Cluster anions: Nonempirical estimate of the electron hydration energy

A model approach describing restricted relaxation of structural fragments of a hydrogen-bond network of water upon the localization of an excess electron is proposed. With the use of nonempirical calculations of water clusters comprising up to 16 molecules, the vertical energies of electron detachment from anionic water structures and the energies of electron hydration by neutral systems are estimated. Based on these data and analysis of the electron density distribution of the highest occupied molecular orbital of the anions, extrapolation estimates of the photoionization energy of the electron removal from the bulk and surface layers of water (4.4 ± 0.2 and 3.0 ± 0.2 eV respectively) and the electron hydration energy (2.6 ± 0.2 eV) are obtained.     

Collisional electron emission at solid surfaces induced by mass selected negative cluster ions

Electron emission efficiency induced by the collision of clusters with a solid surface was measured as a function of cluster size. Emitted electron energy distribution for the impact of mass selected negative ion clusters or mass selected neutral clusters was also measured in the energy region of 0–5 eV. The difference in the shape of the electron spectra was observed depending on the size and charge of the clusters.     

Electronic decay following ionization of aqueous Li+ microsolvation clusters

Ionization in the energetical range between 35 eV and 75 eV of aqueous Li(+) microsolvation clusters may initialize several different electronic decay processes. Electronic decay following H(2)O 2s ionization in a cationic cluster is reported. Li ionization probes the efficiency of electron transfer mediated decay (ETMD) processes. We report estimated ETMD lifetimes in the range of 20-100 fs for clusters with one to five water monomers. Furthermore, tertiary electron emission may occur via a combined cascade of electron transfer mediated decay and intermolecular Coulombic decay.     

The first ionization potential of uranium and thorium measured in a d.c. arc plasma

Tha Saha relationship was used to derive the ionization potentials of uranium and thorium from measurements of temperature or of electron density in a plasma in thermodynamic equilibrium. Introducing into the plasma elements with well defined ionization potentials, such as Ba, Al, V, Cr, Zr, Mo, Cu, Si and some of the rare-earths, as matrices, the temperature and electron density were measured in the central region of the arc plasma. A relation was established between the different ionization potentials and the plasma temperature or the electron density. From this relation the values of 6.3 ± 0.3 and 7.5 ± 0.3 eV were found for the ionization potentials of U and Th respectively.     

Electron-impact ionization of helium clusters close to the threshold: appearance energies

We have investigated the ionization threshold behavior of small helium cluster ions (cluster size n=2-10) formed via electron-impact ionization of neutral helium droplets and derive appearance energies for mass-selected cluster ions using a nonlinear least-square-fitting procedure. Moreover, we report magic numbers in the mass spectrum observed at the electron energy of 70 eV. The apparatus used for the present measurements is a hemispherical electron monochromator combined with a quadrupole mass spectrometer. Our experiment demonstrates that helium clusters are not only exclusively formed via direct ionization above the atomic ionization potential but also indirectly via autoionizing Rydberg states. The present results are compared with previous electron-impact and photoionization results.     

Geometrical Structures and Electronic Properties of Copper-Doped Aluminum Clusters

Using density function theory (DFT), the Cu-doped Aln (n=1?15) clusters have been stud-ied. The electron a±nity, ionization potential, Mulliken population analysis of Cu, mean polarizability, polarizability anisotropy, dipole moments and HOMO-LUMO gaps have also been calculated on the basis of optimized geometries. The results indicate that there is magic numbers in copper-doped aluminum clusters and electronic characteristic depended on the size of clusters. As n=13, the electron affinity and ionization potential of cluster changed more than 0.3 and 0.6 eV respectively, compared with neighborhood clusters.     

Dynamics and fragmentation of van der Waals clusters: (H2O)n, (CH3OH)n, and (NH3)n upon ionization by a 26.5 eV soft x-ray laser

A tabletop soft x-ray laser is applied for the first time as a high energy photon source for chemical dynamics experiments in the study of water, methanol, and ammonia clusters through time of flight mass spectroscopy. The 26.5 eV/photon laser (pulse time duration of approximately 1 ns) is employed as a single photon ionization source for the detection of these clusters. Only a small fraction of the photon energy is deposited in the cluster for metastable dissociation of cluster ions, and most of it is removed by the ejected electron. Protonated water, methanol, and ammonia clusters dominate the cluster mass spectra. Unprotonated ammonia clusters are observed in the protonated cluster ion size range 2< or =n< or =22. The unimolecular dissociation rate constants for reactions involving loss of one neutral molecule are calculated to be (0.6-2.7)x10(4), (3.6-6.0)x10(3), and (0.8-2.0)x10(4) s(-1) for the protonated water (9< or =n< or =24), methanol (5< or =n< or =10), and ammonia (5< or =n< or =18) clusters, respectively. The temperatures of the neutral clusters are estimated to be between 40 and 200 K for water clusters (10< or =n< or =21), and 50-100 K for methanol clusters (6< or =n< or =10). Products with losses of up to five H atoms are observed in the mass spectrum of the neutral ammonia dimer. Large ammonia clusters (NH(3))(n) (n>3) do not lose more than three H atoms in the photoionization/photodissociation process. For all three cluster systems studied, single photon ionization with a 26.5 eV photon yields near threshold ionization. The temperature of these three cluster systems increases with increasing cluster size over the above-indicated ranges.     

Electron impact fragmentation of size-selected krypton clusters

Clusters of krypton are generated in a supersonic expansion and size selected by deflection from a helium target beam. By measuring angular distributions for different fragment masses and time-of-flight distributions for fixed deflection angles and fragment masses, the complete fragmentation patterns for electron impact ionization at 70 eV are obtained from the dimer to the heptamer. For each of the neutral Kr(n) clusters studied, the main fragment is the monomer Kr(+) ion with a probability f(n)(1) > 90%. The probability of observing dimer Kr(2)(+) ions is much smaller than expected for each initial cluster size. The trimer ion Kr(3)(+) appears first from the neutral Kr(5), and its fraction increases with increasing neutral cluster size n, but is always much smaller than that of the monomer or dimer. For neutral Kr(7), all possible ion fragments are observed, but the monomer still represents 90% of the overall probability and fragments with n > 3 contribute less than 1% of the total. Aspects of the Kr(n) cluster ionization process and the experimental measurements are discussed to provide possible reasons for the surprisingly high probability of observing fragmentation to the Kr(+) monomer ion.     

Ionization potentials of large sodium doped ammonia clusters

In a continuous neat supersonic expansion ammonia clusters are generated and doped with sodium atoms in a pickup cell. Thus clusters of the form Na(NH(3))(n) are produced that are photoionized by a tunable dye laser system. The ions are mass analyzed in a reflectron time-of-flight mass spectrometer, and the wavelength dependent ion signals serve for the determination of the ionization potentials (IP) of the different clusters in the size range 10< or =n< or =1500. Aside from a plateau for 10< or =n< or =17 and smaller steps at n=24, 35, and 59 on the average a continuous decrease of the IP with cluster size is observed. The IPs in this size range are linear with (n+1)(-13) and extrapolate to IP(n=infinity)=1.66+/-0.01 eV. The slope is consistent with a dielectric continuum model of the solvated electron and the dielectric constant of the solid. The extrapolated IPs are compared with results obtained for negative ammonia cluster ions and metallic solutions in liquid ammonia. Differences are explained by the presence of counterions and their various distances from the solvated electron.     

Photodetachment of electrons from tetracyanoethylene negative ions

A crossed beam experiment using a surface ionization negative ion source and a xenon are lamp has allowed the relative cross section for photodetachment of an electron from a negative ion of tetracyanoethylene to be determined for photons in the energy range 2.1—3.5 eV (590—350nm). The threshold energy was found to be 2.03 ± 0.07 eV.     

TPEPICO studies near ionization threshold of argon and krypton clusters

Single photon ionization of argon and krypton clusters has been studied in the region between threshold and the ionization potential of the corresponding atom. Synchrotron radiation from the electron storage ring BESSY is used to ionize the clusters; threshold-photo-electron-photoion-coincidence (TPEPICO)-time-of-flight technique is used to detect ions correlated with the emission of zero-kinetic-energy-electrons. The spectra of the clusters in the range ofn=2 to 15 are discussed in view of the extensive fragmentation taking place in these systems. In order to characterize the properties of the clusters a method using scaling laws is applied. The principles and the deduction of Hagena's scaling parameter Γ* are briefly reviewed. Using Γ* an experimentally derived mean cluster size for molecular beams can be assigned. This allows one to clearly demonstrate the systematic variations of the measured spectra due to cluster fragmentation. As a general feature it is observed that, in the range studied, the peak in the measured ionization rate for a cluster ion (fragment) of a given size shifts to higher photon energies as the mean cluster size is increased.     

Electron impact ionization of small silver and copper clusters

Using a quadrupole mass spectrometer, relative cross sections for electron impact ionization of neutral Ag _n and Cu _n clusters withn=1 ... 4 have been measured for electron energies between threshold and 125 eV. From the results, the following ionization energies were obtained: Ag₂: 7.26±0.1 eV, Ag₃: 6.19±0.2 eV, Ag₄: 6.33±0.3 eV, Cu₂: 7.46±0.15 eV, Cu₃: 6.14±1.0 eV, Cu₄: 7.00±0.6 eV. With only two exceptions, these values agree with other data published for Ag₂, Cu₂, Cu₃ and Cu₄.     

The Kohn-Sham density of states and band gap of water: from small clusters to liquid water

Electronic properties of water clusters (H2O)(n), with n=2, 4, 8, 10, 15, 20, and 30 molecules were investigated by sequential Monte Carlo/density-functional theory (DFT) calculations. DFT calculations were carried out over uncorrelated configurations generated by Monte Carlo simulations of liquid water with a reparametrized exchange-correlation functional that reproduces the experimental information on the electronic properties (first ionization energy and highest occupied molecular orbital-lowest unoccupied molecular orbital gap) of the water dimer. The dependence of electronic properties on the cluster size (n) shows that the density of states (DOS) of small water clusters (n>10) exhibits the same basic features that are typical of larger aggregates, such as the mixing of the 3a1 and 1b1 valence bands. When long-ranged polarization effects are taken into account by the introduction of embedding charges, the DOS associated with 3a1 orbitals is significantly enhanced. In agreement with valence-band photoelectron spectra of liquid water, the 1b1, 3a1, and 1b2 electron binding energies in water aggregates are redshifted by approximately 1 eV relative to the isolated molecule. By extrapolating the results for larger clusters the threshold energy for photoelectron emission is 9.6+/-0.15 eV (free clusters) and 10.58+/-0.10 eV (embedded clusters). Our results for the electron affinity (V0=-0.17+/-0.05 eV) and adiabatic band gap (E(G,Ad)=6.83+/-0.05 eV) of liquid water are in excellent agreement with recent information from theoretical and experimental works.     

Surface induced reactions of cluster ions

First results are presented from a new apparatus, consisting of a supersonic beam for generating neutral clusters, a variable energy electron gun for ionizing the clusters, and a tandem mass spectrometer set-up for studying surface induced reactions of mass and energy selected cluster ions. Rare gas cluster ions, fragment ions from SF₆, benzene ions and benzene cluster ions have been investigated so far. Cluster ion dissociation, intracluster ion molecule reactions and surface reactions with adsorbed hydrocarbons have been shown to be important reaction channels for these ion-surface collision at energies ranging from a few eV to 500 eV. The surface induced fragmentation spectrum is demonstrated to be a useful tool for probing binding energy and structure of cluster ions.     

Nonergodicity in electron capture dissociation investigated using hydrated ion nanocalorimetry

Hydrated divalent magnesium and calcium clusters are used as nanocalorimeters to measure the internal energy deposited into size-selected clusters upon capture of a thermally generated electron. The infrared radiation emitted from the cell and vacuum chamber surfaces as well as from the heated cathode results in some activation of these clusters, but this activation is minimal. No measurable excitation due to inelastic collisions occurs with the low-energy electrons used under these conditions. Two different dissociation pathways are observed for the divalent clusters that capture an electron: loss of water molecules (Pathway I) and loss of an H atom and water molecules (Pathway II). For Ca(H(2)O)(n)(2+), Pathway I occurs exclusively for n >or= 30 whereas Pathway II occurs exclusively for n 相似文献   

Ejection of cluster ions as a result of electron impact ionization of argon clusters

Time evolution of mass distribution of argon cluster ions has been studied in pulsed jets (pulse duration≈0.4 ms) by retarding potential method. The ejection of cluster ions of the order of 180 atoms from cluster ions of the order of 500 atoms has been revealed. The ejection has a threshold character as a function of the time elapsed after electron impact ionization. The threshold time was about 20 μs. Delayed non-thermal fragmentation of the cluster ions testifies that the self-trapped ion, formed in argon clusters as a result of ionization, can live for a long time in its excited state without dissipation of the excess energy to the cluster. This ion should be located on the surface of the cluster. Energetic analysis of the fragmentation is consistent with the size of the ejected clusters. A model of the observed time evolution of the delayed fragmentation is herein proposed.     

Ionization potentials and electron affinities of Cu,Ag, and Au: Electron correlation and relativistic effects

The electron correlation and relativistic effects on ionization potentials and electron affinities of Cu, Ag, and Au are investigated in the framework of the coupled cluster method and different 1-component approximations to the relativistic Dirac-Coulomb Hamiltonian. The first-order perturbation approach based on the massvelocity and Darwin terms is found to be sufficiently accurate for Cu and Ag while it fails for Au. The spin-averaged Douglas-Kroll no-pair method gives excellent results for the studied atomic properties. The ionization potentials obtained within this method and the coupled cluster scheme for the electron correlation effects are 7.733(7.735) eV for Cu, 7.461(7.575) eV for Ag, and 9.123(9.225) eV for Au (experimental values given in parentheses). The calculated (experimental) electron affinity results for Cu, Ag, and Au are 1.236(1.226), 1.254(1.303), and 2.229(2.309) eV, respectively. There is a marked relativistic effect on both the ionization potential and electron affinity of Ag which sharply increases for Au while Cu exhibits only a little relativistic character. A similar pattern of relativistic effects is also observed for electric dipole polarizabilities of the coinage metal atoms and their ions. The coupled cluster dipole polarizabilities of the coinage metal atoms calculated in this article in the Douglas-Kroll no-pair formalism (Cu: 46.50 au; Ag: 52.46 au; Au: 36.06 au) are compared with our earlier data for their singly positive and singly negative ions. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 557–565, 1997     

Surface induced ionization of neutral water clusters

The paper presents the results of study of a new phenomenon — the formation of cluster ions of both signs in the scattering of neutral water clusters by solid surfaces and the appearance of a current to the target. The experiments were carried out by the nozzle molecular beam technique. The probabilities of the charge removal from the surface were measured in dependence on incident angle, cluster size, and target material. The model of the process has been proposed which incorporates 1) formation of an ion pair during ion dissociation of a vibrationally excited molecule in a water cluster colliding with the surface; 2) asymmetric neutralization of an ion pair by surface; 3) inertial removal of a cluster ion from the surface. However the measured angular and energy distribution of emitted charged particles testify to more complex mechanisms of ionization and scattering because the spatial pattern of ion emission is of rainbow character and in some cases the direction of inertial removal (along the tangential component of initial velocity) does not dominate.     

Unisized two-dimensional platinum clusters on silicon(111)-7x7 surface observed with scanning tunneling microscope

Uni-sized platinum clusters (size range of 5-40) on a silicon(111)-7 x 7 surface were prepared by depositing size-selected platinum cluster ions on the silicon surface at the collision energy of 1.5 eV per atom at room temperature. The surface thus prepared was observed by means of a scanning tunneling microscope (STM) at the temperature of 77 K under an ambient pressure less than 5 x 10(-9) Pa. The STM images observed at different cluster sizes revealed that (1) the clusters are flattened and stuck to the surface with a chemical-bond akin to platinum silicide, (2) every platinum atom occupies preferentially the most reactive sites distributed within a diameter of approximately 2 nm on the silicon surface at a cluster size up to 20, and above this size, the diameter of the cluster increases with the size, and (3) the sticking probability of an incoming cluster ion on the surface increases with the cluster size and reaches nearly unity at a size larger than 20.     

Ionization potential of clusters in liquids

Time-resolved observations of the fast electron transfer from an electron donor to metal ions adsorbed on metal clusters in solution have shown that a critical size of the cluster is required to make it capable of accepting electrons. The threshold is attributed to a size dependent redox potential of the cluster, increasing with the nuclearity (in contrast with the ionization potential in the gas phase which decreases when n increases): it corresponds to the nuclearity for which the cluster redox potential becomes more positive than the potential of the electron donor acting as a monitor.New data of redox potentials (or IP) of Ag_n clusters (hydroquinone as monitor) and Cu_n cluster (sulfonatopropylviologen anion as monitor) are derived. The influence of n and of the solvation or the ligand is discussed.