  Low energy reaction dynamics can strongly depend on the internal structure of
the reactants. The role of rotationally inelastic processes in cold collisions
involving polyatomic molecules has not been explored so far. Here we address
this problem performing a merged-beam study of the He*+CHF$_3$ Penning
ionization reaction in a range of collision energies $E/k_B$=0.5--120 K. The
experimental cross sections are compared with total reaction cross sections
calculated within the framework of the quantum defect theory. We find that the
broad range of collision energies combined with the relatively small rotational
constants of \chfs makes rotationally inelastic collisions a crucial player in
the total reaction dynamics. Quantitative agreement between theory and
experiment is only obtained if the energy-dependent probability for rotational
excitation is included in the calculations, in stark contrast to previous
experiments where classical scaling laws were able to describe the results.
