ABSORPTION-MEASUREMENTS OF AR-CO B-TYPE ROTATIONAL TRANSITIONS WITH ASUPERSONIC JET MILLIMETER-WAVE SPECTROMETER

Pure rotational b-type transitions of Ar-CO were observed in the frequ ency range from 70 to 103 GHz using a combination of millimeter-wave a bsorption spectroscopy with supersonic jet expansion. The millimeter-w ave radiation was generated by precision tunable synthesizers equipped with backward wave oscillators. Altogether 14 transitions belonging t o Q and R branches with K-s = 1 - 0 have been measured with an accurac y of about 50 kHz. Our results confirm the microwave-millimeter-wave d ouble resonance experiments of Jager and Gerry [J. Chem. Phys. 102, 35 87-3592 (1995)] and demonstrate the sensitivity of the direct absorpti on technique for the detection of van der Waals clusters with a small dipole moment. (C) 1996 Academic Press, Inc.