Theoretical investigation of the ultrafast NeNePo spectroscopy of Au 4 and Ag 4 Clusters

Ultrafast ground state nuclear dynamics of Au₄ and Ag₄ is theoretically explored in the framework of negative ion - to neutral - to positive ion (NeNePo) pump-probe spectroscopy based on the ab initio Wigner distribution approach. This involves the preparation of a nonequilibrium neutral ensemble by pump induced photodetachment of a thermal anionic ground state distribution, gradient corrected DFT classical trajectory simulations “on the fly” on the neutral ground state, and detection of the relaxation process of the ensemble in the cationic ground state by a time-delayed probe pulse. In Au₄, the initially prepared linear structure is close to a local minimum of the neutral state giving rise to characteristic vibrations in the signals for probe wavelength near the initial Franck-Condon transition. A timescale of ∼1 ps for the structural relaxation towards the stable rhombic D_2h neutral isomer was determined by the increase of the signal for probe wavelength in vicinity of the vertical ionization energy of the rhombic structure. In contrast, the relaxation dynamics in Ag₄ is characterized by normal mode vibrations since both the initially prepared anionic ground state and the neutral ground state have rhombic minimum geometries. Thus, time-resolved oscillations of pump-probe signals are fingerprints of structural behaviour which can be used experimentally for the identification of particular isomers in the framework of NeNePo spectroscopy. Received 22 December 2000     

Doublet and quartet states of Li 2 −

Electronic states of the molecular lithium anion are investigated by configuration-interaction calculations. Comparison with the analogously computed potential energy curves for the lowest singlet and triplet states of the neutral Li₂ shows that in addition to the well-known stable ground state X there also exist metastable excited states of Li ₂ ^- . Within the quartet sector, two candidates for such long-lived states are identified and their spectroscopic properties studied. Received 23 March 1999     

Structural properties and reactivity of bimetallic silver-gold clusters

Bimetallic silver-gold clusters are well suited to study changes in metallic versus ionic properties involving charge transfer as a function of the size and the composition. We present structures, ionization potentials (IP) and vertical detachment energies (VDE) for neutral and charged bimetallic Au_nAg_m ( 2(n + m)5) clusters obtained from density functional level of theory. In the stable structures of these clusters Au atoms assume positions which favor charge transfer from Ag atoms. In clusters with equal numbers of hetero atoms (n = m = 1- 4) heteronuclear bonding is preferred to homonuclear bonding, giving rise to large values of ionization potentials. For larger clusters (n=m=5, 10) stable structures do not favor neither hetero bonding nor segregation into the single components, although they exhibit more metallic than ionic features. This remains valid also for Au₈Ag₁₂ cluster characterized by strong charge transfer to gold subunit. The influence of doping of pure gold clusters with silver atoms on VDE and IP values is discussed in context of their reactivity towards O₂ and CO molecules. As a starting point we consider reactivity towards CO and O₂ molecules on the example of AgAu^- dimer. The results show that the catalytic cycle can be fullfilled.     

Optical absorption of isolated silver cluster-tryptophan: A joint experimental and theoretical study

We present a joint experimental and theoretical gas phase study of photoabsorption and photofragmentation of silver cluster-biomolecule complexes. We demonstrate on the example of [ Trp.Ag₃] ⁺ that binding of the metal cluster to a biomolecule leads to a significant enhancement of the photoabsorption in comparison with [Trp.H]⁺ and [Trp.Ag]⁺. This enhancement arises due to the coupling between the excitations in the metallic subunit with charge transfer excitations between silver cluster and tryptophan. Our experimental studies show that silver clusters up to eleven atoms can be bound to tryptophan and we present first results on the photofragmentation of the Trp.Ag₁₁ ⁺ complex cation, in which properties of cluster subunit remain preserved.     

Optical properties of small silver clusters supported at MgO

We present structural and optical properties of silver clusters Ag_n (n=2, 4, 6, 8) at two model support sites of MgO, stoichiometric MgO(100) and F_S-center defect, based on density functional theory and embedded cluster model. Our results provide the mechanism responsible for the absorption and emission patterns due to the specific interaction between the excitations within the cluster and the support site which is strongly cluster size and structure dependent. We propose Ag₄ at stoichiometric site as well as Ag₂, Ag₄ and Ag₆ at F_S-center defects as good candidates for the emissive centers in the visible regime.     

Dynamical aspects and the role of IVR for the reactivity of noble metal clusters towards molecular oxygen

Here, we present the dynamical aspects and the role of internal energy redistribution (IVR) in the reactivity of noble metal clusters towards O₂. We show on the example of Ag₃O₂ ^- / Ag₃O₂ / Ag₃O₂ ⁺ that NeNePo spectroscopy carried out under zero electron kinetic energy (ZEKE) conditions can be a powerful tool to investigate the geometry relaxation and IVR induced by photodetachment in real time. Furthermore, we demonstrate that difference in the reactivity of Ag₆ ^- and Au₆ ^- towards O₂ can be attributed to different nature of the IVR process. Dissipative IVR in Ag₆ ^- favors fast complex stabilization, whereas resonant IVR found for Au₆ ^- might be an important factor determining the catalytic activity of Au₆ ^- cluster in the CO oxidation.     

Reactivity of anionic gold oxide clusters towards CO: experiment and theory

In this paper, we present results from our joint experimental and theoretical study of the reactivity of anionic gold oxide clusters Au_2,3O_1-4 ^- towards CO. We provide clear evidence that, although O–O bond weakening/dissociation is important to enable CO oxidation, the presence of atomic oxygen can be favorable but is not always sufficient. Furthermore, we show that with the addition of a single gold atom the reactivity channels can be changed. As a consequence, in contrast to CO oxidation in the case of anionic gold dimer oxides, association of CO or replacement of O₂ by CO become the dominant reaction channels for Au₃O_n ^-. This demonstrates the nonscalable properties of gold clusters in the size regime in which each atom counts.     

$ {\rm{V}}_2 {\rm{O}}_5^ + $ {\rm{V}}_2 {\rm{O}}_5^ + " border="0"> reaction with C2H4: theoretical considerations of experimental findings

Density functional calculations have been performed for the reactions towards ethylene considering atomic and molecular oxygen loss, oxygen transfer and association reactions. The oxygen transfer channel to ethylene is energetically favourable in contrast to the oxygen loss. This is in agreement with the experimental results [1] which show that does not lose atomic oxygen during the collision induced dissociation at thermal energies. A radical cation mechanism based on structure-reactivity relation of cluster is proposed to explain oxygen transfer channel indicating that this reaction is size selective.     

Ab initio molecular dynamics study on Ag (n = 4, 5, 6)

Ab initio Molecular Dynamics (MD) method, based on density functional theory (DFT) with planewaves and pseudopotentials, was used to study the stability and internal motion in silver cluster Ag_n, with n =4-6. Calculations on the neutral, cationic and anionic silver dimer Ag₂ show that the bond distance and vibrational frequency calculated by DFT are of good quality. Simulations of Ag₄, Ag₅, and Ag₆ in canonical ensemble reveal distinct characteristics and isomerization paths for each cluster. At a temperature of 800 K, an Ag₄ has no definite structure due to internal motion, while for Ag₅ and Ag₆the clusters maintain the planar structure, with atomic rearrangement observed for Ag₅ but not for Ag₆. At a temperature of 200 K, Ag₄ can exist in two planar structures whilst Ag₅ is found to be stable only in the planar form. In contrast Ag₆ is stable in both planar trigonal and 3D pentagonal structures. Micro-canonical MD simulation was performed for all three clusters to obtain the vibrational density of states (DOS). Received 5 May 1999 and Received in final form 20 August 1999     

Kinetics and equilibrium of small metallic clusters: Ab initio confinement molecular dynamics study of 4

The ab initio molecular dynamics (AIMD) [1] is combined with the heuristic, successive confinement method of surveying a potential energy surface (PES) [2], thereby offering a framework for the simulation study of kinetics and equilibrium properties of metallic clusters. This approach is applied to the study of Au₄, a cluster possessing a simple but specific PES, which consists of very shallow and deep basins and due to this presents a challenge to the conventional AIMD methods. Among other things, the probabilities of the transitions between isomers have been found, and on this basis, both the time-dependent and equilibrium populations of the isomers have been calculated for the conditions typical of the NeNePo experiments [3] in the femtosecond pump-probe spectroscopy.     

Theoretical study of structural properties and dissociation pathways of FCN and ClCN

Geometrical parameters, harmonic frequencies and molecular properties of FCN, ClCN, their cations and the isomers of FCN and ClCN are studied in detail using ab initio MP2, CCSD and CCSD(T) methods. The dissociation energy of FCN and ClCN in various dissociation channels has been investigated. Both ground and metastable state of the fragmented atoms are considered in their dissociation pathways. The isomerization energy of FCN and ClCN and the NBO charges of FCN, ClCN, their cations and isomers are analyzed.     

Large-scale molecular dynamics simulations of high energy cluster impact on diamond surface

Large-scale molecular dynamics simulations with high acceleration energy on a diamond surface were performed in order to investigate the surface erosion process. Accelerated argon or CO₂ clusters (∼960 atoms, 100 keV/cluster) impacted on the (111) surface of diamond which consisted of more than 1,000,000 carbon atoms. A typical hemispherical crater appeared about 0.7 ps after the impact, and two or three-layered shockwaves were formed and propagated to certain directions, but the crater was immediately filled up with the fluidized hot carbon material due to the collective elastic recovery before the reflection of the shockwave. The impact energy of the cluster was at first transferred mainly as kinetic energy of the diamond surface in a short time, and the potential energy was activated later. The activated carbon and oxygen atoms from the impact cluster stimulated the evaporation from the diamond surface for the CO₂ cluster impact while the evaporation seemed to be suppressed by the argon atoms themselves for the argon cluster impact. Received 22 November 2000     

Theoretical exploration of ultrafast spectroscopy of small clusters

We present the ultrafast multistate nuclear dynamics involving adiabatic and nonadiabatic excited states of non-stoichiometric halide deficient clusters (Na_nF_n-1) characterized by strong ionic bonding and one-excess electron for which the “frozen ionic bonds” approximation has been justified allowing to consider the optical response of the single excess electron in the effective field of the other electrons. We combined the Wigner-Moyal representation of the vibronic density matrix with the ab initio multi state molecular dynamics in the ground and excited electronic states including the nonadiabatic couplings calculated “on the fly” at low computational demand. This method allows the simulation of femtosecond pump-probe and pump-dump signals based on an analytical formulation, which utilizes temperature dependent ground state initial conditions, an ensemble of trajectories carried out on the electronic excited state as well as on the ground state after the passage through the conical intersection in the case of nonadiabatic dynamics and for probing either in the cationic state or in the ground state. The choice of the systems we presented has been made in order to determine the timescales of the fast geometric relaxation leaving the bonding frame intact as during the dynamics in the first excited state of Na₄F₃, and of the bond breaking processes leading to conical intersection between the first excited state and the ground state as in Na₃F₂. The former is the smallest finite system prototype for an surface F-center of bulk color centers. The latter allows to study the photo isomerization in full complexity taking into account all degrees of freedom. In the case of Na₄F₃ after the fast geometric relaxation in the excited state leading to deformed cuboidal structure without breaking of bonds, different types of internal vibrational redistribution (IVR) processes have been identified in pump-dump signals by tuning the dump laser. In contrast, from the analysis of the pump-probe signals of Na₃F₂ cluster, the timescales for the metallic and the ionic bond breaking, as well as for the passage through conical intersection have been determined. Finally the conditions under which these processes can be experimentally observed have been identified. Received 22 December 2000     

Initial growth of single-walled carbon nanotubes on supported iron clusters: a molecular dynamics study

Molecular dynamics simulations were used to study the initial growth of single-walled carbon nanotubes (SWNTs) on a supported iron cluster (Fe₅₀). Statistical analysis shows that the growth direction of SWNTs becomes more perpendicular to the substrate over time due to the weak interaction between carbon nanotube and the substrate. The diameter of the nanotube also increases with the simulation time and approaches the size of the supported iron cluster.     

Structural and vibrational properties of C36 and its oligomers (C36) M = 2, 3, 4 by tight-binding molecular dynamics

Non-orthogonal tight-binding molecular-dynamics is employed to calculate structural and vibrational properties of C₃₆ and its oligomers (C₃₆) _{M = 2, 3, 4} . The lowest energy configuration of the C ₃₆ cage is confirmed to have D _6h symmetry. For the dimer, too, the D _2h structure reported in the literature is found. The vibrational spectrum is identified with the power spectrum of the displacement autocorrelation function. Additional vibrational properties are extracted from the dynamical matrix. For the monomer, fair agreement with available ab initio calculations is achieved, with comparatively smaller deviations in the Raman-frequencies than for published semi-empirical calculations. The features of the vibrational modes are correlated with the structural properties of the oligomers. Received 24 November 2000 and Received in final form 24 August 2001     

Classical dynamical simulation of spontaneous alloying

“Spontaneous alloying” observed by Yasuda, Mori et al. for metallic small clusters is simulated using classical Hamiltonian dynamics. Very rapid alloying occurs homogeneously and cooperatively starting from the solid phase of the cluster if the heat of solution is negative and the size of cluster is less than a critical size. Analysis of 2D models reveals that the alloying rate obeys an Arrhenius-type law, which predicts the alloying time much less than second at room temperature. Evidences manifesting that the spontaneous alloying proceeds in the solid phase without melting are also presented. The simulation reproduces the essential features of the experiments. Received: 2 March 1998 / Revised: 21 May 1998 / Accepted: 28 May 1998     

A DFT study of electron or hole localization in a peptide containing asparagin

The mechanisms of protein degradation induced by ionisation are of great interest for radiobiology, improvement of mass spectroscopy and industrial processes such as radio sterilisation. Sequences containing asparagin are very sensitive especially if surrounded by glycine. Very few techniques allow a satisfying understanding of the processes induced by creation of an anionic or cationic site in a peptide. We used the methods of quantum chemistry (DFT/B3LYP with 6-31G^* basis set) to characterise the geometry modifications induced in the cations or in the anions derived from peptide Gly Asn Gly. The cationic sites are localised mostly close to the first peptidic bond and induce a lengthening of the Ca-C(O) bond. Conversely the anionic sites are localised on a carbonyl function. Implications are discussed considering the radiolytic products and the proposed mechanisms. Received 21 January 2002 Published online 13 September 2002     

Decay reactions of rare gas cluster ions: Kinetic energy release distributions and binding energies

We have carried out measurements on metastable fragmentation of mass selected argon cluster ions which are produced by electron impact ionization of a neutral argon cluster beam. From the shape of the fragment ion peaks (MIKE scan technique) one can deduce information about the distribution of kinetic energy that is released in the decay reaction. In this study, for Ar ₅ ⁺ to Ar ₁₅ ⁺, it is Gaussian and thus we can calculate from the peak width the mean kinetic energy release 〈KER〉 of the corresponding decay reactions. Using finite heat bath theory we calculate from these data the binding energies of the decaying cluster ions. Received 20 November 2000     

Fragmentation of water clusters: Molecular-dynamics simulation study

The fragmentation of water clusters, [(H ₂ O)_n;n = 2-8], have been investigated by using molecular-dynamics simulation method. In the simulations a polarizable-dissociable potential energy function for water has been used. Particular attention has bee paid to investigate the effect of structural properties and cluster size on the fragmentation. Received 27 April 2000 and Received in final form 6 October 2000     

The role of enhanced aromatic -electron donating aptitude of the tyrosyl sidechain with respect to that of phenylalanyl in intramolecular interactions

An exhaustive ab initio and DFT search for energetically stable conformers from the topologically possible set was undertaken on the N-acetyl-phenylalanyl-N-methylamide and N-acetyl-tyrosyl-N-methylamide systems. The geometries of all 81 phenylalanyl and 162 tyrosyl possible rotamers, described under the rules outlined by Multi-Dimensional Conformational Analysis (MDCA), were attempted at each of the RHF/3-21G, RHF/6-31G(d) and B3LYP/6-31G(d) levels of theory. A total of 32 and 66 stable conformational minima were found for the phenylalanyl and tyrosyl amino acid diamides, respectively, at the B3LYP/6-31G(d) level. From the tyrosyl set, 33 unique conformers emerge when the orientation of the A _i ³ dihedral angle (p-OH orientation) is disregarded. A total of 31 conformers were common to both sets and showed nearly identical geometries. The comparison of the optimized DFT geometries of the two systems showed near by perfect linear fits with R² values of 0.9997, 0.9994, 0.9997, and 0.9996 for the φ _i , ψ _i , A _i ¹ , and A _i ² dihedral angles, respectively. Relative energies of the matching 31 conformers also fitted to a linear plot with an R² value of 0.9985. The geometric centroid of the aromatic ring in the sidechain of both systems was found to be within 4.1 ?of the H and O atoms of the peptide groups, in 21 and 2 of the conformers, respectively. None of the non-matching conformers showed any such interaction distance ≤4.1 ?. Received 1st February 2002 / Received in final form 28 May 2002 Published online 13 September 2002