Atoms routinely emit light when electrons surrounding the nucleus jump to a lower energy level. Energy levels in the nucleus also vary, sometimes by as much as thousands or millions of electron volts. When an excited nucleus relaxes, it usually emits invisible gamma radiation.
Physicists George Irwin of Idaho State University in Pocatello and Kinney Kim of North Carolina Central University in Durham reported in the August 11 Physical Review Letters that they had discovered an element that was the exception to the rule. When Irwin and Kim excited Thorium-229 they found the nucleus' excited state was only 3.5 eV higher than the ground state. The energy was low enough so it excited only the outer electrons. That excitation emerged as green light.
The discovery, the scientists say, could open up studies of the interaction between an atom's nucleus and electrons.