Metastable decay and binding energies of van der Waals cluster ions

In this work the appearance potentials for the metastable decay channel of a series of van der Waals dimer ions are presented. Ionization and metastable dissociation is achieved by resonance-enhanced two-photon absorption in a linear reflection time-of-flight mass spectrometer. From the appearance potentials the binding energy of the neutral dimers is obtained and from the additionally measured ionization potentials binding energies of the dimer cations are achieved. The contribution of charge transfer resonance interaction to the binding in cluster ions is evaluated by investigation of several homo-and heterodimers of aromatic components and the heterodimer benzene/cyclohexane as an example for a dimer consisting of an aromatic and a nonaromatic component.     

Metastable dissociation dynamics of molecular cluster ions

Metastable uni-cluster dissociation for several hydrogen-bonded and van der Waals cluster ions are observed via resonance-enhanced two-photon ionization reflectron time-of-flight (TOF) mass spectrometry. All of the cluster ions studied show evaporation of a single molecule from the respective parent cluster ions as dominant metastable decay processes. Furthermore, the averaged metastable evaporation rate constants (k _evap) of these cluster ions in a fixed time domain of 0.2–50 µs are obtained by analyzing the relative intensity of metastable ion peaks due to evaporation in the acceleration and the field-free drift regions of the TOF mass spectrometer. An intensity anomaly in some of the observed metastable ion peaks, indicative of magic number stability of the cluster ion, is also presented.     

Ab-initio calculations of excited rare gas alkali ions

The excimer-like ions formed by combining a rare gas ion with an alkali atom were investigated by an ab initio HF-CI calculation. The resulting four bound excited states¹Σ⁺,¹Π,³Σ⁺ and³Π were subjected to a semiempirical spin orbit (SO)-coupling yielding eight fine structure levels with angular momenta Ω=2, 1(3) and 0(4). For (NeLi)⁺, (NeNa)⁺, (ArLi)⁺, (ArNa)⁺, (ArK)⁺ and (KrLi)⁺ the wavelengths for the five allowed transitions to the ground state were calculated. The results are in close agreement with the experimental results. Also other spectroscopic properties as binding energies, transition moments and vibrational quanta are given.     

Potential curves of the rare gas alkali ions

The first excited states of the rare gas alkali ions have been calculated using Hellmanntype model potentials. The predicted decay energies agree with the results of a recent measurement within 2%.     

Multiphonon decay of excited states of rare earth ions in crystals

Although the relaxation of excited electronic states through radiationless multiphonon processes has been well established by experiment, there are several serious problems concerning the current theoretical model in which the electron-phonon interaction is treated as a small perturbation, and the multiphonon transitions are attributed to high order expansion terms of perturbation theory. Here, we approach the problem in the framework of the adiabatic approximation. The rate of the multiphonon process is expressed in the form of the linear response time correlation function which is then evaluated by means of the saddle point approximation. The resulting expression predicts an approximately exponential dependence of the rate on the transition gap and a temperature dependence that agrees quantitatively with experiment.     

Reactivity of niobium-carbon cluster ions with hydrogen molecules in relation to formation mechanism of Met-Car cluster ions

It is known that a niobium-carbon Met-Car cluster ion (Nb 8C 12 (+)) and its intermediates (Nb 4C 4 (+), Nb 6C 7 (+), etc.) are selectively formed by the aggregation of the Nb atoms in the presence of hydrocarbons. To elucidate the formation mechanism, we prepared Nb n C m (+) with every combination of n and m in the gas phase by the laser vaporization technique. The reactivity of Nb n C m (+) with H 2 was examined under the multiple collision condition, finding that Nb n C m (+) between Nb 2C 3 (+) and Nb 8C 12 (+) are not reactive with H 2. On the basis of the H 2 affinity of Nb n C m (+) experimentally obtained, we propose a dehydrogenation-controlled formation mechanism of niobium-carbon Met-Car cluster ions.     

Metastable decay of peptide ions on a Fourier transform mass spectrometer equipped with an external ion source

Metastable decay rates of two peptides, RPPGFSPF and PKPQQFFGLM, were determined from ions produced in an external matrix-assisted laser desorption/ionization source with a Fourier transform mass spectrometer. An isolation and subtraction method that gives difference spectra was employed to monitor the product formation and metastable decays. The dependence of metastable decay rates on laser fluences and matrixes was demonstrated.     

Observation and investigation of metastable decay series of (N2) n + cluster ions

N₂ cluster ions are produced by electron impact ionization of a supersonic N₂ cluster beam and analyzed with a double focussing sector field mass spectrometer. It is found that metastable N₂ cluster ions lose more than one N₂ molecule in the μs time regime and decay predominantly via sequential series (N₂) _n ⁺ *→(N₂) _n?1 ⁺ *→...→N ₂ ⁺ , evaporating a single monomer in each of these successive decay steps. The metastable decay rates determined in detail for cluster sizes 2≤n≤6 andn=20 lie between 1 and 10⁶s^?1. These rates(i) depend strongly on the time elapsed after ion formation and on the respective parent cluster ion size, and(ii) exhibit a quasiperiodic pattern in magnitude.     

Heteronuclear diatomic transition-metal cluster ions in the gas phase: Reactivity and thermochemistry of AgFe+

The gas-phase chemistry of AgFe⁺ was studied by using Fourier transform ion cyclotron resonance mass spectrometry. AgFe⁺ is unreactive with alkanes but reacts with cyclic and linear (C4–C8) alkenes. The primary reactions are dominated by dehydrogenation and condensation. In addition, cluster splitting is observed in the reaction of AgFe⁺ with benzene. Secondary reactions generally involve cluster splitting with the loss of Ag, although AgFeC₅H ₆ ⁺ is observed to dehydrogenate cyclopentene to yield AgFeC₁₀H ₁₂ ⁺ . Ion-molecule reactions, collision-induced dissociation, and photodissociation experiments were used to determine the bond energiesD°(Fe⁺–Ag)=53±7 kcal/mol andD°(Ag⁺–Fe)=46±7 kcal/mol. These values in turn were used to calculateH _f (AgFe⁺)=296±7 kcal/mol andIP(AgFe)=6.5±0.3 eV. Related chemical and physical properties of CuFe⁺ are presented for comparison.     

Photodissociation dynamics of the Kr-HBr cluster: the effect of the rare gas atom substitution

The ultraviolet photolysis dynamics of Kr-HBr(v=0) is investigated by means of wave packet calculations, focusing on the fragmentation pathway Kr-HBr+ variant Planck's over 2pi omega-->H+Kr-Br. Photolysis is simulated by starting from two different cluster initial states, namely the ground van der Waals (vdW) and an excited vdW bending state, associated with the Kr-H-Br and Kr-Br-H isomers, respectively. The results show that, for the two initial states of the cluster, the Kr-Br product yield is lower than that of Ar-Br radicals found in previous studies on Ar-HBr photolysis. Despite this decrease, the Kr-Br yield is found to be still rather high, in particular for the initial excited vdW state of Kr-HBr(v=0). In addition, the Kr-Br product state distributions exhibit a remarkably higher excitation (mainly rotational) than the corresponding Ar-Br distributions. The lower yield and higher excitation of Kr-Br as compared to Ar-Br, are attributed to a larger share of the energy available for the radical going to internal excitation in the case of the Kr-Br product. The different partition of the energy available for Kr-Br also causes significant deviations in the photolysis behavior of Kr-HBr when compared to that of Ar-HBr, in the case of the initial excited vdW state of both clusters. A common feature of the photodissociation of Kr-HBr and Ar-HBr is the manifestation of quantum interference effects in the Kr-Br and Ar-Br rotational state distributions, in the form of pronounced structures of supernumerary rotational rainbows.     

Oxidation of CO by aluminum oxide cluster ions in the gas phase

Small aluminum oxide cluster cations and anions, produced by laser vaporization, were investigated regarding their reactivity toward CO and N2O employing guided-ion-beam mass spectrometry. Clusters with the same stoichiometry as bulk alumina, Al2O3, exhibited atomic oxygen transfer products when reacted with CO, suggesting the formation of CO2. Anionic clusters were less reactive than cations but showed higher selectivity towards the transfer of only a single oxygen atom. Cationic clusters, in contrast, exhibited additional products corresponding to the sequential transfer of two oxygen atoms and the loss of an aluminum atom. To determine if these stoichiometric clusters could be generated from oxygen-deficient species, clusters having a stoichiometry with one less oxygen atom than bulk alumina, Al2O2, were reacted with N2O. Cationic clusters were found to be selectively oxidized to Al2O3(+), while anionic clusters added both one and two oxygen atoms forming Al2O3(-) and Al2O4(-). The oxygen-rich Al2O4(-) cluster exhibited comparable reactivity to Al2O3(-) when reacted with CO.     

Direct fission versus sequential evaporation mechanism of sputtered caesium iodide cluster ions

A triple stage hybrid mass spectrometer was scanned for the trapping of the reaction intermediates of the over-all loss of two CsI-moieties from size-selected caesium iodide cluster ions of the general formula Cs (CsI)⁺ _n . In addition to appropriate MS/MS/MS-methods an electrically floated collision cell has been applied to trap intermediates of unimolecular and collision-induced evaporations. In comparison with other experimental findings the features of the evaporation mechanism are discussed.     

A novel desorption mechanism of (AgnIn−1)+ cluster ions

Silver iodide clusters have been generated by inert gas condensation technique, ionized via electron impact, and mass analyzed in a reflectron equipped time-of-flight mass spectrometer. The mass spectra are dominated by cluster ions of the composition (Ag_nI_n?1)⁺. Additional kinetic energy analysis is applied to detect metastable decay of the cluster ions for n up to 30. In contrast to alkali halides, where halogen and monomer desorption had been observed, the main decay channel of silver iodide cluster ions is the loss of a neutral trimer Ag₃I₃.     

Adsorption of cadmium ions from aqueous solutions using nano-montmorillonite: kinetics,isotherm and mechanism evaluations

With increasing industrial development, heavy metal pollution, e.g., cadmium (Cd) pollution, is increasingly serious in soil and water environments. This study investigated the sorption performance of nano-montmorillonite (NMMT) for Cd ions. Adsorption experiments were carried out to examine the effects of the initial metal ion concentration (22.4–224 mg/L), pH (2.5–7.5), contact time (2–180 min) and temperature (15–40 °C). A simulated acid rain solution was prepared to study the desorption of Cd adsorbed on NMMT. After the adsorption or desorption process, the supernatant was analyzed using a flame atomic absorption spectrometry method. The Cd removal rate increased as the pH and contact time increased but decreased as the initial metal ion concentration increased. The maximum adsorption capacity was estimated to be 17.61 mg/g at a Cd²⁺ concentration of 22.4 mg/L. The sorption process can be described by both the Langmuir and Freundlich models, and the kinetic studies revealed that the pseudo-second-order model fit the experimental data. The Cd desorption rate when exposed to simulated acid rain was less than 1%. NMMT possesses a good adsorption capacity for Cd ions. Additionally, ion exchange was the main adsorption mechanism, but some precipitation or surface adsorption also occurred.