On the λ-state of nitrogen on tungsten surfaces

Abstract

The desorption of nitrogen at high coverage from a tungsten surface has been studied using ultra-high vacuum thermal desorption mass spectrometry. The high coverage λ-states of nitrogen were prepared by activation of nitrogen gas by electron bombardment. Experiments using non-activated nitrogen gas also showed population of the λ-states, though with a very small sticking probability of 10−7. Nevertheless, this result shows that the λ-states are thermodynamically stable with respect to gaseous N2. Slow heating desorption experiments with pumping show little resolution between the λ- and β-states and some evidence for depletion of the β-states resulting from population of the λ-state. Since kinetic analysis of the slow desorption spectra appeared impossible, fast heating experiments in a closed system were performed. The fast desorption spectra were computer fitted to several desorption models. While little discrimination between these models was possible, all demanded considerable depletion of the β-state to achieve a satisfactory fit. Kinetic analysis of the slow desorption data, ignoring the depleted β-state, gave excellent agreement with the computer fitted data. Some evidence is presented to show that the extent of β-depletion is a function of heating rate. Population of the β- and λ-states with different isotopes led to complete scrambling on desorption showing that both states are atomic and exist on the same crystal planes. It is suggested that the λ- and β-states represent different desorption mechanisms rather than different desorption states. The evidence for depletion of the “β-state” by population of the “λ-state” suggests that the λ-desorption mechanism is an immobile adjacent atom-pair desorption in contrast to the mobile bimolecular recombination β-mechanism. The λ-desorption mechanism is discussed and a rate equation is derived for the process. No exact demarcation between the λ- and β-mechanisms will exist, but the variation in adatom translational mobility with adatom density will play an important role in the desorption mechanism. In order to explain the complete isotopic mixing between the λ- and β-“states” it is necessary to invoke rotational mobility for the adatom pairs prior to desorption.