EVAPORATION OF ATOMS 



341 



Fig. 10 shows On as a function of i/T (calculated through Table I) for 

 a large range of values of [io- 



Theory of adsorption by isolated active spots 



If the isolated active areas are all alike and each is capable of holding 

 only one adatom, the average life Xa of an adatom on the active area is 



TABLE I 



Data for Calculation of Evaporation Rates 



Logio V = A - B/T 



(Note that here common logs are used. To obtain values of Aa and Ba for use in 

 Eqs. (6) to (id) multiply the tabulated values by 2.303.) 



independent of Oa. Since the probability per second for the evaporation of 

 any adatom is i/To, the total rate of evaporation from the active areas 



is Oa/Xa. 



In a steady state this must be balanced by the rate of arrival of adatoms 

 from the vapor phase. Since a for the surface as a whole is unity, we must 

 assume that an incident atom whose path is directed towards an elementary 

 space of the active area which is already occupied by an adatom, condenses 

 in an adjacent vacant normal space. The total rate of arrival into the active 



