ROTATIONAL SPECTRA OF EXCITED $\Sigma$ VIBRATIONAL STATES OF Ar-HC1 AND Ar-$DC1^{\ast}$

$^{\ast}$ Work supported by NSF and PRF. $^{1}$ K. L. Busarow, G. A. Blake, K. B. Laughlin, B. C. Cohen, Y. T. Lee, and R. J. Saykally, J. Chem. Phys. $89$, 1268 (1988). $^{2}$ R. L. Robinson, D. -H. Gwo, and R. J. Saykally, J. Chem. Phys. $87$, 5156 (1987) $^{3}$ R. S. Ruoff, T. D. Klots, T. Emilsson, and H. S. Gutowsky, J. Chem. Phys., in press.Author Institution: Noyes Chemical Laboratory, University of IllinoisThe $\Sigma$ bend and stretching vibration-rotation spectra of Ar-HC1 have been characterized recently$^{1,2}$ by far infrared spectroscopy. Using the rotational constants reported for $Ar-H^{35}C1/^{37}C1$ we were able to find pure rotational transitions of the $\Sigma$ states by Fourier transform microwave spectroscopy with supersonic expansonic expansion. Analysis of their $^{35}C1/^{37}C1$ isotope effects shows that the H in HC1 is pointed predominantly away from the Ar. Our observation of the rotational transitions was aided by the dependence on carrier gas of the population of excited states in the beam,$^{3}$ with He giving a better signal than Ne which is better than Ar. About 20,000 FID's are needed to get a spectrum. Using this approach we have observed the $\Sigma$ bending mode of $Ar-D^{35}C1$ and $Ar-D^{37}C1$, determining B, D, and $eq^{Q}_{aa}$ to be 1727.9915 MHz, 39.8 kHz, and -28.247 MHz, respectively, and 1686.4781 MHz, 37.8 kHz, and -22.346 MHz. The larger centrifugal distortion constants for Ar-DC1 compared to the 31.6 and 30.5 kHz for the Ar-HCl $species^{1}$ show that deuterium substitution causes the dimer to spend more time in the shallower secondary potential minimum. The B values indicate a more compact structure for the Ar-DC1 dimers. Further analysis will be provided along with the results of searches in progress for Ar-DC1 in other excited states