Fluorescence occurs when an atom or molecule absorbs a photon, promoting a ground state electron to an excited state, and the electron returns very quickly to the ground state, emitting a new photon. The new photon is usually of a longer wavelength because some of the energy is lost to vibrational decay. The quantum yield of the fluorescence process is the ratio of the number of photons emitted to the number absorbed. I have read several times now that the quantum yield can be less than 1, but not greater than 1. I'm not really understanding this. First, I know that fractions of photons can't be emitted, so I'm assuming that to get a quantum yield of 0.5, for example, there must be multiple decay pathways back to the ground state. Say one is a decay that produces no photon, while another decay path is through photon emission, and they are equally likely. Is that right? But then I run into another problem. Would it be impossible for an electron to decay by two successive radiation-producing drops in energy? That seems possible to me. The absorbed high-energy photon would then produce two lower energy photons. But that would give a quantum yield of two, which I've read is impossible. In summary, I'm afraid that I have some fundamental misunderstanding of fluorescence that is confusing me. I haven't learned many details behind the process, so I'm not looking for a very technical explanation yet, although I do have an undergraduate background in quantum mechanics to work with. But it would be very much appreciated if someone could explain where my misunderstanding of the basics is leading me astray. Thanks!