The Far Infrared Spectrum of the NO Dimer

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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−1region and assigned to intermolecular vibrations of thecis–planar ON–NO complex. The strongest is a typebband at 239.36 cm−1, which is assigned as the ν2(symmetric bending) fundamental. A second typebband at 134.50 cm−1, partly obscured by2Π3/2←2Π1/2transitions of the NO monomer, is assigned as the ν3(intermolecular stretch) fundamental. A third typebband at 351.38 cm−1can then be assigned as the ν2+ ν3combination band. Finally, a very weak typeaband at 429.14 cm−1is assigned as the ν6(antisymmetric bending) fundamental. We report here detailed analyses of the ν2and ν2+ ν3bands, which yielded upper state rotational parameters, centrifugal distortion parameters, and band origins. Somewhat less extensive analyses for ν3and ν6give 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.