Electronic-shell-structure effects inCs n +
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Abstract:
Relative mass abundances of singly charged cesium-cluster ions (${\mathrm{Cs}}_{\mathit{n}}^{+}$) extracted from a liquid-metal ion source show sharp decremental steps at n=3, 5, 9, and 21 and a pronounced minimum at ${\mathrm{Cs}}_{10}^{+}$. In the region of n>9, ${\mathrm{Cs}}_{13}^{+}$ shows a maximum in the relative abundance. The time-of-flight technique is used to obtain the mass spectrum. The prominent features observed at n=9 and 21 are interpreted using the shell structure of simple metal clusters. ${\mathrm{Cs}}_{9}^{+}$ and ${\mathrm{Cs}}_{21}^{+}$, which contain 8 and 20 valence electrons, respectively, form closed-shell configurations with enhanced relative stabilities.Keywords:
Valence electron
Caesium
Electron shell
The energies and intensities of the $2^{3}P_{1}\ensuremath{\rightarrow}1^{1}S_{0}$, and $2^{1}P_{1}\ensuremath{\rightarrow}1^{1}S_{0}$ transitions in He-like sulfur ions, and of the $2^{2}P\ensuremath{\rightarrow}1^{2}S$ transition in H-like sulfur ions have been studied as a function of the thickness and electron density of the solid through which the ions travel. The thickness dependence of the x-ray intensities was analyzed in terms of a three-component model description of $K$-shell vacancy production and decay. Cross sections for electron excitation or ionization and capture deduced from this analysis were used to establish the energies of the x-ray peaks for complete emission in vacuum (i.e., outside the target). Energy shifts were obtained by comparing the peak energies for emission in thick targets to those for emission in vacuum. The results show that the energy shifts increase approximately linearly with the square root of the valence electron density of the target and are in good agreement with theoretical expectations.
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Valence shell electronic energies EN of atoms and ions, interpreted as multiplet averages, can be calculated accurately within the framework of a theory developed in terms of nuclear–electronic interaction energies Vne. The results point to a constant EN/Vne ratio, both for neutral atoms and charged species.
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Abstract A review article is presented relating to the concept of valence‐change in the mass spectra of metal‐containing compounds. It is found that the modes of ion dissociation in these spectra are markedly dependent on the oxidation states normally assumed by the metal concerned and it is postulated that electron‐transfer may be possible between the complexed metal atom and its ligands in the ion, such that the odd‐ or even‐electron character of the ion is inter‐changeable. Ion reactions such as the consecutive loss of two radicals are normally of low probability in the mass spectra of organic compounds, but are often observed in the mass spectra of metal‐containing compounds and can be rationalized in terms of the valence‐change concept. Convincing evidence for valence‐change in some spectra is provided by the occurrence of reactions leading to the bare metal ion, or to the loss of neutral fragments containing the metal atom in a lower oxidation state than in the precursor molecule. Further applications of the concept may be found in the rationalization of the mass spectra of inorganic and organometallic compounds.
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Here we report that the odd electron z•-type ions formed by the electron-based peptide dissociation methods (electron capture or transfer, ECD or ETD) have distinctive chemical compositions from other common product ion types. Specifically, b-, c-, and y-type ions have an odd number of atoms with an odd valence (e.g., N and H), while z•-type ions contain an even number of atoms with an odd valence. This tenet, referred to as the valence parity rule, mandates that no c-type ion shall have the same chemical composition, and by extension mass, as a z•-type ion. By experiment we demonstrate that nearly half of all observed c- and z•-type product ions resulting from 226 ETD product ion spectra can be assigned to a single, correct, chemical composition and ion type by simple inspection of the m/z peaks. The assignments provide (1) a platform to directly determine amino acid composition, (2) an input for database search algorithms, or (3) a basis for de novo sequence analysis.
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Unequivocal evidence for significant target outer-shell excitation accompanying multiple-electron capture, in slow collisions of highly charged ions with many-electron atoms, has been obtained by means of simultaneous Auger-electron and cold-target recoil-ion momentum spectroscopic measurements. For the 28 keV ${}^{15}{\mathrm{N}}^{7+}+\mathrm{Ar}$ collision system, it is found that target excitation accompanies about $40%$ of all double-electron capture collisions. The evidence supports the predictions of the molecular classical overbarrier model by Niehaus [A. Niehaus, J. Phys. B 19, 2925 (1986)].
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Recoil
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We present calculations of the electronic stopping power for hydrogen molecular ions ${\mathrm{H}}_{2}^{+}$ in solids. These calculations include contributions from both valence electrons and inner-shell electrons of target atoms with the linear dielectric theory and the local-density approximation method. The screened Coulomb potential is used to describe the repulsion process of the two protons. The values of the stopping-power ratio Q for ${\mathrm{H}}_{2}^{+}$ at the high velocities are decreased, obviously due to the contributions of the inner-shell electrons. The theoretical results are compared with some experimental data.
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The correlation energies of the 1s22s2 inner cores of the first-row atoms B, C, ···, Ne are found to be very different from those of the corresponding four-electron Be-like ions, B+ to Ne6+, due to the exclusion effects of the outer 2p electrons. Whereas the 2s2 correlation, ε(2s2), in the 1s22s2 ions increases from —1.13 eV in Be to —3.2 eV in Ne6+, the 2s2 correlation in the neutral atoms decreases from —1.13 eV in Be to —0.27 eV in Ne. The many-electron theory was used for the nonempirical 2s2 calculations and included the use of the r12 coordinate. With these theoretical ε(2s2) values the correlation of a 2p electron with the 1s22s2 inner core is found to be large, ∼—1 eV. Also the 2p2 correlation comes out about —1 eV. The results show that core energies will, in general, depend strongly on the state and number of the outer-shell electrons and that intershell correlation interactions may be appreciable. Implications for π-electron systems and the ligand-field theory of inorganic complexes are discussed.
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We have measured the total cross section for photodetachment of the Na- ion over the photon energy range 30-51 eV. Electron detachment leads predominantly to the production of Na+ ions in this energy range. The structures in the measured cross section are associated with correlated processes involving the detachment or excitation of a 2p core electron, processes which are often accompanied by the excitation of one or more valence electrons. The most prominent feature in the cross section is a strong resonance associated with the excitation of a 2p electron from the core and a 3s valence electron. As in previous experiments on double excitation of valence electrons, electron correlation is seen to play an important role in the dynamics of negative ions.
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