Controversy on the first ionization potential of PuO2 (nearly) settled by new experimental evidence.
The first ionization potential of the PuO2 molecule was for a long time considered to be 4-5 eV higher than that of UO2. This feature could hardly be explained by the most advanced "ab initio" calculations, which, on the other hand, provide satisfactory results for other actinide oxides. From recent experiments, performed with different techniques, a lower ionization potential of approximately 7 eV was measured, in better agreement with the theoretical predictions. Our recent experiments, where thermally produced ions were measured, make it possible to formulate an accurate relation between the ionization potential of PuO2 and that of PuO: I0(PuO2) = I0(PuO) + 0.42 +/- 0.005 eV. The present uncertainty of I0(PuO) leads to the final assessment, 6.2 < or = I0(PuO2) < or = 6.6 eV, whereby the upper limit is more in line with the aforementioned recent measurements. Considering the still existing uncertainties, one can conclude that these results remove major doubts on the validity of the current theoretical predictions. However, the very small ionization cross section of PuO2 by low-energy electron collisions, which led to the previous spurious assessment of the ion appearance potential, has still an unexplained cause.
Capone
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European Commission, European Institute for Transuranium Elements, POB 2340, D 76125 Karlsruhe, Germany.
Colle
J Y
JY
Hiernaut
J P
JP
Ronchi
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