THE ADSORPTION OF NH3 ON RH(111)

The adsorption of NH3 on Rh(111) has been investigated by temperature programmed desorption, work function measurements, low energy electron diffraction and secondary ion mass spectrometry. TPD indicates the ex istence of three distinct desorption states; alpha(1)-, alpha(2)- and beta-NH3 with peak maxima at 320, 155 and 130 K, respectively. For the most strongly chemisorbed state, alpha(1)-NH3, the desorption energy is 81.5 kJ mol(-1). A (2 x 2) LEED pattern is observed after complete filling of the alpha(1)- and alpha(2)-NH3 states, which is attributed to saturation of the first adsorption layer, corresponding to an NH3 c overage of 0.25 ML. The sticking coefficient for NH3 adsorption at 120 K, is on the order of unity and independent of coverage during fillin g of the first adsorption layer. NH3 adsorption causes a significant w ork function decrease of 2.4 eV below the value of clean Rh(111). The initial work function decrease corresponds to an average dipole per NH 3 molecule of 1.9 D, i.e. higher than that of the NH3 molecule in the gas phase. SIMS spectra of NH3 on Rh(111) contain Rh(NH3)(+) as the pr edominant ammonia-derived cluster ion. The intensity of the Rh(NH3)(+) cluster ion behaves strongly non-linear with the NH3 coverage and ind icates some changes in the NH3 adlayer at a coverage of approximate to 0.12 ML.