  The contact of water with semiconductors typically changes its surface
electronic structure by oxidation or corrosion processes. A detailed knowledge
- or even control of - the surface structure is highly desirable, as it impacts
the performance of opto-electronic devices from gas-sensing to energy
conversion applications. It is also a prerequisite for density functional
theory-based modelling of the electronic structure in contact with an
electrolyte. The P-rich GaP(100) surface is extraordinary with respect to its
contact with gas-phase water, as it undergoes a surface reordering, but does
not oxidise. We investigate the underlying changes of the surface in contact
with water by means of theoretically derived reflection anisotropy spectroscopy
(RAS). A comparison of our results with experiment reveals that a water-induced
hydrogen-rich phase on the surface is compatible with the boundary conditions
from experiment, reproducing the optical spectra. We discuss potential reaction
paths that comprise a water-enhanced hydrogen mobility on the surface. Our
results also show that computational RAS - required for the interpretation of
experimental signatures - is feasible for GaP in contact with water double
layers. Here, RAS is sensitive to surface electric fields, which are an
important ingredient of the Helmholtz-layer. This paves the way for future
investigations of RAS at the semiconductor-electrolyte interface.
