Rovibrational spectroscopy of the v Ä 6 manifold in 12 C 2 H 2 and 13 C 2 H 2

We recorded rovibrational spectra of the 006 level of C2H2 and the 2131 1 level of C2H2 in the ground electronic state using a two-photon sequential double resonance technique with a resolution of 15 MHz. Owing to the g/u symmetry of acetylene, the levels that we observe are inaccessible from the ground state by single photon techniques, and observation of these levels is reported here for the first time. Upper state rotational constants were derived from whole band fits of the observed lines, and compare favorably with expected values. Both spectra exhibit signs of local perturbations, and a density of states analysis leads us to believe that we are observing couplings to the full density of vibrational states one would expect from acetylene in this energy region. Despite the high resolution of our spectrometer, and the high excitation energy, no evidence for acetylene hydrogen permutation exchange isomerization ~which is predicted to proceed through the vinylidene minimum on the potential! has been observed, implying that the rate of exchange isomerization is more than four orders-of-magnitude below the rate predicted by RRKM ~Rice, Ramsperger, Kassel, and Marcus! theory. © 2000 American Institute of Physics. @S0021-9606~00!01640-8#