Lanthanide photoionization

As derived from Fig. 4, we plot next in Fig. 5 the augmentation in the /" ground-state energy (relative to the external reference) which arises between successive elements. This is what in Fig. 1 was made to serve as one-electron-like energy for the electron of configuration/ , to describe processes involving /-electron loss such as photoionization, or intracation / - d transitions. This seems feasible because both the p- and d-bands and the work function are all relatively invariant in energy across any particular series of lanthanide compounds. 

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 Rydberg series and photoionization thresholds obtained have permitted the accurate determination of ionization limits for uranium, (1) neptunium, and ten lanthanides. Q)... 

Oscillator strengths or absorption cross sections may be obtained by applying saturation spectroscopy techniques to multistep photoionization spectroscopy. A few transitions in uranium have been studied.One of the advantages of saturation spectroscopy is that it can be applied to any one of the steps in the schemes shown in Fig. 2. The disadvantages are that the experimental requirements are severe (laser-atomic beam interaction area,-frequency,-band width and-polarization) and interpertation of the data can be complex. A detailed discussion will not be given because little application has been made to the lanthanides and actinides. We will discuss in the Autoionization section the determination of photoionization cross sections by a saturation method. 

In this review of multistep laser photoionization of the lanthanides and actinides, we hope that we have introduced the reader to a number of laser techniques for determining spectroscopic properties of these elements. We have undoubtedly overlooked some techniques and some papers on the subjects we did cover. The importance of laser methods in studying the spectroscopy of the lanthanides and actinides is well established and future applications should greatly expand our knowledge of these elements. 

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) ... 

To understand the origin of low-energy satellites in the 3d spectra of light lanthanides, their implication for a dynamical picture of core level photoionization in the presence of semilocalized screening orbitals to establish their relation to the ground state electronic structure, see fig. 1 (Wertheim and Campagna 1978). 

The study of Rydberg spectra and ionization thresholds of ten lanthanides and actinides has been reported by PAISNER et al. [8.80]. In these experiments, high-lying states were accessed by time-resolved stepwise excitation using pulsed dye lasers tuned to resonant transitions. Atoms excited to levels within 1000 cm of the ionization limit were then photoionized by 10.6 ym radiation from a pulsed CO2 laser. The measurements allowed the accurate determination of ionization thresholds from Rydberg convergences to within 0.0005 eV. 

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