Absorption measurements of Ar-CO b-type rotational transitions with a supersonic jet millimeter-wave spectrometer

Pure rotational b-type transitions of Ar-CO were observed in the frequency range from 70 to 103 GHz using a combination of millimeter-wave absorption spectroscopy with supersonic jet expansion. The millimeter-wave radiation was generated by precision tunable synthesizers equipped with backward wave oscillators. Altogether 14 transitions belonging to Q and R branches with K-s = 1 - 0 have been measured with an accuracy of about 50 kHz. Our results confirm the microwave-millimeter-wave double resonance experiments of Jager and Gerry [J. Chem. Phys. 102, 3587-3592 (1995)] and demonstrate the sensitivity of the direct absorption technique for the detection of van der Waals clusters with a small dipole moment. (C) 1996 Academic Press, Inc.