"Essentially, when electrons
escape from a metal, they do not
do so alone: they take darkons
with them. If they do not escape,
the darkons do not escape either.
This phenomenon is known as the
darkon threshold. If an electron
performs an amount of work W to
escape from a metallic surface,
then the condition for hindrance of
darkon emission (remember, one
wishes to conserve, not get rid of,
darkons) is"