324 PHENOMENA, ATOMS, AND MOLECULES 



Becker ^ has studied the evaporation of atoms, ions, and electrons from 

 caesium films on tungsten and made measurements of 6. He attributed the 

 maximum electron emission to a single complete layer of caesium atoms 

 and the subsequent decrease in emission to the formation of a second layer 

 of caesium atoms. 



The present investigation was undertaken for the purpose of determin- 

 ing quantitatively the dependence of the rates of neutral atom evaporation 

 (Va), ion emission (vp), and electron emission (Vg) on 6, filament tem- 

 perature T, and the field strength ; Va, Vp and v^ represent the number of 

 atoms, ions, or electrons evaporating per unit area, per unit time. To 

 facilitate correlation of Vp and Vg with 6, the greater part of the work has 

 consisted in establishing ^ as a function of caesium pressure and filament 

 temperature. 



In the steady state, 



a[^a = Va ; ( I ) 



that is, the number of atoms evaporating per unit area per unit time (vo) 

 is equal to the number condensing, which is equal to the number arriving 

 per unit area per unit time (\ia), multiplied by the condensation coefficient 

 a. Evidence regarding the nature of a will be given (Section XI) and it 

 will be shown that under our experimental conditions a is equal to unity. 

 Therefore the experiments which give ^ as a function of jXa also give v^ as 

 a function of 6. 



Let us now postulate that Va, Vp and Vg are functions of 6 and T only. 

 The distribution of atoms is assumed to be uniform and independent of the 

 way (e.g., by evaporation or condensation) in which 6 was reached. This 

 "surface phase postulate" will be discussed and justified in Section XI. 

 Determinations of Vg and Vp at various pressures then enable us to express 

 Ve and Vp as functions of 6 and T. 



II. APPARENT AND TRUE FILAMENT SURFACE AND 



DEFINITION OF e 



The apparent area Sa of the filament surface is that calculated from the 

 dimensions of the filament {nld). Actually after heating to 2900° during 

 aging, a tungsten filament becomes etched and develops ^ dodecahedral 

 crystal faces (no) in which there are 1.425 X 10^^ tungsten atoms per 

 cm-. The actual surface area 5" is thus greater than Sa (see Fig. i). 



It so happens that crystals of metallic caesium ^ have the same type 



^ J. A. Becker, Phys. Rev., 28, 341 (1926) ; Jour. Amcr. Electrochem. Soc, 55, 



153 (1929). 



'' I. Langmuir, Phys. Rev., 22, 374 (1923). 



^ Simon and Vohsen, Zeits. f. physik. Chemie, 133, 165 (1928). 



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