Adsorption of Strontium and of Barium on Tungsten

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Abstract
Sr or Ba was deposited on W ribbon receivers by evaporation from source filaments in which a chemical reaction produced the metal at least 99.7% pure. The receiving surfaces could be cleaned at will by heating to high temperatures; repeated tests were thus possible. By thermionic measurements, adsorption and desorption were observed independently under nonequilibrium conditions. By using radioactive Sr isotopes, the deposit corresponding to any thermionic activation was measured. For each ribbon, thermionic activation was apparently a reversible function of the amount adsorbed. Our experimental methods give information on kinetics, rather than equilibria. The adsorption mechanism more nearly resembles that proposed in Langmuir's later papers rather than that of his earlier theories but treatment of desorption by the theory of absolute reaction rates is considerably more satisfactory. This accounts for desorption kinetics over the experimental temperature range by only two disposable constants, the activation energy for desorption, (a.e.d.), and a repulsive energy of interaction between adatoms. The work function [open phi] for a monolayer of Sr is ∼2.2 v and for Ba is ∼1.9 v. The thermionic activation is explained in terms of polarized adatoms, rather than by the usual assumption of partial ionization, which has no experimental justification for Ba or Sr films. The theory proposed gives a reasonable value for the polarizability of Sr and of Ba, which when substituted into the theory of Prosen, Sachs, and Teller predicts a heat of physical desorption agreeing well with the observed a.e.d. This also agrees with the heat of desorption (h.d.) determined by the usual method of applying the Clapeyron equation to kinetics of this kind. We therefore believe that this adsorption should be called ``physical,'' rather than ``chemical,'' although the mean a.e.d.'s are, for Sr 77.4, and for Ba 80.7 kcal/mole, considerably larger than for most examples of chemisorption.