We report on the absorption, resonance Raman, and Raman excitation profile spectra of mass selected lutetium dimers in argon matrices. Two broad absorption regions were found between 415-545nm with peaks at 418nm, 433nm, 452nm, 472nm, 505nm, 540nm, and between 635-660nm, with peaks at 640nm, 652nm. Resonance Raman spectra were obtained by excitation into one of these regions with the visible light of an Ar ion laser between 458-514nm. Two progressions were assigned to lutetium dimers. We interpret them to represent two distinct electronic states: a ground state X and an excited state A. For the ground (X) state, we obtain w_e = 121.6 (8) cm^-1 with w_ex_e = 0.16(10) cm^-1, leading to a spectroscopic dissociation energy of 2.9(18) eV, and force constant k_e = 0.76(1) mdyne/Å. The lowest excited electronic state (A) has an origin at 210.7 cm^-1, and almost identical vibrational parameters. Comparison of spectroscopic properties of lanthanide dimers is discussed, and evidence is presented that lutetium should occupy the position normally given to lanthanum in the periodic table.

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