Can they? Normally[1] emission of a photon would be a result of an electron moving from a high-energy shell to a low-energy shell. Going from two Cl to one Cl_2 is also a transition to a lower-energy state, but it's not immediately obvious that this would be able to generate a photon in the same way as an electron shell transition. You can do the molecule merge with atoms that already had their electrons in the lowest state... can't you?

[1] According to my grade-school chemistry. :/