Experimental search for scattering resonances in near cold molecular collisions

Abstract

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.

Keywords:

• scattering resonances
• partial waves
• near cold regime
• elastic collisions
• inelastic collisions
• reactive collisions
• crossed beams
• merged beams

Acknowledgements

The authors gratefully acknowledge the Agence Nationale de la Recherche (contracts ANR-BLAN-2006-0247 and ANR-12-BS05-0011-02) and the Conseil Régional d’Aquitaine (contract 2007.1221) for financial support.