A surface Penning ionization study of the CO/Ni(111) system

Abstract

Metastable He(2 ¹S) and Ne(³P_2,0) beams were used to probe the electronic properties of Ni(111) and CO/Ni(111) surfaces. The metastable atoms collide with the surface and transfer their electronic excitation energy, causing electron ejection. With the Ni(111) surface, the metastable is first resonantly ionized and the ion is subsequently Auger neutralized, giving an electron energy spectrum which is similar to the corresponding low kinetic energy ion neutralization spectrum (INS). Differences between the metastable quenching spectrum and the INS spectrum were observed, and are discussed in terms of the differences in the mechanisms and the ion kinetic energies. With a CO/Ni(111) surface, the CO eliminates direct interaction of the metastable atom with the metal and the surface Penning ionization electron spectrum (SPIES) is obtained. The SPIES spectrum is much more surface sensitive than the corresponding UPS spectrum, since there is no background due to primary electron emission from the metal which dominates the UPS spectrum. SPIES peaks corresponding to electron emission from the 4σ, 1π+5σ, and 2π* orbitals of CO were observed and investigated as a function of surface coverage and temperature. Under certain coverage and temperature conditions a state of CO is found (associated with a weakly bound state in the thermal desorption spectrum) which has enhanced 1π+5σ and 2π* peaks and this is attributed to tilting of the CO molecules in this state with respect to the surface.