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.

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The Quantum cascade (QC) laser is an entirely new type of semiconductor device in which the laser wavelength depends on the band-gap engineering. It can be made to operate over a much larger range than lead salt lasers, covering significant parts of both the infrared and submillimetre regions, and with higher output power. In this tutorial review we survey some of the applications of these new lasers, which range from trace gas detection for atmospheric or medical purposes to sub-Doppler and time dependent non-linear spectroscopy.

CH4 and C2H2 molecules (and their interconversion) in hydrocarbon/rare gas/H2 gas mixtures in a microwave reactor used for plasma enhanced diamond chemical vapor deposition (CVD) have been investigated by line-of-sight infrared absorption spectroscopy in the wavenumber range of 1276.5−1273.1 cm−1 using a quantum cascade laser spectrometer. Parameters explored include process conditions [pressure, input power, source hydrocarbon, rare gas (Ar or Ne), input gas mixing ratio], height (z) above the substrate, and time (t) after addition of hydrocarbon to a pre-existing Ar/H2 plasma. The line integrated absorptions so obtained have been converted to species number densities by reference to the companion two-dimensional (r,z) modeling of the CVD reactor described in Mankelevich et al. [J. Appl. Phys. 104, 113304 (2008)] . The gas temperature distribution within the reactor ensures that the measured absorptions are dominated by CH4 and C2H2 molecules in the cool periphery of the reactor. Nonetheless, the measurements prove to be of enormous value in testing, tensioning, and confirming the model predictions. Under standard process conditions, the study confirms that all hydrocarbon source gases investigated (methane, acetylene, ethane, propyne, propane, and butane) are converted into a mixture dominated by CH4 and C2H2. The interconversion between these two species is highly dependent on the local gas temperature and the H atom number density, and thus on position within the reactor. CH4→C2H2 conversion occurs most efficiently in an annular shell around the central plasma (characterized by 1400