M. REZAEI, S. SHEYBANI-DELOUI, N. MOAZZEN-AHMADI, Department of Physics and Astronomy, University of Calgary, 2500 University Dr., N.W., Calgary, AB T2N 1N4, Canada; K. H. MICHAELIAN, Natural Resources Canada, CanmetENERGY, 1 Oil Patch Drive, Suite A202, Devon, AB T9G 1A8, Canada; A. R. W. MCKELLAR, National Research Council of Canada, Ottawa, ON K1A 0R6, Canada.

A broad-band (2135 to 2200 cm-1) infrared spectrum is recorded using a tunable quantum cascade laser to probe a supersonic jet expansion of a dilute mixture of carbon monoxide in helium, giving an effective rotational temperature of about 2.5 K. Structure in the region of 2144 cm-1 ~is assigned to the previously elusive CO trimer. In spite of interference from the CO dimer and some remaining unexplained details, there is strong evidence for a planar, cyclic, C-bonded trimer structure, with C3h symmetry and 4.42 Å ~intermolecular separation, in agreement with theoretical calculations.\footnote[2]G.W.M. Vissers et al., J. Chem. Phys. \textbf122, 054306 (2005); T.A. Ford, Spectrochim. Acta A \textbf64, 1151 (2006). A modest vibrational blue-shift of +0.85 cm-1 ~is observed for the trimer, as compared to +0.71 cm-1 ~for the C-bonded form of the dimer (and -1.28 cm-1 ~for the O-bonded form).

Analysis of the new dimer spectrum reveals the first known excited state (vCO = 1) levels with A+ symmetry, and establishes that resonant vibrational splittings are small (<0.2 cm-1) for both the C-bonded and O-bonded dimer isomers. The dimer spectrum extends over a surprisingly large range, with somewhat reduced intensity above 2150 cm-1. A total of 28 new rotational level stacks having A- symmetry are assigned for vCO = 1 on the basis of combination differences, adding to the 8 stacks previously known,\footnote[3]M.D. Brookes and A.R.W. McKellar, J. Chem. Phys. \textbf111, 7321 (1999); K.A. Walker et al., J. Chem. Phys. \textbf113, 6618 (2000). and extending up to 51 cm-1 ~above the vCO = 1 origin. For the first 13 stacks we can establish a correspondence between vCO = 0 and 1, and give labels in terms of values for K, for the low frequency intermolecular geared-bend mode, and for the intramolecular mode (in phase or out of phase C-O stretch).