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Physical Review A

Magnetically induced circular polarization

K.L. Stricklett, D. J. Burns, P. D. Burrow

Published Dec 1, 1987• 2 citations
Publisher

Abstract

Helium atoms excited to an aligned (1s3d${)}^{3}$D state are produced by electron impact parallel to a magnetic field. The initial alignment of the atomic state evolves into an orientation with finite angular momentum along the magnetic field direction and thus gives rise to a circularly polarized component in the (1s3d${)}^{3}$D\ensuremath{\rightarrow}(1s2p${)}^{3}$P radiation propagating parallel to the magnetic field. Circular-polarization fractions of a few percent are observed, in general agreement with theory. The measured polarization fraction is found to be pressure dependent, indicating that secondary excitation and cascade processes must be included for a complete description.

DOI
10.1103/physreva.36.5280

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arXiv:1810.10851•Physical Review A•Oct 10, 2018•21 citations
Helium atoms excited to an aligned (1s3d)3{)}^{3})3D state are produced by electron impact parallel to a magnetic field. The initial alignment of the atomic state evolves into an orientation with finite angular momentum along the magnetic field direction and thus gives rise to a circularly polarized component in the (1s3d)3{)}^{3})3D\ensuremath{\rightarrow}(1s2p)3{)}^{3})3P radiation propagating parallel to the magnetic field. Circular-polarization fractions of a few percent are observed, in general agreement with theory. The measured polarization fraction is found to be pressure dependent, indicating that secondary excitation and cascade processes must be included for a complete description.
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.
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.
The polarization of light resulting from the excitation of helium by electron impact has been measured as a function of electron energy. We report here our results in the energy threshold region for the following transitions: $3 ^{1}P\ensuremath{\rightarrow}2 ^{1}S (\ensuremath{\lambda}=5016 \mathrm{\AA{}})$, $3 ^{3}P\ensuremath{\rightarrow}2 ^{3}S (\ensuremath{\lambda}=3889 \mathrm{\AA{}})$, and $4 ^{1}D\ensuremath{\rightarrow}2 ^{1}P (\ensuremath{\lambda}=4922 \mathrm{\AA{}})$. The results reported here are in substantial agreement with the results previously reported by McFarland. Also, no dependence of the polarization on the magnetic field parallel to the electron beam was observed.
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.
Cross sections for relaxation of circular and linear polarization of 62P326^{2}P_{\frac{3}{2}}62P23​​ cesium atoms, induced in collisions with ground-state He, Ne, Ar, and Xe, have been determined by Zeeman scanning techniques. A fluorescence cell containing low-pressure cesium vapor together with a buffer gas was irradiated with circularly or linearly polarized 8521-\AA{} Cs resonance radiation, producing excited and polarized Cs atoms which became depolarized in subsequent collisions with the buffer-gas atoms. The resulting depolarization of the cesium resonance fluorescence was monitored in relation to buffer-gas pressure (0-10 Torr) and to the strength of the ambient magnetic field (0-10 kG). At low fields at which the nuclear spin $\stackrel{\ensuremath{\rightarrow}}{\mathrm{I}}$ is coupled to the electronic angular momentum $\stackrel{\ensuremath{\rightarrow}}{\mathrm{J}}$, the measurements yielded depolarization cross sections Q(circ.)Q(\mathrm{circ}.)Q(circ.) and Q(lin.)Q(\mathrm{lin}.)Q(lin.), which agree within experimental error with theoretical values calculated for spin-coupled conditions. At kilogauss fields with $\stackrel{\ensuremath{\rightarrow}}{\mathrm{I}}$ and $\stackrel{\ensuremath{\rightarrow}}{\mathrm{J}}$ decoupled, the experiments produced cross sections for disorientation and disalignment, which are also in satisfactory agreement with the appropriate theoretical predictions. The results indicate that the nuclear spin has a more pronounced effect on the collisional relaxation of the atomic dipole than of the quadrupole.
Circular Rydberg states with n=70n=70n=70 have been prepared in helium using a modified version of the crossed-fields method. This approach to the preparation of high-nnn circular Rydberg states overcomes limitations of the standard crossed-fields method which arise at this, and higher, values of nnn. The experiments were performed with atoms traveling in pulsed supersonic beams that were initially laser photoexcited from the metastable 1s2s3S11s2s\phantom{\rule{0.16em}{0ex}}^{3}S_{1}1s2s3S1​ level to the 1s73s3S11s73s\phantom{\rule{0.16em}{0ex}}^{3}S_{1}1s73s3S1​ level by resonance-enhanced two-color two-photon excitation in a magnetic field of 16.154 G. These excited atoms were then polarized using a perpendicular electric field of 0.844 V/cm, and transferred by a pulse of microwave radiation to the state that, when adiabatically depolarized, evolves into the n=70n=70n=70 circular state in zero electric field. The excited atoms were detected by state-selective electric field ionization. Each step of the circular state preparation process was validated by comparison with the calculated atomic energy-level structure in the perpendicular electric and magnetic fields used. Of the atoms initially excited to the 1s73s3S11s73s\phantom{\rule{0.16em}{0ex}}^{3}S_{1}1s73s3S1​ level, $\ensuremath{\sim}80%$ were transferred to the n=70n=70n=70 circular state. At these high values of $n, \mathrm{\ensuremath{\Delta}}n=1$ circular-to-circular Rydberg state transitions occur at frequencies below 20 GHz. Consequently, atoms in these states, and the circular state preparation process presented here, are well suited to hybrid cavity QED experiments with Rydberg atoms and superconducting microwave circuits.