ROTATIONAL SPECTROSCOPY OF SMALL CLUSTERS WITH A FLYGARE FOURIER TRANSFORM MICROWAVE SPECTROMETER

$^{\ast}$Work suported by NSF and PRF. $^{1}$ T. J. Balle, E. J. Campbell, M. R. Keenan, and W. H. Flygare, J. Chem. Phys. 71. 2723 (1979). $^{2}$ E. J. Campbell, W. G. Read, and J. A. Shea, Chem, Phys, Lett, 94, 69 (1983). $^{3}$ T. J. Balle and W. H. Flygare, Rev., Sci Instr. 52, 1 (1981). Address: Noyes Chemical Laboratory, University of Illinois, Urbana, IL 61801.Author Institution:The Flygare-Balle microwave $spectrometer^{1}$ has been very useful in the study of hydrogen bonded and van der waals dimers. Recently we have been able to extend its application to the study of trimers and tetramera such as $Ar_{2}-HX, Ar_{3}-HX$, and $(HCN)_{3}$. Detection and characterization of other small clusters seem feasible. This has been made possible by a continuing series of practical refinements in the Mark II Flygare $spectrometer.^{2}$ They include the following: (a) The cavity is used in a reflection mode rather than in $transmission.^{3}$ This arrangement permits much better recovery of the molecular signal, resulting in a sensitivity improvement from 2 to more than 100-fold depending on frequency. (b) Externally adjustable skimmer slits have been installed, permitting almost complete elimination of Doppler broadening, at a modest sacrifice or signal strength (c) A sample prepration system has been devised which allows precise fingertip control of the sample composition and pressure. The stock gases are handled through capillary Teflon tubes and diluted to the desired concentration with carrier gas only two seconds before being pulsed into the spectrometer. This permits quick and easy optimization of sample composition and eliminates the problem of adsorption and isotope exchange with plumbing walls