Image Potential Surface States Localized at Chemisorbed Dielectric−Metal Interfaces

Abstract

The work reported herein involves an extensive examination of alkanethiol self-assembled monolayers (SAMs) on roughened Au and Ag surfaces by surface-enhanced electronic Raman scattering. The observation of a novel resonance Raman-like process in these systems at excitation energies between 1.7 and 2.0 eV is described in detail. At excitation energies between these limits, a series of intense bands appear superimposed upon the normal surface-enhanced Raman vibrational spectrum of the film. The experimental evidence for an electronic Raman scattering process which leads to these bands is presented. The energy level diagram derived from these observed electronic transitions has led to the development of a model based on electronic Raman scattering between image potential surface states (IPSs). While the experimental Raman data indicates that the IPS electron exists within the dielectric film, quantum mechanical modeling of these image potential states for chemisorbed alkanethiol SAMs on roughened metal surfaces strongly argues that the electron is located in close proximity to the headgroup region of the film. Additionally, Raman excitation profiles for CH3(CH2)9SH SAMs on Au have provided evidence for both a filled, lower electronic state within the metal band gap (from which the IPS levels are filled via optical pumping prior to the Raman scattering event) and an upper electronic state; the energetic separation between the two states is 3.70 ± 0.03 eV. Furthermore, an analysis of these excitation profiles places the upper edge of the lower electronic state and the lower edge of the upper electronic state at approximately 3.60 and 7.29 eV above the clean Au(100) Fermi level, respectively. Thus, these states can be located within the surface band diagram of Au(100), and such a diagram is presented for the CH3(CH2)9SH/Au(100) system. The energetic locations of these states within the metal band structure indicate that these states are not intrinsic to the metal, and are consistent with those previously observed by two-photon photoelectron spectroscopy for alkanethiol films on Cu(111).