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-pro grammed desorption (TPD) studies have been performed to establish the relationship between adsorbate structure and binding energy in a monol ayer of hydrocarbons on a Cu(100) surface. Fourteen different saturate d and monounsaturated hydrocarbons were studied. The activation energy for desorption of these compounds has been found to be dependent on t he following factors: (1) the length of the saturated hydrocarbon line ar chain. (2) the presence and location of a double bond: (3) the cycl ic versus acyclic nature of the hydrocarbons, and (4) the accessibilit y of the CHn groups for creating van der Waals interactions with the s urface. Similar to previous observations on other surfaces, our result s show that an increase of the linear hydrocarbon chain length by one methylene group increases the binding energy of a hydrocarbon by 1.5 k cal/mol. Our results also indicate that the presence of a double bond in a position where overlap between K-orbitals of a hydrocarbon and d- orbitals of the metal is significant (double bond parallel to the Cu(1 00) surface) increases the binding energy of an olefin molecule by 0.7 5 kcal/mol with respect to that of the corresponding saturated hydroca rbon. (C) 1998 Published by Elsevier Science B.V.