Physical Review LettersUltrahigh-Resolution Spectroscopy: Photon Echoes in Yalo3 : Pr3+ and La F3 : Pr3+ .R. M. Macfarlane, Robert M. Shelby, R. L. ShoemakerPublished Jan 14, 1980• 0 citationsPublisherDOI10.1103/physrevlett.44.110