ELECTRON-EMISSION PROPERTIES OF LASER PULSED GAAS NEGATIVE ELECTRON-AFFINITY PHOTOCATHODES

The emitted electron energy distribution of a laser pulsed GaAs negative electron affinity (NEA) photocathode was measured with a retarding grid analyzer. The total electron energy distribution was found to depend strongly on laser intensity and cathode thickness. A model based on the separation of the excess photogenerated carriers within the cathode structure has been developed and tested to explain this behavior. Pulsed electron emission patterns were obtained by directing the emitted electrons to a phosphorous screen. The patterns indicate that uniform emission occurs and that the emission develops uniformly as the cathode is activated to a NEA condition. An extrapolated plot of the angular intensity FWHM versus V(B)1/2 indicates that the pulsed cathode possesses a natural semicone angle of approximately 10-degrees. The angular distribution is narrow because of the electron refraction which occurs upon emission due to differences in the effective mass and vacuum rest mass of the electrons.