Electronic excitations of benzene, pyridine, and pyrazine adsorbed on Ag(111)

Abstract

Using high resolution electron energy loss spectroscopy (EELS), we have investigated the electronic excitations of benzene, pyridine, and pyrazine adsorbed on an Ag(111) surface under ultrahigh vacuum conditions. The molecular orientation of the adsorbed molecules is also deduced from a study of their vibrational spectra. Several electronic excitations are observed for each molecule which can be assigned as metal-perturbed intramolecular transitions. Both singlet and triplet (π,π*) states are observed. The adsorption on the metal surface imparts minimal shifts on the transition energies of these excitations from their gas phase values. The observed energy shifts for these excitations are in dissagreement with the predictions of classical image field theory. In addition to the above ’’intramolecular’’ excitations, we find, in the case of pyridine and pyrazine, a low energy (∼2−2.5 eV), broad, onset-like feature, which does not have a free molecule analog. Our studies show that this transition is present only for molecules in direct contact with the surface and is assigned to a metal-molecule charge-transfer (CT) state. The possible involvement of these CT states in the surface enhanced Raman scattering (SERS) process is discussed. It is suggested that resonant excitation of these CT states enhances the Raman cross section of the adsorbed species. The short range enhancement in SERS thus could be accounted for by the enhancement resulting from CT state excitation. Finally, the excited state lifetime of adsorbed 1B2u pyrazine is deduced from a study of vibronic broadening as a function of the number of adsorbed molecular layers. Very short T2 ∼5×10−15 s (first layer) and ∼3×10−14 s (second layer) relaxation times are deduced. Although these lifetimes are comparable to those predicted by classical theory, other processes of energy transfer to the metal (such as electron–hole pair formation near the metal surface), which are not included in the classical theory, may contribute to this lifetime.