Vibrational and electronic spectroscopy of pyridine and benzene adsorbed on the Rh(111) crystal surface

We report the vibrational and electronic spectra for pyridine and benzene adsorbed on the Rh(111) crystal surface obtained by high-resolution electron energy loss spectroscopy (HREELS). Low-energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and optical second harmonic generation (SHG) have also been used to provide complementary information. Pyridine adsorption on Rh(111) was studied over the 77–450 K temperature range. At 77 K, multilayers of pyridine are observed with a vibrational spectrum similar to that of liquid pyridine. Between 185 and 230 K, HREELS and TDS indicate that both physisorbed and chemisorbed pyridine species are present on the surface. The physisorbed species desorbs at 295 K, while the chemisorbed species is stable until it decomposes on the surface at 400 K. We propose that the chemisorbed species is an α-pyridyl complex as thermal desorption spectroscopy indicates partial dehydrogenation of this pyridine surface species. Electronic energy loss spectra for both benzene and pyridine adsorbed at 310 K show only a weak transition centered at ∼4 eV. The absence of prominent π→π* transitions, which are readily observed for multilayers of benzene adsorbed on Rh(111), implies that the π orbitals are strongly involved in the chemisorption bond of these molecules with the Rh(111) surface.