The far infrared spectrum of the NO dimer

The far-infrared spectrum of the NO dimer in the gas phase has been observe d for the first time using a long-path (180 m) cooled (99 K) absorption cel l and a Bomem Fourier transform spectrometer. Four weak vibration-rotation bands were detected in the 120-450 cm(-1) region and assigned to intermolec ular 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) fu ndamental. 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 ver y 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, centrif ugal distortion parameters, and band origins. Somewhat less extensive analy ses 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 differ ent from previous determinations based on condensed phase spectra.