Actinide ionization potentials

Discussion of Actinide Ionization Potentials. The ionization potentials of actinides determined by laser techniques are given in Table IV together with values determined by surface ionization, appearance potential and semi-emperical methods. For uranium, all values are low compared with the values determined by laser techniques with the exception of the surface ionization value by Smith and Hertel.(z ) The spectroscopic values by Sugar jL were obtained from the 5f 7s - 5f 7s8s intervals interpolated from intervals known for the higher actinides. Except for Sugar s value, all the neptunium ionization potential values are low relative to the more accurate values determined by laser methods. The Rydberg series values are the preferred ionization potentials. 

At that time there were few other known actinide ionization potentials, although several sets of lanthanide potentials (/] through l ) had already been published. Sugar s values for /j and I2 are given in Table 17.4 (with more recent values of for U and Np). High-temperature properties of An ions have been calculated [354]. 

Among efforts to calculate actinide ionization potentials from first principles, the set of Carlson et al. [87] is often cited. However, their /j values are about 10 % smaller than Sugar s [85] and we have chosen to adjust them (see below). 

Quite recently. Sugar [93] derived new actinide 12 values that are cited in Table 17.4. As of the date of writing, the only independent 13 values are those of Carlson et al. [87], but since we note that Carlson s individual /, values are consistently smaller than spectroscopic values or thermochemical sums, the / entries in Table 17.4 are Carlson s values multiplied by a normalizing factor 1.055. A new set of actinide ionization potentials, especially for the interesting higher potentials, is included in Table 17.4 [346]. 

In contrast to the lanthanide 4f transition series, for which the normal oxidation state is +3 in aqueous solution and in solid compounds, the actinide elements up to, and including, americium exhibit oxidation states from +3 to +7 (Table 1), although the common oxidation state of americium and the following elements is +3, as in the lanthanides, apart from nobelium (Z = 102), for which the +2 state appears to be very stable with respect to oxidation in aqueous solution, presumably because of a high ionization potential for the 5/14 No2+ ion. Discussions of the thermodynamic factors responsible for the stability of the tripositive actinide ions with respect to oxidation or reduction are available.1,2... 

Moore, C. E. Ionization Potentials and Ionization Limits Derived from the Analyses of Optical Spectra, NSRDS-NBS 34 National Bureau of Standards Washington. DC, 1970, except for the data on the actinides, which are from The Chemistry ofthe Actinide Elements, Katz, J. J. Seaborg, G. T. Morss, L. R., Eds. Qiapman and Hall New York, 1986 Vol. 2. 

In spite of considerable similarities between the chemical properties of lanthanides and actinides, the trivalent oxidation state is not stable for the early members of the actinide series. Due to larger ionic radii and the presence of shielding electrons, the 5f electrons of actinides are subjected to a weaker attraction from the nuclear charge than the corresponding 4f electrons of lanthanides. The greater stability of tetrapositive ions of actinides such as Th and Pu is attributed to the smaller values of fourth ionization potential for 5f electrons compared to 4f electrons of lanthanides, an effect that has been observed in aqueous solution of Th and Ce (2). Thus, thorium... 

Relativistic effects on the valence electrons are already evident by comparing the electropositive character of Fr and Ra with that of their preceding homologues. The ionization potentials of both elements are not lower than those of their homologues Cs and Ba, respectively, as expected by extrapolation, but the ionization potential of Fr is about the same as that of Cs and the ionization potential of Ra is somewhat higher than that of Ba. The influence of relativistic effects on the properties of the actinides is evident also from the tendency of the heavier actinides to form lower oxidation states. For example, Es already prefers the oxidation state Es2+. 

Corrected on the first ionization potential, the elemental composition at cosmic ray sources as compared to the solar and local galactic composition exhibits the underabundance of H and He by a factor of 10 (if normalized to Fe). It also demonstrates higher Pt/Pb ratio by a factor of 5 and higher actinides abundance, the ratio (Z > 88)/(74 < Z < 87) by a factor of 3. These anomalies may be an indication that cosmic rays arise from supernova material (synthesized in the r-process) mixed with the interstellar gas. 

We will discuss the application of multistep laser excitation and ionization to determine the physical properties mentioned above in the lanthanides and actinides with emphasis on the determination of accurate ionization potentials. The discussion will point out how the laser techniques can circumvent many of the experimental obstacles that make these measurements difficult or impossible by conventional spectroscopy. The experimental apparatus and techniques described can be employed to measure all the properties and they are typical of the apparatus and techniques employed generally in multistep laser excitation and ionization. We do not claim completeness for literature cited, especially for laser techniques not involving photoionization detection. 

Ionization potentials of atoms are usually obtained by the determination of a photoionization threshold or more accurately by the observation of long Rydberg progressions. With the exception of a few of these elements with simple spectra, obtaining such measurements for lanthanides and actinides is difficult if not impossible by conventional spectroscopy. Therefore, very accurate ionization limits were not available for the majority of these elements.( 6)... 

Ionization potentials of 6.1941(5) eV for uranium i and 6.2657(6) eV for neptunium ) have been derived from observed Rydberg series using laser techniques and the method described above. These are the most accurate ionization potentials available for actinide elements. Series converging to the first excited state and to the ground state of the ion were observed for both elements. In the case of neptunium, the presence of two series converging to limits 24 cm - - apart (see Fig. 6) helps to confirm the unpublished value. ) for the interval between the two lowest levels of neptunium. 

Experimental values for heavier actinides where configuration interaction is less important (americium and higher) would be very valuable as they would yield the slope for the 5fN7s2 5fN7s ionization potentials for the second half of the series, N>7. This would allow the determination of extrapolated values for the ionization potentials of actinides beyond einsteinium where experimental values cannot be obtained because materials with sufficiently long half-lives are not available. 

The use of autoionizing Rydberg levels converging to excited states of the ion to determine ionization potentials has been discussed above. If autoionization resonances as narrow as those found in gadolinium exist in the actinides, it should be possible to determine the isotope shifts and hfs of such features. (isotope shifts for actinides range up to 0.4 cm l per mass unit and odd atomic number actinides exhibit hfs with total widths of 4 to 6 cm l and hfs component spacing of 0.2 cm l or more for some transitions). 

Techniques of stepwise laser excitation and photoionization have been applied to study spectroscopic properties of neutral atoms of lanthanides and actinides. The spectroscopic properties that can be determined include the ionization potential, energy levels, isotope shifts, hyperfine structure, lifetimes of energy levels, branching ratios and oscillator strengths. We discuss the laser methods used to obtain these properties (with emphasis on ionization potentials) and give examples of results obtained for each. The ionization potentials obtained by laser techniques are in eV Ce, 3.3387(4) ... 

Martin, W. C. Hagan, L. Reader, J. and Sugar, J., "Ground levels and ionization potentials for lanthanide and actinide atoms and ions," Jj Phys. Chem. Ref. Data, 1974, 771-780. 

Resonance ionization MS is a sensitive and accurate method for determining the ionization potential of atoms. Erdmann et al. (1998) recently reported improved measurements of the ionization potentials for 9 actinide elements by RIMS. 

Erdmann, N., Nunnemann, M., Eberhardt, K., Herrmann, G., Huber, G., Kohler, S., Kratz, J. V., Passler, G., Peterson, J. R., Trautmann, N., and Waldek, A. 1998. Determination of the first ionization potential of nine actinide elements by resonance ionization mass spectroscopy (RIMS). J Alloys Compd 271, 837-840. 

Figure 3.8 Data from Schmalzried, H. (1974) Solid State Reactions, Academic Press, New York, p. 109. Table 3.5 Data from Moore, C.E. (1970), Ionization Potentials and Ionization Limits Derived from the Analysis of Optical Spectra, NSRDS-NBS 34, National Bureau of Standards, Washington, D.C. Data on the actinides is from Seaborg, G.T (1968) Ann. Rev. Nucl. Sci. 18, 53 and references therein.

The ionization potential of an element is one of its fundamental properties. It is known that the first ionization potential of heavy elements depends on relativistic effects. The Mainz group, in Germany, systematically determined the first ionization potential of the actinide elements from Ac through Es using laser spectroscopy as shown in Table 18.12 (Becke et al. 2002). O Figure 18.24 shows the comparison of ionization potentials between lanthanide and actinide atoms (Moore 1971 Becke et al. 2002). The atomic level structure of Fm (2.7 x 10 ° atoms) with a half-life of 20.1 h was studied for the first time by the method of resonance ionization spectroscopy. Two atomic levels were identified at wave numbers (25,099.8 0.2) cm and (25,111.8 0.2) cm (Sewtz et al. 2003). 

First ionization potentials of actinide elements (Becke et al. 2002)... 

Optical resonance excitation fi-om the atomic ground state up to final ionization can follow a number of different pathways (O Fig. 54.7). Typical ionization potential is 6 eV for all the alkaline earths, rare earths, and actinides. Due to the high optical cross section (cr = l2n) of the order of 10 cm, all resonant optical excitation steps between bound atomic states (typical excited state hfetime of 10 s) can be saturated with continuous-wave as well as pulsed laser systems. Nomesonant ionization into the continuum has a relatively low cross section in the range of cm and is thus difficult to saturate with continuous-wave lasers. 

Hydration enthalpies have also been derived from a Born-Haber cycle (e.g., Morss 1971) for ions and An if all other terms are known. Recently, this approach has been updated (Schoebrechts et al. 1989) with new lanthanide ionization potentials and has been extended to the actinide ions using experimental results for the... 

At a time when little was known about ionization potentials of lanthanide ions as well as about thermochemistry of non-tripositive lanthanide speeies, Johnson (1969a) showed that differences in the third ionization potentials /j of the lanthanides are primarily responsible for many of their apparent oxidation-reduction anomalies. In a subsequent paper (Johnson 1969b) he compared and contrasted the relative stabilities of the di-, tri- and tetrapositive oxidation states of the lanthanides and actinides, pointing out how much less is the change in ionization potential for actinides than lanthanides at the half-filled shell (see fig. 4). He elaborated (Johnson 1974) on the first paper by systematizing the properties of the dipositive lanthanide ions in conjunction with those of the alkaline-earth metal ions. 

The above cycles were applied only to rare earths. A simplified cycle, such as the F(M) relationship, must be used for most actinides, since only the first ionization potentials have been derived from experiments for only a few actinides (Morss 1986). 

Alternatively, Bratsch and Lagowski (1985a, b, 1986) proposed an ionic model to calculate the thermodynamics of hydration AGj, A/fJ and ASj using standard thermochemical cycles. This model is based on the knowledge of the values of quantities such as the enthalpy of formation of the monoatomic gas [A/f (M )], the ionization potential sum for the oxidation state under consideration and the crystal ionic radius of the metal ion. This approach, however, is difficult to apply for the actinides since the ionization potentials are, for the most part, unavailable. To overcome this problem, the authors back-calculated an internally consistent set of thermochemical ionization potentials from selected thermodynamic data (Bratsch and Lagowski 1986). The general set of equations developed are ... 

Differences in lanthanide and actinide hydration thermodynamics have been discussed by Bratsch and Lagowski (1986) who attributed the difierences to relativistic effects in the actinides which cause changes in the energies of the s, p, d, and f orbitals. For example, the first and second ionization potentials of the electrons of the 7s state of the actinides are higher than those of the 6s state of the lanthanides whereas the third ionization potentials are similar for both families and the fourth ionization potential is lower for the actinides than the lanthanides. The small decrease in IP3 and IP4 for the f configuration in the actinides results in smooth variations in the relative stabilities of the adjacent oxidation states across the actinide series while the greater spatial extension of the 5f orbitals increases the actinide susceptibility to environmental efiects (Johnson 1982). 

A number of efforts have been made to calculate ionization-potential sums from thermochemical data and appropriate Born-Haber cycles. When an isostructural set of compounds is used, and covalence/repulsion corrections are made from a systematic lanthanide-actinide comparison, such sums can be quite reliable, as has been repeatedly demonstrated for the trivalent lanthanides [88]. For example, Morss [89] was able to estimate the sum of the first three ionization energies (/i +I2 + I3) for Pu as... 

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