In attempt to correlate electronic properties and chemical composition of atomic hydrogen cleaned GaAs(I 0 0) surface, high-resolution photoemission yield spectroscopy (PYS) combined with Auger electron spectroscopy (AES) and mass spectrometry has been used. Our room temperature investigation clearly shows that the variations of surface composition and the electronic properties of a space charge layer as a function of atomic hydrogen dose display three successive interaction stages. There exists a contamination etching stage which is observed up to around 250 L of atomic hydrogen dose followed by a transition stage and a degradation stage which is observed beyond 700 L of exposure. In the first stage, a linear shift in the surface Fermi level is observed towards the conduction band by 0.14 eV, in agreement to the observed restoration of the surface stoichiometry and contamination removal. The next stage is characterized by a drop in ionization energy and work function, which quantitatively agrees with the observed Ga-enrichment as well as the tail of the electronic states attributed to the breaking As-dimers. As a result of the strong hydrogenation, the interface Fermi level E-F-E-v has been pinned at the value of 0.75 eV what corresponds to the degradation stage of the GaAs(1 0 0) surface that exhibits metallic density of states associated with Ga-As antisites defects. The results are discussed quantitatively in terms of the surface molecule approach and compared to those obtained by other groups. (c) 2006 Elsevier B.V. All rights reserved.