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The infrared spectrum of the hydroxyl radical OH, between 1850 and 9000 cm-1 , has been measured with a Fourier transform spectrometer. The source, a hydrogen-ozone diffusion flame, was designed to study the excitation of rotation-vibration levels of the OH Meinel bands under conditions similar to those in the upper atmosphere which produce the nighttime OH airglow emission. Twenty-three bands were observed: nine bands in the Ap = 1 sequence, nine bands in the Ap = 2 sequence, and five bands in the Ap = 3 sequence. A global nonlinear least-squares fit of 1696 lines yielded molecular parameters with a standard deviation of0.003 cm-1. Term values are computed, and transition frequencies in the Ap = 3, 4, 5, 6 sequences in the near-infrared are predicted. Subject headings: laboratory spectra-molecular processes 351

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A novel high-intensity source of jet-cooled molecular radicals is described based on the combination of (i) slit supersonic expansions with (ii) pulsed electric discharges. The electrode bias configuration effectively confines the discharge to a region upstream of the supersonic expansion, which results both in efficient rotational cooling (Trot≈25 K) and high radical densities (>1014/cm3). In conjunction with direct absorption laser probe methods, this discharge source provides a general technique for high-resolution IR studies of jet-cooled radicals. Performance of the slit discharge system is demonstrated on v=1←0 rovibrational transitions in jet-cooled OH radicals, which indicate sub-Doppler linewidths (Δν≈100 MHz) when probed along the slit expansion axis. The enhanced spectral resolution of the slit discharge geometry is utilized to probe the v3=1←0 asymmetric CH stretch vibration–rotation spectra of CH3 radical. Under sub-Doppler conditions, spin–rotation splittings are fully resolved and nuclear hyperfine splittings partially resolved in all of the transitions, permitting the first measure of Fermi contact interactions [af″=−65.5(9) MHz, εbb″=−354(5) MHz, af′=−65(2) MHz, εbb′=−353(2) MHz] and therefore both the sign and magnitude of spin-polarization effects for CH3 under isolated gas-phase conditions. The results permit direct comparison with high level ab initio calculations, and highlight a clear trend in spin-polarization effects between condensed and gas-phase behavior.