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Physical Review Letters

Nuclear Magnetic Resonance of Diatomic Alkali Molecules in Optically Pumped Alkali Vapors

R. Gupta, W. Happer, George Moe, W. Park

Published Mar 18, 1974• 32 citations
Publisher

Abstract

NMR signals from diatomic alkali molecules have been observed in optically pumped cesium and rubidium vapors. The nuclear relaxation times were found to be strongly dependent on the temperature and buffer-gas pressure. The relaxation time seems to be determined by at least two and possibly three major factors: motional narrowing of the molecular hyperfine structure, breakup of the diatomic molecules by collisions with buffer-gas atoms, and possibly chemical exchange of free atoms with atoms bound in the molecules.

DOI
10.1103/physrevlett.32.574

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A new optical method for studying the spin relaxation of optically aligned rubidium vapor is described. In this method, the pumping radiation, consisting of circularly polarized ${D}_{1}$ resonance radiation, is suddenly shut off and then turned on again after a known time interval. The relaxation which takes place during the interval of darkness causes the vapor to become more opaque to the pumping radiation at a rate which is determined by the relaxation time. A large degree of alignment, as well as a relaxation time of about 80 milliseconds, were observed in a closed-off evacuated cylindrical glass cell completely lined, except for $\frac{1}{200}$ of the wall area, with a thin film of tetracontane (${\mathrm{C}}_{40}$${\mathrm{H}}_{82}$). The variation of relaxation time with buffer gas pressure was studied in this cell and in an unlined glass cell. From the observations with the unlined cell, diffusion coefficients for rubidium in neon and argon of 0.31 ${\mathrm{cm}}^{2}$/sec and 0.24 ${\mathrm{cm}}^{2}$/sec, respectively, can be deduced. Observed cross sections for disorientation collisions between aligned ground-state rubidium atoms and neon, argon, krypton, and xenon atoms are 5.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}23}$ ${\mathrm{cm}}^{2}$, 3.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}22}$ ${\mathrm{cm}}^{2}$, 5.9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}21}$ ${\mathrm{cm}}^{2}$, and 1.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}20}$ ${\mathrm{cm}}^{2}$, respectively. In the evacuated tetracontane-lined cell, the relaxation time decreased by 30% for a tenfold increase in rubidium vapor pressure. An explanation for this relatively weak dependence is suggested. The longest observed relaxation time was approximately 0.4 seconds in a tetracontanelined cell filled with neon to a pressure of 3 cm Hg.

Physical Review•Aug 15, 1959•279 citations
NMR signals from diatomic alkali molecules have been observed in optically pumped cesium and rubidium vapors. The nuclear relaxation times were found to be strongly dependent on the temperature and buffer-gas pressure. The relaxation time seems to be determined by at least two and possibly three major factors: motional narrowing of the molecular hyperfine structure, breakup of the diatomic molecules by collisions with buffer-gas atoms, and possibly chemical exchange of free atoms with atoms bound in the molecules.
The spin-exchange rates due to binary collisions and due to the formation of van der Waals molecules between 83Kr^{83}\mathrm{Kr}83Kr nuclei and optically pumped rubidium-metal vapor have been determined using a technique that relies on the simultaneous observation of frequency shifts of nuclear magnetic resonance lines of noble-gas atoms and corresponding frequency shifts of electron paramagnetic resonance lines of alkali-metal atoms. In contrast, earlier studies have relied on the observation of the spin-relaxation rate of either the alkali-metal atoms or the noble-gas atoms.
The hyperfine relaxation of a population difference, produced by optical pumping, between the F=1F=1F=1 mF=0{m}_{F}=0mF​=0 and F=2F=2F=2 mF=0{m}_{F}=0mF​=0 energy levels of the ground state of Rb87{\mathrm{Rb}}^{87}Rb87, has been studied in cells containing neon, argon, helium, or krypton as a buffer gas. The relaxation times T1{T}_{1}T1​ and T2{T}_{2}T2​ have been measured as a function of the rubidium vapor pressure in the cell. The relaxation rate due to spin exchange in Rb87{\mathrm{Rb}}^{87}Rb87-Rb87{\mathrm{Rb}}^{87}Rb87 collisions has been found equal to 65 ${\mathrm{sec}}^{\ensuremath{-}1}$ at 44\ifmmode^\circ\else\textdegree\fi{}C, and the diffusion coefficients of Rb87{\mathrm{Rb}}^{87}Rb87 atoms in He, Ne, and Ar evaluated as 0.6, 0.31, and 0.22 cm2{\mathrm{cm}}^{2}cm2 ${\mathrm{sec}}^{\ensuremath{-}1}$, respectively. The cross section for Rb87{\mathrm{Rb}}^{87}Rb87-Ne and Rb87{\mathrm{Rb}}^{87}Rb87-Ar collisions are found to be 1.6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}22}$ cm2{\mathrm{cm}}^{2}cm2 and 9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}22}$ cm2{\mathrm{cm}}^{2}cm2. These values are in general agreement with values previously obtained by other methods. The extrapolated values of 1T2\frac{1}{{T}_{2}}T2​1​ for zero vapor pressure of rubidium are 185, 237, and 318 ${\mathrm{sec}}^{\ensuremath{-}1}$ for Ne, He, or Ar, respectively. Relaxations by the pumping light or by the magnetic field inhomogeneities are also discussed.
The spin relaxation of optically pumped rubidium vapor induced by rubidium-rare-gas atomic collisions has been investigated in an ultraclean environment. The experiment was performed under conditions such that the partial pressure of nonrare-gas impurities was less than 1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ Torr, and that the time between Rb-Rb spin exchange collisions was short compared to the spin relaxation time. The cross sections (in cm2{\mathrm{cm}}^{2}cm2) for disorientation of rubidium were found to be: He, 3.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}25}$; Ne, 3.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$; Ar, 1.1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}22}$; Kr, 7.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}21}$; Xe, 1.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$. The fact that these cross sections are considerably smaller than previously reported values is attributed primarily to the low degree of impurity contamination in the present work. When compared with already existing values for cesium and sodium, the disorientation cross sections of these three alkali metals are seen to have a remarkably similar dependence on buffer-gas atomic number. The new experimental values are also compared with recent theoretical calculations.
We present a comprehensive theory of nuclear spin polarization of 3He{}^{3}\mathrm{He}3He and 129Xe{}^{129}\mathrm{Xe}129Xe gases by spin-exchange collisions with optically pumped alkali-metal vapors. The most important physical processes considered are (1) spin-conserving spin-exchange collisions between like or unlike alkali-metal atoms; (2) spin-destroying collisions of the alkali-metal atoms with each other and with buffer-gas atoms; (3) electron-nuclear spin-exchange collisions between alkali-metal atoms and 3He{}^{3}\mathrm{He}3He or 129Xe{}^{129}\mathrm{Xe}129Xe atoms; (4) spin interactions in van der Waals molecules consisting of a Xe atom bound to an alkali-metal atom; (5) optical pumping by laser photons; (6) spatial diffusion. The static magnetic field is assumed to be small enough that the nuclear spin of the alkali-metal atom is well coupled to the electron spin and the total spin is very nearly a good quantum number. Conditions appropriate for the production of large quantities of spin-polarized 3He{}^{3}\mathrm{He}3He or 129Xe{}^{129}\mathrm{Xe}129Xe gas are assumed, namely, atmospheres of gas pressure and nearly complete quenching of the optically excited alkali-metal atoms by collisions with N2{\mathrm{N}}_{2}N2​ or H2{\mathrm{H}}_{2}H2​ gas. Some of the more important results of this work are as follows: (1) Most of the pumping and relaxation processes are sudden with respect to the nuclear polarization. Consequently, the steady-state population distribution of alkali-metal atoms is well described by a spin temperature, whether the rate of spin-exchange collisions between alkali-metal atoms is large or small compared to the optical pumping rate or the collisional spin-relaxation rates. (2) The population distributions that characterize the response to sudden changes in the intensity of the pumping light are not described by a spin temperature, except in the limit of very rapid spin exchange. (3) Expressions given for the radio-frequency (rf) resonance linewidths and areas can be used to make reliable estimates of the local spin polarization of the alkali-metal atoms. (4) Diffusion effects for these high-pressure conditions are mainly limited to thin layers at the cell surface and at internal resonant surfaces generated by radio-frequency magnetic fields when the static magnetic field has substantial spatial inhomogeneities. The highly localized effects of diffusion at these surfaces are described with closed-form analytic functions instead of the spatial eigenmode expansions that are appropriate for lower-pressure cells.
It has been demonstrated that optical pumping of alkali atoms (rubidium and cesium) results to a highly efficient indirect optical orientation of metastable He (2 3 S 1 ) atoms. However, in an attempt to perform the inverse experiment (i.e. to receive spin-polarization of alkali atoms upon optical pumping of metastable helium atoms) the result was quite unexpected: the magnetic resonance signal of Cs or Rb atoms was very weak (several hundred times weaker compared to conventional optical pumping of helium in the presence of cesium (rubidium)). We propose an explanation for the unusually small ratio of the magnetic resonance signals from Cs (Rb) and metastable He atoms under condition of optical orientation of He atoms. Except of Cs atoms we are the first to have observed magnetic resonance signals from atoms of 85 Rb and 87 Rb isotopes when using the indirect optical orientation in conditions of helium-rubidium gas discharge plasma. It is shown that the anomaly in the amplitude ratios of the observed MR signals can be explained by the unidirectional action of three different factors (the presence of a large nuclear spin of alkali atoms, depolarization of alkali atoms between collisions, and spin exchange between atoms of rubidium isotopes (for the He-Rb system).
A new optical method for studying the spin relaxation of optically aligned rubidium vapor is described. In this method, the pumping radiation, consisting of circularly polarized D1{D}_{1}D1​ resonance radiation, is suddenly shut off and then turned on again after a known time interval. The relaxation which takes place during the interval of darkness causes the vapor to become more opaque to the pumping radiation at a rate which is determined by the relaxation time. A large degree of alignment, as well as a relaxation time of about 80 milliseconds, were observed in a closed-off evacuated cylindrical glass cell completely lined, except for 1200\frac{1}{200}2001​ of the wall area, with a thin film of tetracontane (C40{\mathrm{C}}_{40}C40​H82{\mathrm{H}}_{82}H82​). The variation of relaxation time with buffer gas pressure was studied in this cell and in an unlined glass cell. From the observations with the unlined cell, diffusion coefficients for rubidium in neon and argon of 0.31 cm2{\mathrm{cm}}^{2}cm2/sec and 0.24 cm2{\mathrm{cm}}^{2}cm2/sec, respectively, can be deduced. Observed cross sections for disorientation collisions between aligned ground-state rubidium atoms and neon, argon, krypton, and xenon atoms are 5.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}23}$ cm2{\mathrm{cm}}^{2}cm2, 3.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}22}$ cm2{\mathrm{cm}}^{2}cm2, 5.9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}21}$ cm2{\mathrm{cm}}^{2}cm2, and 1.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}20}$ cm2{\mathrm{cm}}^{2}cm2, respectively. In the evacuated tetracontane-lined cell, the relaxation time decreased by 30% for a tenfold increase in rubidium vapor pressure. An explanation for this relatively weak dependence is suggested. The longest observed relaxation time was approximately 0.4 seconds in a tetracontanelined cell filled with neon to a pressure of 3 cm Hg.