Electronic structure of small copper clusters

An all-electron ab initio LCAO -MO SCF calculation has been carried out for the electronic structure of small copper clusters (Cu_n, n = 2–6). The basis set superposition error occurring in the calculation, the equilibrium configuration of Cu₃, the bond energy in the clusters, and the localized d-hole in excited and ionized states of Cu₂ are closely examined.     

Electronic spectroscopy of para-fluorotoluene clusters

We report (1 + 1) resonance-enhanced multiphoton ionization spectra for clusters of para-fluorotoluene (pFT)_n (n = 1–11). After n = 2, the spectra appear to have converged in appearance, suggestive of a dimer chromophore, with weak bonding for subsequent additions of pFT molecules. The spectra also indicate dramatic, vibrational-mode-sensitive changes of oscillator strength between the monomer and the clusters. We also briefly describe the results of probing different parts of the expansion, and varying the laser power.     

Far-infrared spectroscopy of small neutral silver clusters

The vibrational spectra of Ag(3) and Ag(4) are recorded in the far-infrared between 100 and 220 cm(-1) using multiple photon dissociation spectroscopy of their complexes with Ar atoms. For Ag(3)-Ar two IR active bands are found at 113 and 183 cm(-1), for Ag(4)-Ar one band at 163 cm(-1) and very weak IR activity at 193 cm(-1) are observed. This, together with recent theoretical studies, allows for a reassignment of the controversial vibrational data reported earlier for the bare Ag(3) cluster. The influence of the number of Ar atoms in the complexes on the frequency of the IR active modes is found to be minor. However, the low-frequency IR-active band of Ag(3) shifts with increasing Ar coverage from 113 cm(-1) for Ag(3)-Ar to about 120 cm(-1) for Ag(3)-Ar(4), the value known for Ag(3) embedded in rare gas matrices.     

High resolution spectroscopy of small metal clusters

The optical absorption spectrum of small lithium clusters has been measured up to Li₈. In Li₃ high resolution Two Photon Ionization (TPI) spectra have been recorded allowing us to determine the geometry and potential surfaces of the ground and excited states. In larger clusters, the excited states are dissociative and the absorption spectra have been obtained by Depletion Spectroscopy. Vibronic resolution is still achieved in Li₄, but not in larger clusters. The measured spectra exhibit a rather small number of transitions to electronically excited states. In Li₇, only one intense band is observed in the blue region, while in Li₈, an intense band is also observed in the blue region and a much weaker band in the red region. All the obtained results are in very good agreement with the ab initio calculation of Bonacic-Koutecky et al. This demonstrates that molecular effects are always present in these small clusters. The semi-classical models of surface plasma resonances are also discussed.     

Electronic spectrum of small silicon clusters in silicon dioxide

Institute of Theoretical and Applied Mechanics, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 4, pp. 23–26, July–August, 1989.     

Electronic correlation and effective interactions in small alkali clusters

The crucial importance of correlation effects versus delocalization, and their nature in small Alkali clusters is analysed from an ab-initio point of view through a detailed investigation of the Li₂ dimer. The role of the external correlation (provided by extended basis sets and large Configuration Interaction calculations) is shown to lower the energy of ionic configurations and to increase their weight in the electronic wavefunction, increasing simultaneously the importance of delocalization versus internal correlation within thes-band. Effective interactions are determined from accurate diabatic calculations on dimers and transfered to clusters via an effective hamiltonian spanned bys orthogonal orbitals. Although not including explicitely thep-band, this model provides results in good agreement with abinitio calculations on Lithium clusters.     

Electronic structure and magnetic properties of small manganese oxide clusters

To investigate the electronic structure and magnetic properties of manganese oxide clusters, we carried out first-principles electronic structure calculations for small MnO clusters. Among various structural and magnetic configurations of the clusters, the bulklike [111]-antiferromagnetic ordering is found to be favored energetically, while the surface atoms of the clusters exhibit interesting electronic and magnetic characteristics which are different from their bulk ones. The distinct features of the surface atoms are mainly attributed to the reduction of Mn coordination numbers and the bond-length contractions in the clusters, which may serve as a key factor for the understanding of physical and chemical properties of magnetic oxide nanoparticles.     

Anion photoelectron spectroscopy and density functional study of small aluminum-vanadium oxide clusters

Small aluminum-vanadium oxide clusters, AlVO(y)(-) (y = 1-3) and Al(x)VO(2)(-) (x = 2, 3), were investigated with anion photoelectron spectroscopy and density functional calculations. The adiabatic detachment energies of AlVO(y)(-) were estimated to be 1.06 ± 0.05, 1.50 ± 0.08, and 2.83 ± 0.08 eV for y = 1, 2, and 3. Those of Al(2)VO(2)(-) and Al(3)VO(2)(-) were estimated to be 1.22 ± 0.08 and 1.25 ± 0.08 eV. Comparison of theoretical calculations with experimental measurement suggests that the most probable structure of AlVO(-) cluster is quasilinear with O atom in the middle. AlVO(2)(-) has an irregular chain structure of Al-O-V-O and a C(2v) cyclic structure very close in energy. The structure of AlVO(3)(-) cluster is evolved from the C(2v) cyclic AlVO(2)(-) structure by adding the third O atom to the V atom. Al(2)VO(2)(-) has a pair of nearly degenerate Al-O-V-O-Al chain structures that can be considered as cis and trans forms. Al(3)VO(2)(-) probably has two low-lying isomers each containing a four-membered ring. The structures of the corresponding neutral clusters are discussed.     

Electronic factors determining the methane bond breaking process on small aluminum clusters

In order to understand the catalytic activity of small metal clusters as a function of their size, we have studied the interaction of CH₄ with Al₄ and Al₅ neutral and charged clusters, as well as neutral thermally expanded clusters in the two lowest lying spin states, using density functional theory. These calculations, via extended search, are used to determine the stable positions of H and CH₃ near the cluster, and the transition state to break the H─CH₃ bond. In order to understand the factors underlying the reactivity of the clusters, we have analyzed the electronic structure at the transition state. By an analysis of the change of the electronic density of states close to the transition state, we identify the orbitals involved in the bond breaking process. In conjunction with our previous studies of Al₂ and Al₃ clusters, we find that the small Al clusters, except for Al₅, lower the CH₃─H dissociation barrier with respect to the gas-phase value, although Al lacks occupied d-orbitals. Still, Al₅ does not catalyze methane bond breaking, which is attributed to the required interaction with low-lying Al sp-states. Furthermore, in all cases where stable methyl-aluminum-hydrides are possible, the recombinative desorption of methane is studied by vibrational analysis and application of transition state theory.     

Determining the dissociation threshold of ammonia trimers from action spectroscopy of small clusters

Infrared-action spectroscopy of small ammonia clusters obtained by detecting ammonia fragments from vibrational predissociation provides an estimate of the dissociation energy of the trimer. The product detection uses resonance enhanced multiphoton ionization (REMPI) of individual rovibrational states of ammonia identified by simulations using a consistent set of ground-electronic-state spectroscopic constants in the PGOPHER program. Comparison of the infrared-action spectra to a less congested spectrum measured in He droplets [M. N. Slipchenko, B. G. Sartakov, A. F. Vilesov, and S. S. Xantheas, J. Phys. Chem. A 111, 7460 (2007)] identifies the contributions from the dimer and the trimer. The relative intensities of the dimer and trimer features in the infrared-action spectra depend on the amount of energy available for breaking the hydrogen bonds in the cluster, a quantity that depends on the energy content of the detected fragment. Infrared-action spectra for ammonia fragments with large amounts of internal energy have almost no trimer component because there is not enough energy available to break two bonds in the cyclic trimer. By contrast, infrared-action spectra for fragments with low amounts of internal energy have a substantial trimer component. Analyzing the trimer contribution quantitatively shows that fragmentation of the trimer into a monomer and dimer requires an energy of 1700 to 1800 cm(-1), a range that is consistent with several theoretical estimates.     

Infrared spectroscopy of small sodium-doped water clusters: interaction with the solvated electron

The measured vibrational OH-stretch spectra of size-selected Na(H2O)n clusters for n=8, 10, 16, and 20 are compared with first-principle calculations, which account for the interaction of the sodium cation, the electron, and the water molecules with the hydrogen-bonded network. The calculated harmonic frequencies are corrected by comparing similar results obtained for pure water clusters with experiment. The experimental spectra are dominated by intensity peaks between 3350 and 3550 cm(-1), which result from the interaction of the H atoms with the delocalized electron cloud. The calculations, which are all based upon the average spectra of the four lowest-energy isomers, indicate that most of the peaks at the lower end of this range (3217 cm(-1) for n=8) originate from the interaction of one H atom with the electron distribution in a configuration with a single hydrogen-bonding acceptor. Those at the upper end (3563 cm(-1) for n=8) come from similar interactions with two acceptors. The doublets, which arise from the interaction of both H atoms with the electron, appear in the red-shifted part of the spectrum. They are with 3369/3443 cm(-1) quite pronounced for n=8 but slowly vanish for the larger clusters where they mix with the other spectral interactions of the hydrogen-bonded network, namely, the fingerprints of the free, the double, and the single donor OH positions known from pure water cluster spectroscopy. For all investigated sizes, the electron is sitting at the surface of the clusters.     

Electronic spectroscopy of toluene-rare-gas clusters: the external heavy atom effect and vibrational predissociation

Toluene-X van der Waals clusters (where X = Ne, Ne2, Ar, Ar2, Kr, Xe) have been investigated by fluorescence excitation spectroscopy in the region of the S1-S0 transition. With the exception of Xe, for each rare-gas studied, we have assigned cluster transitions in the region of all the strong monomer vibrational bands up to 1000 cm(-1) above the origin band. We have further investigated the S1 relaxation dynamics for each vibrational level of each complex, via their fluorescence decay profiles. Clustering with neon has little appreciable effect on the vibrationless S1 lifetime. By contrast, the clusters with argon and krypton exhibit markedly shorter fluorescence lifetimes compared with the monomer. The effect is so severe in the case of toluene-Xe clusters that no fluorescence signals were observed. We interpret these results in terms of an external heavy atom effect in which the rate of intersystem crossing in toluene is influenced by the cluster partner. For clusters built upon excited S1 vibrational levels, the situation is potentially complicated by intramolecular vibrational redistribution and vibrational predissociation (VP). The majority of the fluorescence decay profiles were satisfactorily modeled using single exponential decays. The emission following pumping of the 37(1) level in the toluene-Kr cluster, however, is an exception. We have modeled the decay of this level with a simple kinetic scheme including VP and determined a predissociation rate of (1.04 +/- 0.54) x 10(7) s(-1).     

Electronic properties of divalent-metal clusters

The transition from van der Waals to metallic bonding expected to occur in divalent-metal clusters (e.g., Be _n , Mg _n , Hg _n ) as a function of cluster size is discussed. Theoretical results for several electronic properties reflecting this transition in Hg _n -clusters are briefly reviewed and compared with available experiments. The limitations of the present theory particularly concerning the role of correlations and van der Waals interactions are discussed and possible improvements are suggested.     

Electronic structure of metal clusters

Photoemission spectra of valence electrons in metal clusters, together with threshold ionization potential measurements, provide a coherent picture of the development of the electronic structure from the isolated atom to the large metallic cluster. An insulator-metal transition occurs at an intermediate cluster size, which serves to define the boundary between small and large clusters. Although the outer electrons may be delocalized over the entire cluster, a small cluster remains insulating until the density of states near the Fermi level exceeds 1/kT. In large clusters, with increasing cluster size, the band structure approaches that of the bulk metal. However, the bands remain significantly narrowed even in a 1000-atom cluster, giving an indication of the importance of long-range order. The core-electron binding-energy shifts of supported metal clusters depend on changes in the band structure in the initial state, as well as on various final-state effects, including changes in core hole screening and the coulomb energy of the final-state charge. For cluster supported on amorphous carbon, this macroscopic coulomb shift is often dominant, as evidenced by the parallel shifts of the core-electron binding energy and the Fermi edge. Auger data confirm that final-state effects dominate in cluster of Sn and some other metals. Surface atom core-level shifts provide a valuable guide to the contributions of initial-state changes in band structure to cluster core-electron binding energy shifts, especially for Au and Pt. The available data indicate that the shift observed in supported, metallic clusters arise largely from the charge left on the cluster by photoemission. As the metal-insulator transition is approached from above, metallic screening is suppressed and the shift is determined by the local environment.     

Electronic structure of Lanthanum-carbon clusters

As the attachment of a metal change the molecular and electronic structure of carbon clusters, the electronic properties as ionization potentials (IP) and electron affinities (EA) for small Lanthanum-carbon clusters LaC _n with n=1–6 have been investigated theoretically. They were studied by density-functional-theory (DFT) within LDA and considering Gradient corrections (GC) for the exchange-correlation potential ( Becke-Perdew). The results for both quantities were obtained in good agreement with the experimental data: odd-even alternating IP’s, and no alternations for the EA’s. The different charge location in the carbon chains or at the La atom can explain the different trends of both quantities, respectively.     

Fragmentation spectroscopy of heterogeneous clusters

Photoionisation experiments were performed with heterogeneous Ar-Xe-clusters produced by supersonic expansion of argon gas with small quantities of xenon added to it. A threshold-electron photoionisation (TEPICO) technique was used to obtain time of flight cluster mass spectra. These mass spectra show particularly strong intensities for Ar₁₂Xe⁺ and Ar₁₈Xe⁺ which are attributed to the extraordinary stabilities of these cluster ions. Maxima in the ionic size distribution around Ar₇Xe⁺ are related to a particular abundance ofneutral Ar₁₂Xe which is fragmented after ionization. These stabilities are explained in terms of geometries consisting of a central Xe atom or ion surrounded by shells of Ar atoms. Filled shells exhibit particular strong bonding because these exhibit the largest number of atom-atom bonds. This conclusion is supported by simple theoretical calculations. The ionization process is discussed in terms of two direct and one indirect ionization channels the latter one proceeding via an intermediate electronic excitation of the Ar component in the neutral cluster.     

Photoelectron spectroscopy of silver clusters

A comparison of x-ray photoemission from Ag clusters deposited on polygraphite and highly oriented pyrolitic graphite shows the influence of the support both on the valence band and on the core 3d level of the metal. Positive shifts have been obtained with respect to the bulk for the Fermi edge and the 3d peaks depending on the number of silver atoms deposited on the substrates. When the deposition is very small (cluster regime) the positive shifts of the binding energies are quite different for different substrates and cannot have a common origin. In contrast with recent work, we show that several effects contribute to these shifts: initial state effects like charge redistribution as well as final state effects like the hole-electron interaction.     

Electronic structure calculations of small Al n (n=2–8) clusters

The electronic states of small Al _n (n=2–8) clusters have been calculated with a relativistic ab-initio MO-LCAO Dirac-Fock-Slater method using numerical atomic DFS wave-functions. The excitation energies were obtained from a ground state calculation of neutral clusters, and in addition from negative clusters charged by half an electron in order to account for part of the relaxation. These energies are compared with experimental photo-electron spectra.