M. Veljković

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.

TEMPERATURE-PROGRAMMED DESORPTION MASS SPECTROMETRIC STUDY OF THE SURFACE PROPERTIES OF GLASSY CARBON

The temperature-programmed desorption (TPD) method combined with mass spectrometric analysis has been applied to the characterization of glassy carbon (GC) and modified glassy carbon surfaces. Five different samples of GC were obtained from a commercial resol type of phenol formaldehyde resin that had undergone several treatments: the samples were either pure GC, GC treated with boron and phosphorus, or GC modified by metal (silver and palladium). Mass spectrometric analysis has shown that the thermal decomposition of surface oxide species results in desorption of H2O, CO and CO2, these being the major gas products. On the basis of the TPD spectra obtained, desorption energies have been calculated using a peak position method. The influence of CO2 oxidation on the surface properties of the samples was also investigated. It was concluded that metal deposition on a glassy carbon surface contributes to increased stability of surface oxide species, the most stable surface being one that has been modified by palladium deposition.

MASS SPECTROMETRIC STUDY OF THE IONIZATION AND DISSOCIATION OF SULPHUR HEXAFLUORIDE BY MONOENERGETIC ELECTRON IMPACT

The ionization and fragmentation of SF6 under electron impact has been examined by means of a trochoidal electron velocity selector and a mass spectrometer. The ionization efficiency curves of the positive ions were recorded and the appearance energies of different electronic states of these ions were determined. The experimental results were combined with auxiliary thermochemical data to evaluate the dissociation energies D°°(F5S–F) = 4.01 eV, D°°(F4S–2F) = 6.39 eV, D°°(F3S–3F) = 9.98 eV, the ionization energy of SF5, = 11.27 eV and heats of formation ΔH°f(SF5+) = 1.81 eV, ΔH°f(SF4+) = 4.12 eV, ΔH°f(SF3+) = 4.11 eV and ΔH°f(SF3) = − 5.13 eV.

Formation of oxygen complexes in controlled atmosphere at surface of doped glassy carbon

The effects of boron and phosphorus incorporation in phenolic resin precursor to the oxidation resistance of glassy carbon have been studied. In order to reveal the nature and composition of the oxygen complexes formed at the surface of doped glassy carbon, under controlled atmosphere, the surface of the samples was cleaned under vacuum up to 1273 K. Specific functional groups, subsequently formed under dry CO2 or O2 atmosphere on the surface of boron-doped and phosphorus-doped glassy carbon samples, were examined using the temperature-programmed desorption method combined with mass spectrometric analysis. Characterization of surface properties of undoped and doped samples has shown that in the presence of either boron or phosphorus heteroatoms, a lower amount of oxygen complexes formed after CO2 exposure, while, typically, higher amount of oxygen complexes formed after O2 exposure. It has been concluded that the surface of undoped glassy carbon has a greater affinity towards CO2, while in the presence of either boron or phosphorus heteroatoms, the glassy carbon surface affinity becomes greater towards O2, under experimental conditions.

Energetics of the ionization and fragmentation of phosphorus oxyfluoride studied by electron-impact spectroscopy

The relative intensities and appearance energies of several positive ions in the mass spectrum of POF3 are reported together with an ionization-efficiency curve for F–, the only negative ion of significant intensity. Probable ion-fragmentation pathways are proposed together with the heats of formation of some positive ions and ionization energies for POF2 and POF. The appearance energies of various electronic states of the observed ions have been determined by deconvolution of the experimental ionization efficiency curves by the energy-distribution difference method.