THE CO-DIMER: NEW STATES AND TUNNELING SPLITTING

Author Institution: I. Physikalisches Institut, University of Cologne; I. Physikalisches Institut, Institute of SpectroscopyNew extensive millimeter-wave measurements of the $^{12}C^{16}O$ dimer have been made and more than 250 new spectral transitions have been observed in the frequency range of 80 - 135 GHz. Joint analysis of these and previous millimeter-wave data yielded in determination and precise location of 34 new energy levels of $A^{+}$ -symmetry and 21 levels of $A^{-}$-symmetry. Some of them belong to already known stacks and others make up 9 new stacks of the dimer. These new energy levels are located at energies from 8 to $18 cm^{-1}$ and in a free rotation limit they correspond to the states with $(j_{1}, j_{2}, K) = (1, 1, 0), (1, 1, 1), (1, 1, 2)$ and (2, 0, 2), where j is a rotational quantum numbers of the CO monomer and K is a projection of the total angular momentum on the intermolecular axis. One newly observed state of $A^{+}$ -symmetry originating at $12 cm^{-1}$ has $K = 0$ and might be tentatively attributed to the lower tunneling component of the stretching vibration of the CO-dimer. The tunneling splitting for many different states of two isotopic modifications of the dimer, $(^{12}C^{16}O)_{2}$ and $(^{13}C^{16}O)_{2}$ was determined, and its dependence on J-, K- values and on isotopic mass was studied. For some states the tunneling splitting increases in $(^{13}C^{16}O)_{2}$ as compared to $(^{12}C^{16}O)_{2})$. Possible explanations of this anomalous behavior will be discussed

MILLIMETER WAVE SPECTRUM OF THE 13C16O^{13}C^{16}O DIMER

References [1] A.R.W. McKellar, J. Chem. Phys. 115, 3571 (2001). [2] J. Tang, A.R.W. McKellar, L.A. Surin, D.N. Fourzikov, B.S. Dumesh, and G. Winnewisser, J. Mol. Spectrosc. 214, 87 (2002).Author Institution: I. Physikalisches Institut, Universit\""{a}t zu K\""{o}ln, Z\""{u}lpicher Str.; Steacie Institute for Molecular Sciences, National Research Council of CanadaThe millimeter wave spectrum of the isotopically substituted CO dimer, $(^{13}C^{16}O)_{2}$ 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 $(^{13}C^{16}O)_{2}$, 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 $(^{13}C^{16}O)_{2}$ to be $3.769 cm^{-1}$, slightly larger than the $(^{12}C^{16}O)_{2}$ 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

The millimeter wave spectrum of the 13C16O dimer

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. Eighty-seven transitions in the 77-180?GHz region have been assigned and analyzed in terms of a model-independent term value scheme involving 57 rotational levels with J=0-8. 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 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 of 3.731?cm-1. A large amount of precise experimental data is now available for the CO dimer, which should lead to greater theoretical insight into its structure and tunneling dynamics.NRC publication: Ye

Millimeter-wave spectra of the CO dimer : three new states and further evidence of distinct isomers

The millimeter-wave spectrum of the normal isotope of the CO dimer, (12C16O)2, has been systematically surveyed in the regions 75-105 and 131-174 GHz, with additional measurements covering the entire 60-176 GHz range. By combining these results and using the technique of combination differences based on previously known energy levels, 14 new rotational levels have been assigned and precisely ([approximate]0.1 MHz) located. They belong to 3 completely new states, 1 with A+ symmetry and 2 with A- symmetry. The position of the lowest energy A- state results in a new and lower value for the effective tunneling splitting of the CO dimer, 3.73 cm-1. The observation of dramatically different intensities for different bands supports the concept of two isomeric forms for (CO)2, the ground state having a larger intermolecular separation ([approximate]4.4 ?) with most likely a C-bonded configuration, and the low-lying (0.88 cm-1) excited state having a smaller separation ([approximate]4.0 ?) and an O-bonded geometry.NRC publication: Ye