The far infrared spectrum of the NO dimer

The far-infrared spectrum of the NO dimer in the gas phase has been observed for the first time using a long-path (180 m) cooled (99 K) absorption cell and a Bomem Fourier transform spectrometer. Four weak vibration-rotation bands were detected in the 120-450 cm(-1) region and assigned to intermolecular vibrations of the cis-planar ON-NO complex. The strongest is a type b band at 239.36 cm(-1) which is assigned as the nu(2) (symmetric bending) fundamental. A second type b band at 134.50 cm(-1), partly obscured by (2)Pi(3/2) <-- (2)Pi(1/2) transitions of the NO monomer, is assigned as the nu(3) (intermolecular stretch) fundamental. A third type b band at 351.38 cm(-1) can then be assigned as the nu(2) + nu(3) combination band. Finally, a very weak type a band at 429.14 cm(-1) is assigned as the nu(6) (antisymmetric bending) fundamental. We report here detailed analyses of the nu(2) and nu(2) + nu(3) bands, which yielded upper state rotational parameters, centrifugal distortion parameters, and band origins. Somewhat less extensive analyses for nu(3) and nu(6) give their rotational parameters and band origins. These results resolve the mystery of the true values of the intermolecular vibrational frequencies of the NO dimer, which turn out to be rather different from previous determinations based on condensed phase spectra.