INTERACTION OF ATOMIC NITROGEN WITH RH(110)

The behaviour of atomic nitrogen layers on a Rh(110) surface produced by dissociative adsorption of NH3 at 430-450 K or by atomization of N2 on a hot filament was studied by means of thermal programmed desorption (TPD), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Combined LEED, TPD and Auger measurements were used to investigate the growth, ordering and stability of the nitrogen layers. Two ordered phases 3 x 1 and 2 x 1 were observed with increasing nitrogen coverage. The transition from a disordered layer at very low coverages, to ordered phases at moderate and high coverages, resulted in apparent higher temperature shift of the N2 TPD traces indicating stabilization of the adlayer. The desorption parameters corresponding to the disordered, 3 x 1 and 2 x 1 phases were evaluated from the desorption data using leading edge lineshape analysis of the TPD traces and desorption at constant temperature. It was found that at moderate and high coverages the desorption is best fitted to a first-order desorption kinetics. The desorption energy increased by approximately 40 kJ/mol as a result of ordering of the layer. Comparison of the TPD, Auger and LEED data for various nitrogen layers indicates that a fraction of nitrogen adatoms penetrates beneath the surface and desorbs after liberation of the surface sites in a wide temperature range (550-800 K). Several models including possible N-induced structural changes of the surface are proposed to explain the structure and stability of the nitrogen layers.