SURFACE CHEMISTRY 47 



by the thermal agitation of the molecules but depends on dF/da, since 

 differences in the value of F over the surface contribute to the mobility. 



We may thus consider that in general the adatoms on solids move over 

 the surface by hopping between elementary spaces. If the adatoms move 

 nearly independently of one another, so that they migrate freely on to all 

 unoccupied portions of the surface, the adsorbed films can be regarded as 

 a 2-dimensional gas in spite of the fact that the atoms tend to occupy 

 definite positions. Such films constitute a 2-dimensional crystalline gas, 

 the crystalline character being imparted by the under l\ing lattice. If attrac- 

 tive forces exist between the adatoms which are sufficiently strong to 

 cause them to form a definite 2-dimensional condensed i)hase in equilibrium 

 with a 2-dimensional vapor phase, we clearly have to do also with a 2- 

 dimensional sohd or liquid. If the mobility is high, this condensed film 

 may have properties characteristic of liquids ; with no mobility at lower 

 temperatures, the conditions will be analogous to that of a 2-dimensional 

 glass. A 2-dimensional solid analogous to ordinary 3-dimensional solids 

 would exist only if the forces exerted by the adatoms on each other can 

 keep them from slipping past one another. 



Condensation-Evaporation Theory of Adsorption on Solids. When 

 molecules of a gas in contact with a solid impinge individually on the 

 solid, they may either condense or may rebound from the solid as though 

 elastically reflected. Those molecules that condense may subsequently 

 evaporate. There is much theoretical and experimental evidence that a 

 true reflection of molecules under these conditions is a rather abnormal 

 occurrence, although the specular reflection of molecular rays from certain 

 crystalline surfaces shows that it sometimes exists. In the great majority 

 of cases, however, the observed phenomena indicate that the larger part 

 of all incident molecules condense on the surface and reach thermal 

 equilibrium with it before they evaporate (26) (27). With such gases as 

 hydrogen and helium accommodation coefficients materially less than unity 

 have been found. In general these occur with gases striking solids at such 

 temperatures that the rate of evaporation may be assumed to be so high 

 that the life of the condensed molecule is of the order of io~^^ seconds, 

 which is about the time required for a molecule to perform a single ther- 

 mal oscillation on the surface. Under these conditions it is not surprising 

 that thermal equilibrium is not reached. 



When the rate at which molecules impinge on the surface is |x, as 

 given by Eq. (4), and v is the rate at which they evaporate (molecules 

 cm~2 sec~^.), then the rate at which they accumulate on the surface is 

 given by 



^- = ««-»•, (10) 



