H. S. P. MÜLLER, I. Physikalisches Institut, Univ. Köln, 50937 Köln; and MPIfR, 53121 Bonn, Germany; B. J. DROUIN, J. C. PEARSON, LINDA R. BROWN, JPL, California Inst. of Technol., Pasadena, CA 91109, USA; I. KLEINER, A. DEHAYEM, J. ORPHAL, P. CHELIN, LISA, Univ. Paris 12 & Paris 7 & CNRS, 94010 Créteil, France; R. L. SAMS, PNNL, Richland, WA 99352, USA.
Recently, interactions between vibrational polyads were studied for propyne, H3C-CCH; in particular those between v10 = 1 at 30 µm with the 15 µm dyad (v9 = 1, v10 = 2), as well as between that dyad and the 10 µm tetrad (v5 = 1, v9 = v10 = 1, v10 = 3, v8 = 1)b. Pronounced effects were caused by v10 = \pm 1, K = 0, l = \pm 3 Fermi-type resonances at K ~ 12. Such resonances had not been found thus far for the isoelectronic methyl cyanide, H3C-CN, molecule despite extensive previous spectroscopic work. As methyl cyanide is also an important interstellar molecule, in particular in hot and dense molecular cores, and as it may play a role in the atmospheres of planets or of Titan, we have recorded extensive rotational and rovibrational spectra up to 1.6 THz and 1500 cm-1, respectively. The present investigations focus on the v8 = 0, 1, and 2 states. The 8 mode in methyl cyanide corresponds to the 10 mode in propyne, and it is at a rather similar energy. While the infrared data pertaining to these states help to constrain their K level structures they do not reach K ~ 14 which are perturbed most; the l = 0 component of 2 8 may be an exception. The pure rotational data on the other hand access K levels well beyond these perturbations which can be easily recognized in the spectra. Since the 9 = (CCH) mode in propyne is missing in methyl cyanide one would expect easier assignments and analyses. However, besides v8 = \pm 1, K = 0, l = \pm 3 Fermi-type resonances around K of 14, additional fairly strong resonances occur at similar K values which are described by v8 = \pm 1, K = \mp 2, l = \pm 1. The latter type of resonance takes even place weakly between v = 0 and v8 = 1; an indication for this was seen previously.\footnoteM. Simecková et al. , J. Mol. Spectrosc. (2004) 123-126. The analyses of interactions between states with v8 \leq 2 have been largely completed. The results will be compared with those in propyne.