The degradation of quantum efficiency in negative electron affinity GaAs photocathodes under gas exposure

The influence of O-2, CO2, CO, N-2, H-2 and CH4 on the stability of the quantum efficiency (QE) of a negative electron affinity gallium arsenide (GaAs) photocathode activated with caesium (Cs) and oxygen (O) has been demonstrated for the first time under an extremely high vacuum condition, a base pressure of 1.5 x 10(-11) mbar, where the influence of the background gas is minimized. It was found that exposure of a GaAs photocathode to N-2, H-2 and CH4 does not affect the QE, whereas exposure to O-2, CO2 and CO leads to a substantial reduction in photocathode QE. It was also found that the QE of photocathodes which have been degraded under O-2 exposure can be recovered to 95% of their initial QE level by the re-caesiation process, while those which have been degraded under exposure to CO and CO2 can only be partly restored to 60-70% of their initial QE levels.