Absorption, fluorescence, and Raman spectra of mass-selected rhenium dimers in argon matrices

We report absorption, laser fluorescence, and Raman spectra for Re₂ in an argon matrix prepared by the mass-selected ion deposition technique. The dirhenium absorption spectrum consists of seven band systems (A-G) extending from the near infrared into the ultraviolet region. For the A system (a simple vibrational progression), we find T₀=10 817(1) cm^-1, ω_e=317.1(5) cm^-1 and ω_exe=1.0(1) cm^-1. A Franck-Condon analysis of the A system intensities predicts that this state has a smaller equilibrium internuclear distance than the ground state (Δr_e=-0.073 Å), in violation of Badger's rule. The B system starts at 13 250 cm^-1 and consists of four overlapping (and possibly perturbed) subsystems, whose average vibrational spacing is 270(11) cm^-1. The C, D, E, and F systems (vibrational spacings in parentheses) are centered at 22 300 cm^-1 (210 cm^-1), 24 500 cm^-1 (195 cm^-1), 29 150 cm^-1 (175 cm^-1), and 32 900 cm^-1 (160 cm^-1), respectively. Weak fluorescence spectra, obtained upon laser excitation into the A system, were characterized by vibrational progressions to the dimer ground (X) state and to a low lying (X') state for which T₀=357.6(5) cm^-1 and ω_e=332.3(2) cm^-1. Raman and fluorescence progressions to the ground state were observed when the B system was excited. These data give ω_e=337.9(49) cm^-1 for the dimer ground state in good agreement with measurements from photodetachment spectra [J. Am. Chem. Soc. 108, 178 (1986)]. We propose likely assignments for the low lying electronic states of Re₂ and discuss our results in terms of the bonding in the other group VIIB dimers, Mn₂ and Tc₂.