  Photon coincidence spectroscopy relies on detecting multiphoton emissions
from the combined atom-cavity system in atomic beam cavity quantum
electrodynamics experiments. These multiphoton emissions from the cavity are
nearly simultaneous approximately on the cavity lifetime scale. We determine
the optimal time for the detection window of photon pairs in two-photon
coincidence spectroscopy. If the window time is too short, some photon pairs
will not be detected; if the window time is too long, too many nearly
coincident independent single photons will be falsely interpreted as being a
photon pair. The paper has been submitted to The European Physical Journal D.
