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Journal of Experimental and Theoretical Physics

Dependence of the differential cross sections for radiative-collisional excitation of metastable atomic states on the polarization of radiation

E. A. Chesnokov

Published Nov 1, 2005• 2 citations
Publisher

Abstract

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.

DOI
10.1134/1.2149056

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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.
A theory of radiative-collisional transitions between the ground and metastable 1 S states of a colliding atom is proposed. The theory uses the uniform quasi-classical approximation generalized to the case of spherically asymmetric interactions. The theory takes into account the angular momentum of an emitted (absorbed) photon and allows one to calculate the total and differential scattering cross sections in a wide range of radiation frequencies including both wings and the center of the line of a forbidden atomic transition. The range of admissible collision energies and intensities of an external radiation field is restricted by the use of the adiabatic approximation, as well as the approximation of a weak coupling between the ground and excited states of a quasi-molecule, the potentials of which are assumed to be monotonically repulsive.
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.
Formulas are derived for the differential and integral cross sections associated with laser-induced excitation transfer. The theory is patterned closely after our recent treatment of laser-induced chemi-ionization. It is found that cross sections specific to single-photon absorption are proportional to the square of the cosine of the angle between the laser polarization and the initial relative velocity of the two colliding atoms. The distortion by a laser of the cross section for an elastic scattering event also is treated. Finally, a cross-section formula is derived for a collision-induced two-photon absorption, mediated by a single intermediate electronic state.
The formal structure of the time dependence of the state of an atom in a perturber bath in an intense classical monochromatic radiation field is studied. The authors allow the cross sections of binary collisions to be affected by the radiation field. The result is used to derive an expression for the collision-broadened saturated fluorescence excitation spectrum and absorption rate in terms of a collisional relaxation operator. This operator can be related to the rate of collisionally aided radiative excitation.
A general formulation for thermal collisions between a Rydberg-state atom and a ground-state rare-gas atom is developed within the framework of the impulse approximation. This formulation allows calculation of cross sections for state-to-state transitions for an arbitrary initial orientation between the Rydberg-state atom and the relative velocity of the two partners of the collision. It also allows a direct computation of these cross sections averaged over all orientations, a situation corresponding to a cell experiment. In this formulation, the differential cross sections with respect to the modulus of the momentum transfer are obtained analytically in terms of rotation matrix elements. Numerical applications are made for the case of a sodium atom in a ``circular'' Rydberg state (\ensuremath{\Vert}n,l,m〉, with l=m=n-1) colliding with helium. The collisional processes are found to be highly anisotropic. In particular, a selection rule may appear, or disappear, depending upon the initial orientation. The extension to the case where an external electric field is present is also discussed, with special emphasis on symmetry properties.