"The adsorption of oxygen molecules on evaporated gallium films has been studied by UV photoelectron spectroscopy between 10 and 300 K. In addition to the oxygen levels, the chemical shift of the Ga 3d core level has been investigated using monochromatized light from a He discharge lamp at ℏω = 40.8 eV. Four different states of oxygen have been found depending on temperature. At 10 K the molecules of the first layer are physisorbed onto which several additional layers can be condensed. The rigid relaxation shifts to smaller binding energies are 2.7 eV for physisorbed and 1.3 eV for condensed oxygen. During warming-up the oxygen reacts with the gallium surface. Between 70 and 130 K an oxygen species develops which is interpreted as chemisorbed molecular oxygen. This is concluded from the valence band UP spectra, the chemical shift of the Ga 3d level, and the work function change. At 300 K oxygen is dissociatively bound and the bulk oxide grows." - D. Schmeisser, K. Jacobi