Electron impact and single photon ionization cross sections of neutral silver clusters

Neutral silver atoms and small clusters Ag _n (n=1...4) were generated by sputtering, i.e. by bombarding a polycrystalline silver surface with Ar⁺ ions of 5 keV. The sputtered particles were ionized by a crossed electron beam and subsequently detected by a quadrupole mass spectrometer. In alternative to the electron impact ionization, the same neutral species were also ionized by single photon absorption from a pulsed VUV laser (photon energy 7.9 eV), and the photoionization cross sections were evaluated from the laser intensity dependence of the measured signals. By in situ combining both ionization mechanisms, absolute values of the ratio σ _e (Ag _n )/σ _e (Ag) between the electron impact ionization cross sections of silver clusters and atoms could be determined for a fixed electron energy of 46 eV. These values can then be used to calibrate previously measured relative ionization functions. By calibrating the results using literature data measured for silver atoms, we present absolute cross sections for electron impact ionization of neutral Ag₂, Ag₃ and Ag₄ as a function of the electron energy between threshold and 125 eV.     

A computational study on CunN0,±1 (n=1–4) clusters by density functional methods

Clusters of the type Cu_nN^0,±1 (n=1–4) are investigated computationally using density functional theory methods. Equilibrium geometries are optimized under the constraint of well-defined point-group symmetries at the B3LYP level employing a pseudo-potential method in conjunction with double-zeta basis sets. In this article, different molecular properties such as total energies, electron affinities, ionization potentials, fragmentation energies and equilibrium geometries of the Cu_nN^0,±1 (n=1–4) clusters are systematically calculated and discussed. In particular, the photoelectron spectra of the anionic Cu_nN⁻¹ (n=2–4) clusters are calculated showing a good agreement with the available experimental results. In addition, Mulliken and natural orbital population analyses, and natural orbital configurations are calculated in order to elucidate the charge distributions in the clusters.     

Absolute cross sections for electron impact ionization of Ag2

The previously measured relative cross section function for electron impact ionization (EII) of neutral Ag₂ has now been calibrated quantitatively by combining the electron impact ionization with in situ non resonant two photon ionization (NR2PI). By comparing the NR2PI saturation intensities measured for Ag ₂ ⁺ and Ag⁺ with the corresponding EII intensities, the ratio between the electron impact ionization cross sections (EIICS) of neutral Ag₂ and Ag was determined to be σ_Ag2/σ_Ag=1.53 for an electron energy of 46 eV. This result agrees well with the geometricn ^2/3-rule \((\sigma X_n \sim n^{2/3} )\) commonly proposed for the dependence of the EIICS of clustersX _n on the cluster sizen.     

Study of small chlorine‐doped potassium clusters by thermal ionization mass spectrometry

The theoretical calculations have predicted that nonmetal‐doped potassium clusters can be used in the synthesis of a new class of charge‐transfer salts which can be considered as potential building blocks for the assembly of novel nanostructured material. In this work, K_nCl (n = 2–6) and K_nCl_n?1 (n = 3 and 4) clusters were produced by vaporization of a solid potassium chloride salt in a thermal ionization mass spectrometry. The ionization energies (IEs) were measured, and found to be 3.64 ± 0.20 eV for K₂Cl, 3.67 ± 0.20 eV for K₃Cl, 3.62 ± 0.20 eV for K₄Cl, 3.57 ± 0.20 eV for K₅Cl, 3.69 ± 0.20 eV for K₆Cl, 3.71 ± 0.20 eV for K₃Cl₂ and 3.72 ± 0.20 eV for K₄Cl₃. The K_nCl⁺ (n = 3–6) clusters were detected for the first time in a cluster beam generated by the thermal ionization source of modified design. Also, this work is the first to report experimentally obtained values of IEs for K_nCl⁺ (n = 3–6) and K_nCl_n?1⁺ (n = 3 and 4) clusters. The ionization energies for K_nCl⁺ and K_nCl_n?1⁺ clusters are much lower than the 4.34 eV of the potassium atom; hence, these clusters should be classified as ‘superalkali’ species. Copyright © 2012 John Wiley & Sons, Ltd.     

Theoretical study of the stability of X N n (n=−1, 0, +1, +2; X=Ag,Cu;N≤25) clusters as a function of size using a non-local density functional formalism

The spherical jellium model and self-consistent Weighted Density Approximation (WDA) to density functional theory have been used to study the stability of X _N ⁿ (n=?1, 0, +1, +2; X=Ag, Cu;N≤25) clusters. The calculated magic numbers coincide with the observed ones. The first (IP1) and second (IP2) ionization potentials of Ag _N and Cu _N as a function of size show the typical oscillations induced by the electronic shell-filling effect. IP1 of Cu _N is about 0.5 eV higher than IP1 of Ag _N in the range studied (N≤25). For both Cu _N and Ag _N , IP1 appears to converge well towards the respective experimental values of the work function. The use of WDA allows us to obtain bound negative clusters of small size or with a nearly empty external shell, which is not possible using the Local Density Approximation (LDA) [1, 2]. However the electron affinity of X _N clusters obtained as the difference of energies of the neutral and the negatively charged clusters, becomes negative forN=2, 3 and 8 (very close to zero forN=8), revealing that WDA needs further refinements.     

The Ionization Energies of Di-n-Alkyl Diacetylenes

The He(Iα) photoelectron spectra and the ionization energies of symmetrically substituted di-n-alkyl-diacetylenes R-(C?C)₂-R (with R ? CH₃, C₂H₅, n-C₃H₇, n-C₄H₉) are presented. The effect of the alkyl substitutents is that the two acetylenic ionization energies, I_v,1 and I_v,2, shift by the same amount, i.e. their difference I_v,2 – I_v,1 remains constant (2.45 ± 0.05 eV). Between 12.5 eV and 17 eV the band system in the photoelectron spectrum of R-(C?C)₂-R is superimposable with that in the spectrum of the corresponding alkane, RH, with the exception of a uniformly small shift of all the bands to higher ionization energy.     

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_nX systems. In the present study, K₂X (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 K₂F, K₂Cl, K₂Br, and K₂I 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 K₂X (X = F, Br, and I) are presented for the first time. The ionization energy of K₂Cl 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. Copyright © 2011 John Wiley & Sons, Ltd.     

Collision-induced dissociation and theoretical studies of Cu+-dimethoxyethane complexes

Collision-induced dissociation of the dimethoxyethane (DXE) complexes with copper ions, Cu⁺(DXE)_n, n = 1 and 2, is studied using kinetic energy dependent guided ion beam mass spectrometry. For Cu⁺(DXE)₂, the only product formed corresponds to endothermic loss of a neutral ligand, while the Cu⁺(DXE) complex dissociates by several competitive channels. The cross-section thresholds for single ligand loss are interpreted to yield 0 and 298 K bond energies for Cu⁺-DXE and (DXE)Cu⁺-DXE after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. We find absolute 0 K bond dissociation energies for these complexes of 2.74 ± 0.08 and 1.87 ± 0.06 eV, respectively. These values are compared with theoretical values obtained using density functional and second order Møller-Plesset perturbation, MP2, theories. We also compare our results with previously studied alkali cation-ether complexes. Although Cu⁺ and all alkali cations have ¹S electronic ground states, the comparison shows different trends for Cu⁺ because of hybridization effects involving the valence d-electrons.     

Binding of ammonia to small copper and silver clusters

We report equilibrium geometries, harmonic frequencies, and thermochemical data for the metal cluster–ammonia complexes Ag_n(NH₃) and Cu_n(NH₃) (n=1,2,3,4), Ag₄(NH₃)₂, and Cu₄(NH₃)₂ calculated by a density functional method. The calculated shifts in ammonia umbrella mode frequency correlate with the observed shifts and the calculated enthalpies of complexation. The preferred site for NH₃ adsorption and the calculated bond enthalpies can be rationalized by considering atomic charges obtained from a natural population analysis and polarization of the metal electron density.     

Sampling of aryldiazonium, anilino, and aryl radicals by membrane introduction mass spectrometry: Thermolysis of aromatic diazoamino compounds

Membrane introduction mass spectrometry (MIMS) is used to sample free radicals generated by thermolysis at atmospheric pressure. This is done by heating the solid sample in a custom-made probe that is fitted with a silicone membrane to allow selective and rapid introduction of the pyrolysates into the ion source of a triple quadrupole mass spectrometer. Phenyldiazonium radical (C₆H₅N ₂ ^· ) and some of its ring-substituted analogs, the methoxy anilino radical CH₃OC₆H₄NH^·, and aryl radicals are generated by gas phase thermolysis of symmetrical aryl diazoamino compounds (ArNH-N₂Ar). The radicals are identified by measurement of their ionization energies (IE) using threshold ionization efficiency data. A linear correlation between the ionization energy of the phenyldiazonium radicals and their Brown σ⁺ values is observed, and this confirms the formation of these species and validates the applicability of MIMS in sampling these radicals. The ionization energies of the aryldiazonium radicals are estimated as IE (p-CH₃O-C₆H₄N ₂ ^· ), 6.74 ± 0.2 eV; IE (p-CH₃-C₆H₄N ₂ ^· ), 7.72 ± 0.2 eV; IE (C₆H₅N ₂ ^· ), 7.89 ± 0.2 eV; IE (m-Cl-C₆H₄N ₂ ^· ), 7.91 ± 0.2 eV; IE (p-F-C₆H ₄ ^· N ₂ ^· ), 8.03 ± 0.2 eV; and IE (m-NO₂-C₆H₄N ₂ ^· ), 8.90 = 0.2 eV. The ionization energies of the aryl radicals are estimated as IE (p-CH₃O-C₆H ₄ ^· ), 7.33 ± 0.2 eV; IE (p-CH₃-C₆H ₄ ^· ), 8.31 ± 0.2 eV; IE (C₆H ₅ ^· ), 8.44 ± 0.2 eV; IE (m-Cl-C₆H ₄ ^· ), 8.50 ± 0.2 eV and IE (p-F-C₆H ₄ ^· ), 8.54 ± 0.2 eV. Also, the ionization energy of the p-methoxyanilino radical (p-CH₃O-C₆H₄NH^·) is estimated as 7.63 ± 0.2 eV.     

Double-ionization energies to ground and excited states of the tungsten hexacarbonyl dication

Tungsten hexacarbonyl was investigated by double-charge-transfer (DCT) spectroscoPy, and the double-ionization energies to ground and electronically excited states of W(CO) ₆ ²⁺ determined. The double-ionization energies corresponding to the first two distinct peaks in the spectra are 22.8 ± 0.3 eV and 28.5 ± 0.3 eV, but numerous overlapping peaks at higher energies are evident. It is shown that the DeI spectra can explain the main features of a previously determined (J. Am. Soc. Mass Spectrom. 1990, 1, 16–27) internal energy distribution curve for W(CO) ₆ ²⁺ ions formed by 70-eV electron ionization of W(CO)₆ molecules.     

ECP-CI study of electronic structure and geometry of small neutral and charged Ag n clusters; Predictions and interpretation of measured properties

The ground state geometries of small neutral Ag _n (n=2–9) and charged Ag _n ^± (n=2–9) clusters have been determined in the framework of the SCF procedure employing a relativistic pseudopotential accounting for core-valence correlation effects (RECP-CVC). Similarities and differences between neutral and charged clusters have been found. Large scale CI for 5s electrons only has been carried out for determining stabilities, ionization potentials (IP) and vertical detachment energies (VDE) of anions. A comparison between predicted and measured observables allows for the tentative structural assignments. In addition, the low lying energies of excited states for the neutral species at the anionic geometries have been calculated to account fully for geometrical and spectroscopic assignment to the photodetachment measurements.     

Experimentally determined and calculated values of the energies required to form doubly charged ions of the bromomethanes CH3Br,CH2Br2 and CHBr3

Two experimental techniques were used to determine the double ionization energies of CH₃Br, CH₂Br₂ and CHBr₃. In one, these energies were measured directly by double-charge-transfer spectroscopy. In the other, charge stripping of [CH₃Br]⁺, [CH₂Br₂]⁺ and [CHBr₃]⁺ ions was investigated and the ionization energies of the singly charged ions were measured. The double ionization energies of the molecules obtained by adding known single ionization energies of the molecules to the single ionization energies of the ions were in good agreement with those determined by double-charge-transfer spectroscopy. The relevant mean values from the two techniques were 28.9 ± 0.5, 27.5 ± 0.5 and 29.1 ± 0.5 eV for the double ionization energy of CH₃Br, CH₂Br₂ and CHBr₃, respectively. The results of ab initio calculations using second-order Møller-Plesset perturbation theory were in good agreement with the observed double ionization energies; they were consistently slightly lower than the experimental values.     

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

Clusters of the type Li_nX (X = halides) can be considered as potential building blocks of cluster‐assembly materials. In this work, Li_nBr (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_nBr (n = 4–7) cluster were detected for the first time. The order of ion intensities was Li₂Br⁺ > Li₄Br⁺ > Li₅Br⁺ > Li₆Br⁺ > Li₃Br⁺. The ionization energies (IEs) were measured and found to be 3.95 ± 0.20 eV for Li₂Br, 3.92 ± 0.20 eV for Li₃Br, 3.93 ± 0.20 eV for Li₄Br, 4.08 ± 0.20 eV for Li₅Br, 4.14 ± 0.20 eV for Li₆Br and 4.19 ± 0.20 eV for Li₇Br. 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_nBr (n = 2–4) are slightly lower than those in the corresponding small Li_n or Li_nH clusters, whereas the IEs of Li_nBr are very similar to those of Li_n or Li_nH 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_nBr (n = 2–7) clusters (because their ions are thermodynamically stable with respect to the loss of lithium atoms in the gas phase) and increasingly contributes toward the development of clusters for practical applications. Copyright © 2012 John Wiley & Sons, Ltd.     

Photofragmentation of Ag4(N2)x +, x = 0—3: N2 binding energies

Photodissociation cross sections and cationic fragments observed upon one-photon dissociation of mass selected Ag₄(N₂)_x ⁺, x = 0—3 have been recorded in the 2.53 to 2.88 eV photon energy range. Measurements allow first order assessments of N₂ ··· Ag₄(N₂)_x-1 ⁺ binding energies as well as of internal excitation levels prior to irradiation.     

Ionic conductivity of double vanadate Ag3Sc2(VO4)3

Ionic conductivity of double vanadate Ag₃Sc₂(VO₄)₃ with the NASICON structure is studied by the method of impedance spectroscopy in the frequency range from 5 to 5 × 10⁵ Hz and in the temperature range of 300–827 K. The vanadate Ag₃Sc₂(VO₄)₃ is prepared in the form of fine crystalline powder by solid-state synthesis from V₂O₅, Sc₂O₃, and AgNO₃ at 1173 K. The conductivity of Ag₃Sc₂(VO₄)₃ ceramic samples σ = 8 × 10^?3 S/cm (at 563 K). The σ vs. T curve demonstrates an anomaly at 563–623 K which corresponds to thermal effects in this temperature range. The values of enthalpy of ion transport activation are ΔH = 0.40 ± 0.05 eV (T < 563 K) and ΔH = 0.30 ± 0.05 eV (T > 623 K). The ionic conductivity of Ag₃Sc₂(VO₄)₃ is due to Ag⁺ ions localized in channels of the framework structure of the NASICON type.     

A contracted bromine basis set for use in calculation of molecular energies

A contracted [9s6p2d] basis set derived from Dunning's (14s11p5d) primitive Gaussian set for bromine has been used in ab initio molecular orbital calculations of the dissociation energies of HBr, CH₃Br, and Br₂, the ionization potentials of Br and HBr, and the electron affinity of Br. The calculated energies are within 0.1 eV of the experimental values. This is similar to the accuracy obtained in a previous study, also using a contracted [9s6p2d] basis set, of the dissociation and ionization energies of the GeH_n, AsH_n, and SeH_n hydrides.     

Applications of spectral-Representation model as a potential method for Cu clusters

We applied the spectral-representation technique developed by Katsuki and Huzinaga as a model potential in calculating the electronic structure of Cu clusters. The characteristics of this potential were closely investigated in Cu and Cu₂. For Cu, Cu₂, Cu₅, Cu₉, and Cu₁₃, we performed all-electron ab initio self-consistent field calculations and model-potential calculations where 3p, 3d, and 4s electrons, and 3d and 4s electrons are treated as valence electrons. The ionization potentials (IPs) given by the all-electron calculations were 6.26, 5.55, 4.52, 4.02, and 4.08 eV for Cu, Cu₂, Cu₅, Cu₉, and Cu₁₃, respectively. The IPs given by the model-potential calculations were 6.25, 5.56, 4.62, 4.09, and 4.23 eV for the 3p-, 3d-, and 4s-valence electrons, and 6.26, 5.68, 4.71, 4.07, and 4.19 eV for the 3d- and 4s-valence electrons. The IPs given by the model-potential calculations agree well with those of the all-electron calculations. We also performed model-potential calculations where only the 4s electrons were treated as valence electrons. The calculated IPs were 6.47, 5.98, 5.38, 4.63, and 4.88 eV for Cu, Cu₂, Cu₅, Cu₉, and Cu₁₃, respectively. These are ca. 0.8 eV higher than the IPs by the all-electron calculation for the larger clusters of Cu₅, Cu₉, and Cu₁₃. The higher IPs originate from the expulsion of the 3d electrons from the valence electrons. We also performed model-potential calculations with 4s electrons for Cu₇₄. The calculated IP is 4.61 eV, which is estimated to be 0.8 eV larger than that obtained by the all-electron calculation. The IPs with correlation corrections are 7.7, 7.4, 6.3, 5.8, 5.9, and 5.6 eV for Cu, Cu₂, Cu₅, Cu₉, Cu₁₃, and Cu₇₄, respectively. Experimental values are 7.73, 7.37, 6.30, 5.37, 5.67, and 5.26 eV. The agreement between the two is fairly good. The electron affinities are also discussed. © 1996 by John Wiley & Sons, Inc.     

Dissociation energy and ionization potential of the molecule CF

The gaseous equilibrium S + CF₂ = SF + CF was studied over the temperature range 1851 to 2232 K by mass spectrometry, and the derived enthalpy change was used to evaluate the heat of formation of CF ΔH₂₉₈ = 58.0 ± 2.4 kcal/mol (2.52 ± 0.10 eV), and the dissociation energy D₀⁰ (CF) = 130.8 ± 2.4 kcal/mol (5.67 ± 0.10 eV). The new thermochemical data indicate a slightly higher stability for CF than earlier determinations. Direct measurement by electron impact yielded a value of 9.17 ± 0.10 eV for the vertical ionization potential of CF, in agreement with an indirect result obtained from the photodissociative ionization of C₂F₄.     

Adsorption of NO2 on Small Silver Clusters with Copper Impurity: A Density Functional Study

Density functional theory calculations were performed to investigate the structural and energetic properties of NO₂ adsorption on small bimetallic Ag _n Cu _m clusters (n?+?m?≤?5). Generally NO₂ is adsorbed in bridge configuration. The adsorbates prefer Cu sites when both Ag and Cu co-exist in the clusters. The adsorption energies and the dissociation energies of the complex clusters increase as the Cu content increases for the given cluster size. Our calculation suggests that the bimetallic Ag _n Cu _m may react with NO₂ dissociatively by way of Ag atom, Ag₂ or AgCu loss. The N–O vibrational properties of the complex clusters were also discussed and analyzed.