15min:

L. A. SURIN, D. FOURZIKOV, G. WINNEWISSER, I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany; JIAN TANG AND A. R. W. MCKELLAR, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada.

The millimeter wave spectrum of the isotopically substituted CO dimer, (¹³C¹⁶O)₂ has been studied for the first time, confirming and extending a recent infrared study [1]. A total of 87 transitions in the 77 - 180 GHz region have been assigned and analyzed with a model-independent term value scheme involving 57 rotational levels with J = 0 to 8. The levels can be classified into 7 ``stacks" which have symmetry classifications of either A⁺/B^- or A^-/B⁺, and K-values of either 0 or 1. For the normal isotope, symmetry and nuclear spin statistics cause alternate rotational levels to be missing, but for (¹³C¹⁶O)₂, all levels are present with an intensity alternation of 1:3 between the A and B symmetries. The four A⁺/B^- stacks have not previously been observed, and the lowest of them establishes the tunneling splitting of (¹³C¹⁶O)₂ to be 3.769 cm^-1, slightly larger than the (¹²C¹⁶O)₂ value [2] of 3.731 cm^-1. Even though a considerable amount of precise experimental data is now available for the CO dimer, we still have little theoretical insight into its structure and tunneling dynamics.

\vskip 0.3 truecm \centerline References \parindent=0pt

[1] A.R.W. McKellar, J. Chem. Phys. \textbf115, 3571 (2001).

[2] J. Tang, A.R.W. McKellar, L.A. Surin, D.N. Fourzikov, B.S. Dumesh, and G. Winnewisser, J. Mol. Spectrosc. \textbf214, 87 (2002).