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Physical Review

Depolarization Cross Sections for the Metastable Noble Gases by Optical Pumping

L. D. Schearer

Published Dec 5, 1969• 14 citations
Publisher

Abstract

The relaxation times of optically oriented metastable atoms of Ne, Ar, and Xe in buffers of noble gases have been measured, and depolarization cross sections obtained. The cross sections range from 0.43 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ for ${\mathrm{Xe}}^{m}\ensuremath{-}\mathrm{Xe}$ collisions to 190 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ for ${\mathrm{Ne}}^{m}\ensuremath{-}\mathrm{He}$.

DOI
10.1103/physrev.188.505

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Jochen Fricke, J. Haas, E. Lüscher +1 more

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Spin relaxation of rubidium atoms in sudden and quasimolecular collisions with light-noble-gas atoms

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Collisional Depolarization of Mercury Resonance Radiation

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The depolarization of mercury resonance radiation (2537 Å), induced in collisions of Hg(63p1) atoms with various buffer gases, has been investigated using the method of delayed coincidences. The experiments yielded the following depolarization cross sections. Hg–He, 37.8 Å2; Hg–Ne, 43.5 Å2; Hg–Ar, 72.9 Å2; Hg–Kr, 101 Å2; Hg–Xe, 144 Å2; Hg–N2, 120 Å2.

Canadian Journal of Physics•Apr 1, 1973•3 citations
The relaxation times of optically oriented metastable atoms of Ne, Ar, and Xe in buffers of noble gases have been measured, and depolarization cross sections obtained. The cross sections range from 0.43 \ifmmode\times\else\texttimes\fi{} 10\ensuremath−16{10}^{\ensuremath{-}16}10\ensuremath−16 cm2{\mathrm{cm}}^{2}cm2 for Xem\ensuremath−Xe{\mathrm{Xe}}^{m}\ensuremath{-}\mathrm{Xe}Xem\ensuremath−Xe collisions to 190 \ifmmode\times\else\texttimes\fi{} 10\ensuremath−16{10}^{\ensuremath{-}16}10\ensuremath−16 cm2{\mathrm{cm}}^{2}cm2 for Nem\ensuremath−He{\mathrm{Ne}}^{m}\ensuremath{-}\mathrm{He}Nem\ensuremath−He.
The 3P2^{3}P_{2}3P2​ metastable atoms of neon have been optically oriented. This represents the first case of the optical pumping of non-SSS state atoms. The optical-pumping process is described, and depolarization cross sections for collisions with ground-state neon and ground-state helium atoms measured. The cross section for collisional depolarization with helium is 4.30 \ifmmode\pm\else\textpm\fi{} 0.22 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2, and with neon is 166 \ifmmode\pm\else\textpm\fi{} 8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2 at 300\ifmmode^\circ\else\textdegree\fi{}K. The metastable 3P2^{3}P_{2}3P2​ atoms of Ar and Xe have also been optically pumped. As the polarizability of the noble-gas buffer increases, the collisional depolarization cross section approaches the diffusion cross section.
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.
Cross sections for random collisional reorientation of J within the sublevels of the 62P326^{2}P_{\frac{3}{2}}62P23​​ excited state of cesium have been measured using the D2{D}_{2}D2​ optical-pumping technique. The measured cross sections ${{\ensuremath{\sigma}}_{M}}^{2}$ are: Cs-He (3.2\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ cm2{\mathrm{cm}}^{2}cm2, Cs-Ne (2.9\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ cm2{\mathrm{cm}}^{2}cm2, Cs-Ar (6.0\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ cm2{\mathrm{cm}}^{2}cm2, Cs-Kr (9.2\ifmmode\pm\else\textpm\fi{}1.9)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ cm2{\mathrm{cm}}^{2}cm2, Cs-Xe (11.3\ifmmode\pm\else\textpm\fi{}2.3)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ cm2{\mathrm{cm}}^{2}cm2. Simplified equations for D1{\mathrm{D}}_{1}D1​ and D2{\mathrm{D}}_{2}D2​ pumping with zero, partial, and complete excited-state mixing are derived for a hypothetical alkali atom with zero nuclear spin. The calculations are extended to cesium atoms with nuclear spin 72\frac{7}{2}27​ pumped by ${\ensuremath{\sigma}}^{+}$ D2{\mathrm{D}}_{2}D2​ radiation. The equilibrium value 〈Sz〉0{〈{S}_{z}〉}_{0}〈Sz​〉0​ and the time dependence 〈Sz(t)〉〈{S}_{z}(t)〉〈Sz​(t)〉 of the electronic spin polarization are computed for various degrees of excited-state mixing. 〈Sz(t)〉〈{S}_{z}(t)〉〈Sz​(t)〉 is found to be nearly single-exponential in the zero- and the complete-mixing cases, but shows a distinct double-exponential behavior in the intermediate region. 〈Sz〉0{〈{S}_{z}〉}_{0}〈Sz​〉0​ is shown to be dependent on the degree of mixing, and to change sign as the degree of mixing increases. Measurements were performed with a new transmission technique by passing a separate ${\ensuremath{\sigma}}^{+}$-polarized D2{\mathrm{D}}_{2}D2​ pumping beam and an alternately ${\ensuremath{\sigma}}^{+}$- and ${\ensuremath{\sigma}}^{\ensuremath{-}}$-polarized D1{\mathrm{D}}_{1}D1​ testing beam through the cesium absorption cell. The cross sections for collisionally induced mixing were evaluated by measuring the buffer pressure at which 〈Sz〉0{〈{S}_{z}〉}_{0}〈Sz​〉0​ passed through zero.
Elastic and depolarizing collisions between excited ytterbium atoms (6s6p 3{}^{3}3P1{P}_{1}P1​ level) and noble gases (He, Ne, and Xe) are investigated using a stimulated-photon-echo technique. The method is sensitive to collisional velocity changes concerning both the total population ${\ensuremath{\rho}}^{0}$ and the alignment ${\ensuremath{\rho}}^{2}$ of the level. More specifically, it enables us to compare precisely the velocity changes associated with elastic scattering and with alignment-preserving scattering. This represents an improvement with regard to previous works. A detailed theoretical discussion of depolarizing collisions in laser spectroscopy is presented. A simplified density-matrix equation is derived under appropriate approximations. Stimulated photon-echo data for elastic scattering are interpreted within the limits of small-angle scattering in a long-range van der Waals potential; total elastic scattering cross sections are derived from comparison between experiment and calculation: ${\mathit{\ensuremath{\sigma}}}_{\mathrm{He}}^{\mathrm{el}}$=220(20) A˚2{\r{}}^{2}˚2, ${\mathit{\ensuremath{\sigma}}}_{\mathrm{Ne}}^{\mathrm{el}}$=320(20) A˚2{\r{}}^{2}˚2, ${\mathit{\ensuremath{\sigma}}}_{\mathrm{Xe}}^{\mathrm{el}}$=1240(60) A˚2{\r{}}^{2}˚2. Total rates for destruction of alignment are measured in an auxiliary standard photon-echo experiment; the corresponding cross sections are ${\mathit{\ensuremath{\sigma}}}_{\mathrm{He}}^{(2)}$=94(3) A˚2{\r{}}^{2}˚2, ${\mathit{\ensuremath{\sigma}}}_{\mathrm{Ne}}^{(2)}$=123(7) A˚2{\r{}}^{2}˚2, ${\mathit{\ensuremath{\sigma}}}_{\mathrm{Xe}}^{(2)}$=300(20) A˚2{\r{}}^{2}˚2. Stimulated photon-echo data for depolarizing collisions are successfully compared with a calculation that we develop starting from a long-range anisotropic model potential.
Spin relaxation of optically pumped Rb85{\mathrm{Rb}}^{85}Rb85 and Rb87{\mathrm{Rb}}^{87}Rb87 atoms colliding with light-noble-gas atoms is shown to be strongly influenced by an anomalous relaxation process which we attribute to the formation and destruction of bound and quasibound Van der Waals molecules. The anomalous relaxation rate has been measured and analyzed in Ne, complementing an earlier study in He. Relative rates of formation of complexes in two- and three-body collisions have been determined. The correlation time for the quasimolecular interaction in Rb-Ne has been found to be 6.7 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ ${p}^{\ensuremath{-}1}$ sec, where ppp is the Ne pressure in Torr. Nuclear-spin-independent cross sections for the relaxation of 〈Sz〉〈{S}_{z}〉〈Sz​〉 in sudden binary collisions of Rb atoms with noble-gas atoms have been measured to be (units of ${10}^{\ensuremath{-}24}$ cm2{\mathrm{cm}}^{2}cm2: $\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}\mathrm{He})=3.1$ $\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}\mathrm{Ne})=19$ $\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}\mathrm{Ar})=630$ $\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}{\mathrm{N}}_{2})=83$. Relaxation in sudden binary collisions is shown to follow the theoretically expected nuclear spin dynamics: two relaxation rates in the ratios 8:1 for Rb87{\mathrm{Rb}}^{87}Rb87 and 18:1 for Rb85{\mathrm{Rb}}^{85}Rb85 have been measured. Diffusion coefficients of Rb in the various buffer gases have been measured to be (units of cm2{\mathrm{cm}}^{2}cm2/ sec, at 305\ifmmode^\circ\else\textdegree\fi{}K): ${D}_{0}(\mathrm{Rb}\ensuremath{-}\mathrm{He})=0.42$, ${D}_{0}(\mathrm{Rb}\ensuremath{-}\mathrm{Ne})=0.235$, ${D}_{0}(\mathrm{Rb}\ensuremath{-}\mathrm{Ar})=0.16$, ${D}_{0}(\mathrm{Rb}\ensuremath{-}{\mathrm{N}}_{2})=0.16$. Nuclear-spin-independent cross sections for the relaxation of 〈Jz〉〈{J}_{z}〉〈Jz​〉 in the 52P125^{2}P_{\frac{1}{2}}52P21​​ state of Rb have been measured to be 3.4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2 for Rb-He, and 5.9 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2 for Rb-Ne. Anomalies, disagreements, and puzzles occurring in earlier measurements of ground-state relaxation in Rb are shown to be largely resolved when considered in the light of the new results.