  We study the initial interaction of adsorbed H2O with P-rich and Ga-rich
GaP(100) surfaces. Atomically well defined surfaces are prepared by
metal-organic vapour phase epitaxy and transferred contamination-free to
ultra-high vacuum, where water is adsorbed at room temperature. Finally, the
surfaces are annealed in vapour phase ambient. During all steps, the impact on
the surface properties is monitored with in-situ reflection anisotropy
spectroscopy (RAS). Photoelectron spectroscopy and low-energy electron
diffraction are applied for further in-system studies. After exposure up to
saturation of the RA spectra, the Ga-rich (2 x 4) surface reconstruction
exhibits a sub-monolayer coverage in form of a mixture of molecularly and
dissociatively adsorbed water. For the p(2 x 2)/c(4 x 2) P-rich surface
reconstruction, a new c(2 x 2) superstructure forms upon adsorption and the
uptake of adsorbate is significantly reduced when compared to the Ga-rich
surface. Our findings show that microscopic surface reconstructions of GaP(100)
greatly impact the mechanism of initial interface formation with water, which
could benefit the design of e.g. photoelectrochemical water splitting devices.
