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

The influence of O 2 , CO 2 , CO, N 2 , H 2 and CH 4 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 × 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 CH 4 does not affect the QE, whereas exposure to O 2 , CO 2 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 CO 2 can only be partly restored to 60–70% of their initial QE levels.