Abstract:
By monitoring the surface directly by Auger electron spectroscopy (AES), adsorption isotherms for nitrogen on polycrystalline tungsten ribbons have been determined in the temperature range 1200–1400 K. These are not of the Langmuir type, but can be linearized by plotting log P(eq) against coverage, θ, and therefore conform to the Temkin equation. The isosteric heat of adsorption (ΔHads) is initially about 370 kJ mol–1, but falls slightly with increasing coverage. Integral molar entropies of adsorption are determined and shown to fall between those calculated for an immobile layer and a 2-dimensional perfect gas.
The cylindrical Auger analyzer has been used in the fast scanning mode to measure absolute rates of desorption and these are shown to be strictly second order below θ= 1.50 × 1014 molecule cm–2. This is taken to indicate that the adsorption of nitrogen is dissociative. The rate constants for desorption yield ΔEdes= 330 ± 50 kJ mol–1 in agreement with the ΔHads values. By considering the measured rates of desorption sticking probabilities have been determined and the mechanism of desorption shown to conform to a simple collision model. The residence times of nitrogen adatoms are calculated and shown to be in accord with the Frenkel equation.
