EVAPORATION OF ATOMS 379 



tungsten ; since A V for metallic caesium and Cs adsorbed on tungsten in 

 the first layer is almost i volt. The heat of evaporation from the second 

 layer is even less than that from metallic caesium. All this is striking 

 evidence of the true monatomic nature of the first layer of Cs on tungsten. 



Mobility and surface diffusion coefficient of adatoms 



Let us consider o adatoms per unit area distributed at random among 

 elementary spaces which are arranged in a square surface lattice, each 

 elementary space being a square of side a so that a^ = i/oi. Let t be the 

 average life of an adatom in a particular space when the 4 adjacent spaces 

 are vacant. The probability per second that an atom in a given space will 

 hop into a given adjacent vacant space is 1/4T. We may take the probability 

 of hopping into an occupied space to be zero. If we may assume that the 

 atoms exert no appreciable forces on one another (except that needed to 

 keep 2 out of a single space), t may be taken to be independent of o. 



The flux q) of atoms per cm of length across a line midway between two 

 adjacent rows {A and 5) of elementary spaces (perpendicular to X 

 axis) is 



(PAB = {a<rA/^T){\ -o-fi/o-i) from A to B 

 and (54) 



(PBA = {cL(x Bi,^r){\ — (ta/ (Ti) from B to A. 



The net flux or drift flux ^d is thus 



iPD = {a/^T){(TA-<yB) = {a'^/^r)d<T/dx. (55) 



The surface diffusion coefficient D may be defined by equating q)^ to 

 D do/dx and thus we find for all values of 9 from o to i 



P=aV4T = l/4(riT. (5^) 



In case we have to deal with a hexagonal surface lattice in which atoms 

 may hop to any one of six adjacent spaces, this equation needs to be 

 modified merely by replacing the 4 in the denominator by 3. For tungsten 

 surfaces which have been highly heated, the atoms are arranged in a surface 

 lattice in which the elementary rectangle of dimensions 3.15 X 4.46A has 

 one atom at each corner and one atom in the center. Each surface atom 

 has thus 4 near neighbors and therefore Eq. (56) should be applicable. 



This equation has been derived on the assumption that the time x during 

 which an atom remains in an elementary space is large compared to a/v, 

 the time required for the passage of an atom from one space to the next. 



