Laser spectroscopy of matrix-isolated metal diatomics

Abstract

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 Fe₂, to determine their bond dissociation energies. The value obtained for Mn₂ confirms it to be a very weakly bound molecule. Moreover we propose that Fe₂ does not possess a large number of states close to the ground state, since the value obtained for it, 1.3^+0.4_–0.1eV, 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 Mn₂, 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 Mn₂ 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; Cr₂ is identified as a likely candidate.