Intermultiplet Transitions in Praseodymium Using Neutron Spectroscopy

Intermultiplet transitions with energies up to 810 meV in praseodymium metal have been observed by neutron spectroscopy. Both dipolar (${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}\to {}_{}{}^3{}_{}{}^{}H_5^{}{}_{}{}^{}$) and, for the first time, nondipolar, interterm (${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}\to {}_{}{}^3{}_{}{}^{}F_{2,3,4}^{}{}_{}{}^{}$) transitions have been identified. The energies of the interterm transitions are significantly reduced by approximately 50 meV from their free-ion values. This gives direct evidence of conduction-electron screening of the (on-site) Coulomb interaction between $4f$ electrons in rare-earth metals.