The mystery of the CO dimer: assignments from variable-temperature jet-cooled infrared spectra

The infrared absorption spectrum of the CO dimer was studied in the vicinity of the CO fundamental band in both axisymmetric and planar supersonic jet expansions. By probing the jet at varying distances downstream, we observed (CO)2 spectra over a range of effective rotational temperatures from about 15 down to 1 K. The temperature dependence of the transitions enabled us to identify one perpendicular and two parallel subbands arising from the lowest rotational levels of the dimer. The effective intermolecular separation for (CO)2 was determined to be about 4.35 إ. The only previous assignment of CO dimer spectra is incompatible with our results.