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Journal of Physics B Atomic Molecular and Optical Physics

Measurement of collisional depolarisation rates in the 4f66s27F levels of Sm by optical field induced circular birefringence

Qihang Gong, Y. H. Zou

Published Jul 14, 1990• 0 citations
Publisher

Abstract

The Hanle resonance investigated as an optical field induced circular birefringence has been applied to studying the collisional depolarisation in the 4f 6 6s 2 7 F J (J not=0) levels of Sm by rare gas perturbers. The measured cross sections for the destruction of orientation in 4f 6 6s 2 7 F J (J=1-6) by He and Ar perturbers are reported. The data for the 7 F 1 level are compared with previous works.

DOI
10.1088/0953-4075/23/13/015

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The Hanle resonance investigated as an optical field induced circular birefringence has been applied to studying the collisional depolarisation in the 4f 6 6s 2 7 F J (J not=0) levels of Sm by rare gas perturbers. The measured cross sections for the destruction of orientation in 4f 6 6s 2 7 F J (J=1-6) by He and Ar perturbers are reported. The data for the 7 F 1 level are compared with previous works.
A polarization technique based on the detection of Zeeman quantum beats in transmission is applied to the determination of multipole relaxation rates in ground or near-ground atomic levels. With an appropriate choice of polarization and geometry, beat signals are obtained whose decay directly yields the relaxation rates of orientation or alignment. The technique has been used to study collisional depolarization in the $4{f}^{6}6{s}^{2}^{7}F_{1}$ near-ground level of Sm I by rare-gas perturbers. Derived cross sections are compared with previous, widely differing determinations.
Measurements of collisional depolarization cross sections play an important role in gaining valuable information about the interaction between inert and rare-gas atoms.A key element in understanding collisional dynamics is the detailed analysis of the polarization of the emitted photons.In order to investigate the collisional cross section of the excited J=1/2 cesium atoms, by collisions with the ground level argon atoms, experimental measurements were made studying circular polarization spectra.Orientation in the J=1/2 level was optically induced by circularly polarized light with a positive helicity.A two-photon double-resonance (6s 2 S 1/2 →6p 2 P 1/2 → 10s 2 S 1/2 ) condition was achieved using nanosecond pulsed dye lasers, and the intensity of the cascade fluorescence was monitored in the presence of argon atoms ranging from 10 -4 torr to 100 torr.
Hanle effect studies using circularly polarized laser beam and linearly polarized laser beam were carried out. Lifetimes, orientation and alignment destruction cross sections were measured for the 5 d 33 , 6 p 22 and 6 p 01 excited levels of xenon. The observed ratio of orientational cross section to alignment cross section for Xe–Xe scattering was 1.1±0.4 for the 5 d 33 level, and 0.55±0.2 for the 6 p 22 level. The ratio for Xe–He scattering was 0.8±0.2 for the 5 d 33 level and 0.6±0.2 for the 6 p 22 level. They are compared with those calculated by Wang and Tomlinson. The observed radiative lifetime was 520±30 ns for the 5 d 33 level, 35±2 ns for the 6 p 22 level and 37±4 ns for the 6 p 01 level. The decay rate of the 5 d 33 level observed in the coherence transfer with spontaneous emission is consistent with the width of the Mania signal.
We report detailed studies of Ramsey-type resonances due to collisional velocity diffusion of Sm atoms within the Doppler-broadened 7F1^{7}\mathrm{F}_{1}7F1​\ensuremath{\rightarrow}7{\mathrm{}}^{7}7F0{\mathrm{F}}_{0}F0​ 154Sm^{154}\mathrm{Sm}154Sm transition at \ensuremath{\lambda}=570.68 nm in the presence of rare-gas perturbers (He,Ne,Ar,Kr,Xe). The experimental technique uses counterpropagating laser fields and relies on coherent resonance Raman processes to excite and detect sublevel coherence optically. A modulated pump laser field is used for velocity-selective resonance Raman excitation of oscillating Zeeman coherence in the J=1 ground state of Sm; the sublevel coherence is monitored by Raman heterodyne detection using a counterpropagating, unmodulated probe laser field. If the pump and probe fields interact with different atomic velocity subgroups, a signal only occurs in the presence of velocity-changing collisions which provide a transport of the Sm atoms with sublevel coherence in velocity space. The sublevel resonance curves turn out to be sensitive to both depolarizing and velocity-changing collisions; the measured line shapes show the characteristic features of Ramsey-type resonances. Our theoretical treatment is based on a novel four-level density matrix calculation; the analysis reveals that the temporal evolution of the collisional velocity-diffusion processes can simply be obtained by means of a Fourier transformation of the measured sublevel resonance curves. In the case of He perturbers the diffusion in velocity space can be described by a Fokker-Planck equation; here, the theoretical fits of the measured sublevel resonance curves directly yield the time constant for collisional thermalization and the cross section for depolarizing collisions. In the case of the heavier rare gases, however, the Fokker-Planck model fails to describe the velocity thermalization; here, the Keilson-Storer kernel yields a better agreement with the experimental data. We derive cross sections for depolarizing and velocity-changing collisions and values for the collisional velocity changes associated with orientation-preserving collisions.
We have measured the orientation of the Ba 6p 1{\mathrm{}}^{1}1P level produced by collision-induced excitation from the ground state by circularly polarized light. The detuning dependence of the far-wing excited-state orientation can be interpreted in terms of reorientation of molecular orbitals which occur during the collision. Effects due to rotational coupling are seen to occur at large blue-wing detunings. We have also determined the collisional rate for destruction of orientation by measuring the pressure dependence of the excited-state orientation.