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Journal of Physics B Atomic Molecular and Optical Physics

Free–free experiments: the search for dressed atom effects

Nicholas L S Martin, C.M. Weaver, B N Kim, B.A. deHarak

Published May 8, 2018• 6 citations

Abstract

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.

DOI: 10.1088/1361-6455/aac31b

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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.

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 investigated the complex index of refraction in the x-ray regime of atoms in laser light. The laser (intensity up to

{10}^{13}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}

, wavelength

800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}

) modifies the atomic states but, by assumption, does not excite or ionize the atoms in their electronic ground state. Using quantum electrodynamics, we devise an ab initio theory to calculate the dynamic dipole polarizability and the photoabsorption cross section, which are subsequently used to determine the real and imaginary part, respectively, of the refractive index. The interaction with the laser is treated nonperturbatively; the x-ray interaction is described in terms of a one-photon process. We numerically solve the resolvents involved using a single-vector Lanczos algorithm. Finally, we formulate rate equations to copropagate a laser and an x-ray pulse through a gas cell. Our theory is applied to argon. We study the x-ray polarizability and absorption near the argon

K

edge over a large range of dressing-laser intensities. We find electromagnetically induced transparency (EIT) for x rays on the Ar $1s\ensuremath{\rightarrow}4p$ pre-edge resonance. We demonstrate that EIT in Ar allows one to imprint the shape of an ultrafast laser pulse on a broader x-ray pulse (duration

100\phantom{\rule{0.3em}{0ex}}\mathrm{ps}

, photon energy

3.2\phantom{\rule{0.3em}{0ex}}\mathrm{keV}

). Our work thus opens new opportunities for research with hard x-ray sources.

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.

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.