MILLIMETER-WAVE SPECTROSCOPY IN A PULSED SUPERSONIC SLIT NOZZLE DISCHARGE

A Doppler-free technique for direct absorption spectroscopy of rotatio nally cold molecular ions in the millimeter wave range is presented. T he method uses a double modulation technique, based on a simultaneous modulation of a frequency doubled microwave sweeper and a pulsed slit nozzle, incorporating a discharge in a high-pressure supersonic expans ion. The performance is demonstrated with the observation of the hyper fine structure of the J = 2 <-- 1 pure rotational transition of N2H+. Further perspectives of the method are discussed. (C) 1998 Elsevier Sc ience B.V. All rights reserved.