  Resonances are among the clearest quantum mechanical signatures of scattering
processes. Previously, shape resonances and Feshbach resonances have been
observed in inelastic and reactive collisions involving atoms or diatomic
molecules. Structure in the integral cross section has been observed in a
handful of elastic collisions involving polyatomic molecules. The present paper
presents the observation of shape resonances in the reactive scattering of a
polyatomic molecule, NH3. A merged-beam study of the gas phase He((3)S1) + NH3
Penning ionization reaction dynamics is described in the collision energy range
3.3 $\mu$eV < E$_{coll}$ < 10 meV. In this energy range, the reaction rate is
governed by long-range attraction. Peaks in the integral cross section are
observed at collision energies of 1.8 meV and 7.3 meV and are assigned to $\ell
= 15,16$ and $\ell = 20,21$ partial wave resonances, respectively. The
experimental results are well reproduced by theoretical calculations with the
short-range reaction probability P$_{sr}$ = 0.035. No clear signature of the
orbiting resonances is visible in the branching ratio between NH3(+) and NH2(+)
formation.
