chapter Sixteen 



THE EVAPORATION OF ATOMS, IONS, 

 AND ELECTRONS FROM CAESIUM 

 FILMS ON TUNGSTEN* 



Precision methods for measuring the number of caesium atoms adsorbed 

 on tungsten are described. With these methods for determining 6 (the 

 fraction of the tungsten surface covered with Cs), the rates of atom, ion 

 and electron emission are measured as functions of 6 and T, the filament 

 temperature. The rate of atom evaporation, Vg, increases rapidly with 6 

 and with T. At low filament temperatures and high pressures of Cs vapor 

 the concentration of adsorbed Cs atoms approaches a limit 3.563 X 10^^ 

 atoms cm"- of true filament surface (one Cs atom for four tungsten atoms) . 

 This film (^=1) exhibits all the characteristics of a true monatomic 

 layer. The formation of a second layer begins only at filament temperatures 

 corresponding to nearly saturated Cs vapor. A theory of the formation of a 

 second and of polyatomic layers is given and experiments supporting it are 

 described. The heat of evaporation (given by the Clapeyron equation) for 

 Cs atoms from clean tungsten is 2.83 volts (65,140 calories), 1.93 volts or 

 44,473 calories at G = 0.67, and 1.77 volts or 40,757 calories at =^ i. 

 The adsorbing tungsten surface after proper aging is homogeneous, except 

 that about 0.5 per cent of it (active spots) can hold Cs more firmly than 

 the rest. The procedure in obtaining electron (vg) and ion (vp) emission 

 for zero field and the large changes in the effect of external field with $ 

 are described. From both Vg and Vp the contact potential Vc is calculated, 

 agreeing, except for very concentrated films, with Vc calculated entirely 

 from data on neutral atom evaporation. At constant temperature the 

 electron emission increases to a maximum at ^ = 0.67 and decreases as 

 ^ = I is approached. The positive ion emission increases rapidly to a maxi- 

 mum at ^^--^o.oi and then decreases. The work function (exponent in 

 Dushman type equation) for electrons at ^ = 0.67 is 1.70 volts (clean 

 tungsten =4.62 volts). The work function for ions is T.91 volts at ^ = o, 

 and 3.93 volts at ^ = 0.67. It is shown by experiment that the saturated 

 ion current from a clean hot (1200-1500° K.) tungsten filament is an 

 * In collaboration with John Bradshaw Taylor. 



322 



