First ionization potentials, lanthanide

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]. 

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

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 lanthanide and actinide atoms (Moore 1971 Becke et al. 2002)... 

Recently, it has also been shown that the addition of a CPP to 4f-in-core PPs of lanthanide atoms leads to a significant improvement of the atomic first and second ionization potentials [95]. A comparison of calculations without and with a CPP is given in Fig. 15. The somewhat larger deviations for, e.g., La, Ce, Gd and Lu, arise partly from the neglect of spin-orbit corrections. The 4f-in-core PP-bCPP approach also leads to slight overall improvements of the molecular constants for lanthanide diatomics. The explicit correlation of the 4f shell in small-core PP calculations is quite tedious and does not lead to significantly better results [231,232]. 

Figure 15. First (IPj) and second (IP2) ionization potentials of the lanthanide elements j La -2jLu. Experimental values are compared to results from 4f-in-core pseudopotential (PP) calculations with and without account of core-valence correlation effects by means of a core polarization potential (CPP) [95].

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. 

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... 

In general, ionization potential decreases while going down the group. Therefore, ionization potential of the elments of second transition series have lower values than those of the elements of first transition series as expected. However, ionization potentials of the elements of third transition series except lanthanum have higher values of ionization potentials due to lanthanide contraction. The atomic radii of the elements of second and third transition series are almost same but atomic numbers differ by 32. Thus, the outer electrons are firmly attached to the nucleus and ionization potential values are very hi. On accoimt of this, the elements of third transition series are almost inert under ordinary conditions. 

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