Knowledge Bank

A spectrum of the carbon dioxide trimer van der Waals species has been recorded near $3613cm-1$ at sub-Doppler resolution using an optothermal (bolometer-detected) molecular-beam color-center laser spectrometer. The trimer transitions are distinguished from the dimer lines in the same region by their pressure and concentration dependence. The trimer spectrum is that of an oblate symmetric top with missing $K=3N\pm 1$ subbands due to the vanishing statistical weights arising from the off-axis zero nuclear spin $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}O$ atoms. A precise fit of the low temperature $(T\sim 1.3K)$ spectrum is obtained with a planar, cyclic structure of $C3h$ symmetry with $B\prime \prime =0.0398088(47)cm-1$ and a carbon-carbon spacing of $4.0382(3)\backslash AA$, No tunneling or perturbations are observed. The angle of the monomer axes away from a tangential $D3h$ structure is estimated to be $-43\circ$ from considerations of the electrostatic quadrupole-quadrupole potential and the van der Waals radil.