No abstract

Abstraet. Spin polarized positive muons injected in matter serve as magnetic probes for the investigation of various properties. The evolution of muon spin polarization rests on the same basis as in conventional magnetic resonance techniques. The background of the technique, different variants of the experimental set-up, and potential and limitations of the muon asa probe are described.

chanics Division (N00014-92-5-1202). Refereaces and Notes(1) Weiske, T.; Whme, D. K.; HruUk, J.; Krltschmer, W.; Schwarz, H. (2) ROSS, M. M.; Callahan, J. H. J. Phys. Chem. 1991, 95, 5720. (3) Caldwell, K. A.; Giblin, D. E.; Hsu, C. S.; Cox, D.; Gross, M. L. J. (4) Campbell, E. E. 9.; Ulmer, G.; Hertel, I. V. Z . Phys. D 1992, 23, 1. (5) Weiske, T.; H d k , J.; mhme, D. K.; Schwarz, H. Helv. Chim. Acra (6) WanHsu, M. T.; Rincon, M. E.; Kemper, P. R.; Bowers. M. T. Chem. Phys. Let?. 1990,174, 223. (28) Lifshitz, C.; Iraqi, M.; Fischer, J. E.; Pews, T. In?. J. Mars Spectrom. Ion Processes 1991, 107, 565. (29) Sandler, P.; Lifshitz, C.; Klots, C. E. Chem. Phys. Lett, submitted for publication. Chem. 1991, 95, 10564. 157.Atomic muonium (Mu, the light isotope of hydrogen) is easily encapsulated inside fullerene cages and acts as a highly sensitive microscopic probe of both the fullerene's rotational dynamics and the anisotropy in its electronic distribution. Zerolfield muon spin relaxation studies of MU@C!~~ as a function of temperature reveal the appearance of the Mu triplet precession upon orientational ordering of the fullerene at 270 K, signaling the presence of an axially symmetric hyperfine interaction. A small 2pz admixture is found in the ground-state wave function of Mu@C~O; as a result, the electron density reproduces perfectly the internal shape of the fullerene skeleton, including the small "pinching" in the equator.Positive muons (p+) may bind an electron to form a muonium atom (Mu = &), the light isotope of hydrogen (mMu -(1/9)mH),l Mu encapsulated inside fullereneZ cages has been recently observed*-5 by transverse-field muon spin rotation (TF-pSR) techniques. Here we present the results of a muon spin relaxation study of the endohedral Mu@Cm complex in zerofleld (ZF-pSR) between 10 and 310 K. A singlafrequency oscillation at Y -0.70 (2) MHz appears on cooling near 270 K and signals the presence of an axially symmetric hyperfine interaction that lifts the Mu triplet degeneracy. The rapid tumbling of the fullmne molecules drastically shws upon orientational ordering, and the M U @ C ,~ rotational correlation time is estimated to be -30 ( 5 ) ns at 200 K. Below 150 K, there is evidence for a completely asymmetric hyperfine matrix, indicating a further change in molecular motion. The results show that muonium, trapped in thecavityof a fullcrenc, can be used as a higbly sensitive micrawpic probe of both the molecule's rotational dynamics and the anisotropy in its electronic distribution.The CT0 samp!e (-98.5% average purity, -500 mg) was prepared as desmbed previously? Sublimation7 at 650 O C followed by extended annealing at 250 OC (22 days) leads to highly crystalline material with no traces of trapped solvent and a facccentered-cubic structure at room temperature. The sample was characterized by powder X-ray and high-resolution neutron diffraction, "C NMR spectroscopy, and prompt y-ray neutron activation analysis. ZF-pSR experiments (52 pT) w m performed in longitudinal geometry8 between 10 and 310 K usin...

The long-range magnetic ordering of Na 4 3ϩ clusters with spin Sϭ1/2 on a body-centered-cubic lattice in sodium electrosodalite has been studied in zero applied magnetic field by positive muons as local magnetic probes. An experimental determination of the temperature dependence of the magnetic order parameter of an s-electron antiferromagnet is presented. The order parameter is measured via the local magnetic field at the muon site (B loc ). The temperature dependence B loc (T) exhibits critical behavior near the Néel temperature T N ϭ(50.3Ϯ0.2) K. The critical exponent ␤ϭ(0.36Ϯ0.1) is close to the predicted value for a threedimensional Heisenberg system. The precession signal is ascribed to muons in a diamagnetic state. A fraction of muons not contributing to the precession signal below T N is ascribed partly to muons in domains which are not magnetically ordered and partly to muons forming a bound state with an unpaired electron.

No abstract