360 PHENOMENA, ATOMS, AND MOLECULES 



A similar equation containing Bp gives the work function (Vp) for ions. 

 The term 2Tm corrects for the simpHfication made in adopting Eq. (26). 

 Also Ve = Vw — Vc, where V^ is the work function (4.622 volts) for pure 

 tungsten and Vc is the contact potential. It is to be noted that 



Fe=(6„«-(5/2)r)/ll,606 volts. 



XI. THE CONDENSATION COEFFICIENT a AND TRANSIENT 



PHENOMENA 



Concept of surface phase 



Each phase in a heterogeneous equilibrium has properties which are 

 uniquely determined by a definite number of parameters such as com- 

 position, temperature, pressure, etc. 



If similar factors determine the properties of adsorbed films of caesium 

 on tungsten, we may expect that all the properties of such a film would be 

 uniquely determined by B and T. On the other hand, one may well conceive 

 of conditions under which the properties would depend on many other 

 factors. For example, it is possible that the surface of the underlying 

 tungsten may vary according to its method of preparation so that the rate 

 of evaporation of caesium atoms from different tungsten surfaces would 

 differ even if 6 and T were the same. Or again, if a caesium film with 

 given 6 is formed in two different ways, as, for example, by condensation 

 on to a bare surface or by evaporation from a more concentrated film, the 

 distribution of Cs atoms over the surface might be different and thus cause 

 variations in the properties. The properties could thus depend upon whether 

 or not the film is in equilibrium with the surrounding Cs vapor. 



It will be very useful, however, to look upon the unique dependence 

 of the properties on 6 and T as an ideal case which may be approached 

 under favorable conditions. Let us, therefore, consider the properties of 

 adsorbed films which conform to the following postulate. 



Surface phase postulate: All the properties of an adsorbed film on an 

 underlying surface of given composition are uniquely determined by , 

 6 and T 

 If this condition is fulfilled, v„, Vp and Vg are functions of 6 and T only, 

 even if the film is not in equilibrium, with the vapor phase. The adsorbed 

 film in equilibrium with caesium vapor is thus a system possessing two 

 degrees of freedom in the sense of the phase rule.-^^ 

 Under non-equilibrium conditions we then have 



d(x/dt = aalla-\-Otpllp—Va—Vp. \^1) 



