The exchange of energy between a platinum surface and helium atoms and its dependence upon the structure of the surface
| Autor: | W. B. Mann |
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| Rok vydání: | 1937 |
| Předmět: | |
| Zdroj: | Proceedings of the Royal Society of London. Series A - Mathematical and Physical Sciences. 161:236-247 |
| ISSN: | 2053-9169 0080-4630 |
| Popis: | In a previous communication (Mann 1934) it was recorded that the accommodation coefficient for helium atoms relative to a platinum surface at 100°C., which had been cleaned by heating for some hours at 1000°C., was considerably lower than the values relative to a normal contaminated surface. The accommodation coefficient rose with time, however, on lowering the temperature of the platinum to 100°C., this rise indicating some process of contamination. If, however, the platinum surface were heated for several hours at 1000°C. in oxygen, the rate of increase of the accommodation coefficient of helium, after flashing, was now greatly decreased; the platinum surface had become much less susceptible to contamination. This difference is shown in fig. 1, where the increase in the accommodation coefficient at 100°C. is plotted against time after flashing the platinum filament both before and after the oxygen treatment. The increase in accommodation coefficient after the oxygen treatment is inappreciable. It was not possible to determine whether this effect was due to the formation of a protective film of oxide or to a levelling of the platinum surface by an etching away of the prominences by combination with oxygen and evaporation of the platinum oxide formed. That a certain amount of such evaporation actually occurred was supported by the observed loss in the mass of the platinum filament during the oxygen treatment. It seemed, however, that an investigation of the surface conditions might well be carried out by the method of electron diffraction, and in this communication experiments will be described in which both the accommodation coefficient of helium relative to a platinum surface and also the nature of the surface have been determined. Apparatus The choice of apparatus suitable for these experiments has been governed by the necessity of carrying out electron-diffraction investigations and energy-loss determinations upon one and the same specimen with as short a time interval as possible between the two. An electron camera of the Thomson-Fraser pattern (Thomson and Fraser 1930) was available and was used, in a horizontal position, after slight alterations had been made to the specimen holder and to the plate chamber. In order to exclude the possibility of contamination of the platinum surface in the process of raising the pressure in the apparatus to atmospheric for the purpose of changing the photographic plates, the precaution was taken of using a plate-changing device which eliminated the necessity of admitting air into the main apparatus. |
| Databáze: | OpenAIRE |
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