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Optics and Spectroscopy

Alignment of atomic states by collisions with slow charged particles

A. G. Petrashen, V. N. Rebane, T. K. Rebane

Published Aug 1, 1984• 0 citations
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Abstract

The alignment of atomic states as the result of collisions with a beam of slow charged particles is investigated. The problem of the collisional reorientation of atomic moments is solved by numerical integration of the equations of the impact parameter method taking into account charge--quadrupole interaction. Calculations are made of the alignment of the /sup 2/P/sub 3//sub ///sub 2/ levels of alkali atoms populated by collisions with selective excitation of the /sup 2/P/sub 1//sub ///sub 2/ level. The linear polarization of light emitted by the /sup 2/P/sub 3//sub ///sub 2/ level attains some tens of percents and corresponds to the predominance of polarization in directions perpendicular to the beam. The efficiency of the resulting alignment depends on the volume density but not on the velocity of the beam of charged particles. Thus the slow drift of ions (and electrons) in a gas-discharge plasma can compete with alignment as the result of fluxes of charged particles. We shall formulate the sum rules for anisotropic collisional relaxation of atomic polarization moments. It follows from these that for the appearance of alignment in intramultiplet mixing under the action of anisotropic collisions we require a nonuniform population of multiplet levels that differs from the more » ratio of their statistical weights. « less

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Optics and Spectroscopy•Nov 1, 2000•0 citations
The alignment of atomic states as the result of collisions with a beam of slow charged particles is investigated. The problem of the collisional reorientation of atomic moments is solved by numerical integration of the equations of the impact parameter method taking into account charge--quadrupole interaction. Calculations are made of the alignment of the /sup 2/P/sub 3//sub ///sub 2/ levels of alkali atoms populated by collisions with selective excitation of the /sup 2/P/sub 1//sub ///sub 2/ level. The linear polarization of light emitted by the /sup 2/P/sub 3//sub ///sub 2/ level attains some tens of percents and corresponds to the predominance of polarization in directions perpendicular to the beam. The efficiency of the resulting alignment depends on the volume density but not on the velocity of the beam of charged particles. Thus the slow drift of ions (and electrons) in a gas-discharge plasma can compete with alignment as the result of fluxes of charged particles. We shall formulate the sum rules for anisotropic collisional relaxation of atomic polarization moments. It follows from these that for the appearance of alignment in intramultiplet mixing under the action of anisotropic collisions we require a nonuniform population of multiplet levels that differs from the more » ratio of their statistical weights. « less
Direct observability of polarization moments of a higher order—octupole orientation of atomic angular momenta (κ=3) and their hexadecapole alignment (κ=4)—from polarization of quadrupole radiation is discussed. The case when atoms are excited through dipole absorption of light producing orientation or alignment of angular momenta with κ=1 or 2 and higher polarization moments are induced by collisions with a beam of particles is considered. It is shown that differences in the laws of transformation of polarization moments of various ranks κ in rotations of coordinate axes allow us to select the pure contribution of the higher polarization moment under study from the signal of polarization of quadrupole emission by excluding the contribution of orientation or alignment with κ=1 or 2. Optimum systems of coordinate axes for the observation of octupole orientation from circular polarization of light and for the observation of hexadecapole alignment from linear polarization of light, in which the pure contribution of the higher polarization moment under study to polarization of quadrupole emission reaches its maximum, are calculated.
A formula has been obtained which describes the dependence of the overall cross section of the inelastic process in experiments with intersecting atomic and light beams on five parameters, two of which depend on the properties of the quasimolecule and the collisional energy, and three others on the degree of linear polarization of the exciting light and two angles which determine the geometry of the experiment. It was shown that measuring the overall cross section by varying the two latter parameters allows us to obtain additional information on the mechanism of the reactions. As an example, the experimentally studied process of spin transfer in Ca(4s5p/sup 1/P/sub 1/)+He and the process of Penning ionization of He(2/sup 1/P/sub 1/)+Cd(Zn) has been considered.
Theoretical study is made of the transfer between orientation and alignment of excited-state atoms by anisotropic collisions with ground-state atoms. The anisotropy considered is due to the anisotropic velocity distribution of atoms excited by a single-mode laser beam. The transfer rate and relaxation rates for each multipole moment are calculated for the excited state with J=1J=1J=1 as a function of detuning of the laser frequency from the center of an absorption line and mass of perturbing ground-state atoms. Experiments to observe the transfer from alignment to orientation and from orientation to alignment in a weak magnetic field are considered, and corresponding signals are analyzed in terms of the transfer and relaxation rates.
Calculations of the collisional relaxation of electronic--nuclear polarization moments of all state of the fine and hyperfine structure of the 3/ital p//sup 2//ital P/ doublet of the sodium atom (/ital J/=1/2 and 3/2, nuclear spin /ital I/=3/2) are carried out. The case of selective monochromatic excitation of the fine level /sup 2//ital P//sub 1/2/, allowing for the overlap of the Doppler shapes of the hyperfine components of the spectral line, is discussed. Such excitation gives rise in the /sup 2//ital P//sub 1/2/ state to four groups of excited atoms differing in values of the velocity projection on the direction of the light beam. The problem of collisional relaxation of the polarization moments is solved for each of these groups of atoms, account being taken of the anisotropy of the velocity distribution and intramultiplet mixing. The alignment induced in the levels of the hyperfine structure of the /sup 2//ital P//sub 3/2/ state, populated by anisotropic collisions with atoms of different inert gases, is calculated, as is the linear polarization of the light of sensitized fluorescence. The calculation results are in good agreement with the experiment.
New experimental results have been obtained in collisions of singly charged ions with Na Rydberg atoms. (a) The distribution of final n states populated by electron capture shows non-monotonic variations of width with incident energy over the range 60-2100 eV. (b) The total capture cross section for alkali ions on Rydberg targets shows peaks and dips over the same energy range, correlated with particular reduced velocities v and the polarization of the exciting laser. (c) {ell}-Changing collision of Na{sup +} on Na(nd) targets, n=26 and 28, at low v preferentially transfer multiple units {h_bar} of orbital angular momentum under single-collision conditions, despite very large impact parameters. (d) Inelastic impacts of Na{sup f+} on Na 36s and 30p states, perturbed by electric fields, populate a strongly field-dependent mixture of nearby Stark levels. A synopsis of recent progress will be given.
The linear polarization of quadrupole emission by the J=2-J 0=0 transition under conditions of laser excitation in a gas medium is studied. Cases of excitation through dipole and quadrupole absorption of monochromatic laser radiation are considered. Taking into account the anisotropy of collisional relaxation, the contributions of polarization moments of the second and fourth ranks, i.e., those with usual and hexadecapole alignments, to the signal of linear polarization of quadrupole emission are calculated. The dependence of this signal on the laser frequency, the density of the gas medium, and the angles determining the orientation of the system of axes of observation of light polarization relative to a laser ray is studied. The numerical calculations of contributions of usual and hexadecapole alignment to the signal of linear polarization are made for the magnetic quadrupole transition J=2-J 0=0 between the states 2p 5(2 P 1 2/0 )3p′[3/2]2 and 2p 5(2 P 1 2/0 )3s 3s′[1/2]0 of neon atoms in the xenon atmosphere.