ME09		15 min4:12

I. PAK, M. HEPP, D. ROTH AND G. WINNEWISSER, I. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany.

A double modulation technique was used to achieve higher sensitivity in absorption measurements with a pulsed supersonic jet. The wavelength of a lead--salt diode laser was modulated at a frequency of 10 kHz and detected at twice the frequency by a lock--in amplifier, reducing significantly the influence of the low frequency excess laser noise. The output of this lock--in amplifier was measured by a boxcar integrator, driven at 80 Hz, the frequency of the pulsed jet. The same technique was applied to the jet absorption measurements in the millimeter wave region using mm--wave synthesizers as radiation sources. The recorded spectra of Ar-CH₄, Kr-CH₄ and Ar-CO molecular van der Waals complexes are presented in order to show the improved sensitivity of the tunable diode laser and mm-wave spectrometers.