  Whereas collisions between atoms and molecules are largely understood,
collisions between two molecules have proven much harder to study. In both
experiment and theory, our ability to determine quantum state-resolved
bimolecular cross sections lags behind their atom-molecule counterparts by
decades. For many bimolecular systems, even rules of thumb -- much less
intuitive understanding -- of scattering cross sections are lacking. Here, we
report the measurement of state-to-state differential cross sections on the
collision of state-selected and velocity-controlled nitric oxide (NO) radicals
and oxygen (O2) molecules. Using velocity map imaging of the scattered NO
radicals, the full product-pair correlations of rotational excitation that
occurs in both collision partners from individual encounters are revealed. The
correlated cross sections show surprisingly good agreement with quantum
scattering calculations using ab initio NO-O2 potential energy surfaces. The
observations show that the well-known energy-gap law that governs atom-molecule
collisions does not generally apply to bimolecular excitation processes, and
reveal a propensity rule for the vector correlation of product angular momenta.
