Ultrafast photodissociation of Kn=3...9 clusters

By means of femtosecond pump and probe technique we investigated the temporal evolution of the photodissociation of excited potassium clusters (3 ≤ n ≤ 9). For three different excitation energies E = 1.47 eV, 2.00 eV, and 2.94 eV we were able to measure the transient multiphoton ionization spectra. In the frame of a simple energy level model the photodissociation process could be analyzed. Fragmentation time constants are estimated to be cluster size- and energydependent in the range of 200 fs up to 10 ps. The results are compared with data received from similar experiments on Nan clusters.     

Photoionization studies of transition metal clusters: Ionization potentials of ScnO (n=5−36)

The photoionization efficiency (PIE) spectra and ionization potentials are reported for scandium cluster monoxides (Sc _n O,n=5–36). As found for other transition metal clusters, strong dependence of ionization potential on cluster size is found for small clusters, with the ionization potentials of larger clusters decreasing relatively smoothly with increasing size. The IPs are 0.6–0.8 eV lower than that predicted by conducting spherical droplet model.Performed at Argonne National Laboratory     

Comparative analysis of the state of lithium and sodium atoms in water clusters

Structures and energetic characteristics of Li(H₂O) _n and Li⁺(H₂O) _n clusters with n = 1–6, 19, and 27 determined in the second order of the Møller-Plesset perturbation theory with 6–31++G(d,p) basis set are analyzed. The electron density redistribution, which takes place upon the electron addition to a Li⁺(H₂O) _n cluster, is found to be provided by hydrogen-bonded water molecules: initially almost neutral molecules, which are most distant from lithium, become negatively charged. The calculated energies of the electron capture by Li⁺(H₂O) _n clusters are approximated with the appropriate electrostatic model, and estimates of the lithium ionization energy in water clusters of various sizes are found. Similar estimates obtained earlier for sodium are made more accurate.     

Doubly charged sodium clusters: plasmon response and photoproduction

The optical response of doubly charged sodium clusters Na _n+2 ⁺⁺ was measured for n = 20, 40, and 58 valence electrons, for which the jellium model predicts spherical clusters. A new experimental scheme was developed which allows to separate doubly charged clusters of even mass from the singly charged with half the mass. The optical spectra are dominated by a plasmon-like resonance which is blue shifted and narrower than that of the singly charged clusters. The smallest doubly charged cluster observed was Na ₉ ⁺⁺ . The photo ionization cross section for singly charged clusters was found to be typically 2.6·10^-19cm² per Na atom for photon energies of around 6 eV, which is a factor of 400 smaller than the maximum in the plasmon absorption in the region of hω=2.6 eV.     

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.     

Ionization of sodium in water clusters

Structures of Na(H₂O)_n and Na⁺(H₂O)_n clusters with n = 1?6, 19, and 28 are determined in the second order of the Møller-Plesset perturbation theory with the use of extended atomic basis set 6–31++G^**. It is found that when the number of molecules is sufficient for the formation of two solvation shells around sodium, a continuous hydrogen-bond network is formed in both neutral and charged clusters, and the orientation of each molecule is determined by the balance between interactions with the neighboring water molecules and with the field of the central particle. In the cations, this field is stronger, and up to the third solvation shell, molecules have a predominant orientation with respect to sodium. In the neutral clusters, with an increase in the number of water molecules, the maximum of the electron density distribution of the highest occupied molecular orbital becomes more distant from the sodium nucleus, being shifted toward the cluster surface. The energy of this orbital accordingly decreases in absolute value approaching 22 kcal/mol inmicroparticles. In the charged clusters, the distribution of the positive charge generally correlates with the character of the highest occupied orbital in the neutral systems, so that with an increase in the number of molecules, the atomic charge of sodium decreases and tends to zero as n → ∞. The ionization potential of sodium changes in inverse proportion to the linear size of the cluster, and should not exceed 1.1 eV in watermicroparticles.     

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.     

Photoionization,Structures, and Energetics of Na-Doped Formic Acid–Water Clusters

The influence of formic acid on water cluster aggregation has been investigated experimentally by mass spectrometry and tunable UV laser ionization applied to Na-doped clusters formed in the supersonic expansion of water vapors seeded with formic acid (FA) as well as theoretically using high level quantum chemistry methods. The mass spectra of Na−FA(H₂O)_n clusters show an enlarging of mass distribution toward heavier clusters with respect to the Na−(H₂O)_n clusters, suggesting similar mass distribution in neutral clusters and an influence of formic acid in water aggregation. Density functional theory and coupled-cluster type (DLPNO-CCSD(T)) calculations have been used to calculate structures and energetics of neutral and ionized Na−FA(H₂O)_n as well as neutral FA(H₂O)_n. Na-doped clusters are characterized by very stable geometries. The theoretical adiabatic ionization potential values match pretty well the measured appearance energies and the calculated first six electronic excited states show Rydberg-type characters, indicating possible autoionization contributions in the mass spectra. Finally, theoretical calculations on neutral FA(H₂O)_n clusters show the possibility of similarly stable structures in small clusters containing up to n=4–5 water molecules, where FA interacts significantly with waters. This suggests that FA can compete with water molecules in the starting stage of the aggregation process, by forming stable nucleation seed.     

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 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.     

Structure and energy of the positively ionized water clusters

Geometry optimization of small (H₂O)_n⁺ clusters (n ≤ 4) at the UHF/4–31 + + G^** level indicates that the cations consist of two fragments: the OH radical and the H_2n−1 O⁺_n−1 ion. The latter can be considered as a thermodynamically stable combination of a distorted H₃O⁺ ion and (n−2) H₂O molecules. The H bond between the fragments becomes weaker with increasing cluster size. Extrapolation of the adiabatic ionization potentials calculated for the (H₂O)_n oligomers (n ≤ 4) at the MP2 level to an infinite cluster size provides the value of approximately 8.7 eV, which can be presumably necessary for the ionization of liquid water in a vacuum. © 1997 John Wiley & Sons, Inc.     

Ionization dynamics of van-der-Waals clusters

The ionization process of homogeneous and heterogeneous van-der-Waals clusters has been investigated using various ionization methods (electron bombardment, charge exchange, photoionization methods), and different analyzing techniques. Direct and indirect ionization processes can be distinguished in the experiments from the shape of the ionization curve which depends on the type of cluster. These features appear differently in homogeneous and heterogeneous systems: Homogeneous systems exhibit characteristic ionization efficiency curves where the direct ionization path appears as a sudden increase in the ionization efficiency while the indirect transition gives rise to a long drawn out tail extending to the true ionization threshold. In heterogeneous clusters the indirect ionization path proceeds via excited states of the component with the larger ionization potential and subsequent energy transfer to the other component. These transitions are shifted and broadened depending on the type of internal interaction. Conclusions are drawn concerning the geometry and the interaction potential inside the cluster. The resolution of the TEPICO (Threshold Electron Photo Ion Coincidence) experiments makes it possible to determine the kinetic energy release of the fragments. It is shown that the results are related to the stabilities of the cluster ions involved in the fragmentation chain. Results are presented for pure rare gas clusters (Ar _n , Kr _n , Xe _n ) and for mixed systems (Ar _n O_2m , Ar _n Xe, Kr _n Xe, (CH₄) _n Ne).     

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.     

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.     

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.     

Threshold photoionization behaviour of (Li2O)Lin clusters produced by a laser vaporization source

We report on the production of small and medium size lithium and lithium oxide clusters by a laser vaporization cluster source. The isotopomeric distribution of natural lithium allowed to identify Li_kO clusters as the most abundant components in the mass spectrum. Photoionization efficiency curves of Li_kO clusters with photon energies from 3.4 to 4.7 eV were measured for 8 ≤ k ≤ 27. Using linear extrapolation of the increase in photoionization efficiency with photon energy, ionization potentials were extracted. With the chemical bond of the O^2- anion to two Li atoms, leaving n = k-2 valence electrons in the (Li₂O)Li_n clusters, clear shell closure effects are present at n = 8 and n = 20.     

Small can be different

Theoretical investigations of size evolutionary patterns for multiply charged anionic metal clusters and solvation of sodium in water clusters are discussed. For Na _N ^Z- clusters, energetic stability and electron decay channels are determined. Formation of a “surface Rydberg-like state” in Na(H₂O) _N , correlating with calculated and measured ionization potentials, is analyzed.     

Evaporation of tetramers in Sb4n clusters and conditions for the formation of Sb2n+1 clusters

Antimony clusters are produced by the inert gas condensation technique. They are found to be built from Sb₄ units. The fragmentation by evaporation of Sb₄ units is studied as a function of the excess energy in the cluster. By this way the binding energy of the Sb₄ units in the cluster is found to be about 1.5 eV, well below the binding energy of a Sb atom in the bulk and in Sb₄(?3eV). The evolution of ionization potentials of Sb_4n clusters confirms that their structure is probably non metallic. Finally the possible metastable character of this Sb_4n structure is discussed.     

Photoelectron spectroscopy of jet-cooled aluminium cluster anions

Aluminium cluster anions (Al _n ^? ) are produced by laser vaporization without additional ionization and cooled by supersonic expansion. Photoelectrons from mass-identified anion bunches (n=2...25) are detached by laser light (hv=3.68 eV) and undergo energy analysis in a magnetic bottle-type time-of-flight spectrometer. The measurements provide information about the electronic excitation energies from ionic ground states to neutral states of the clusters. In contrast to bulk aluminium these cluster photoelectron spectra partially have well-resolved bands which originate from low-lying excited bands. For small clusters, especially the aluminium dimer and trimer, quantum-chemical calculations will be compared to the measurements. The electron affinity size dependence of larger clusters shows conclusive evidence for “shell” effects.