  High quantum yield, low transverse energy spread and prompt response time
make GaAs activated to negative electron affnity, an ideal candidate for a
photocathode in high brightness photoinjectors. Even after decades of
investigation, the exact mechanism of electron emission from GaAs is not well
understood. We show that a nanoscale surface roughness can affect the
transverse electron spread from GaAs by nearly an order of magnitude and
explain the seemingly controversial experimental results obtained so far. This
model can also explain the measured dependence of transverse energy spread on
the wavelength of incident light.
