Structure and stability of CN adlayers on Rh(110)

The formation and stability of CN adlayers on Rh(110), formed by dissociati ve adsorption of C2N2 at 373 K, have been studied as a function of coverage and temperature by low-energy electron diffraction (LEED), X-ray photoelec tron spectroscopy(XPS), and thermal desorption spectroscopy (TDS). Two diff erent CN adsorption states have been distinguished by their different C Is and N Is XPS core-level binding energies. The CN-I state is exclusively occ upied up to a surface coverage of 0.5 monolayers (ML), where a well-ordered c(2 x 2) LEED pattern is observed. The CN-II state becomes additionally po pulated at higher coverages from 0.5 ML to the saturation coverage of 0.87 ML. At CN saturation, a c(4 x 2) LEED structure is formed. Desorption of CN as molecular C,N, occurs only for surface coverages >0.5 ML and appears to be mainly derived from the CN-II state. The onset of C-N bond rupture is i ndicated at similar to 450-550 K, depending on the CN coverage; the resulti ng N-ad desorbs at similar to 580 K from the more crowded surface and in th e range of similar to 650-950 K, whereas C-ad remains at the Rh surface and cannot be desorbed thermally. (C) 2000 Elsevier Science B.V. All rights re served.