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1.
The timescale for the coupling of electronic and vibrational excitation in isolated fullerenes is determined by recording positive ion time-of-flight mass spectra on excitation with ultrashort laser pulses at 790 nm of the same fluence but different pulse durations. The coupling leads to the onset of a delayed ionisation “tail” on the parent fullerene ion peak. This occurs for a pulse duration of 500-1000 fs, depending on laser fluence. Received 20 October 2000  相似文献   

2.
We review some of the ways clusters offer special kinds of insights both into properties of bulk matter and properties unique to small systems. We then survey some of the tantalizing open questions that lie ahead in cluster science.  相似文献   

3.
New experimental data is reported for the absolute cross sections for the fusion reaction channel in single gas-phase collisions between fullerenes. The experimental data is compared with the results of quantum mechanical and classical molecular dynamics simulations as well as with simple models. Quantum molecular dynamics simulations are in very good quantitative agreement with the experimental data. The overall dynamical behaviour can be well-described qualitatively in the framework of simple models. Received 2 October 2000  相似文献   

4.
We present two complementary techniques that provide detailed diagnostics of supersonic beams involving several species. First, surface scattering, together with quadrupole mass spectrometer detection, yields the monomer percentage for each species within the beam. Second, analyses of beam profiles for different masses after scattering by a buffer gas permit determination of mixed cluster presence and, if any, of cluster sizes and compositions. The two techniques are applied to supersonic expansions of an argon-nitrogen mixture. We discuss the results that provide new insight in binary nucleation processes. Received: 6 October 1997 / Revised: 4 November 1997 / Accepted: 13 November 1997  相似文献   

5.
Reactive accelerated cluster erosion (RACE) of single crystal artificial diamond has been used to fabricate various nano- and microstructures. Carbondioxide clusters of about 1000 molecules are accelerated to 100 keV to act as the eroding agent. Using movable shadow masks, the accelerated cluster beam may erode staircase structures acting as an optical grating. A cycloid gear has been generated via a stationary nickel mask. Non-reactive accelerated cluster erosion using argon clusters will be considered for comparison. Received 30 November 2000  相似文献   

6.
Photoionization of rare gas clusters in the innervalence shell region has been investigated using threshold photoelectron and photoion spectrometers and synchrotron radiation. Two classes of states are found to play an important role: (A) valence states, correlated to dissociation limits involving an ion with a hole in its innervalence ns shell, (B) Rydberg states correlated to dissociation limits involving an ion with a hole in its outervalence np shell plus an excited neutral atom. In dimers, class A states are “bright”, that is, accessible by photoionization, and serve as an entrance step to form the class B “dark” states; this character fades as the size of the cluster increases. In the dimer, the “Mulliken” valence state is found to present a shallow potential well housing a few vibrational levels; it is predissociated by the class B Rydberg states. During the predissociation a remarkable energy transfer process is observed from the excited ion that loses its innershell electron to its neutral partner. Received: 10 February 1998 / Revised: 17 July 1998 / Accepted: 31 July 1998  相似文献   

7.
We make a theoretical study of the shake-up of the 1s photoemission of C60. The method takes into account the N-body reactions of the π and σ electrons which appear during the formation of the photoemission hole on one carbon atom. We analyze the origin of the satellite in the spectra due to transitions between N-body states. Our calculation shows that the satellite spectra is essentially given by N-body transitions which involve the creation of one or two electron-hole pairs. The method has been applied also to C48N12. The situation is more complex. The spectra of the two most stable species have been investigated. Moreover the influence on the spectra of the position of the hole created on the carbon atom in C48N12 has been examined (all the carbon positions are not equivalent for some isomers).  相似文献   

8.
Optimized structures and cohesive energies of small mercury clusters (HgN; N = 3–7, 13, 19) are calculated with the spin-orbit diatomics-in-molecules method. The theory takes into account the effect of s-p mixing which tends to enhance the binding energies in the ground state. It is shown that excimer clusters have significantly short optimum bond lengths and their atomic geometries differ considerably from those in the ground state. Excitation energy gap depends sensitively on both cluster size and nearest-neighbor separation. Numerical results are compared with other theories and experiments.  相似文献   

9.
Theoretical and experimental information on the shape and morphology of bare and passivated gold clusters is fundamental to predict and understand their electronic, optical, and other physical and chemical properties. An effective theoretical approach to determine the lowest-energy configuration (global minimum) and the structures of low energy isomers (local minima) of clusters is to combine genetic algorithms and many-body potentials (to perform global structural optimizations), and first-principles density functional theory (to confirm the stability and energy ordering of the local minima). The main trend emerging from structural optimizations of bare Au clusters in the size range of 12-212 atoms indicates that many topologically interesting low-symmetry, disordered structures exist with energy near or below the lowest-energy ordered isomer. For example, chiral structures have been obtained as the lowest-energy isomers of bare Au28 and Au55 clusters, whereas in the size-range of 75-212 atoms, defective Marks decahedral structures are nearly degenerate in energy with the ordered symmetrical isomers. For methylthiol-passivated gold nanoclusters [Au28(SCH3)16 and Au38(SCH3)24], density functional structural relaxations have shown that the ligands are not only playing the role of passivating molecules, but their effect is strong enough to distort the metal cluster structure. In this work, a theoretical approach to characterize and quantify chirality in clusters, based on the Hausdorff chirality measure, is described. After calculating the index of chirality in bare and passivated gold clusters, it is found that the thiol monolayer induces or increases the degree of chirality of the metallic core. We also report simulated high-resolution transmission electron microscopy (HRTEM) images which show that defects in decahedral gold nanoclusters, with size between 1-2 nm, can be detected using currently available experimental HRTEM techniques.  相似文献   

10.
11.
The spectra of deep inner-core excited mixed rare-gas clusters were recorded by using electron ion coincidence (EICO) and multi-hit momentum imaging (MHMI) techniques. The EICO spectra for Ar99Kr1 clusters reveal that singly charged ions are emitted from the inner-core excited clusters in addition to the multiple charged ions. The dependence of the EICO spectra on photon energy and cluster size suggests that the holes created through vacancy cascade on the krypton atoms are transferred to the surrounding atoms, and that the singly charged ions are the primary product of the krypton photoabsorption. Charge localization is suggested for the inner-core excited mixed rare-gas clusters from the analysis of the EICO peak width. The MHMI measurements give us direct evidence for the strong charge migration from X-ray absorbing atoms to surrounding atoms. The photon energy dependence of the PSD image for fragment ions suggests that the momentum of the fragment ions depends on the number of charges generated by the vacancy cascade.  相似文献   

12.
In this work we present results from a theoretical study on the properties of sodium clusters. The structures of the global total-energy minima have been determined using two different methods. With the parameterized density-functional tight-binding method (DFTB) combined with a genetic-algorithm we investigated the properties of NaN clusters with cluster size up to 20 atoms, and with our own Aufbau/Abbau algorithm together with the embedded-atom method (EAM) up to 60 atoms. The two sets of results from the independent calculations are compared and a stability function is studied as function of the cluster size. Due to the electronic effects included in the DFTB method and the packing effects included in the EAM we have obtained different global-minima structures and different stability functions.  相似文献   

13.
The influence of doping of Li-clusters by electronegative O and C atoms on the ionization potentials was investigated. Experimentally, we report ionization potentials for bare Lin clusters deduced from photoionization efficiency spectra. The values are compared with the results for LinO and LinC clusters. Observed differences are largely attributed to a quantum size effect caused by the segregated molecular part around the impurity, which changes the electron work function. Theoretically, the Fermi and exchange-correlation energies which enter the work function, are calculated in the frame of the augmented plane wave (APW) method by taking explicitly into account the presence of the molecular core. The other contribution to the work function, the moment of the double layer at the cluster surface, is computed by solving the corresponding Poisson's equation. Received 9 September 1999 and Received in final form 7 February 2000  相似文献   

14.
The geometries and electronic properties of the most stable small Aun clusters with n=2 to 20 are presented. An intensive search for low-energy minima of Aun clusters was carried through using a density-functional tight-binding method combined with genetic algorithms for an unbiased global structure optimization. The structural and energetic properties of the small gold clusters are compared with those of planar Aun clusters with n=5 to 15.  相似文献   

15.
The geometrical structure of ground state Ban clusters (n =2-14) has been predicted from various types of calculations including two ab initio approaches used for the smaller sizes namely HF+MP2( n =2-6), DFT (LSDA)( n =2-6, 9) and one model approach HF+pairwise dispersion used for all sizes investigated here. The lowest energy configurations as well as some isomers have been investigated. The sizes n =4, 7 and 13 are predicted to be the relatively more stable ones and they correspond to the three compact structures: the tetrahedron, the pentagonal bipyramid and the icosahedron. The growth behavior from Ba7 to Ba13 appears to be characterized by the addition of atoms around a pentagonal bipyramid leading to the icosahedral structure of Ba13 which is consistent with the observed size-distribution of barium clusters. Values for vertical ionization potentials calculated for n =2-5 at the CI level are seen to be in quite good agreement with recent measures. Received: 14 May 1997 / Received in final form: 2 February 1998 / Accepted: 27 February 1998  相似文献   

16.
Condensation of nano-droplets in a supersaturating vapor decomposes in two steps: the formation of a nucleation center, also called critical nuclei or nucleation seed, and the growth sequence, by accretion of further atoms on the nucleation center. These two steps have been investigated separately through the clustering of homogeneous particles Nan and heterogeneous particles NanX in a helium buffer gas (X = (Na2O)2 or (NaOH)2). The growth sequence is analyzed with preformed molecules X injected in a supersaturating sodium vapor and driving production of NanX clusters. Cluster distribution mean sizes are controlled by sodium concentration and by the condensation cell effective length. The signal intensities observed for homogeneous and heterogeneous clusters are proportional to the homogeneous and heterogeneous nucleation center numbers respectively. We can measure the efficiency for the homogeneous nucleation center production versus sodium concentration. This process is the onset of the condensation phase transition.  相似文献   

17.
The structures, binding energies, and electronic properties of Cn and NaCn (n=2–12) clusters have been systematically investigated using density functional theory (DFT). A number of previously undiscovered isomers of NaCn clusters are reported, including fan-like, linear and three-dimensional structures. Moreover, NaCn clusters with even n are found to be more stable than those with odd n, in contrast with the case of Cn clusters.  相似文献   

18.
    
We have investigated segregation phenomena in Cu–Au bimetallic clusters with decahedral structures at 100 K and 300 K, based on the second-moment approximation of the tight-binding (TB-SMA) potentials by using Monte Carlo method. The simulation results indicate that there are three regions (split, three-shell onion-like and core-shell region) at 100 K and two regions (split and core-shell) at 300 K with the structure of decahedral clusters, as the chemical potential difference Δμ changes. It is found that the structure of decahedral clusters undergoes a division into smaller clusters in the split region. In the core-shell structure, Au atoms are enriched in surface and Cu atoms occupy the core of the clusters because of the different surface energy of Cu and Au. The Au atoms are enriched in the surface shell, and the Cu atoms are in the middle shell, while a single Au atom is located in the center to form the three-shell onion-like structure. The structure and binding energy of smaller clusters after splitting are also discussed. The Au atoms generally lie on the surface of the smaller clusters after splitting.  相似文献   

19.
Geometries, electronic states and electron affinities of AlmAsn and AlmAs n (m+n=2–5) clusters have been examined using four hybrid and pure density functional theory (DFT) methods. Structural optimization and frequency analyses are performed using a 6-311+G(2df) one-particle basis set. The geometries are fully optimized with each DFT method independently. The three types of energy separations reported in this work are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The calculation results show that the singlet structures have higher symmetry than that of doublet structures. The best functional for predicting molecular structures was found to be BLYP, while other functionals generally underestimated bond lengths. The largest adiabatic electron affinity, vertical electron affinity and vertical detachment energy, obtained at the 6-311+G(2df)/BP86 level of theory, are 2.20, 2.04 and 2.27 eV (AlAs), 2.13, 1.94 and 2.38 eV (AlAs2), 2.44, 2.39 and 2.47 eV (Al2As), 2.09, 1.80 and 2.53 eV (Al2As2), 2.01, 1.57 and 2.36 eV (AlAs3), 2.32, 2.11 and 2.55 eV (Al2As3), 2.40, 1.45 and 3.26 eV (AlAs4), 1.94, 1.90 and 2.07 eV (Al4As), respectively. However, the BHLYP method gives the largest values for EAad and EAvert of Al3As and EAad of Al3As2, respectively. For the vibrational frequencies of the AlnAsm series, the B3LYP method produces good predictions with the average error only about 10 cm-1 from available experimental and theoretical values. The other three functionals overestimate or underestimate the vibrational frequencies, with the worst predictions given by the BHLYP method.  相似文献   

20.
Various dissociation channels of silver bromide cluster ion Ag2Br+ and silver cluster ion Ag3 + were observed in high-energy collisionally-activated dissociation (CAD) using a Cs target. The fragment patterns of the high-energy CAD were compared with those of the metastable dissociation and low-energy CAD. The difference in the fragment patterns between the high-energy CAD and the other dissociation methods was explained in terms of the internal energy distributions. The dissociation mechanisms of neutral silver bromide cluster Ag2Br and silver cluster Ag3 were also investigated by charge inversion mass spectrometry using the Cs target. While the fragment ions AgBr- and Ag2 - were dominantly observed in the charge inversion spectrum of Ag2Br+, the undissociated ion Ag3 - was observed as a predominant peak in the case of Ag3 +. The dissociation behavior of Ag2Br* can be explained on the basis of the calculated thermochemical data. Contrary to this, the predominant existence of the undissociated Ag3 - cannot be explained by the reported thermochemical data. The existence of undissociated Ag3 - suggests that the dissociation barrier is higher than the internal energy of Ag3 * (theoretical: 1.03 eV, experimental: 2.31 eV) estimated from the ionization potentials of Ag3 and Cs.  相似文献   

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