Knowledge Bank

A broad-band (2135 to 2200 \wn) 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 \wn ~is assigned to the previously elusive CO trimer. \textbf{138}, 071102 (2013).} 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 \AA ~intermolecular separation, in agreement with theoretical calculations. \textbf{122}, 054306 (2005); T.A. Ford, \textit{Spectrochim. Acta A } \textbf{64}, 1151 (2006).} A modest vibrational blue-shift of +0.85 \wn ~is observed for the trimer, as compared to +0.71 \wn ~for the C-bonded form of the dimer (and $-$1.28\backslash wn fortheO-bondedform).Analysisofthenewdimerspectrumrevealsthefirstknownexcitedstate(v$_{CO}$ = 1) levels with $A+$ symmetry, and establishes that resonant vibrational splittings are small ($<$0.2 \wn) 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 \wn. A total of 28 new rotational level stacks having $A-$ symmetry are assigned for v = 1 on the basis of combination differences, adding to the 8 stacks previously known, \textbf{111}, 7321 (1999); K.A. Walker et al., \textit{J. Chem. Phys.} \textbf{113}, 6618 (2000).} and extending up to 51 \wn ~above the v = 1 origin. For the first 13 stacks we can establish a correspondence between v = 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).