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 $\ensuremath{\theta}$ (the fraction of the tungsten surface covered with Cs), the rates of atom, ion and electron emission are measured as functions of $\ensuremath{\theta}$ and $T$, the filament temperature. The rate of atom evaporation, ${\ensuremath{\nu}}_{a}$, increases rapidly with $\ensuremath{\theta}$ and with $T$. At low filament temperatures and high pressures of Cs vapor the concentration of adsorbed Cs atoms approaches a limit 3.563\ifmmode\times\else\texttimes\fi{}${10}^{14}$ atoms ${\mathrm{cm}}^{\ensuremath{-}2}$ of true filament surface (one Cs atom for four tungsten atoms). This film ($\ensuremath{\theta}=1$) 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 $\ensuremath{\theta}=0.67, \mathrm{and} 1.77$ volts or 40,757 calories at $\ensuremath{\theta}\ensuremath{\simeq}1$. The adsorbing tungsten surface after proper aging is homogeneous, except that about 0.5 percent of it (active spots) can hold Cs more firmly than the rest. The procedure in obtaining electron (${\ensuremath{\nu}}_{e}$) and ion (${\ensuremath{\nu}}_{p}$) emission for zero field and the large changes in the effect of external field with $\ensuremath{\theta}$ are described. From both ${\ensuremath{\nu}}_{e}$ and ${\ensuremath{\nu}}_{p}$ the contact potential ${V}_{c}$ is calculated, agreeing, except for very concentrated films, with ${V}_{c}$ calculated entirely from data on neutral atom evaporation. At constant temperature the electron emission increases to a maximum at $\ensuremath{\theta}=0.67$ and decreases as $\ensuremath{\theta}=1$ is approached. The positive ion emission increases rapidly to a maximum at $\ensuremath{\theta}\ensuremath{\simeq}0.01$ and then decreases. The work function (exponent in Dushman type equation) for electrons at $\ensuremath{\theta}=0.67$ is 1.70 volts (clean tungsten=4.62 volts). The work function for ions is 1.91 volts at $\ensuremath{\theta}=0, \mathrm{and} 3.93$ volts at $\ensuremath{\theta}=0.67$. It is shown by experiment that the saturated ion current from a clean hot (1200-1500\ifmmode^\circ\else\textdegree\fi{}K) tungsten filament is an accurate measure (experimental error of about 0.2 percent) of the number of atoms striking the filament per second. The condensation coefficient ($\ensuremath{\alpha}$) for atoms striking a tungsten filament is proved by experiment to be unity from $\ensuremath{\theta}=0$ to nearly 1. The important bearing of this fact and of the experimentally observed existence of surface migration or diffusion on the mechanism of evaporation and condensation in dilute and concentrated films is discussed. In addition surface migration is correlated with irregular ion evaporation rates occurring when two phases (dilute and concentrated films of Cs) exist on the tungsten surface. Transient effects in which $\ensuremath{\theta}$ changes with time are studied and entirely explained by the observed rates of evaporation and condensation. This and other facts are used to justify a surface phase postulate according to which all the properties of the adsorbed film are uniquely determined by $\ensuremath{\theta}$ and $T$.