Ok, I have this homework problem that is driving me nuts, and I only have a short period of time to get it. The question is, Estimate the temperature of the universe when it has cooled enough that photons no longer disassociate the deuteron. There was another part that asked for the temperature when the hydrogen atom is no longer dissociated. For that problem I used the Ionization Energy of the ground state of electron(13.6 eV), and used the following approach: 13.6eV=kT, where k=8.62e-5eV/K. I got that part right, so I figured that I could use the Ionization energy of deuterium (14.9eV) for the 2nd part, but this doesn't give me the right answer. Does anyone know why this is so and if there's another approach to the problem?