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Optics and Spectroscopy

Change in the polarization characteristics of an ensemble of excited barium atoms in collisions with induced dipoles

A. G. Petrashen

Published Mar 1, 2005• 0 citations
Publisher

Abstract

The change in the polarization characteristics of an ensemble of barium atoms in collisions with dipoles is considered in the approximation of long-range interaction. Certain considerations concerning the quantitative characteristics of the polarized emission that can be used for diagnostics of parameters of the colliding particles are presented.

DOI
10.1134/1.1890505

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Journal of the Optical Society of America B•Jan 27, 2026•0 citations
The change in the polarization characteristics of an ensemble of barium atoms in collisions with dipoles is considered in the approximation of long-range interaction. Certain considerations concerning the quantitative characteristics of the polarized emission that can be used for diagnostics of parameters of the colliding particles are presented.
The collision-induced absorption and emission coefficients for electric-dipole forbidden atomic transitions are calculated for weak radiation fields. The approximations used are valid for $\ensuremath{\hbar}\ensuremath{\omega}$ near $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}$, the atomic energy differences. The example case of $S\ensuremath{-}D$ transitions induced by a spherically symmetric perturber (e.g., a noble-gas atom) is treated in detail and compared to measurements. The case of "radiative collisions," in which both colliding atoms change their state, is included in the theory and is also compared to experiment.
A theoretical analysis is made of the polarization of atomic states (inhomogeneous distribution over magnetic sublevels) induced by collisions with particles polarized by a laser field. The transfer of polarization may be observed by investigating the polarization of the spontaneous radiation emitted from atoms. The polarization tensor of such spontaneous radiation is calculated. A close relationship between the components of this tensor and the characteristics of an elementary scattering event is established.
The effect of atomic collisions on the optical characteristics of an ensemble of excited atoms is well known [ 1 , 2 ]. The evolution of laser techniques and the resulting interest in various nonlinear-spectroscopy methods have suggested that atomic collisions have a significant effect on the lasing mode of gas lasers and the shape and width of various nonlinear signals. For a long time attention has been focused on the important role of elastic collisions, among other things, and various methods have been proposed for describing such collisions [ 3 ].
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.
Once created, an ensemble of atoms in their identical Rydberg states immediately experiences significant variations of its structure and corresponding properties. The variation is caused by the inevitable radiation transitions of highly excited states into different states of higher or lower energies. In particular, the change of a uniform definite-state population for another kind of population of different states will change the long-range interaction between atoms, even transforming the van der Waals interaction, Δ E vdW =− C 6 / R 6 , into dipole–dipole interaction with a different dependence on the interatomic distance, Δ E d−d =− C 3 / R 3 . In this paper, the most important—dipole radiation transitions from Rydberg states to higher- or lower-energy | n ′ 1 P 1 ⟩ states—are considered, both spontaneous and induced by the blackbody radiation (BBR) of the environment. The total transition probability rate, which is usually determined as the natural width, equals the imaginary part of the Rydberg-state energy Γ n S /2=−Im E n S . The probabilities of the | n ′ 1 P 1 ⟩-state populations W n S ( n ′ P, T ), which depend on the BBR temperature T , on the lifetime scale are also rather important optical characteristics of the ensemble of the Rydberg-state atoms. The value of W n S ( n ′ P, T ) is determined as the ratio of a particular transition probability rate Γ n S→ n ′ P sp +Γ n S→ n ′ P BBR ( T ) to the sum of all possible radiative transition rates, which include spontaneous and BBR-induced rates, where the latter depend on the temperature T of the environment. Numerical values of W n S ( n ′ P, T ) are calculated and presented in wide regions of the principal quantum numbers n , n ′ , and BBR temperatures T .