A Test of the Classical "Momentum Transfer" Theory of Accomodation Coefficients of Ions at Cathodes

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Abstract
On the classical theory of momentum transfer at impact of a moving particle with a stationary particle, one of us suggested that the accommodation coefficient for positive gas ions striking a metal cathode should be less than unity only if the mass of the metal atom exceeds that of the gas ion. To test this hypothesis, momentum experiments of the type already reported for helium ions striking a molybdenum cathode have been continued for argon ions striking molybdenum and aluminum, respectively. The transfer of momentum was measured by the deflection of a delicate pendulum, whose bob consisted of the metal cathode under investigation, the surrounding argon atmosphere being strongly ionized by a discharge between supplementary electrons. The experimental data made possible the computation of the accommodation coefficient for the positive ions and also the fraction of the current carried by electrons at the cathode. The accommodation coefficient for argon ions on molybdenum was 0.8, in good agreement with the value obtained by Compton and Van Voorhis by thermal measurements, while for argon ions on aluminum the accommodation coefficient was unity, in accordance with the above prediction. The classical theory would be expected to hold in such cases where the energy of the impinging particle greatly exceeds the thermal energy of the cathode. Three cases are analyzed: first, impact by large ions which do not penetrate into the cathode; second, impact by smaller ions which may penetrate only a few layers into the cathode; and third, impact of very small high speed ions which may penetrate many layers of atoms in the cathode. The experimental values of accommodation coefficients are shown to conform sufficiently closely to the predictions of this analysis to indicate that the phenomena of loss of energy and momentum are at least approximately described by the postulates of this analysis.