Rao, K. Umar (1988)
Positron interactions at low-dimensional condensed surfaces.
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A microprocessor control system for automatic data acquisition in a photon spectroscopy system was developed. The control process performed Doppler broadening measurements over the temperature range 4.2-600K with simultaneous recording when necessary of pressure. After work on the chalcogenide semiconductor (Pb,Ge)Te, Doppler broadening studies concentrated on the phenomenon of gas condensation on cooled graphite substrates. This class of adsorbent, of proven homogeneity was found ideal to explore the interaction of positrons at surfaces. By measurement of positronium, adlayer formation of methane, krypton, argon, nitrogen and oxygen was followed. All gases produced peaks in positronium emission with an investigation of the substrate further revealing an underlying thermal variation. Sound evidence in the form of accurate simultaneous pressure measurements and isotherms was found for a coverage of one half at the maximum in positronium. These studies of gas physisorption were conducted under a variety of temperatures and with pressures embracing atmospheric to under one Torr. Although results for the gases were of the same form each produced different maximum levels of positronium which appeared to relate to substrate-adsorbate binding energy. Maximum emission in the case of argon showed a temperature dependence however this requires verification. Oxygen was a special case as it yielded 12[percent] para-positronium at half-coverage. The coupling of this to a low 1.5[percent] ortho-positronium emission was attributed to chemical quenching. A convolution procedure based on the superposition of an additional para-positronium derived narrow gaussian to the usual gaussian and inverted parabola components was employed for an estimate of the positronium energy. This was used to attack the question of whether positronium is bound at the surface or is created flying.
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Institution: University of London, Royal Holloway and Bedford New College (United Kingdom).