Resonant two-photon ionization spectroscopy was used to study jet-cooled Ni2 produced by pulsed laser ablation of a nickel target in the throat of a supersonic nozzle using argon as the carrier gas. Spectral regions previously investigated using helium as the carrier gas were reinvestigated, and the improved cooling achieved was found to suppress transitions arising from an Ω=4 state that had been thought to be the ground state. Seven new vibronic progressions were assigned, with spectroscopic constants determined for the excited states. The predissociation threshold in Ni2 was reinvestigated, and a revised value for the binding energy is given as D○0(Ni2)=2.042±0.002 eV. The ionization energy of Ni2 was found to be 7.430±0.025 eV, and from this result and the revised bond dissociation energy of the neutral, the binding energy of the cation was calculated to be D○0(Ni+2)=2.245±0.025 eV. Similarly, D○0(Ni−2)=1.812±0.014 eV is obtained using D○0(Ni2) and the electron affinities of Ni and Ni2. Twenty bands were rotationally resolved, all originating from a lower state of Ω″=0+g or 0−u which we argue is the true ground state, in agreement with ligand field and ab initio theoretical studies. The rotational analysis also yielded a ground state bond length of 2.1545±0.0004 Å for 58Ni2.

Dispersed fluorescence spectroscopy has been used to study jet-cooled AgAu and Pt 2. Fluorescence resulting from the excitation of five bands of the A←X 1 ⌺ ϩ system of AgAu was dispersed, and 51 measured ground state vibrational levels were fit to provide ground state vibrational constants of e Љϭ198.22Ϯ0.11 cm Ϫ1 and e Љx e Љϭ0.512Ϯ0.002 cm Ϫ1. A Franck-Condon calculation was performed using the experimental values of the ground and excited state vibrational frequencies and anharmonicities, providing an estimate of the change in bond length upon excitation of the A←X system of ⌬r e ϭ0.214Ϯ0.005 Å. Fluorescence resulting from four different excitations of Pt 2 was dispersed, providing vibrational constants for the ground and two low-lying excited states. Ground state vibrational constants of e ϭ222.3 cm Ϫ1 and e x e ϭ0.62 cm Ϫ1 were obtained, based on the analysis of 16 measured ground state vibrational levels. In addition, a low-lying excited state was located at T 0 ϭ2877 cm Ϫ1 , with e ϭ197 cm Ϫ1. This state perturbed the ground state, from which it was deduced that it has the same symmetry as the ground state. A comparison to theoretical calculations suggests that both states have 0 g ϩ symmetry. Finally, a metastable state of Pt 2 lying at an unknown energy was determined to have e ϭ211 cm Ϫ1 , e x e ϭ0.4 cm Ϫ1 .

A resonant two-photon ionization study of the jet-cooled RuC molecule has identified the ground state as a Σ+1 state arising from the 10σ211σ25π42δ4 configuration. The Δi3 state arising from the 10σ211σ25π42δ312σ1 configuration lies very low in energy, with the Δ33 and Δ23 components lying only 76 and 850 cm−1 above the ground state, respectively. Transitions from the X 1Σ+, Δ33, and Δ23 states to the Π23, Π13, Φ33, Φ43, Φ31, and Π11 states arising from the 10σ211σ25π42δ36π1 configuration have been observed in the 12 700–18 100 cm−1 range, allowing all of these states to be placed on a common energy scale. The bond length increases as the molecule is electronically excited, from r0=1.608 Å in the 2δ4, X 1Σ+ state, to 1.635 Å in the 2δ312σ1, Δ3 state, to 1.66 Å in the 2δ36π1, Π3 and Φ3 states, to 1.667 Å in the 2δ36π1, Φ1 and 1.678 Å in the 2δ36π1, Π1 state. A related decrease in vibrational frequency with electronic excitation is also observed. Hyperfine splitting is observed in the 2δ312σ1, Δ33 state for the Ru99(I=5/2)12C and Ru101(I=5/2)12C isotopic combinations. This is analyzed using known atomic hyperfine parameters to show that the 12σ orbital is roughly 83% 5sRu in character, a result in good agreement with previous work on the related RhC and CoC molecules.

Resonant two-photon ionization spectroscopy of jet-cooled tantalum carbide, TaCThe first optical investigation of the spectra of diatomic PdC has revealed that the ground state has ⍀ϭ0 ϩ , with a bond length of r 0 ϭ1.712 Å. The Hund's case ͑a͒ nature of this state could not be unambiguously determined from the experimental data, but dispersed fluorescence studies to be reported in a separate publication, in combination with a comparison to theoretical calculations, demonstrate that it is the 2␦ 4 12 2 , 1 ⌺ 0 ϩ ϩ state, which undergoes spin-orbit mixing with a low-lying 2␦ 4 12 1 6 1 , 3 ⌸ 0 ϩ state. An excited 3 ⌺ ϩ state with r e ϭ1.754Ϯ0.003 Å (r 0 ϭ1.758 Ϯ0.002 Å) and ⌬G 1/2 ϭ794 cm Ϫ1 is found at T 0 ϭ17 867 cm Ϫ1 . Although only the ⍀ϭ1 component of this state is directly observed, the large hyperfine splitting of this state for the 105 Pd 12 C isotopomer implies that an unpaired electron occupies an orbital that is primarily of 5s character on Pd. Comparison to ab initio calculations identifies this state as 2␦ 4 12 1 13 1 , 3 ⌺ 1 ϩ . To higher wavenumbers a number of transitions to states with ⍀ϭ0 ϩ have been observed and rotationally analyzed. Two groups of these have been organized into band systems, despite the clear presence of homogeneous perturbations between states with ⍀ϭ0 ϩ in the region between 22 000 and 26 000 cm Ϫ1 .

In a spectroscopic investigation of jet-cooled Rh2 by the resonant two-photon ionization method, an abrupt predissociation threshold is observed in a dense set of vibronic levels at 19 405±4 cm−1. Based on the high density of states expected in the rhodium dimer, the sharp definition of the predissociation threshold that is observed, and the validation of a similar conclusion in the case of V2, it is argued that predissociation occurs as soon as the energy of the separated ground state atoms is exceeded. On this basis the bond energy of Rh2 is assigned as D0(Rh2)=19 405±4 cm−1=2.4059±0.0005 eV. This value is compared to the results of other experiments and to theoretical calculations. The bond energy of disilver, D0(Ag2)=1.65±0.03 eV, provides a particularly useful standard of comparison, and suggests that 4d contributions to the bond energy of Rh2 amount to at least 0.76 eV.