First Determination of the Ionization Potential of Actinium and First Observation of Optical Transitions in Ferminm

Abstract

For the detennination of the first ionization potential of actinium, 227Ac was electrodeposited on a Ta backing and covered with ---1 J.!m Zr. From this filament, Ac atoms were evaporated at 2 1250°C. By resonant excitation with UV light of 388.67 nm and subsequent excitation with light of ca 568 nm, Ac was ionized in an external electrical field.By determining the ionization thresholds as a function of the electrical field strength and by extrapolation to zero field strength, the first ionization potential of 43398(3) cm-1 == 5.3807(3) eV was measured. About 1 ng of 255Fm, half life 20.1 h, was prepared at ORNL by milking from 255Es produced in the High Flux Isotope Reactor and shipped to Mainz. Here, a fraction of 2.7 . 10 10 atoms was electrodeposited on a Ta backing and covered with ---1 11m Ti. From this filament, Fm atoms were evaporated at~1000°C in the buffer gas of an optical cell and resonantly excited and ionized with two beams of an excimer-dye-Iaser combination. The ions were identified by mass-selective detection employing the Ion-Guide-Quadrupole-Mass Separation (IGQMS) technique. Two resonant transitions were found at energies of25099.8 cm-1 and 25111.8 cm-I •

Ion Mobility Measurements and Ion Chemical Reaction Studies at Heavy Elements in a Buffer Gas Cell

“…A drift time difference of (0.07 AE 0.02) ms was measured between plutonium and americium, at a total drift time of (1.88 AE 0.01) ms. With these numbers equation (3) yields a relative contraction of the ionic radius of americium compared to that of plutonium of (3.1 AE 1.3)%, taking r Ar = 191 pm [9] and r Pu = 196 pm (calculations from Desclaux [10]). The result is in accord with the contraction of the Cf þ relative to the Fm þ ion, which was measured in a previous experiment [1,5,6]. Relativistic calculations for the differences of the atomic radii predict a contraction of same magnitude.…”

“…To corroborate the effect of the actinide shrinkage as observed in our previous measurements on Cf þ and Fm þ [1,5,6], drift time measurements were performed for 243 Am and 239 Pu [7]. Am and Pu atoms were evaporated from an electrically-heated filament containing both elements, see Figure 1.…”

“…It was first demonstrated in [1] that such relativistic effects for heavy elements can also be studied by a measurement of ionic radii. These can be extracted from ion mobility measurements in inert buffer gases like argon.…”

Ultratrace analysis of long-lived radionuclides by resonance ionization mass spectrometry (RIMS)

“…4 explains how the ionization thresholds W th depend on the field strength F. By plotting the threshold energy as a function of F 1/2 and by extrapolation to zero field strength, the ionization potential IP is obtained as shown in Fig. 5 for actinium [2]. It has a value of 43,398 ± 3 cm -1 .…”

First successful ionization of Lr (Z = 103) by a surface-ionization technique

“…[1][2][3][4] Recently the atomic level structure of fermium (Fm, Z = 100) was also investigated by RIMS using a sample of 2.7 × 10 10 atoms of 255 Fm with a half-life (T 1/2 ) of 20.1 h. 5,6 The elements heavier than Fm, however, must be produced at accelerators using reactions of heavy ions with heavy target materials. Moreover, both half-lives and cross sections of the isotopes of the still heavier elements are rapidly decreasing.…”