Adsorption of tetracyanoethylene on a nickel(111) surface studied by Auger electron spectroscopy, thermal desorption spectroscopy, and Raman spectroscopy

We have studied the adsorption, subsequent chemistry, and charge transfer characteristics of tetracyanoethylene (TCNE) on a Ni(111) surface under ultrahigh vacuum (UHV) conditions. In these experiments we have employed laser Raman scattering, thermal desorption spectroscopy, and Auger electron spectroscopy. At room temperature, a stable film that is more than one monolayer thick can be grown on the surface. Desorption from this film begins at 680 K. Desorption is complete by 900 K leaving a clean Ni(111) surface. Our experiments lead us to postulate the direct desorption of a negatively charged species of the stoichiometry of TCNE from the room temperature film. Low-temperature exposure to a clean Ni(111) surface results in condensation of TCNE on top of the above-described film. We have identified the (TCNE)^- anionic radical within the condensed layers via the observation of the characteristic Raman active modes and the associated excitation profiles of this species. Heating the condensed layer results in desorption of neutral TCNE in competition with strong chemisorption on top of the multilayer film. The resulting chemisorbed TCNE desorbs at ∼650 K.