Infrared spectroscopy and ab initio potential energy surface for Ne-C2H2 and Ne-C2HD complexes

The rotationally resolved spectra of Ne-C2H2 and Ne-C2HD were measured in the region of the asymmetric C-H stretch (nu(3)) band of the acetylene monomer. The transitions in the Ne-C2H2 spectrum are substantially broadened by vibrational predissociation, while those of Ne-C2HD are quite narrow. This difference is attributed to the fact that in the former dissociation proceeds through a "'doorway'' state, related to a Fermi resonance involving the bending vibrations of C2H2. In C2HD this Fermi resonance is absent. The potential energy surface (PES) for the Ne-acetylene complex has been computed using symmetry-adapted perturbation theory. This PES has been fit to an analytic form and applied in calculations of the rovibrational energy levels of Ne-C2H2 and Ne-C2HD. From these levels and calculated transition intensities we generated the near-infrared spectra of these complexes in the region of the nu(3) band. These complexes may be considered as nearly free internal rotors. For Ne-C2H2 the results obtained from the ground state PES gave semiquantitative agreement with. the measured spectrum. For Ne-C2HD we could assign all of the (much sharper) lines in the experimental spectrum and obtain the nu(3) excited state interaction potential from a fit of the calculated spectrum to the experimental one. The ground state ab initio potential was not altered in this fit; the excellent agreement between the calculated and measured infrared spectrum for Ne-C2HD demonstrates that our Ne-acetylene potential is quite accurate. (C) 1998 American Institute of Physics, [S0021-9606(98)01644-4].