We present the absorption (scattering depletion spectroscopy) spectrum of lanthanum dimers, resonance Raman spectra and Raman excitation spectra of lanthanum dimers in an argon matrix prepared by the mass-selected ion deposition technique. The absorption spectrum of La₂ shows two broad transitions centered at 490 nm and 540 nm. Resonance Raman spectra, obtained by excitation into the first band with an argon ion laser, display triplet structure which appear to be a “site” based set of progressions, with the lowest wavenumber lines at 234.5 (5) cm^-1, 466.4 (6) cm^-1, 696.1 (9) cm^-1, and 925.1 (7) cm^-1. These data give w_e = 236.0(8) cm-1 with w_ex_e = 0.9(2) cm ^–1. The resulting spectroscopic dissociation energy is De = 1.8(3) eV. The resulting force constant (2.28 mdyne/Å) is anomalously larger than expected by nearby trends in the periodic table. This may be explained by detailed consideration of atomic promotion energies.