Thermochemistry of surface emission

Similarly, adsorption of ions (n+) onto a metal surface leads to a heat of adsorption of Q,. Generally, Q is about 2-3 eV and is greater than Q, which itself is about 1 eV. The difference between Q, and is the energy required to ionize neutrals (n ) on a metal surface so as to give ions (n+) or vice versa. This difference, Q - Q, can be equal to, greater than, or less than the difference, I - ( ), between the ionization energy (1) of the neutral and the ease with which a metal can donate or accept an electron (the work function, ( )). Where Q, - Q, I - ( ), the adsorbed 

Values of the average work function ( (), electronvolts) for the commonly used filament metals. The melting points of the metals are also shown to give some guidance as to the maximum temperature at which they can be used. Normally, the practical maximum would lie a few hundred degrees below the melting point to prevent sagging of the filament. 

Therefore, the ratio of the number of ions to the number of neutrals desorbing from a heated filament depends not only on the absolute temperature but also on the actual surface coverage of ions and neutrals on the filament (C, C ) and crucially on the difference between the ionization energy and work function terms, I and (j). This effect is explored in greater detail in the following illustrations. 

Clearly, the lower the ionization energy with respect to the work function, the greater is the proportion of ions to neutrals produced and the more sensitive the method. For this reason, the filaments used in analyses are those whose work functions provide the best yields of ions. The evaporated neutrals are lost to the vacuum system. With continued evaporation of ions and neutrals, eventually no more material remains on the filament and the ion current falls to zero. 

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