A Study of NbMo and NbMo^- by Anion Photoelectron Spectroscopy

Photoelectron spectra of the niobium-molybdenum diatomic anion, obtained at 488 and 514 nm, display vibrationally resolved transitions from the ground state and one excited electronic state of the anion to the ground state and one excited electronic state of the neutral molecule. The electron affinity of NbMo is measured to be 1.130 ± 0.005 eV. Its ²Δ_3/2 spin-orbit component is observed to lie 870 ± 20 cm^-1 above its previously identified ²Δ_5/2 ground state. For ⁹³Nb⁹⁸Mo, vibrational energies measured for levels up to v = 4 for the ²Δ_5/2 and ²Δ_3/2 states give harmonic frequency and anharmonicity constant values of ω_e = 492 ± 12 cm^-1 and ω_ex_e = 8.0 ± 3.2 cm^-1, the former value corresponding to a force constant of 6.80 ± 0.35 mdyn/Å. These two vibrational parameters suggest a bond dissociation energy that is too low by at least a factor of 3, indicating that the ground state potential energy curve of NbMo deviates markedly from a Morse potential at higher energies. An excited electronic state of NbMo, assigned as a ²Σ⁺ state, is observed at 2900 ± 25 cm^-1 (T₀). Vibrational energies up to v = 8 in this excited state give values of ω_e = 544 ± 8 cm^-1 and ω_ex_e = 1.9 ± 1.2 cm^-1 for ⁹³Nb⁹⁸Mo. The former value corresponds to a high vibrational force constant of 8.30 ± 0.25 mdyn/Å. Both doublet states of the neutral molecule are accessed from the anion ground state, which is assigned as ¹Σ⁺. For the ⁹³Nb⁹⁸Mo^- anion, the fundamental vibrational frequency (ΔG_1/2) is 484 ± 15 cm^-1. Electron affinity data indicate that the bond dissociation energy of NbMo^- is 0.213 ± 0.005 eV greater than that of neutral NbMo, whose previously reported value then gives D₀ = 4.85 ± 0.27 eV for the anion. An excited state of the anion lying 3050 ± 25 cm^-1 (T₀) above its ground state is assigned as ³Δ, and the energies of its spin-orbit components above the ³Δ₃ lowest energy level are measured to be 450 ± 20 cm^-1 (³Δ₂) and 1100 ± 20 cm^-1 (³Δ₁). Their uneven spacing suggests that the energy of the ³Δ₂ level is lowered by interaction with a higher energy Ω = 2 anion state. The vibrational frequency (ΔG_1/2) for the ³Δ₁ and ³Δ₂ states is measured to be 433 ± 20 cm^-1. Bond length differences among the observed states are estimated from Franck-Condon fits to vibrational band intensity profiles. When combined with the previously reported NbMo bond length, these provide bond length estimates for the ground state of the anion (1.940 ± 0.025 Å) and for the observed excited states. These species offer extreme examples of multiple metal-metal bonding, with formal bond orders of 5¹/₂ for the ²Δ ground and ²Σ⁺ excited doublet states of NbMo, 6 for the singlet ground state of the anion, and 5 for its low-lying triplet state. The relationships among their bonding properties and those of related multiply bonded transition metal dimers are discussed.