Millimeter-wave spectra of the CO dimer: Three new states and further evidence of distinct isomers

Abstract The millimeter-wave spectrum of the normal isotope of the CO dimer, ( 12 C 16 O) 2 , has been systematically surveyed in the regions 75–105 and 131–174 GHz, with additional measurements covering the entire 60–176 GHz range. By combining these results and using the technique of combination differences based on previously known energy levels, 14 new rotational levels have been assigned and precisely (≈0.1 MHz) located. They belong to 3 completely new states, 1 with A + symmetry and 2 with A − symmetry. The position of the lowest energy A − state results in a new and lower value for the effective tunneling splitting of the CO dimer, 3.73 cm −1 . The observation of dramatically different intensities for different bands supports the concept of two isomeric forms for (CO) 2 , the ground state having a larger intermolecular separation (≈4.4 A) with most likely a C-bonded configuration, and the low-lying (0.88 cm −1 ) excited state having a smaller separation (≈4.0 A) and an O-bonded geometry.