15min:

M. REZAI, N. MOAZZEN-AHMADI, Department of Physics and Astronomy, University of Calgary, 2500 University Dr., N.W., Calgary, AB T2N 1N4, Canada; A. R. W. MCKELLAR, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, ON K1A 0R6, Canada; BERTA FERNÁNDEZ, 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; DAVID FARRELLY, Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322-0300.

Remarkably, there are no previously published experimental spectra of the helium-acetylene van der Waals complex. Apparently, infrared spectra of He-C₂H₂ 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.\footnote[1] T. Slee, R.J. Le Roy, and C.E. Chuaqui, Mol. Phys. \textbf77, 111 (1992); R. Moszynski, P.E.S. Wormer, and A. van der Avoird, J. Chem. Phys. \textbf102, 8385 (1995). Here, we study rare gas-C₂D₂ complexes in the ₃ region ( 2439 cm^-1) using a rapid-scan tuneable diode laser spectrometer to probe a pulsed supersonic slit-jet expansion. The He-C₂D₂ assignment problem is readily apparent: most of the absorption is piled-up in a very narrow region around 2440.85 cm^-1, close to the R(0) line of the C₂D₂ 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 C₂D₂. 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 Fernández.\footnote[2] R. Munteanu and B. Fernández, J. Chem. Phys. \textbf123, 014309 (2005). So far the results are limited to He-C₂D₂ transitions which correlate with the monomer R(0) transition.

Ne-C₂D₂ also shows a free-rotation pile-up of lines near R(0) which makes assignment tricky. In contrast, Ar-C₂D₂ exhibits more conventional behavior and a normal asymmetric rotor analysis is possible.