STRUCTURAL AND ELECTRONIC-PROPERTIES OF RH OVERLAYERS ON MO(110)

The structure, thermal stability, surface electronic, and CO chemisorptive properties of Rh overlayers [04 monolayers (ML)] on Mo(110) were studied with use of Auger-electron spectroscopy, low-energy electron diffraction, and ultraviolet photoemission spectroscopy. Rh grows in the Frankvan der Merwe mode at room temperature and undergoes a transition from the Kurdjumov-Sachs (KS) to the Nishiyama-Wassermann orientation near 1 ML. The Rh monolayer in the KS orientation is stable up to the desorption temperature of about 1000°C. States near -2.5 eV below EF with Rh character and states near -0.9 eV with Mo character are found to grow in intensity for Rh coverages up to 1 ML, and this is attributed to a bonding-antibonding pair of Rh(4d)-Mo(4d)-derived interface states. At coverages beyond 2 ML, a surface state (resonance) at -0.9 eV develops and is completely established at 4 ML. CO dissociation on the Mo substrate decreases linearly with Rh coverage while molecular CO adsorbs on the Rh covered surface and increases with the Rh thickness. The direction of electronic charge transfer between an adatom and a metallic substrate is shown to be determined by the sign of J=(I+A)/2-, where A and I are the affinity and ionization level of the adatom and is the substrate work function. This criterion is used to explain the observed initial work-function changes of Rh/Mo(110) and other previously studied systems. © 1990 The American Physical Society.