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International Laser Science Conference

Experimental determination of the spin-rotation coupling in Na-Xe molecules

M. Hou, B. Feng, J. Zhang, Y. She, Z. Mi, T. CHEN

Published Jan 1, 1986• 0 citations

Abstract

A theory for calculating the spin–rotation coupling coefficient in alkali-metal–noble-gas van der Waals molecules was recently presented.1 It assumes the spin–rotation interaction in alkali-metal–noble-gas molecules to be mainly due to the spin–orbit interaction of the alkali-metal valence electron within the core of the noble gas, and predicts that the spin–rotation constant γ should depend on the spin–orbit interaction in the noble gas and be almost independent of the spin–orbit interaction of the alkali-metal atom. Experimentally, only a few of the possible pairs of the different alkali-metal atoms with different noble-gas atoms have been investigated. By measuring the sodium spin relaxation rate as a function of the helium buffer-gas pressure and the width of the sodium magnetic slowing-down curve, we report in this paper our experimentally determined value of the spin–rotation interaction constant in Na–Xe molecules γ = (1.1 ± 0.3)MHz. The measured value is nearly the same as was measured in K–Xe, Rb–Xe, and Cs–Xe.1 This result shows a reasonable agreement with the theoretical prediction.

DOI: 10.1364/ils.1986.ff6

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By analyzing the measured spin-relaxation transients of

^{129}\mathrm{Xe}

nuclear spins and alkali-metal-atomic spins in mixtures of alkali-metal-atom vapors, Xe gas, and larger amounts of

{\mathrm{N}}_{2}

gas, we have determined the three-body formation rates and the spin-transfer probabilities for alkali- metal--noble-gas van der Waals molecules. Three parameters, in addition to the spin quantum numbers of the alkali-metal and noble-gas nuclei, are needed to predict the spin-transfer rates. These parameters, which we have determined from experimental measurements, are x=\ensuremath{\gamma}N/\ensuremath{\alpha}, the ratio of the spin-rotation interaction \ensuremath{\gamma}N to the spin-exchange interaction \ensuremath{\alpha};

{p}_{0}

, the third-body pressure for which the molecular breakup rate ${\ensuremath{\tau}}^{\mathrm{\ensuremath{-}}1}$ is equal to the spin-rotation frequency \ensuremath{\gamma}N/h; and Z, the three-body rate constant for forming van der Waals molecules.

The theory of spin exchange between optically pumped alkali-metal atoms and noble-gas nuclei is presented. Spin exchange with heavy noble gases is dominated by interactions in long-lived van der Waals molecules. The main spin interactions are assumed to be the spin-rotation interactions $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{\mathrm{N}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}$ between the rotational angular momentum $\stackrel{\ensuremath{\rightarrow}}{\mathrm{N}}$ of the alkali-metal---noble-gas pair and the electron spin $\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}$ of the alkali-metal atom, and the contact hyperfine interaction $\ensuremath{\alpha}\stackrel{\ensuremath{\rightarrow}}{\mathrm{K}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}$ between the nuclear spin $\stackrel{\ensuremath{\rightarrow}}{\mathrm{K}}$ of the noble-gas atom and the electron spin $\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}$. Arbitrary values for

K

and for the nuclear spin

I

of the alkali-metal atom are assumed. Precise formal expressions for spin transfer coefficients are given along with convenient approximations based on a perturbation expansion in powers of ${(\frac{\ensuremath{\alpha}}{\ensuremath{\gamma}N})}^{2}$, a quantity which has been shown to be small by experiment.

We present a detailed theoretical analysis of the noble-gas nuclear-spin relaxation due to the anisotropic magnetic-dipole hyperfine interaction between the noble-gas nucleus and alkali-metal valence electron vis \`a vis the already well-understood (spin-conserving) isotropic magnetic-dipole hyperfine interaction in alkali-metal--noble-gas systems. We find that, for all pairs in which the noble gas is not helium, the predicted spin-relaxation rate from the anisotropic interaction does not exceed 2.5% of the rate from the isotropic interaction, thereby not appreciably limiting the maximum noble-gas nuclear polarization attainable via spin-exchange collisions with polarized alkali-metal atoms. For alkali-metal--helium pairs, we predict that the anisotropic interaction has a slightly larger relative effect, perhaps limiting the nuclear polarization to $\ensuremath{\sim}95%$ of the electronic polarization in the Rb-

{}^{3}\mathrm{He}

system; however, our confidence in the helium results is limited by a lack of knowledge of the interatomic potentials necessary for the calculation.

We report a study, by molecular beam methods, of the alkali–rare gas van der Waals molecule KAr. Paramagnetic potassium–argon molecules were produced in a supersonic molecular beam, and their magnetic hyperfine structure probed by resonance techniques. The principle interaction studied is the spin–rotational coupling of the spin angular momentum h/S of the alkali valence electron and the rotational angular momentum h/N of the molecule. The measured value of the interaction constant γ in the (v=0, N=0) molecular state is 240(10) kHz. Limits have been set on the variation of γ with R, the internuclear separation, over the range R=9 to 13 bohr.