Suzana R. Veličković

Formation and ionization energies of small chlorine-doped lithium clusters by thermal ionization mass spectrometry

RATIONALE Theoretical calculations have shown that the first ionization energy of clusters of the type Li(n) Cl (n ≥2), with more than eight valent electrons, is lower than that of alkali metal atoms; hence they are named superalkali. Superalkali clusters can mimic the chemical behavior of alkali metals and may be used as building blocks of new cluster-assembled materials. There is currently no reliable experimental proof of this kind of clusters and such proof is required. METHODS The Li(n) Cl (n = 2-6) clusters were produced by a thermal ionization source of modified design, and mass selected by a magnetic-sector mass spectrometer. The modification pertains to the replacement of the side filaments by a cylinder in the triple-filament thermal ionization source. The sample, which is LiCl salt, was pressed into a ring and placed on the inner wall of the cylinder. RESULTS It was observed that the ions of clusters with an even number of lithium atoms (Li(2) Cl(+) , Li(4) Cl(+) , Li(6) Cl(+) ) are more stable than the odd-numbered ones (Li(5) Cl(+) , Li(3) Cl(+) ). The ionization energies were determined to be 3.98 ± 0.25 eV for Li(2) Cl, 4.10 ± 0.25 eV for Li(3) Cl, 3.90 ± 0.25 eV for Li(4) Cl, 4.01 ± 0.25 eV for Li(5) Cl, and 4.09 ± 0.25 eV for Li(6) Cl. The presence of a halogen atom reduces the ionization energy of the metal clusters. CONCLUSIONS The thermal ionization source of modified design presents a suitable simple way to obtaining and measuring the ionization energies of very small lithium monochloride clusters. Clusters Li(n) Cl, n = 4 to 6, were detected for the first time.

Formation of positive cluster ions LinBr (n = 2–7) and ionization energies studied by thermal ionization mass spectrometry

Clusters of the type Li(n)X (X = halides) can be considered as potential building blocks of cluster-assembly materials. In this work, Li(n)Br (n = 2-7) clusters were obtained by a thermal ionization source of modified design and selected by a magnetic sector mass spectrometer. Positive ions of the Li(n)Br (n = 4-7) cluster were detected for the first time. The order of ion intensities was Li(2)Br(+) > Li(4)Br(+) > Li(5)Br(+) > Li(6)Br(+) > Li(3)Br(+). The ionization energies (IEs) were measured and found to be 3.95 ± 0.20 eV for Li(2)Br, 3.92 ± 0.20 eV for Li(3)Br, 3.93 ± 0.20 eV for Li(4)Br, 4.08 ± 0.20 eV for Li(5)Br, 4.14 ± 0.20 eV for Li(6)Br and 4.19 ± 0.20 eV for Li(7)Br. All of these clusters have a much lower ionization potential than that of the lithium atom, so they belong to the superalkali class. The IEs of Li(n)Br (n = 2-4) are slightly lower than those in the corresponding small Li(n) or Li(n)H clusters, whereas the IEs of Li(n)Br are very similar to those of Li(n) or Li(n)H for n = 5 and 6. The thermal ionization source of modified design is an important means for simultaneously obtaining and measuring the IEs of Li(n)Br (n = 2-7) clusters (because their ions are hermodynamically stable with respect to the loss of lithium atoms in the gas phase) and increasingly contributes toward the development of clusters for practical applications.

Ionization energies of K2X (X = F, Cl, Br, I) clusters

The electronic structure and properties of dipotassiummonohalides are important for understanding the unique physical and chemical behavior of M(n)X systems. In the present study, K(2) X (here X=F, Cl, Br, I) clusters were generated in the vapor over salts of the corresponding potassium halide, using a magnetic sector thermal ionization mass spectrometer. The ionization energies obtained for K(2)F, K(2)Cl, K(2)Br, and K(2)I molecules were 3.82 ± 0.1 eV, 3.68 ± 0.1 eV, 3.95 ± 0.1 eV, and 3.92 ± 0.1, respectively. These experimental values of ionization energies for K(2) X (X=F, Br, and I) are presented for the first time. The ionization energy of K(2)Cl determined by thermal ionization corresponds to previous results obtained by photoionization mass spectrometry, and it agrees with the theoretical ionization energy calculated by the ab initio method. The presently obtained results support previous theoretical predictions that the excess electron in dipotassiummonohalide clusters is delocalized over two potassium atoms, which is characteristic for F-center clusters.

Mass spectrometric study of the structures and ionization potential of LinI (n = 2, 4, 6) clusters

We report a combined experimental and theoretical investigation of small heterogeneous clusters of lithium iodide. Apparatus based on magnetic sector instrument, where Knudsen cell is placed into the ionization chamber, provides suitable conditions for the detection of both the ionic and neutral components of LinI (n = 2, 4, 6) clusters. The clusters Li4I and Li6I were detected experimentally for the first time. Ionization potentials determined by the electron impact ionization mass spectrometry were (4.69 ± 0.25) eV for Li2I, (4.86 ± 0.25) eV for Li4I and (4.96 ± 0.25) eV for Li6I. The ionization potential of Li2I corresponds to previous experimental data obtained by thermal ionization mass spectrometry. The first theoretical data of the ionization potential and structure of the above mentioned clusters are presented in this work. A comparison with the experimental ionization potentials provides evidence for the theoretically calculated geometrical structures of the small heterogeneous clusters of lithium iodide.

Characterization of the Ag43Cu37Zn20 alloy surface after potentiostatic polarization using LDI-TOF mass spectrometry

Abstract This work is the first to demonstrate the possibilities of direct analysis of the corrosion film formed on Ag43Cu37Zn20 alloy using laser desorption/ionization (LDI) on a commercial matrix-assisted LDI time of flight mass spectrometry (MS) instrument. In the LDI mass spectra measured from the water solution of the corrosion film, the ion species Zn+, Zn(H2O)+, AgH+, Cu2H+, Cu2O(OH)+, Cu3O(OH)+ (high intensity), and Cu(OH)(H2O)+, ZnCl(OH)(H2O)+, AgCl+, CuCl+ (low intensity) were identified. The LDI mass spectra of the propan-2-ol solution of the corrosion film contains the ions Zn5Cl(OH)3(H2O)11+, Zn5Cl2(OH)8(H2O)6+, and Zn5Cl2(OH)(H2O)14+. Mass spectra were observed at laser energies in the range of 1340–1860 arbitrary units in positive ion mode. Previously, Raman spectra and X-ray diffraction (XRD) analysis of the same anodic film have shown similar results as LDI MS – that the anodic film contains the mixture of Ag, Cu2O, CuCl, Zn5Cl2(OH)8·H2O, and β-Zn(OH)Cl. LDI MS could be a complementary method for XRD and Raman, as demonstrated in this investigation. The LDI MS method greatly reduces the analysis time and amount of the sample compared to traditional methods. For this reason, the LDI MS method can be a useful tool for the fast qualitative screening of corrosion films.