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Journal of Physics B Atomic and Molecular Physics

On the magnetic-field effect in the collisional depolarisation of the2P1/2state of the alkalis

E. E. Nikitin, M. Ya. Ovchinnikova

Published Feb 14, 1978• 2 citations
Publisher

Abstract

The influence of an external magnetic field upon the depolarisation cross section of the 2 P 1/2 state of the alkalis in collisions with noble-gas atoms is estimated for a strong spin-orbit coupling ( Delta epsilon >>h(cross)/ tau 0 , mu e B, where Delta epsilon is the spin-orbit splitting of the 2 P j doublet, tau 0 is the collision time and mu e B is the Zeeman splitting). The relative correction to the depolarisation cross sections appears to be less than 10 -2 (B/10 kOe) 2 . This casts serious doubts on the suggested interpretation of the experimentally found large-field effect.

DOI
10.1088/0022-3700/11/3/018

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Physical Review•Dec 14, 1964•62 citations
The influence of an external magnetic field upon the depolarisation cross section of the 2 P 1/2 state of the alkalis in collisions with noble-gas atoms is estimated for a strong spin-orbit coupling ( Delta epsilon >>h(cross)/ tau 0 , mu e B, where Delta epsilon is the spin-orbit splitting of the 2 P j doublet, tau 0 is the collision time and mu e B is the Zeeman splitting). The relative correction to the depolarisation cross sections appears to be less than 10 -2 (B/10 kOe) 2 . This casts serious doubts on the suggested interpretation of the experimentally found large-field effect.
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.
The cross sections for disorientation of 42P1/2 potassium atoms, induced in collisions with noble gas atoms, have been determined in zero magnetic field by studying the depolarization of K(7699 Å) resonance fluorescence in relation to noble gas pressures. Potassium vapor at low density, mixed with a noble gas in a fluorescence vessel, was irradiated with circularly polarized 7699 Å potassium resonance radiation and the resulting resonance fluorescence, observed in an approximately backward direction, was analyzed with respect to circular polarization. The variation of the degree of polarization with gas pressure was interpreted on the basis of a 'J randomization' model for the collisions and yielded the following disorientation cross sections which are appropriately corrected for the effect of nuclear spin: K–He, 24 ± 4 Å2; K–Ne, 21 ± 3 Å2; K–Ar, 37 ± 5 Å2; K–Kr, 51 ± 7 Å2; K–Xe, 69 ± 9 Å2. The cross sections are significantly smaller than values obtained previously in kilogauss fields.
The cross sections for disorientation of magnetically oriented 62P126^{2}P_{\frac{1}{2}}62P21​​ cesium atoms, induced in collisions with noble gases, have been determined using Zeeman-scanning techniques at kilogauss magnetic fields. The ${m}_{J}=\ensuremath{-}\frac{1}{2}$, ${m}_{I}=\ensuremath{-}\frac{7}{2}$ hyperfine Zeeman substate was populated selectively by irradiating the cesium-vapor-noble-gas mixture with cesium-resonance radiation, and collisional transitions to the mJ=+12{m}_{J}=+\frac{1}{2}mJ​=+21​, ${m}_{I}=\ensuremath{-}\frac{7}{2}$ substate were monitored by measurements of the relative intensities of the ${\ensuremath{\sigma}}^{\ensuremath{-}}$ and ${\ensuremath{\sigma}}^{+}$ components in resonance fluorescence observed in the direction parallel to the magnetic field. The following cross sections were obtained: Cs-He, 11.8 A˚2{\mathrm{\AA{}}}^{2}A˚2; Cs-Ne, 4.7 A˚2{\mathrm{\AA{}}}^{2}A˚2; Cs-Ar, 10.7 A˚2{\mathrm{\AA{}}}^{2}A˚2; Cs-Kr, 37.9 A˚2{\mathrm{\AA{}}}^{2}A˚2; Cs-Xe, 71.7 A˚2{\mathrm{\AA{}}}^{2}A˚2.
An account of recent work is given by the author and co-workers on the experimental determination of cross sections for excitation transfer between the (2)P((1/2)) and (2)P(3/2)resonance levels in potassium, rubidium and cesium, induced by inelastic collisions with alkali atoms and inert gas atoms in their ground states. Forty values of cross sections were obtained in a series of sensitized fluorescence experiments, in which alkali vapors and vapor-gas mixtures were irradiated with one component of the appropriate alkali resonance doublet, and the relative intensities of both components appearing in the fluorescent light were measured. The experiments indicate that the collisional interaction between an alkali atom in a (2)P state and a ground state inert gas atom probably occurs between the inert gas atom and the (2)P alkali valence electron which behaves like a quasi-free particle.
By applying an external magnetic field, we have determined the population of the magnetic sublevels of photodissociated atoms from the intensities of atomic fluorescence lines split by the Zeeman effect. A preferential population of the magnetic sublevels of the atomic fragment Na(3 2P3/2), which arises from the direct photodissociation through the D1Π(continuum) state of the NaK molecule, has been observed. The results are explained by assuming the separability of electronic and nuclear motion during the dissociation. The observed results could be rationalized by the sum of the photodissociation amplitudes caused by excitation of the P, Q and R branches. The population of the magnetic sublevels of Rb(5 2P3/2) atoms produced by predissociation of the C1Πu state of the Rb2 molecule by the repulsive molecular state c3Σ+u has been determined from the Zeeman spectra of the emission Rb(5 2P3/2→ 5 2Sm′1/2). The observed results are interpreted by including the spin-orbit coupling between an excited level of the C1Πu state and levels of the c3Σ+u state and the effect of the external magnetic field.
Cross sections for noble-gas-induced Rb spin disorientation are calculated, taking into account second-order cross terms in the spin-orbit and K\ifmmode\cdot\else\textperiodcentered\fi{}L couplings, as initially proposed by Bernheim. Contrary to Bernheim, it is argued that spin-orbit effects are negligible in first order. Using simple wave functions, it is shown that the spin-orbit coupling provided by the electric field of the Rb ionic core, when the valence electronic orbital is deformed in short-range encounters, is sufficient to yield the observed relaxation rates. For He, the calculated disorientation cross section shows agreement to within a factor of 6 with the experimentally determined cross section for depolarization of optically oriented Rb vapor. Moreover, if, for all Rb-rare-gas pairs, we assume the same proportionality between the short- and calculated long-range energies at the kinetic radius, agreement to within a factor of 6 for Ne, A, Kr and Xe is also obtained.