The infrared spectrum of the CO dimer

The infrared spectrum of (¹²C¹⁸O)₂ has been studied for the first time using a tunable diode laser spectrometer in the 2095 cm⁻¹ region to probe a pulsed supersonic jet expansion. Very dilute gas mixtures of CO in He were used, resulting in small consumption of ¹²C¹⁸O sample gas, as well as cold and simple spectra. The results were analyzed using a term value scheme to obtain model-independent energies for 7 rotational levels belonging to 2 stacks in the lower state, v_CO=0, and 22 levels belonging to 7 stacks in the upper state, v_CO=1. The two ground state isomers of the CO dimer were found to be separated by only 0.639 cm⁻¹ for (¹²C¹⁸O)₂. These results provide a foundation for future studies of the millimeter wave spectrum.