Experimental search for scattering resonances in near cold molecular collisions

We review the experimental findings on scattering resonances in molecular collisions occurring in the energy domain equivalent to temperatures of a few Kelvin. We discuss the conditions required to observe shape (or orbiting) and Feshbach resonances, the crucial factor being in all cases the number of partial waves involved in the process. This implies operating with supersonic beams with matched and low velocities, high velocity resolution and low beam-intersection angle. This is exemplified for elastic, inelastic and reactive collisions, where different strategies, thence experimental arrangements, are used. We review the first observation of resonances in elastic collisions from the 1970s and the recent results in inelastic and reactive collisions. The most striking result is obtained for O2 + para-H2 inelastic collisions for which rotational excitation occurs through a pure quantum process, yielding a well-resolved structure in the integral cross section assigned to definite well-separated partial waves.