  In this paper, we present a 3-D one step photoemission model that can be used
to calculate the quantum efficiency and momentum distributions of electrons
photoemitted from ordered single crystal surfaces close to the photoemission
threshold. Using Ag(111) as an example, we show that the model can not only
calculate the quantum efficiency from the surface state accurately without
using any ad hoc parameters, but also provides a theoretical quantitative
explanation of the vectorial photoelectric effect. This model in conjunction
with other band structure and wave function calculation techniques can be
effectively used to screen single crystal photoemitters for use as electron
sources for particle accelerator and ultrafast electron diffraction
applications.
