We examine the resonance Raman spectroscopy of the heteronuclear dimers VCo and VFe, using a mass-selected cluster source. Cluster cations are produced in a sputtering source and mass filtered with a Wien filter, then neutralized and deposited in an Ar matrix at low temperatures. For VCo we obtain the resonance Raman spectra in Ar matrix excited with 457.9, 488.0, 496.5, 501.7, and 514.5 nm laser lines. We observe Raman shifts of 459 and 916 cm(-1), and we assign these lines to the fundamental and first overtone of the ground state. The resulting force constant is k(e) = 3.42 mdyne/A. Resonance Raman spectra of VFe, obtained at the same five excitation wavelengths, display Raman shifts at 423 and 836 cm(-1), and we assign these to the fundamental and first overtone of the ground-state vibration. The resulting force constant is k(e) = 2.94 mdyne/Angstrom. We compare these observed force constants and configurations with those of other V-X dimers (X = Ti-Ni), as well as related isoelectronic species. The value for VFe, with 13 valence electrons, is considerably lower than its neighbors. This molecule most likely has a (2)Sigma(+) ground state, stemming from a (3dsigma)(2)(3dpi)(4)(3ddelta)(4)(4ssigma)(2)(4ssigma*)(1) configuration. The higher force constant for VCo with 14 valence electrons indicates either a (3dsigma)(2)(3dpi)(4)(3ddelta)(4)(3ddelta*)(2)(4ssigma)(2) configuration with a (3)Sigma state, or a (3dsigma)(2)(3dpi)(4)(3ddelta)(4)(3ddelta*)(1)(4ssigma)(2)(4ssi gma*)(1) configuration with a (3)Delta state. (C) 2003 American Institute of Physics.