NEXAFS and TPD studies of molecular adsorption of hydrocarbons on Cu(100): segmental correlations with the heats of adsorption

Near-edge X-ray absorption fine structure (NEXAFS) and temperature-programmed desorption (TPD) studies have been performed to establish the relationship between adsorbate structure and binding energy in a monolayer of hydrocarbons on a Cu(100) surface. Fourteen different saturated and monounsaturated hydrocarbons were studied. The activation energy for desorption of these compounds has been found to be dependent on the following factors: (1) the length of the saturated hydrocarbon linear chain; (2) the presence and location of a double bond; (3) the cyclic versus acyclic nature of the hydrocarbons; and (4) the accessibility of the CHn groups for creating van der Waals interactions with the surface. Similar to previous observations on other surfaces, our results show that an increase of the linear hydrocarbon chain length by one methylene group increases the binding energy of a hydrocarbon by 1.5 kcal/mol. Our results also indicate that the presence of a double bond in a position where overlap between π-orbitals of a hydrocarbon and d-orbitals of the metal is significant (double bond parallel to the Cu(100) surface) increase the binding energy of an olefin molecule by 0.75 kcal/mol with respect to that of the corresponding saturated hydrocarbon.