Towards the elusive Efimov state of the ^4He_3 molecule through a new   atom-optical state-selection technique
  Excited states and excitation energies of weakly bound systems, e.g. atomic few-body systems and clusters, are difficult to study experimentally. For this purpose we propose a new and very general atom-optical method which is based on inelastic diffraction from transmission gratings. The technique is applicable to the recently found helium trimer molecule ^4He_3, allowing for the first time an investigation of the possible existence of an excited trimer state and determination of its excitation energy. This would be of fundamental importance for the famous Efimov effect. 
Gerhard C. Hegerfeldt and Thorsten Koehler