Molecular Spectra As a Tool in Assigning Carbon 1s Photoelectron Spectra of Physisorbed Overlayers

• Published in: Journal of physical chemistry. C 2010-09, Vol.114 (36), p.15383-15393
• Main Authors: Zahl, Maria G, Myrseth, Velaug, Andersen, Trine H, Harnes, Jarle, Borg, Anne, Sæthre, Leif J, Børve, Knut J
• Format: Article
• Language: English
• Subjects: C: Surfaces, Interfaces, Catalysis
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp1003352

Carbon 1s photoelectron spectra were measured for 1,1-dichloroethene and 1,3-cyclohexadiene adsorbed on Si(111)-7×7 and for gaseous cyclopentene and 1,1-dichloroethene. We explore closely how the molecular spectra of 1,1-dichloroethene and 1,3-cyclohexadiene change upon physisorption and show how gas-phase X-ray photoelectron spectra can be a useful tool for interpretation of spectra of physisorbed molecules. When assigning spectra of adsorbed states, it is frequently assumed that carbon 1s ionization energies (IE) group according to the hybridization state of the carbon atom and, moreover, that sp3-hybridized carbons have lower ionization energies than do sp2-hybridized carbons. These assumptions are explored by reviewing gas-phase spectra where it is possible to assign spectral peaks to individual carbon atoms without ambiguity. In cyclopentene, the carbon 1s ionization energies do group according to hybridization, albeit with the sp3 carbons at higher IE than the sp2 carbons. In propene, the 1s ionization energy of the central, unsaturated carbon atom groups together with that of the methyl rather than the terminal sp2 carbon, almost 0.5 eV higher than the IE of the latter. These findings are backed by a sizable data set, showing unequivocally that the practice of assigning spectra based on the assumption that sp2 carbons appear at higher C1s ionization energies than do sp3 carbons is unjustified.