High stability of negative electron affinity gallium arsenide photocathodes activated with Cs and NF₃

In this paper we report the first time demonstration under extremely high vacuum (XHV) conditions of the influence of O 2 , CO 2 , CO, N 2 , H 2 and CH 4 on the quantum efficiency (QE) of negative electron affinity (NEA) gallium arsenide (GaAs) photocathodes activated with Cs and NF 3 . The photocathodes were exposed to a small quantity (<0.25 Langmuirs) of each gas species under test in a vacuum chamber with a typical base pressure of mbar, thereby minimising the influence of the residual gas in the photocathode response. It was found that exposure to N 2 , H 2 and CH 4 does not affect the QE of the photocathodes, whereas exposure to O 2 and CO 2 lead to a substantial reduction in the QE of the photocathodes. Only small degradation in the QE under CO exposure was observed. Compared to those activated with Cs and O 2 in our previous study [ 1 ], photocathodes activated with Cs and NF 3 are more stable, especially under exposure to CO.