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Journal of Physics B Atomic Molecular and Optical Physics

Alignment to orientation conversion for a collisionally excited He3D state in an external magnetic field

H Avci, H -P Neitzke

Published Feb 14, 1989• 6 citations
Publisher

Abstract

The mixing of state multipoles for an atomic state with unresolved fine structure in a magnetic field is investigated in its simplest case: the conversion of alignment to orientation. The combined action of the external field and the spin-orbit coupling results in a build-up of orientation and hence in the emission of circularly polarised light although the excitation process is axially symmetric and is invariant with respect to the reversal of the symmetry axis and thus the system could only show alignment initially. The case investigated is the collisional excitation of the He 3 3 D state by He + ions. After correction of some errors in the theoretical literature the agreement between the calculated and the observed circular polarisation of the fluorescence radiation from the collisionally excited He atoms is very good. Owing to possible objections in two previous experiments this is the first unambiguous demonstration of the alignment to orientation conversion effect.

DOI
10.1088/0953-4075/22/3/013

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The mixing of state multipoles for an atomic state with unresolved fine structure in a magnetic field is investigated in its simplest case: the conversion of alignment to orientation. The combined action of the external field and the spin-orbit coupling results in a build-up of orientation and hence in the emission of circularly polarised light although the excitation process is axially symmetric and is invariant with respect to the reversal of the symmetry axis and thus the system could only show alignment initially. The case investigated is the collisional excitation of the He 3 3 D state by He + ions. After correction of some errors in the theoretical literature the agreement between the calculated and the observed circular polarisation of the fluorescence radiation from the collisionally excited He atoms is very good. Owing to possible objections in two previous experiments this is the first unambiguous demonstration of the alignment to orientation conversion effect.
Excited-state angular-momentum alignment--orientation conversion for atoms with hyperfine structure in the presence of an external magnetic field is investigated. Transversal orientation in these conditions is reported. This phenomenon occurs under Paschen Back conditions at intermediate magnetic-field strength. Weak radiation from a linearly polarized diode laser is used to excite Rb atoms in a cell. The laser beam is polarized at an angle of $\ensuremath{\pi}/4$ with respect to the external magnetic-field direction. Ground-state hyperfine levels of the 5S1/2{5S}_{1/2}5S1/2​ state are resolved using laser-induced fluorescence spectroscopy under conditions for which all excited 5P3/2{5P}_{3/2}5P3/2​ state hyperfine components are excited simultaneously. Circularly polarized fluorescence is observed to be emitted in the direction perpendicular to both to the direction of the magnetic field B\mathbf{B}B and direction of the light polarization E.\mathbf{E}.E. The obtained circularity is shown to be in quantitative agreement with theoretical predictions.
We report a detailed investigation of excited-state alignment-to-orientation conversion in the presence of an external magnetic field. This counterintuitive phenomenon occurs under intermediate-coupling conditions. A weak, linearly polarized, cw laser beam was used to excite and align the Na 3P3/2{\mathit{P}}_{3/2}P3/2​ state in an atomic beam along the z direction. The degree of circular polarization of the resulting fluorescence was detected along the z direction as a function of magnetic-field strength. The spectrally integrated transitions originating from individual F levels of the 3S1/2{\mathit{S}}_{1/2}S1/2​ state yield a maximum circular-polarization fraction of \ensuremath{\sim}40%; integrating the circular polarization over all the allowed 3S1/2{\mathit{S}}_{1/2}S1/2​-3P3/2{\mathit{P}}_{3/2}P3/2​ transitions gives rise to an \ensuremath{\sim}8% effect. The results are predicted by the Breit formula, which is in excellent agreement with our observations.
The alignment-orientation process is exactly treated for an alkali-like atom with purely electronic couplings. Analytic expressions for the circular polarization of the optical emission are given for specific types of anisotropic excitation in terms of the magnetic field and the spin-orbit parameter. No advanced formalism is used. It is shown how the hyperfine interaction with the electron orbital motion operates in a manner very similar to the purely electronic spin-orbit coupling, again leading to orbital orientation and to circularly polarized optical emission. Detailed computed results for alignment-orientation conversion for neutral helium are summarized, as are the results of experiments which directly exhibit circularly polarized emission due to the alignment-orientation effect in gases.
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.
We studied alignment-to-orientation conversion caused by excited-state level crossings in a nonzero magnetic field of both atomic rubidium isotopes. Experimental measurements were performed on the transitions of the D2{D}_{2}D2​ line of rubidium. These measured signals were described by a theoretical model that takes into account all neighboring hyperfine transitions, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. In the experiments, laser-induced fluorescence components were observed at linearly polarized excitation and their difference was taken afterwards. By observing the two oppositely circularly polarized components, we were able to see structures not visible in the difference graphs, which give deeper insight into the processes responsible for these signals. We studied how these signals are dependent on intensity and how they are affected when the exciting laser is tuned to different hyperfine transitions. The comparison between experiment and theory was carried out fulfilling the nonlinear absorption conditions. The theoretical curves described the experimental measurements satisfactorily, reproducing even small features in the shapes of the curves.
We report the experimental observation of alignment to orientation conversion in the 7D3/2 and 9D3/2 states of Cs in the presence of an external dc electric field and without the influence of magnetic fields or atomic collisions. Initial alignment of angular momentum states was created by two-step excitation with linearly polarized laser radiation. The appearance of transverse orientation of angular momentum was confirmed by the observation of circularly polarized light. We present experimentally measured signals and compare them with the results of a detailed theoretical model based on the optical Bloch equations. The effect is odd under time reversal and should be taken into account in ever more sensitive searches for an electron electric dipole moment.