  Molecular dynamics simulations, with full Coulomb interaction are used to
model short-pulse photoemission from a finite area in a microdiode. We
demonstrate three emission regimes, source-limited emission, space-charge
limited emission for short-pulses, and space-charge limited emission for the
steady state. We show that beam brightness is at a maximum during transition
from the source-limited emission regime to the space-charge limited emission
regime for short pulses. From our simulations it is apparent that the important
factor is the emitter spot size when estimating the critical charge density for
short-pulse electron emission.
