Spectroscopy of mass‐selected niobium trimers in argon matrices

The absorption (scattering depletion) spectrum and Raman spectra for Nb₃ in an argon matrix prepared by the mass selected ion deposition technique have been obtained. The absorption spectrum in the visible region shows three overlapping transitions, centered at 20 300 cm⁻¹ (A), 18 800 cm⁻¹ (B), and 17 000 cm⁻¹ (C), respectively. Resonance Raman spectra obtained with excitation into these bands display two distinct fundamental frequencies at 227.4±2.9(e′) and 334.9±2.8(a₁^′) cm⁻¹ which indicate that the ground state of the Nb₃ molecule has a nearly equilateral triangular geometry (D_3h). The f_r (bond stretch) and f_rr (stretch–stretch interaction) force constants for Nb₃ are determined to be 1.95 and 0.05 mdyn/Å, respectively. The Raman excitation profiles bear a strong resemblance to the triniobium absorption spectrum, but the peaks seem more closely spaced than the absorption spectrum. Both a₁^′ and e′ vibration are observed with the same intensity near the peak A, while the a₁^′ vibration dominates the profile at the peak B. Neither displays much intensity near the peak C.