Study of clean and CO-covered Ge(111) surfaces by photoemission and ion-neutralizaiton spectroscopies

Chemisorption of carbon monoxide (CO) on the Ge(111) 2×8 surface was investigated using ultraviolet photoemission spectroscopy (UPS) and ion-neutralization spectroscopy (INS). It was found that CO molecules are not adsorbed on the Ge(111) surface unless they are activated. Once activated by hot tungsten filaments, CO molecules are chemisorbed nondissociatively at room temperature with a small sticking coefficient (∼0.001). Both UPS and INS show that CO saturates the (111) surface to produce four CO-associated features at $E-E_{\mathrm{VAC}}=-7.0,-9.5,-12.0,\mathrm{and}-14.2\mathrm{}$ eV in the surface electronic density of states. The feature at highest energy is identified as an enhanced Ge $p$-like valence band due to CO molecules on the surface. The remaining three peaks at -9.5, -12.0, and-14.2 eV are related to the $5\sigma$, $1\pi$, and $4\sigma$ orbitals of the free CO molecule, respectively. These spectral features are significantly different from those for CO adsorption on a number of transition metals. Simultaneous use of INS and UPS provides us with complementary information on the present chemisorption system, on the basis of which we propose that the CO molecule occupies upon adsorption the threefold interstitial site with the carbon atom toward the bulk and the oxygen atom toward the vacuum.