The distribution of oxide species in the Cs ∕ O activation layer on InP(100) negative electron affinity photocathodes

The atomic arrangement of Cs oxides in the activation layer of an InP photocathode is investigated using angle dependent photoemission spectroscopy. Two distinct peaks in the O 1 s core level and in valence band spectra have led to the discovery of two molecular oxygen species incorporated in the thin activation layer: Cs peroxide ( Cs 2 O 2 ) and Cs superoxide ( Cs O 2 ) . The different angular dependences of these oxides observed in the photoemission spectra are caused by different vertical locations of the oxygen molecules in each Cs oxide in the activation layer. The thickness of the activation layer, which is about 7 Å , suggests lateral distribution of Cs peroxide and Cs superoxide. The quantum efficiency of InP photocathodes in our ultra high vacuum system decreases with time due to the chemical transformation of the Cs oxides and subsequent substrate oxidation, as deduced from an observation of the peak evolution in the photoemission spectra, and supported by the thermodynamic stability of Cs superoxide as compared to Cs peroxide when there is residual oxygen around.