Infrared spectroscopy of CO trapped in an argon matrix revisited

The infrared spectra of carbon monoxide in an argon matrix at cryogenic temperatures have been reinvestigated by FTIR spectroscopy over a wide range of experimental conditions, and new assignments are suggested for the peaks at 2136.7, 2138.5, and 2140.1 cm−1. The former two peaks are assigned to the CO monomer, while the last peak is attributed to the CO dimer. A temperature dependence, observed for the absorption at 2138.5 and 2140.1 cm−1, is explained by the coupling between librational modes and the lattice phonons. The increase in the CO dimer concentration upon annealing suggests the rearrangement of Ar toward the closely packed structure.