Negative electron affinity (NEA) photocathode electron sources operating within sealed off tubes (closed vacuum) and demountable vacuum systems (open vacuum), suffer from different factors that affect the initial high performance and longevity. To aid in the understanding of the factors affecting an open vacuum system, the chemistry of a cesium and oxygen (Cs/O) adlayer was investigated, focusing, on the role of oxygen during the NEA co-deposition process (activation) and operation of a GaN (0001) emitter. Synchrotron radiation photoemission spectroscopy and a focused mercury arc discharge lamp are used for analyzing surface chemical species and total quantum yield, respectively, as the photocathode ages. The results for the activation within a demountable vacuum system indicated: (1) oxygen concentration increased over time on the Cs/O activated surface independent of the starting substrate as observed in related studies of InP (100) and GaAs (100), (2) the dominant initial oxygen species is assigned to an ion of molecular oxygen (di-oxygen) in the Cs/O activation layer having -2 charge state that changes to the -1 charge state as the quantum yield decays, and (3) the chemical changes of the Cs/O adlayer are not accompanied by a significant loss of cesium from the surface, but instead an increase in oxygen from the open vacuum system occurred. We present a chemical model for the activation layers and their transformation over time consistent with observations (1) through (3). The quantum efficiency (QE) from the GaN (0001) photocathode remained constant within a few percent over a 10 h period at greater than or equal to 20% QE and decayed by less than a factor of 2 over the subsequent 7 h as the chemical changes at the surface took place. The slowly decreasing monotonic yield gives preliminary evidence that GaN is a prospect for a more robust emitter. A long lived photocathode of this type offers near constant quantum yield over several hours when operating within open vacuum conditions by simply adjusting laser power with a feedback circuit. On the other hand, this decay can most certainly be avoided in sealed tubes by maintaining a pristine closed off environment with the proper over pressure of Cs. A detailed account of the effects of the chemical changes in the Cs/O adlayer on the resulting emission properties for GaAs and GaN (open vacuum) are discussed in a separate report. (C) 2003 American Vacuum Society.