ACCURATE DETERMINATION OF THE FIRST IONIZATION-POTENTIAL OF ACTINIDES BY LASER SPECTROSCOPY

A new method is described for the precise determination of the first ionization potential of elements which are available only in small amounts such as the heavier actinides. It is based on resonance ionization mass spectroscopy (RIMS) in the presence of an external electric field. Extrapolation of the ionization thresholds obtained with different electric field strengths to field strength zero leads directly to the first ionization potential. With samples of 10(12) atoms of Np-237 and Am-243 experimental values for the first ionization potential of neptunium of IPNp = 6.2655(2) eV and of americium of IPAm = 5.9738(2) eV were obtained. This technique was also applied to thorium yielding a value of IPTh = 6.3067(2) eV. In addition the precision of the method was confirmed by the convergences of Rydberg series of americium measured by means of RIMS.