LOW RESOLUTION MICROWAVE OPTOACOUSTIC SPECTROSCOPY IN THE 22-29 GHZ RANGE

Abstract

A microwave spectrometer has been constructed based on the optoacoustic effect using an amplitude modulated klystron source in order to study the absorption profiles in the 1-10 torr region. Using known $NH_{3}$ inversion and $CH_{3}OH$ rotational lines, sensitivity has been studied as a function of pressure, phase angle, modulation frequency, and acoustic isolation of the detector. S/N$>$2 has been observed for the $CH_{3}OH$ line at 27880MHz (-10 log $\gamma=68, 1.6 \times 10^{-7}$ $cm^{-1})^{1}$. Between modulation frequencies of 1-8 kHz, no acoustic resonance effects have been observed in the cell. Preliminary Stark spectra indicate two weak b-type pile-ups for $CF_{3}CH_{2}OH [\nu\sim(2A-B-C) (k_{-1}+\frac{1}{2})$ for a near prolate rotor]. Analysis of the optoacoustic (true absorption) spectrum shows a strong band of greater than 400 MHz width in the same region as one of the Stark bands.