Adsorbate induced contact charging: pure and OH-substituted benzoic acids adsorbed on wide band gap insulators

Adsorbates on insulating surfaces have a large influence on the properties of contact charging; especially the influence of organic acids is well known. In order to clarify their specific role, we investigated the electronic structure of benzoic acid (BA), 2-hydroxybenzoic acid (salicylic acid, SA) and 4-hydroxybenzoic acid (p-SA), adsorbed on epitaxial NaCl(100) and KCl(100) films with UV photoelectron spectroscopy and electron energy loss spectroscopy in combination with theoretical calculations. The spectra of both the occupied and the unoccupied electronic states are dominated by non-bonding molecular orbitals. Assignments of molecular orbitals have been made by a combination of DFT, HF-SCF and CIS calculations. All three adsorbates generate characteristic electronic states within the band gaps of the alkali halides, thus reducing and effectively determining the band gap at the surface. As shown by our experimental data and also by the calculations, the phenolic OH group of the hydroxybenzoic acids interacts with the acid group and leads to characteristic molecular orbital shifts so that the effective band gap, i.e. the separation between HOMOs and LUMOs of the acids, can be tuned from 1 eV for SA to 2.2 eV for p-SA. The impact of this effect will be discussed in the context of contact charging between NaCl and KCl.