TRANSVERSE ENERGY SPREAD OF PHOTOELECTRONS EMITTED FROM GAAS PHOTOCATHODES WITH NEGATIVE ELECTRON-AFFINITY

The mean transverse energy (MTE) of electrons emitted from GaAs photoc athodes was measured systematically using a new method based on adiaba tic transverse expansion of an electron beam in a spatially decreasing magnetic field. Electrons with energies above the conduction band min imum are found to be thermalized with the lattice temperature of the c athode while electrons having suffered energy losses prior to their em ission show enhanced transverse energies. For (Cs,O) and (Cs,F) activa tion layers on the same cathode the same MTE is found. By cooling the cathode with liquid nitrogen, the MTE of the high-energy electrons was reduced to approximate to 14 meV. (C) 1997 American Institute of Phys ics.