Knowledge Bank

New extensive millimeter-wave measurements of the $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C16O$ 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 $18cm-1$ and in a free rotation limit they correspond to the states with $(j1,j2,K)=(1,1,0),(1,1,1),(1,1,2)$ 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 $12cm-1$ 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, $(12C16O)2$ and $(13C16O)2$ was determined, and its dependence on J-, K- values and on isotopic mass was studied. For some states the tunneling splitting increases in $(13C16O)2$ as compared to $(12C16O)2)$. Possible explanations of this anomalous behavior will be discussed.