Knowledge of rotational energy transfer (RET) involving carbon monoxide (CO) molecules is crucial for the interpretation of astrophysical data. As of now, our nearly perfect understanding of atom-molecule scattering shows that RET usually occurs by only a simple “bump” between partners. To advance molecular dynamics to the next step in complexity, we studied molecule-molecule scattering in great detail for collision between two CO molecules. Using advanced imaging methods and quasi-classical and fully quantum theory, we found that a synchronous movement can occur during CO-CO collisions, whereby a bump is followed by a move similar to a “do-si-do” in square dancing. This resulted in little angular deflection but high RET to both partners, a very unusual combination. The associated conditions suggest that this process can occur in other molecule-molecule systems.

Collision between two carbon monoxide (CO) molecules at a collision energy of 1460 cm^-1 has been investigated by measuring velocity-map ion images for rotationally excited CO molecules using a crossed molecular beam apparatus in combination with a VUV (Vacuum Ultra-Violet) REMPI (Resonance Enhanced Multi Photon Ionization) scheme. Partially resolved pair-correlated differential cross sections (PC-DCSs) were extracted for analysis of the radius-dependent angular distribution of the image using an onion-peeling inversion procedure.  The experimental results of this analysis were compared with advanced theory, in particular with the predictions of QCT (Quasi-Classical Trajectory) calculations, and good agreement between experiment and theory was found. An extraordinary energy transfer channel is revealed, featuring forward scattering while both CO product molecules are highly rotationally excited, which defies prediction from prevailing models of inelastic scattering. An extraordinary energy transfer channel is revealed, featuring forward scattering while both CO product molecules are highly rotationally excited, which defies prediction from prevailing models of inelastic scattering.  Simulation by quasi-classical trajectories on an accurate intermolecular potential energy surface and opacity function analysis show that these unusual product events start with the large C ends of CO colliding with each other, with immediate and large transfer from translational to rotational energy.  This forward-scattered-symmetric-excitation behaviour was also cinfirmed by Quantum Close-Coupling calculations.