ADSORBED STATES OF CO ON THE SI(100)-K SURFACE - ELECTRON-ENERGY-LOSSSPECTROSCOPY AND THERMAL-DESORPTION STUDIES

Adsorbed states of CO on the Si(100)-K surface have been studied using high-resolution electron energy-loss spectroscopy and thermal desorpt ion spectroscopy. Three adsorbed states of CO (alpha, alpha(0), and be ta) were found on clean Si(100)-c(4 x 2) formed at 90 K. alpha(beta)-C O is characterized by the C-O stretch energy of 260 (-) meV and therma l desorption at 190 (415) K. The preadsorbed K atoms block the adsorpt ion sites of alpha-CO and weaken the Si-C bond of the coadsorbed alpha -CO. These results indicate that alpha-CO is predominantly bonded by t he donation of 5 sigma electrons of CO into the pi surface state loca lized mainly at the down atom of a Si dimer. The preadsorbed K atoms b lock some adsorption sites of beta-CO, but have little effect on the c hemical bond between beta-CO and Si. These results indicate that the a dsorption sites of beta-CO are the defects on Si(100). alpha(0)-CO is characterized by the thermal desorption at 235 K, and is also associat ed with defects. Redshift is observed of the C-O stretch energy of alp ha-CO with increasing K coverage, which is mainly attributed to the vi brational Stark effect caused by the preadsorbed K adatoms. (C) 1998 E lsevier Science B.V.