The ultraviolet–visible spectra of diatomic, triatomic, and higher nickel clusters

The uv–visible spectra of Ni2 and Ni3 trapped in argon matrices are reported. Three bands were observed for the former, two with ν00 at 377 nm and 529 nm respectively and ω0 approximately 330 cm−1 and one continuous band centered at 410 nm. Two bands of Ni3 were identified, a continuous one at approximately 420 nm and one with ν00 at 480 nm and ω0 of 200 cm−1. The spectra of higher, uncharacterized clusters were also observed. These showed one intense, broad band centered at approximately 335 nm. Nickel colloid formed upon prolonged warmup of the matrix and its spectrum compares well with that calculated from Maxwell-Garnett theory. The HOMO → LUMO transition is found to increase in energy as one progresses from Ni2 to bulk nickel. The observed spectral transitions are compared with reported values computed by various approximate molecular orbital calculations, and several discrepancies are found between the predicted and observed energy differences.