Lanthanides ionization potentials

Discussion of Lanthanide Ionization Potentials. A summary of accurate ionization potentials of the lanthanides is given in the last two columns of Table III. For comparison, values from electron impact and the semi-emperical spectroscopic values are given in columns 2 and 3. Although their uncertainties are much larger, the agreement is quite good. 

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... 

Cerium is strongly electropositive having a low ionization potential for the removal of the three most weakly bound electrons. The trivalent cerous ion [18923-26-7] Ce ", apart from its possible oxidation to Ce(IV), closely resembles, the other trivalent lanthanides in behavior. 

Let us now consider MMCT for the case in which the donating ion is a lanthanide ion with a partly filled 4/ shell M(/")M(d°)CT. The trivalent lanthanide ions with a low fourth ionization potential are Ce, Pr ", Tb ". Their optical absorption spectra show usually allowed 4f-5d transitions in the ultraviolet part of the spectrum [6, 35]. These are considered as MC transitions, although they will undoubtedly have a certain CT character due to the higher admixture of ligand orbitals into the d orbitals. In combination with M(d°) ions these M(/") ions show MMCT transitions. An early example has been given by Paul [36] for Ce(III)-Ti(IV) MMCT in borosilicate glasses. The absorption maximum was at about 30000 cm ... 

Symbol Nd atomic number 60 atomic weight 144.24 a rare earth lanthanide element a hght rare earth metal of cerium group an inner transition metal characterized by partially filled 4/ subshell electron configuration [Xe]4/35di6s2 most common valence state -i-3 other oxidation state +2 standard electrode potential, Nd + -i- 3e -2.323 V atomic radius 1.821 A (for CN 12) ionic radius, Nd + 0.995A atomic volume 20.60 cc/mol ionization potential 6.31 eV seven stable isotopes Nd-142 (27.13%), Nd-143 (12.20%), Nd-144 (23.87%), Nd-145 (8.29%), Nd-146 (17.18%), Nd-148 (5.72%), Nd-150 (5.60%) twenty-three radioisotopes are known in the mass range 127-141, 147, 149, 151-156. 

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 Potentiate and Ionization Limits Derived from the Analyses of Optical Spectra, NSRDS-NBS 34 National i Bureau of Standards Washington, DC 1970 and personal communication. Data for the lanthanides and actinides from Martin, W. C. . Hagan, L. Reader, J. Sugar, J.J.Phys. Chem.Ref. Data 1974,3.771 and Sugar J.J. Opt. Soc. Am. 1975,65,1366.. .., ... 

Deviations from this rule may occur for elements to the right of the lanthanides in the periodic table. Here the nuclear charge has increased much more than in the previous row of the periodic table, because of the additional lanthanide elements, and the ionization potentials of many of these elements are in fact higher than the potentials of their family members in preceding rows of the periodic table (compare this to the lanthanide contraction, p. 52). 

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... 

The Saha-Langmuir equation has been used to obtain both ionization potentials [25] and work functions [26]. Measuring ion beam intensities at several different temperatures and plotting their logarithms vs. 1/7" yield a straight line whose slope is ( - f)/k. If either or / is known, the other is readily calculated. Hertel introduced a method of measuring ionization potentials that was independent of the work function of the surface, using instead as reference an element of known ionization potential he applied it in the determination of the first ionization potentials of the lanthanide elements [27]. 

Deviations from regular smooth variation of properties of lanthanides occur at quarter-, half- and three-quarter filled 4/ configurations which have been attributed to tetrad effect. This effect has been attributed to small changes in Racah parameters when the ligands around the metal change during the reactions. The half-filled shell effect and the quarter-and three-quarter shell effects are caused by changes in El and 3 in the theoretical ionization potential expressions for /" ions [4],... 

Like copper, silver and gold have a single s electron outside the completed d shell, but in spite of the similarity in electronic structures and ionization potential, the chemistries of Ag, Au, and Cu differ more than might be expected. There are no simple explanations for many of the differences although some of the differences between Ag and Au may be traced to relativistic effects on the 6s electrons of the latter. The covalent radii of the triad follow the trend Cu < Ag Au, i.e., gold has about the same or a slightly smaller covalent radius than silver in comparable compounds, a phenomenon frequently referred to as relativistic contraction (c/. lanthanide contraction). 

Usually, the plot of Goldschmidt s ionic radii produced no inflection around the gadolinium region although Bommer 12) later supported Klemm s diad theory by plotting the cell constant (a) for the C-type lanthanide oxides. To support his diad theory Klemm has (13) also pointed out that while La(III) and Lu(III) possess empty (4 f°) and completely filled (4/14)/-shell respectively, Gd(III) has a half-filled shell (4/7), and that usually a break is observed around the half-filled shell. He plotted (13) the ionization potentials for the M - M+ reaction for the series B—Ne and Al-Ar and showed that a break does exist in the N—0 and P—S region. In Fig. 2 several plots are made using the newly acquired data (14). 

The ground terms and the ionization potentials (IP) for the processes in the case of the lanthanides are well documented (14). However, the thir ionization potentials... 

M2+(MIII) - M3+(MIV)] for the lanthanides excluding La, Ce, Pr and Yb are not known at present. Faktor and Hanks (77) have recently calculated, using a Born-Haber cycle, the third ionization potentials (I3) for all lanthanides except Pm. 

Fig. 36. Plots of the (a) observed second ionization potentials (Ij) (14) and (b) sum of second and third IP s against the L-values of the monovalent lanthanide ions (Mil).

Fig. 37. Plot of the estimated third ionization potentials (77) against the L-values of the divalent lanthanide ions (Mill). The observed values are shown as crosses and the postulated values for Pm and Er as open squares.

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)... 

The same arguments apply to the study of ionization thresholds. While some success has been possible for the elements with simpler electronic structure (ytterbium, europium, and thulium),(34>35) for the remainder of the lanthanides it is nearly an impossible task to unravel the spectra originating from the many populated metastable levels to accurately determine the ionization potential with confidence.(36)... 

Table III. Summary of Values are lanthanide first ionization potentials, in eV (1 eV = 8065.479 cm-1) ...

Figure 9. Normalized ionization potentials of the lanthanides plotted as a function of number of f electrons. Only the cerium and gadolinium points required normalization to the 4P6s2 4fN6s + e process (3).

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