A student asked me a question last night that stumped me. Suppose we have an atom in its first excited state A, and it's going to decay electromagnetically to the ground state B, losing energy E. We would expect this to occur through the emission of a photon with energy E. But what prevents it from emitting, say, two photons, each with energy E/2? Obviously this can't occur as a two-step process, since there isn't any atomic state mid-way between A and B. But I don't see why it can't occur as a one-step process. I certainly don't think we observe this in reality. My experience in research was in gamma-ray spectroscopy, and we never even considered such a possibility. Is such a process possible in principle, but just very unlikely? If so, how would one estimate the probability? Are there conditions under which it might be observable?