Local electronic structure of alkanethiol on Pt(111) studied by metastable atom electron spectroscopy

Electron emission spectra resulting from thermal collision of He*(2(3)S) metastable atoms with alkanethiols (R-SH; R=CH3, C2H5, C3H7) on Pt(111) at 50-140 K were measured to clarify the local electronic structures. Upon chemisorption the lowest-lying band due to the nonbonding (ns) orbital composed of S 3p is strongly suppressed relative to the higher lying bands, indicating that the molecules are really bound to the metal through the S atom with the terminal methyl group outside the surface. Further, the n(s)-derived band reveals a long tail extending to the Fermi level due to the S 3p-Pt 5d coupling, in contrast to the cases of gaseous and condensed molecules. The clear appearance of the Fermi edge in such interface states indicates that the chemisorbed molecules can be regarded as "metallic" in nature, although the condensed multilayer film is an insulator with a wide HOMO-LUMO gap. (c) 2007 Elsevier B.V. All rights reserved.