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Physical Review A

Theory of collisional transfer between orientation and alignment of atoms excited by a single-mode laser

T. Manabe, T. Yabuzaki, Takuya Ogawa

Published Nov 1, 1979• 19 citations
Publisher

Abstract

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 $J=1$ 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.

DOI
10.1103/physreva.20.1946

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In this study we explored the angular momentum alignment-to-orientation conversion occurring in various alkali metals -- K, Rb, Cs. We used a theoretical model that is based on Optical Bloch equations and uses the density matrix formalism. Our model includes the interaction of all neighboring hyperfine levels, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. Additionally we simulated signals where the ground- or the excited-state coherent processes were switched off allowing us to determine the origins of obtained signals. We also performed experiments on Cs atoms with two laser beams: linearly polarised Cs D1 pump and circularly polarized Cs D2 probe. We used the pump beam to create angular momentum alignment in the ground state and observed the transmission signal of the probe beam as we changed the magnetic field. Full analysis of the experimentally obtained transmission signal from a single circularly polarized probe laser component is provided.

•Dec 16, 2020•0 citations
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.
The first experimental evidence of transfer from alignment to orientation of laserexcited atoms due to anisotropic collisions is reported. The experiment has been made with respect to neon atoms in the 2p42{p}_{4}2p4​ state, excited by a single-mode laser, which collide with the ground-state neon atoms, and good agreement between experiment and theory is found.
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.
We report observations of inelastic collisions of atoms excited by a weak monochromatic laser field whose frequency is close to the resonance frequency of the transition from the ground state to an excited state. Our experiments use sodium atoms in a gas cell with the laser frequency detuning from resonance ($|\ensuremath{\Delta}|<6$ GHz) much smaller than the inverse of a collision duration. In this frequency region, our results may be interpreted in terms of inelastic collisions involving virtually excited atoms. The inelastic collision rate is determined by measuring the ratio of fluorescence from the collisionally populated level to emission from the laser populated level. By studying this emission ratio as a function of perturber pressure we show that the inelastic collision rate constant for virtually excited atoms is the same as the thermally averaged rate constant for level-to-level population transfer. Measurements of the emission ratio as a function of the laser detuning from resonance explicitly show how the correlation of collision rates with excitation frequency due to the Doppler effect is lost as the laser is tuned more than several Doppler widths from line center. The collisions of virtually excited atoms fundamentally limit experiments utilizing the Doppler effect to obtain velocity-dependent excited-state collision rates.
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.
In this study we explored the angular momentum alignment-to-orientation conversion occurring in various alkali metals -- K, Rb, Cs. We used a theoretical model that is based on Optical Bloch equations and uses the density matrix formalism. Our model includes the interaction of all neighboring hyperfine levels, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. Additionally we simulated signals where the ground- or the excited-state coherent processes were switched off allowing us to determine the origins of obtained signals. We also performed experiments on Cs atoms with two laser beams: linearly polarised Cs D1 pump and circularly polarized Cs D2 probe. We used the pump beam to create angular momentum alignment in the ground state and observed the transmission signal of the probe beam as we changed the magnetic field. Full analysis of the experimentally obtained transmission signal from a single circularly polarized probe laser component is provided.