Polarization behavior, temperature dependence, and vibronic structure of the 23,000-cm-1 absorption system in the electronic spectra of tetrakis[.mu.-(acetato-O,O')]dimolybdenum(Mo-Mo) and related compounds. Emission spectrum of tetrakis[.mu.-(trifluoroacetato-O,O')]dimolybdenum(Mo-Mo) at 1.3 K