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Journal of Physics B Atomic Molecular and Optical Physics

Effect of target dressing in free - free transitions in a bichromatic laser field

Sándor Varró, F. Ehlotzky

Published Feb 28, 1997• 16 citations
Publisher

Abstract

The influence of laser dressing of the atomic target on free - free transitions in a bichromatic laser field of variable relative phase is investigated. For simplicity, hydrogen is taken as target atom and only the elastic, non-resonant case is considered. Exchange effects are neglected and the scattering process is treated to the lowest-order Born approximation. It is found that at very small scattering angles the dressing effects have some influence on the phase dependence of the scattering cross sections in a bichromatic field.

DOI
10.1088/0953-4075/30/4/023

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The influence of laser dressing of the atomic target on free - free transitions in a bichromatic laser field of variable relative phase is investigated. For simplicity, hydrogen is taken as target atom and only the elastic, non-resonant case is considered. Exchange effects are neglected and the scattering process is treated to the lowest-order Born approximation. It is found that at very small scattering angles the dressing effects have some influence on the phase dependence of the scattering cross sections in a bichromatic field.
We consider electron scattering by hydrogen atoms in a bichromatic linearly polarized laser field of frequencies ω and 3ω and we discuss the coherent control of the scattering process as a function of the relative phase φ between the two components of the radiation field. The electrons are assumed to have sufficiently high energies in order to permit the description of the scattering process in the first-order Born approximation. The scattered electrons, embedded in the bichromatic laser field of moderate power, as well as the laser dressing of the hydrogen atoms in their ground state are described by time-dependent perturbation theory. We discuss the angular dependence of the nonlinear differential scattering cross sections for the low values N = ±2 of absorbed or emitted laser photons as a function of the relative phase φ between the two radiation field components. We also compare our results with those obtained from the assumption of a static potential describing the target atom.
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.
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.
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.
Photoelectric effect in atomic hydrogen in the presence of bichromatic laser fields composed of a fundamental frequency and its second or third harmonics is investigated. The expression of the S-matrix for the laser-assisted photoelectric effect is derived in the length gauge, which can simplify the elements of the matrix. A general dressed wavefunction of the atom is derived in perturbation theory. The continuum state of the ejected electron is described by a Coulomb–Volkov wavefunction. Numerical results of photoionization cross section for the distinct second component are obtained and compared with the monochromatic field dressing case. It is shown that the photoionization cross section is modified notably by introducing the second component of field, especially when the colliding system absorbs or emits a considerable number of laser photons.
Process of photon scattering on atoms is considered at energy above the ionization threshold. Influence of the resonant structure in the continuum, induced by a laser field due to coupling with the level 41 S in helium, on the cross sections of elastic and inelastic scattering of a probe radiation on atom in the meta-stable state 21 S is theoretically investigated. In the rotating wave approximation approach dependences of resonant scattering cross sections on the probe and laser radiation characteristics are obtained. The frequency and polarization dependences of the scattering cross sections to the final states of atom 11 S, 21 S and 41 S are calculated.