Positive muons injected into solid C6p, K4C6p, and K6C6p form vacuumlike muonium (tt e ) with a (6-12)% probability. Observation of coherent spin precession of muonium establishes that all three materials are nonmagnetic The experiment was performed on the M15 and M13 beamlines at TRIUMF which provide nearly 100% spinpolarized positive muons with a momentum of 28 MeVjc. The starting material of high-purity C60 powder was prepared using standard techniques [2] and showed no detectable infrared lines attributable to solvent molecules. High-resolution powder x-ray diA'raction yielded a crystallite size of greater than 1500 A and high-performance liquid chromatography showed better than 99.5% pure C6p with no trace of C600. The process for K doping is described elsewhere [20]. In order to prevent any exposure to air, the K4C60 and K6C6p powdered samples were

We discuss the possibility of capturing carbon dioxide from the flue gas of a coal-fired electrical power plant by cryogenically desublimating the carbon dioxide and then preparing it for transport in a pipeline to a sequestration site. Various other means have been proposed to accomplish the same goal. The problem discussed here is to estimate the "energy penalty" or "parasitic energy loss,' defined as the fraction of electrical output that will be needed to provide the refrigeration and that will then not be deliverable. We compute the energy loss (7.9-9.2% at 1 atm) based on perfect Carnot efficiency and estimate the achievable parasitic energy loss (22-26% at 1 atm) by incorporating the published coefficient of performance values for appropriately sized refrigeration or liquefaction cycles at the relevant temperatures. The analyses at 1 atm represent a starting point for future analyses using elevated pressures.

Semi-insulating GaAs (100) was exposed to hydrogen atoms that were produced in a remote microwave plasma. The flux of hydrogen atoms was determined by isothermal calorimetry. Hydrogen atom partial pressures between 3 and 51 mTorr were found to etch GaAs continuously at rates that ranged from 3 to 40 nm/min at tern B eratures above 200 "C. The rate coefficients could be fitted to the Arrhenius equation, k= 105.2'o. nm min-* Torr-' e-".a5'o.05 eVfltT, between 200 and 360 "C. The surfaces etched crystallographically and were found to become arsenic deficient. The possible intluence of hydrogen atom absorption by GaAs is considered. 0 1995 American Institute of Physics.

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