Rattle and hum

Neutron scattering experiments on endofullerenes containing either water or hydrogen molecules, carried out at the Institut Laue-Langevin (ILL), in Grenoble, France, have revealed the first known quantum selection rule in molecules, a law of nature that prevents certain transitions between different quantum states. The use of endofullerenes, referred to rather whimsically as baby rattle molecules, could take quantum studies of molecules out of their infancy by providing a unique testing ground for quantum theory. The experiments, undertaken by Mark Johnson and colleagues showed that various forbidden transitions were absent from the spectra, confirming the existence of a molecular selection rule.