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, CO2, CO, N-2, H-2 and CH4 on the quantum efficiency (QE) of negative electron affinity (NEA) gallium arsenide (GaAs) photocathodes activated with Cs and NF3. 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 1.5x10(-11) mbar, thereby minimising the influence of the residual gas in the photocathode response. It was found that exposure to N-2, H-2 and CH4 does not affect the QE of the photocathodes, whereas exposure to O-2 and CO2 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 NF3 are more stable, especially under exposure to CO.