The resonance Raman spectrum obtained from manganese containing argon matrices previously assigned to the dimer [M.Moskovits, D.P.DiLella, and W.Limm, J. Chem. Phys. 80, 626 (1984)] is shown to belong to a zero order D3h trimer subject to Jahn–Teller distortions. The actual resonance Raman spectra of the dimer in both argon and krypton matrices are reported and discussed along with the visible absorption spectra. The resonance Raman of manganese dimer in krypton gives the 1Σ+g ground state constants of ωe =76.4 cm−1 and ωexe=0.53 cm−1. Trimer constants from the resonance Raman spectra in argon are (ωs−2xs)=196.79 cm−1, ωa=130.15 cm−1, Dωa =2.95 cm−1, xa=0.06 cm−1, and xas=0.79 cm−1. Independent values of ωs and xs could not be determined from our spectral data.

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Resonance Raman and optical absorption spectra of matrix-isolated transition-metal diatomics have provided valuable vibrational data for these molecules. We have performed LeRoy-Bernstein analysis on four such diatomics, Sc2, Ti2, Mn2 and Fez, to determine their bond dissociation energies. The value obtained for Mn, confirms it to be a very weakly bound molecule. Moreover we propose that Fe, does not possess a large number of states close to the ground state, since the value obtained for it, 1.3?:' : eV, is close to the quantity calculated from mass-spectral data by assuming a small density of states close to the ground state. The spectrum of Mn,, which is known to be an antiferromagnetically coupled pair of Mn atoms, is very sensitive to temperature as a result of the presence of low-lying spin states resulting from the antiferromagnetic coupling. The temperature variation of the resonance Raman spectrum of this molecule suggests that there is rapid equilibration, not only among ground spin states but also among excited spin states, implying that Mn. is an antiferromagnetic dimer in at least one of its excited states. The trend in the force constants of the first-row transitionmetal diatomics is discussed in terms of the interplay between s and d bonding. A suggestion is made that near the centre of the first row one might encounter a diatomic with a double-minimum ground-state potential; Cr, is identified as a likely candidate.

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