Exploring the weakest bond: Structure and ionization of the Helium Dimer, Trimer and the search for the Efimov State

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Molecules made of only two Helium atoms (Dimers, He2) and three atoms (Trimers, He3) are extremely fragile, floppy and diffuse quantum objects. The Helium dimer has the by far longest and weakest bond ever observed. It spreads over 100 times the atomic diameter. For the trimer an excited halo state, a so called Efimov state, is predicted but not yet found experimentally. The fascinating quantum nature of the dimer and trimer resulted in a large body of often controversial theoretical work. Experimentally so far only the existence of the dimer and ground state trimer and the mean value of the internuclear distance could be established. The current project suggests a completely new approach to study these objects: we will ionize them with synchrotron radiation, short laser pulses and free electron laser beams to explore their ionization dynamics, image their structure via Coulomb explosion imaging and search for the elusive Efimov state. These unique studies will become possible by putting the dimer and trimer in a COLTRIMS reaction microscope, a multiparticle imaging system, to which the applicant has made major contributions. We will obtain a direct image of the geometrical structure and internuclear distances of the ground state, allowing us to settle the theoretical controversies and providing the most sensitive and background free signal for a search for the Efimov state.