Experimental evidence for the existence of two neighboring Pr I energy levels for which J is not a good quantum number

We report the discovery of two previously unknown energy levels of neutral praseodymium (Pr I) having odd parity, found during systematic laser spectroscopic recordings of the Pr I spectrum. From unusual intensity ratios of the observed hyperfine structure components we have to conclude that for both levels the quantum number J is not a good quantum number. J is a mixture of values 15/2 and 13/2, and the observed hf patterns of the investigated transitions depend strongly on the combining upper energy level. Moreover, the levels have a metastable character; no transitions to lower lying even energy levels could be found. The intensity distribution within the transition on which the levels were discovered can be roughly explained by a model which assumes transition probability transfer between levels having J = 15/2 and J = 13/2.