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Remarkably, there are no previously published experimental spectra of the helium-acetylene van der Waals complex. Apparently, infrared spectra of He-C$_2$H$_2$ were recorded around 1990 in Roger Miller's lab, but a detailed rotational assignment was not possible even with the help of two extensive sets of theoretical predictions. \textbf{77}, 111 (1992); R. Moszynski, P.E.S. Wormer, and A. van der Avoird, \textit{J. Chem. Phys.} \textbf{102}, 8385 (1995).} Here, we study rare gas-C$_2$D$_2$ complexes in the \nub{3} region ($\sim$2439 $\wn$) using a rapid-scan tuneable diode laser spectrometer to probe a pulsed supersonic slit-jet expansion. The He-C$_2$D$_2$ assignment problem is readily apparent: most of the absorption is piled-up in a very narrow region around 2440.85 $\wn$, close to the $R(0)$ line of the C$_2$D$_2$ monomer. This pile-up is a signature of very weak anisotropy in the helium-acetylene intermolecular potential, leading to almost free internal rotation of the C$_2$D$_2$. We are able to achieve a convincing rotational assignment with the help of theoretical energy level calculations based on the intermolecular potential surface of Munteanu and Fernandez. \textbf{123}, 014309 (2005).} So far the results are limited to He-C$_2$D$_2$ transitions which correlate with the monomer $R(0)$ transition. Ne-C$_2$D$_2$ also shows a free-rotation pile-up of lines near $R(0)$ which makes assignment tricky. In contrast, Ar-C$_2$D$_2$ exhibits more conventional behavior and a normal asymmetric rotor analysis is possible.


Author Institution: Department of Physics and Astronomy, University of Calgary; 2500 University Dr., N.W., Calgary, AB T2N 1N4, Canada; Steacie Institute for Molecular Sciences, National Research; Council of Canada, Ottawa, ON K1A 0R6, Canada; Department of Physical Chemistry and Center for Research in; Biological Chemistry and Molecular Materials (CIQUS), University of; Santiago de Compostela, E-15782 Santiago de Compostela, Spain; Department of Chemistry and Biochemistry, Utah State University; Logan, UT 84322-0300