Stretch and relax! - Losing 1 electron switches magnetism on in dichromium
HZB
Dramatic effect on magnetism
The team studied the effect of x-ray magnetic circular dichroism on free Cr2+ ions that were stored at 18 K in a dedicated cryogenic ion trap. This effect gives direct experimental insight into spin coupling and localization of the relevant valence electrons. For their experiments, the scientists used the unique Nanocluster Trap experimental station that is available at beamline UE52-PGM of the BESSY II synchrotron radiation source at Helmholtz-Zentrum Berlin.
Localisation of ten valence electrons
Even though only one out of twelve electrons is removed when ionizing Cr2 the molecule reacts dramatically, with complete localization of all ten 3d electrons and with maximum spin coupling. This turns an archetypal antiferromagnet ferromagnetic. "It's a dramatic effect we see," says team leader Tobias Lau. "Its particular spin configuration can be interpreted as a result of indirect exchange coupling, where the two groups of localized electrons "talk" to each other via a single bonding electron as a messenger that controls the parallel alignment of all their spins," says Vicente Zamudio-Bayer who conducted this work as part of his PhD thesis at HZB and TU Berlin and who now continues his research as a postdoc in the Freiburg group.
Almost the same bonding energy
While in the neutral molecule all twelve valence electrons participate in bonding and create a short, unusual sextuple bond, the cation is only bound by one single electron with an almost doubled bond distance but almost the same bond energy. These significantly different bonding situations illustrate the fragile and untypically weak multiple bond in dichromium. They can be visualized as a change from a short and tight multiple bond to which all valence electrons contribute, to a long and loose single bond with all electrons except one localized at both ends. Combining their new results with earlier findings, the scientists can now even give relative energies of the excited states that have caused much confusion in the correct description of this molecular ion, a fact that will facilitate future theoretical approaches.
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