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 nu2 (symmetric bending) fundamental. A second type b band at 134.50 cm-1, partly obscured by 2Pi3/2 <-- 2Pi1/2 transitions of the NO monomer, is assigned as the nu3 (intermolecular stretch) fundamental. A third type b band at 351.38 cm-1 can then be assigned as the nu2 + nu3 combination band. Finally, a very weak type a band at 429.14 cm-1 is assigned as the nu6 (antisymmetric bending) fundamental. We report here detailed analyses of the nu2 and nu2 + nu3 bands, which yielded upper state rotational parameters, centrifugal distortion parameters, and band origins. Somewhat less extensive analyses for nu3 and nu6 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. Copyright 1999 Academic Press.