Rovibrational spectroscopy of the v=6 manifold in (C2H2)-C-12 and (C2H2)-C-13

We recorded rovibrational spectra of the 006(+) level of (C2H2)-C-12 and th e 2131 (1)1(-1) level of (C2H2)-C-13 in the ground electronic state using a two-photon sequential double resonance technique with a resolution of 15 M Hz. Owing to the g/u symmetry of acetylene, the levels that we observe are inaccessible from the ground state by single photon techniques, and observa tion of these levels is reported here for the first time. Upper state rotat ional constants were derived from whole band fits of the observed lines, an d compare favorably with expected values. Both spectra exhibit signs of loc al perturbations, and a density of states analysis leads us to believe that we are observing couplings to the full density of vibrational states one w ould expect from acetylene in this energy region. Despite the high resoluti on of our spectrometer, and the high excitation energy, no evidence for ace tylene hydrogen permutation exchange isomerization (which is predicted to p roceed through the vinylidene minimum on the potential) has been observed, implying that the rate of exchange isomerization is more than four orders-o f-magnitude below the rate predicted by RRKM (Rice, Ramsperger, Kassel, and Marcus) theory. (C) 2000 American Institute of Physics. [S0021-9606(00)016 40-8].