Millimeter-wave detection of doubly excited bending mode in the CO–N2 van der Waals complex

A millimeter-wave intracavity OROTRON spectrometer in combination with a pulsed supersonic pin-hole jet expansion has been used to record a new subband of the CO–N 2 weakly bound van der Waals complex in the 100–150 GHz region. Seven lines were assigned to the K = 0–0, ( j CO , j N2 ) = (2, 0)–(1, 0) “hot band” transitions to a thus far unobserved K = 0 state at 9.336 cm −1 above the ground state. This is the highest in energy van der Waals mode detected so far, and this mode may be interpreted as a doubly excited bending vibration of the CO– ortho N 2 nuclear spin isomer. Hyperfine structure caused by two 14 N nuclei was partly resolved for the measured lines, and the quadrupole coupling constant, associated with the N 2 subunit, together with rotational and centrifugal distortion parameters were precisely determined for the new K = 0, ( j CO , j N2 ) = (2, 0) state. The high lying energy levels of this state provided a sensitive test for the recent ab initio potential energy surfaces.