15min:

MATTHEW D. BROOKES AND A. R. W. MCKELLAR, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada; T. AMANO, Department of Physics and Astronomy, Ibaraki University, Mito 310, Japan.

We are studying the pure rotational spectrum of the NO dimer, (NO)₂. The dimers are produced using a continuous supersonic jet source, and probed using a mm-wave spectrometer based on harmonic multiplication. Microwave radiation (75-100 GHz) from a Gunn oscillator is multiplied (× 3 or × 4) in a commercial tripler, focussed through the jet by teflon lenses, and detected by a helium-cooled InSb bolometer. The frequency scan and data collection are under microcomputer control. The computer also turns the jet off and on at intervals of 5 scans (about 5 sec), and automatically subtracts the background and sample spectra.

Previously, 4 rotational transitions of (NO)₂ in the 0 to 23 GHz microwave region had been reported. Currently, we have measured about 80 new transitions in the frequency range from 227 to 382 GHz, with J values from 5 to 16, and K_a values from 2 to 8. The underlying ¹⁴N hyperfine structure is partially resolved for some of the transitions. The observed line positions are quite well predicted by the existing molecular parameters from our analysis of the ₁ infrared band, but the new data will obviously allow a considerably more precise set of parameters to be derived. These new mm-wave results do not directly address the two great mysteries of the NO dimer, namely the locations of the intermolecular vibrational modes and of the low-lying electronic states. We still hope to address the former mystery by means of long-path, low temperature FT spectroscopy in the far-ir region.