Concentration modulation spectroscopy with a pulsed slit supersonic discharge expansion source

Frequency modulated discharges are combined with pulsed slit supersonic exp ansions to provide an intense, long path source of jet-cooled radicals/mole cular ions for direct absorption laser spectroscopy. The high modulation fr equency (100 kHz) permits lock-in detection of the modulated absorbance sig nals at sensitivities down to 7.3 x 10(-7) absorbance/Hz(1/2) (within 40% o f the shot noise limit) and provides excellent discrimination for radical o r molecular ion spectra in the presence of much higher precusor concentrati ons. The sensitivity of these methods is demonstrated with direct absorptio n spectral data on sample jet-cooled molecular ions (H-3(+)) and hydrocarbo n radicals (CH3CH2). (C) 2001 Elsevier Science B.V. All rights reserved.