Emission, absorption, and excitation spectroscopy has been used for a detailed analysis of the optical transitions of Hg-2 trapped in cryogenic matrices. Upon excitation of electronic states correlating to the P-3(1) or the P-1(1) asymptote, fast nonradiative relaxation leads to emission from the lowest excited A0(g)(+) state in all matrices, which decays monoexponentially in 1 ms in Ne, 280 mu s in Ar, and 12 mu s in Xe. In addition, electronically unrelaxed emission of Hg-2 is reported in neon and in xenon matrices and attributed to the B1(g) state in neon and to the B1(g) state and the C0(u)(-) or A0(g)(-) states in xenon. The results are rationalized by assuming: (a) that population of the excited states occurs mainly close to the asymptotic limit, where branching is determined by nonadiabatic coupling and energetics, that are strongly environment dependent, and (b) that in Xe matrices the Hg-2 states correlating to the P-3(1) and P-3(0) asymptotic limits are stabilized in different configurations, as a result of the very different solvation properties of the atomic P-3(1) and the P-3(0) state. Further emission bands are found in the vicinity of the dimer transitions, which we attribute to Hg-3 and to site effects on Hg-2. In particular, electronically unrelaxed emission from excited states of Hg-3 is reported.