EVAPORATION OF ATOMS 323 



accurate measure (experimental error of about 0.2 per cent) of the number 

 of atoms striking the filament per second. The condensation coefficient (a) 

 for atoms striking a tungsten filament is proved by experiment to be unity 

 from ^ = o to nearly i. The important bearing of this fact and of the ex- 

 perimentally observed existence of surface migration or diffusion on the 

 mechanism of evaporation and condensation in dilute and concentrated 

 films is discussed. In addition surface migration is correlated with irregular 

 ion evaporation rates occurring when two phases (dilute and concentrated 

 films of Cs) exist on the tungsten surface. Transient effects in which 

 changes with time are studied and entirely explained by the observed rates 

 of evaporation and condensation. This and other facts are used to justify 

 a surface phase postulate according to which all the properties of the 

 adsorbed film are uniquely determined by 6 and T. 



I. INTRODUCTION 



Caesium films on tungsten form an important and interesting case of 

 adsorption. The adsorbed atoms are held on the tungsten by such large 

 forces that remarkable changes in the thermionic properties of the tungsten 

 can be observed. As the tungsten surface can be completely cleaned and 

 the supply of caesium atoms can be controlled, this system is capable of 

 furnishing quantitative information regarding adsorption and the electrical 

 properties of surface films. 



Films of alkali metal atoms on tungsten have been the subject of several 

 investigations. Langmuir and Kingdon ^ found that caesium atoms ad- 

 sorbed on tungsten greatly increased the electron emission of the tungsten 

 and showed that caesium atoms striking a hot clean tungsten filament were 

 converted into positive ions which could be collected by a plate at a negative 

 potential. Both the electron and ion emissions were functions of the tem- 

 perature (T) of the filament and of the fraction (6) of the surface covered 

 with caesium. The electron emission increased to a maximum at a certain 

 value of 6 and decreased with further increase in 6. The positive ion 

 emission became appreciable only for low values of ^ ; at higher values of $, 

 caesium films evaporated only as neutral atoms. Only rough estimates of 6 

 were made in this work. 



Recently Langmuir ^ has treated the properties of caesium adsorbed 

 on tungsten in a general discussion of evaporation, condensation, and 

 adsorption, and has given a partial derivation of equations used in this 

 paper. 



M, Langmuir and K. H. Kingdon, Science, 57, 58 (1923); Proc. Roy. Soc, 

 A107, 61 (1925)- 



^ I. Langmuir, Jour. Amer. Chem. Soc, 54, 2798 (1932). 



