.18.685

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A recent experiment in which an electron scattering off an atom at a resonance energy is excited to a higher resonance energy by absorption of a laser photon is analyzed. There is more than one mechanism for the reaction, but the cross section, as a function of the laser frequency, is shown to have only one peak for each pair of electron-atom resonance states which can be connected by absorption of a photon. The width of the peak is the width of the upper resonance state.
Dressed states of an atom in a classical laser field of frequency close to an atomic transition frequency are obtained, which are a considerable improvement over the usual two-level rotating-wave approximation. These wave functions are used in a calculation of the cross section for elastic electron-atom scattering in a laser field. Numerical calculations for electron-hydrogen scattering demonstrate that scattering involving absorption or emission of photons can be strongly influenced by coupling to the nonresonant states, regardless of the field strength. Inclusion of the counter-rotating terms in the calculation is shown to be essential for extending the applicability of the theory to scattering in nonresonant fields, where a first-order perturbation-theoretic treatment of the atom-field interaction would be expected to be quite justified.
We have calculated the excitation spectra of a single electron in the excited state of a three-level atom interacting with two off-resonant laser fields at high intensities. We have also considered the process where one laser photon is absorbed by the ground state, as well as a Raman-type process where one photon of one of the laser fields is emitted while another photon of the other laser field is absorbed simultaneously by the second excited state of the atom. The net excitation spectra of the three processes in question consist of two peaks having always positive intensities indicating induced absorption (attenuation) and a third peak which has positive, zero and negative intensities for values of the Rabi frequencies , and , respectively. When , the Raman-type process prevails over the other two, and the resulting peak has a negative intensity indicating that significant amplification is likely to occur at the corresponding frequency.
Abstract Experiments on free–free electron scattering, specifically the absorption or emission of 1.17 eV photons from a Nd:YAG laser field by an unbound electron when it is scattered by an atom or molecule, are reviewed. For large scattering angles such experiments are well described by a simple analytical theory that is independent of the properties of the target. At small scattering angles this theory breaks down for targets with a high dipole polarizability α , and an additional term needs to be incorporated in the scattering amplitude. This term is proportional to the dipole polarizability, and hence introduces the properties of the target into the free–free cross section—i.e., the laser field ‘dresses’ the atom. A progress report is given of free–free experiments designed to look for such ‘dressed atom’ effects during the electron-impact excitation of argon in the presence of a laser field; the lowest excited states of argon have α ≈ 300 atomic units.
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.