  Considering the electron states inside and outside the solid, we derive a
formula of photoemission intensity. A general theoretical way to determine
electronic structures of solids from ARPES experiments is outlined. It is shown
that the spectral function inside the solids cannot be measured directly by
ARPES, the effects of free electron states on the electronic structure observed
by ARPES measurements must be considered, and the results from ARPES
experiments cannot be understood until these results have been made consistent
with a theoretical calculation.
