Ultraviolet/high resolution electron energy loss spectroscopy of CO on Pt(110) and benzene on Pt(110)

Recently there has been some controversy about how CO and benzene adsorb on platinum. With CO, there is disagreement over whether there is back donation of electrons from the metal to the CO 2 π* orbital. With benzene, there is disagreement, over whether benzene adsorbs in a distorted state. In this article we use a standard high resolution electron energy loss (HREELS) spectrometer with modified electronics to measure the equivalent of a ultraviolet (UV) spectrum for two different systems: CO on Pt(110) and benzene on Pt(110) and benzene on Pt(110). In the case of CO, the UV spectrum shows peaks at 5.6 and 8.2 eV that may shift slightly with coverage. By comparison, gas-phase CO shows peaks at 6.2 and 8.3 eV. The difference between the gas-phase peak positions and those on the surface are indicative of the antibonding orbitais being stabilized, as one would expect from the Blyholder model. With benzene we observe two different spectra: a first monolayer spectrum with a broad peak centered at 4.9 eV, and a multilayer spectrum with peaks at 0.75, 3.82, 4.71, 6.20, and 6.84 eV. The multilayer spectrum matches the spectrum of condensed benzene, while the first monolayer spectrum is quite different and resembles that of a diene. Together, these results show that UV/HREELS spectroscopy provides useful information about adsorbates on surfaces.