## 1 Introduction  
In the field of organic electronics, chemically bonded self-assembled monolayers (SAMs) have been used to change the electronic properties of a huge variety of interfaces [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. They have allowed the realization of n-type organic transistors by screening interface traps [3], they have been used to shift turn-on voltages of devices by several dozens of volts via the introduction of polar and reactive groups into transistor channels [4; 5; 6; 7], and they have been employed for tuning injection barriers by adsorption on electrode surfaces, changing contact resistances by orders of magnitude [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. Especially for the latter applications, the shift in the (electrostatic) energy landscape generated by the SAM is of crucial importance. On the one hand, this shift is a consequence of the intrinsic dipoles of the adsorbed molecules due to polar groups (like carboranes, pyrimidines, aromatic sulfones and sulfides, or esters) embedded into the molecular backbones [15; 17; 19; 20; 21; 22; 23] or due to polar tail group substituents (like halogens, amines, nitriles, nitro groups, pyrimidines, alcohols, ethers, or carboxylic acids) [9; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38]. On the other hand, there is always an interface dipole localized in the immediate contact region between the substrate and the adsorbed molecule, as discussed in a variety of experimental and theoretical studies [11; 25; 27; 28; 39; 40; 41; 42; 43; 44]. This interface dipole consists of the surface dipole of the (metal) substrate, the dipole associated with the (typically polar) docking group, and the bond dipole due to interfacial charge rearrangements following bond formation. The superposition of the electric fields due to the interface dipoles of all adsorbed molecules causes a step in the electrostatic energy, as explained in detail in several review papers [31; 40; 42; 45; 46]. This step in electrostatic energy triggers a shift in the position of the vacuum level above the surface and, consequently, as the most common observation, changes the sample work function [8; 11; 14; 15; 16; 17; 19; 21; 22; 25; 27; 28; 31; 32; 35; 39; 41; 43; 44; 47; 48; 49; 50; 51; 52]. Additionally, theoretical as well as experimental studies reveal that it modifies the interfacial energy-level alignment, i.e., the positions of the electronic states of the adsorbed monolayer relative to those of the substrate [27; 28; 31; 39; 40; 42; 45; 46; 47; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62]. A change in the interfacial level alignment has an immediate impact on charge-transport through the adsorbed layer [53; 54; 55; 56; 57; 58; 59; 60; 61; 62] and also on the positions of the core levels of the adsorbate [42]. The latter has been primarily discussed for embedded dipole SAMs, with a focus on shifts caused by esters [20; 63; 64] or pyrimidines [21; 22; 48; 49]. Notably, [PERSON] et al. also used it to assess the level alignment in mixed monolayers consisting of flat-lying donor and acceptor molecules [65]. Changes in core level binding energies can be probed by X-ray photoelectron spectroscopy (XPS) [66; 67; 68; 69; 70], which is one of the default techniques for characterizing SAMs, used, for example, for analyzing the quality of self-assembled monolayers and to verify bonding [67; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82]. In this paper we will argue that the electrostatic core level shifts make XPS an ideally suited tool for probing interface dipoles. For supporting that claim, we will use state of the art quantum-mechanical simulations to determine core level shifts encountered in biphenyl-based SAMs attached to an Au(111) substrate by a variety of chemically different docking groups. To the best of our knowledge, a corresponding set of experimental XPS data with systematically varied docking groups and consistent film structures and morphologies does not exist yet.