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Optics and Spectroscopy

Manifestation of higher polarization moments of the electronic shell of an atom in the collisional population transfer between the hyperfine-structure levels

V. N. Rebane, T. K. Rebane

Published Jan 1, 2002• 0 citations
Publisher

Abstract

The contributions of the relaxation of electronic polarization moments of various orders κ to the probabilities for the population transfer between the hyperfine-structure levels of an atom under the action of collisions are studied. It is shown that the contributions of higher electronic polarization moments with κ > 2 are comparable with the contributions of the orientation (κ=1) and the alignment (κ=2) or even exceed them in magnitude. This opens up the possibility of studying the collisional relaxation of higher polarization moments which are usually not observed and not involved in the emission and absorption of light under dipole transitions. The bounds for the allowable values of the rate constants for the collisional relaxation of higher polarization moments are obtained from the condition for nonnegativity of the total probabilities of population transfer between different hyperfine-structure levels under the action of collisions.

DOI
10.1134/1.1446572

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Optics and Spectroscopy•Nov 1, 2000•0 citations
The contributions of the relaxation of electronic polarization moments of various orders κ to the probabilities for the population transfer between the hyperfine-structure levels of an atom under the action of collisions are studied. It is shown that the contributions of higher electronic polarization moments with κ > 2 are comparable with the contributions of the orientation (κ=1) and the alignment (κ=2) or even exceed them in magnitude. This opens up the possibility of studying the collisional relaxation of higher polarization moments which are usually not observed and not involved in the emission and absorption of light under dipole transitions. The bounds for the allowable values of the rate constants for the collisional relaxation of higher polarization moments are obtained from the condition for nonnegativity of the total probabilities of population transfer between different hyperfine-structure levels under the action of collisions.
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.
The induction of higher polarization moments-the octupole orientation and the hexadecapole alignment—in the case of two-step excitation of atoms through dipole transitions and their manifestation in the polarization of dipole emission under conditions of anisotropic collisions are studied. For all channels of the two-step excitation process J 0→J 1 J, the efficiencies of production of higher polarization moments with integer and half-integer angular momenta from J=2 to J=7 through the absorption of linearly or circularly polarized light are calculated. The rates of collisional relaxation and interconversion of higher polarization moments, for both orientation and alignment, as functions of frequency detuning of the laser line from the center of the Doppler profile are calculated. The theory is illustrated by an example of two channels of excitation of the atomic state J=2 by circularly polarized light: J 0=0→J 1=1J=2 and J 0=3→J 1=2 (or 3)→J=2. In the first case, the octupole orientation enhances or attenuates the signal of circular polarization depending on the laser frequency detuning. In the second case, it represents the only source producing orientation and circular polarization of the emitted light.
Coefficients are calculated for the alignment transfer between levels of hyperfine structure (hfs) of atoms with electron momenta j=1, 1.5, and 2 and nuclear spins i=0.5, 1, 1.5, 2, 2.5, 3, and 3.5 under the influence of nonresonance collisions with atoms of a foreign gas. The results are tabulated. A concrete application of the results is illustrated in calculating numerical values of the effective cross sections for collisional transfer of alignment between hfs levels of the first excited nsup2sup 2sup2Psub3sub 3sub3/sub2sub 2sub2 state of rubidium and cesium atoms. (AIP)
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.
It is found that a sufficiently strong electric field can substantially decrease the relaxation rate of high-frequency polarization of a gas corresponding to the resonance dipole-dipole interaction of its molecules. The corresponding decrease in the integrated cross section for intermolecular Weisskopf collisions is calculated using the two-level approximation for the description of quantum dynamics of internal (vibrational) degrees of freedom of molecules, whereas their translational and rotational motions are treated classically. It is shown that the effect is caused by the Stark change of dipole moments of molecules and the ordering of their orientations in a constant electric field. As a result, for a certain magnitude of this field, the contributions of constant and high-frequency dipole moments of a molecule to the change in its density matrix upon a collision are counter-balanced to a considerable extent. The conditions required for the experimental observation of the effect are formulated.
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.