How to study the elusive Efimov state of the He-4(3) molecule through a new atom-optical state-selection technique

Excited states and excitation energies of weakly bound systems, e.g., atomi c few-body systems and clusters, are difficult to study experimentally. For this purpose we propose a new and very general atom-optical method which i s based on inelastic diffraction from transmission gratings. The technique is applicable to the recently found helium trimer molecule He-4(3), allowin g for the first time an investigation of the possible existence of an excit ed trimer state and determination of its excitation energy. This would be o f fundamental importance for the famous Efimov effect.