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Journal of Physics B Atomic Molecular and Optical Physics

Monoenergetic electron-beam stimulated light-dressing-induced resonance in a helium atom

Hicham Agueny, A. Makhoute

Published Nov 14, 2017• 2 citations
Publisher

Abstract

Abstract We present a theoretical study of energetic electron-beam excitation of ‘six-level helium atom’ embedded in a laser field. The angular distribution of the scattered electron-beam, which contains all information about the combined interactions, is analyzed and exhibits a resonant peak in the vicinity of the 1 sns – 1 snp ( n = 2, 3) transitions. Here, we show that this resonant feature can be stimulated by means of the transferred momentum between the collision partners during the scattering process. By manipulating the latter parameter, one can enhance the resonant peak or lead to its attenuation and even to its cancellation. Our findings are supported by additional calculations based on the time-dependent perturbation theory, where a spurious divergence is expected at a resonant region.

DOI
10.1088/1361-6455/aa9aa3

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Abstract We present a theoretical study of energetic electron-beam excitation of ‘six-level helium atom’ embedded in a laser field. The angular distribution of the scattered electron-beam, which contains all information about the combined interactions, is analyzed and exhibits a resonant peak in the vicinity of the 1 sns – 1 snp ( n = 2, 3) transitions. Here, we show that this resonant feature can be stimulated by means of the transferred momentum between the collision partners during the scattering process. By manipulating the latter parameter, one can enhance the resonant peak or lead to its attenuation and even to its cancellation. Our findings are supported by additional calculations based on the time-dependent perturbation theory, where a spurious divergence is expected at a resonant region.
We present theoretical study of energetic electron-impact excitation of "six-level helium atom" embedded in a laser field. During the scattering process, the combined interactions lead to a change in the kinetic energy and the angular distribution of the scattered electron. The latter contains all information about the combined interactions. The analysis of the angular distribution reveals the emergence of a peak profile. This is will be discussed in terms of quantum-path interferences in bound-bound electronic transitions. Furthermore, the sensitivity of the phenomenon to the collision dynamics will also be discussed.
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 We theoretically investigate quantum virtual path interference caused by the dynamic Stark effect in bound–bound electronic transitions. The effect is studied in an intermediate resonant region and in connection with the energetic electron impact excitation of a helium atom embedded in a weak low-frequency laser field. The process under investigation is dealt with via a Born–Floquet approach. Numerical calculations show a resonant feature in laser-assisted cross sections. The latter is found to be sensitive to the intensity of the laser field dressing. We show that this feature is a signature of quantum beats which result from the coherent mixture of different quantum virtual pathways, and that excitation may follow in order to end up with a common final channel. This mixture arises from the dynamic Stark effect, which produces a set of avoided crossings in laser-dressed states. The effect allows one to coherently control quantum virtual path interference by varying the intensity of the laser field dressing. Our findings suggest that the combination of an energetic electron and a weak laser field is a useful tool for the coherent control of nonadiabatic transitions in an intermediate resonant region.
The weak-field soft-photon theory for free - free transitions is generalized to include atomic potentials which support resonances. The present formulation is not restricted by geometrical constraints. In particular, it may be applied in situations where the laser polarization is perpendicular to the electron momentum transfer. In this special geometry, former theories have predicted that laser-assisted electron - atom scattering resonances should appear as pure Breit - Wigner peaks on a vanishingly small background. The present theory includes higher-order terms and shows that the background amplitude may compete with the resonance part to produce characteristic Fano-type resonance signals. Considering tunable radiation fields it is shown that the laser may induce resonance-interference effects. The shapes of overlapping resonances are in general determined by an interplay between resonance scattering amplitudes and a laser-controlled contrast parameter. Characteristic spectral features in the laser-induced overlap between fine-structure components are uniquely related to the distinctly different interference properties of direct and spin-flip amplitudes.
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.
When an atom is driven by a laser field with a photon energy that is nearly resonant with an atomic transition, it can undergo periodic population transfers between the ground and excited state, known as Rabi oscillations [1], [2]. The aim of this work is to study photoionization from atoms for parameters accessible to a free-electron laser (FEL) [3]. Using time dependent perturbation theory to model photoionization from a Rabi oscillating atom, we have shown that in addition to the expected pathway of ionization from the excited state, non-resonant two-photon ionization will have a comparable amplitude at intensities on the order of 1013W/cm2[3].