After each step of exposure, XPS and UPS (hv(HeII) = 40.81 eV) measurements were conducted subsequently (Fig. ). The XP spectra clearly show a core level shift of In 3d 5/2 and P 2p towards lower binding energies of about 0.27 eV after cooling the surface. This shift cannot be only explained by a freeze-out of acceptor states, shifting the Fermi level into the VBM at this temperature. Moreover, the cryogenic temperature pronounces source-induced surface photovoltages because Shockley-Read-Hall recombination over surface states is reduced as described elsewhere . After the step-wise adsorption of water, both core levels reveal a shift toward higher binding energies and get damped by the growing water layer, rising the O 1s signal at ≈ 533 eV. As the shift shows a linear dependence on the water coverage even in the multi-layer regime (> 1 L), we attribute it to a charging effect of the surface when the insulating ice-layer charges due to the emission of photoelectrons. However, the water adsorption itself does not lead to any spectral changes in the core level lines, indicating a chemically immune surface upon water adsorption. After warming up the surface back to room temperature, the water fully desorbs from the surface and no O 1s emission remains to be observed. At the same time, the In and P core levels shift back to their initial energy positions and no spectral changes are visible. With the HeII UP spectra, a very surface sensitive In 4d emission spectrum can be measured (Fig. ). After cooling, the 4d core level shifts about ∆E = 0.33 eV towards the Fermi level, similar as observed from the XP core levels. With higher water coverage, a characteristic water feature arises in the valence band with 3 features which can be attributed to the 1b 1 (6.5 eV), 3a 1 (8.5 eV) and 1b 2 (12.5 eV) highest occupied molecular orbitals of molecular water . The water features shift to higher binding energies with increasing coverage, indicating charging of the water layer similar as also observed by XPS. In contrast to the XP spectra, the In 4d line seems to be unaffected by the water layer. Only for coverages > 1 L, the In 4d line slightly shifts to higher binding energies, confirming that all core level shifts are source dependent and can be related to the charging of the topmost ice layer. Even though the photon flux of the monochromatized HeII-source (≈ 10 11 Photons/s/mm 2 ) is in the range of the XPS-source, the HeII measurement is less susceptible to charging as the lower photon energy leads to less photoionization. After the desorption of the ice layers, the UP spectra reveal only slight spectral changes. However, at 8.0 eV and 9.9 eV, two features appear in the difference spectra, which might be assigned to 1π and 3σ bonds of adsorbed OH groups , indicating a dissociation of adsorbed H 2 O. Dissociative water adsorption has been reported for the InP(110) as well as for the In-rich InP(001) surface . However, the weak signal in UPS and no signal in the XPS O 1s line indicates that OH is only present at a few surface sites far below a monolayer coverage. Furthermore, the LEED pattern still exhibits the 2×1 surface reconstruction, indicating that there are no structural changes after the desorption of water. As a conclusion from our experimental data, we state that starting from a defective P-rich InP(001) surface, almost no electronic interaction with chemical bond formation to water is observed. In order to verify our experimental findings, we determined the water adsorption configuration for the ideal 2×2-2D-2H and the hydrogen deficient surfaces computationally. The potential energy surface (PES) for single H 2 O molecules adsorbed on the InP(001) 2×2-2D-2H surface is shown in Fig. (left). Interestingly, the P dimer is not a favorable bonding site for water. This agrees with earlier findings for In-rich InP surfaces , where water adsorbs at In rather than on P sites. Here, the molecule preferably sits in the trench between the P dimer rows. In Figure , the equilibrium bonding geometry of a surface adsorbed water molecule is shown together with the calculated charge redistribution upon adsorption. There is a slight charge accumulation between the water proton and the P dimer up-atom, and a somewhat weaker charge accumulation between water oxygen and second-layer In atom, leading to a weak physisorption. However, neither the atomic structure nor the electronic properties of the InP(001) 2×2-2D-2H surface are strongly affected by the water adsorption. In particular, water adsorption does not give rise to additional gap states.