EVAPORATION OF ATOMS 371 



Let us postulate several different possible mechanisms for evaporation 

 and see whether or not. when they are reversed, they yield reasonable 

 mechanisms for condensation and whether tliese are consistent with the 

 experimental fact that a = i . 



We first need to dehne some terms which will help to make our con- 

 cepts more precise. In a state of equilibrium the atoms near a plane surface 

 may be divided in general into four groups : 



(i) Adsorbed atoms or adafoins. These are the atoms on the surface 

 which contribute to 6. 



(2) Incident atoms, or atoms which are moving towards the surface 

 from remote regions. The paths described by the nuclei of such atoms are 

 called incident paths. 



(3) Emergent atoms, or atoms which are receding from the surface 

 along paths (emergent paths) that will carry them to remote regions. 



(4) Hopping atoms, or atoms whose nuclei describe paths (hopping 

 paths) that originate and terminate on the surface. 



We may define the remote region as that region which lies outside of 

 the range of the surface forces, where the paths of the atoms are straight 

 lines. The region closer to the surface where the paths are curved we shall 

 call the force sheath. 



When the nucleus of an incident, or a hopping atom, approaches to a 

 definite point close to the original surface, the atom either becomes an 

 adatom or it starts to describe a new path (emergent, or hopping). Let us 

 call this definite point the terminus of the path. Similarly, each emergent 

 and hopping path has an origin. The straight paths of the incident and 

 emergent atoms in the remote region, if extended as straight lines to their 

 intersections with an ideal plane at the adsorbing surface, give points which 

 we shall call the flight termini and flight origins. 



When equilibrium prevails, the concentration of atoms, their directions 

 of motion, and their velocities are governed by the laws of the Maxwell- 

 Boltzmann distribution (M.B.D.) throughout the force sheath as well as 

 the remote region. Thus, all the paths (incident, emergent and hopping) 

 that pass through any point have a spherically symmetrical distribution 

 of directions at that point. The concentrations of atoms must vary in accord 

 with the Boltzmann equation : 



w = «o exp (— F^/^T), (46) 



where Ve is the increase in potential energy of an atom when it passes from 

 a region where the concentration is wq to one at which it is n. 



We see, then, that the flight termini and flight origins must be uniformly 



