Jet-cooled molecular radicals in slit supersonic discharges: Sub-Doppler infrared studies of methyl radical

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 (T-rot approximate to 25 K) and high radical densities (>10(14)/cm(3)). 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 rovibrarional transitions in jet-cooled OH radicals, which indicate sub-Doppler linewidths (Delta v approximate to 100 MHz) when probed along the slit expansion axis, The enhanced spectral resolution of the slit discharge geometry is utilized to probe the v(3)= 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 [a(f)(n)=-65.5(9) MHz, epsilon(bb)'' = -354(5) MHz, a(f)' = -65(2) MHz, epsilon(bb)' = -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. (C) 1997 American Institute of Physics.