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Bulletin of the Lebedev Physics Institute

Angular distribution and polarization of photons during resonant above-threshold scattering on an atom

Грызлова Елена Владимировна, Alexandr I. Magunov, С. И. Страхова

Published Dec 1, 2010• 0 citations
Publisher

Abstract

The effect of the resonant structure in the continuous spectrum induced by an external laser field due to the coupling of continuum states with the 1s4s 1 S level in helium on cross sections of elastic and inelastic scattering of probe radiation on an atom in the metastable 1s2s 1 S state at an energy above the ionization threshold is theoretically studied. Explicit expressions for the angular distribution and the degree of polarization of scattered photons as functions of probe and laser radiation characteristics are derived in the rotating wave approximation. Frequency, angular, and polarization dependences of the cross sections of scattering into final 1s 2 1 S, 1 s2s 1 S, and 1s4s 1 S atomic states are calculated.

DOI
10.3103/s1068335610120055

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The effect of the resonant structure in the continuous spectrum induced by an external laser field due to the coupling of continuum states with the 1s4s 1 S level in helium on cross sections of elastic and inelastic scattering of probe radiation on an atom in the metastable 1s2s 1 S state at an energy above the ionization threshold is theoretically studied. Explicit expressions for the angular distribution and the degree of polarization of scattered photons as functions of probe and laser radiation characteristics are derived in the rotating wave approximation. Frequency, angular, and polarization dependences of the cross sections of scattering into final 1s 2 1 S, 1 s2s 1 S, and 1s4s 1 S atomic states are calculated.
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.
The rotating wave approximation is used to obtain parametric expressions for the resonance cross section for the atomic ground state ionisation by linearly polarised probe radiation in the vicinity of an autoionisation state coupled resonantly to another autoionisation state through circularly polarised laser radiation. Calculations are made for the 2s2p 1 P and 2s3d 1 D states of the helium atom. It is shown that the structure of the photoionisation cross-section spectrum formed for circularly polarised laser radiation differs qualitatively from the structure formed in the case of linear polarisation. The dependence of this structure on the intensity and frequency of laser radiation and the direction of polarisation of the probe radiation is studied.
We calculate the total and differential cross sections for radiative-collisional excitation of the metastable 2 1 S state of He atoms at collisions with Ne atoms in external radiation fields of various frequencies and polarizations. The calculations are performed for a thermal collision energy of E = 10 −3 atomic units and light intensity of I = 1 MW cm −2 , which corresponds to a single photon absorption by a quasi-molecule during the collision. Both the differential and total cross sections are shown to depend strongly on the relative orientation of the radiation polarization vector and the initial relative velocity vector of the colliding atoms. We analyze the azimuthal scattering asymmetry related to the orientation of the angular momentum of the absorbed photon.
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.
A compact expression is derived for the cross section of scattering of arbitrarily polarized light by oriented atomic systems, in which the dependence on the geometric parameters and on the Stokes parameters preassigning the state of partial polarization of incident radiation is explicitly separated. It is found that the cross section of any photoprocess accompanied by photon absorption (stimulated emission) contains the sum of the products of the circular and linear dichroisms of the process by the respective Stokes parameters. The effect of the atomic orientation and of the dissipation of light energy on the polarization singularities and angular distribution of scattered light is investigated. In particular, it is demonstrated that, in the case of an open dissipation channel, the angular distribution remains dependent on the atomic orientation even in the case of zero degree of circular polarization of scattered radiation.