## 3 Results and Discussion  
### Energetics of a Metal-SAM Interface  
For understanding the correlation between XP spectra and interface dipoles, it is useful to first consider how interface dipoles impact the electronic properties of a system consisting of an inorganic substrate and a chemically bonded monolayer (formed by upright-standing molecules). For the sake of simplicity, in the following we will focus on metal substrates. In this way, further complications due to band-bending effects [35; 63; 121; 122] in semiconducting substrates (and their modification by the adsorbate layer) can be avoided.  
A schematic representation of the electronic structure of the metal-SAM interface is shown in Figure 1 for the case of a SAM in which an interface dipole occurs (virtually all SAMs) and in which also a functional tail group is used bearing a tail group dipole. Due to the superposition of the fields generated by the dipoles in the densely packed monolayer (i.e., due to collective electrostatic effects) [27; 31; 40; 42; 45; 46; 63; 123; 124], steps in the electrostatic energy occur. The steps due to the interface dipole, ID, and due to the tail group dipole, TD, are indicated by vertical blue arrows. The sum of these two energy shifts causes the change in substrate work function due to monolayer adsorption, \(\Delta\Phi\). Conversely, only the interface dipole is responsible for the change in injection barriers, \(\Delta\)IB, which determines the final electron- and hole-injection barriers, EIB and HIB, respectively. The core levels are shifted in line with the frontier levels. Consequently, the interface dipole should result in a shift of the positions of the core levels relative to the Fermi level of the substrate, \(\Delta\)EC\({}_{\text{1s}}\), that is equivalent to \(\Delta\)IB. This shift is directly translated into a modification of the kinetic energies of the electrons, E\({}_{\text{kin}}\), and, thus, in a shift of the XP spectra.  
In this context it is important to stress that the above considerations disregard energetic shifts due to interfacial charge transfer processes, which could significantly modify the energies of strongly localized orbitals (i.e., the core levels). Indeed, for the atoms in a SAM that are in the immediate vicinity of the interface, such a charge transfer typically modifies the core level binding energies [68; 69; 125; 126], which has also been observed for simulations on SAMs [90]. In experiments on extended, upright standing molecules it will, however, be often inconsequential for the measured spectra, as the atoms in the immediate vicinity of the interface mostly affected by these \"chemical\" shifts hardly contribute to the measured spectra, especially when using primary photon energies of a few hundred eV in synchrotron measurements. This is a consequence of the finite escape depth of the photoelectrons. The situation changes for experiments with typical lab sources with primary photon energies of, e.g., 1253.6 eV (Mg-K\(\alpha\)) and 1486.6 eV (Al-K\(\alpha\)), for which the attenuation length can be larger than the typical thickness of SAMs. Thus, for properly resolving the electrostatic shifts discussed her, the said synchrotron experiments are preferable (also because of the often higher energy resolution).  
Finally, it should be mentioned that in the absence of an energetic shift due to a tail group dipole (or when comparing differently docked systems with identical tail group dipoles), the above considerations imply that SAM-induced changes of the substrate work function should directly correlate with shifts in core level binding energies. Below, we will test under which circumstances such a situation is encountered at a metal-SAM interface.