共查询到20条相似文献,搜索用时 546 毫秒
1.
N. Pontius P.S. Bechthold M. Neeb W. Eberhardt 《Applied physics. B, Lasers and optics》2000,71(3):351-356
A femtosecond photo-electron experiment is described which provides excellent conditions for measuring time-resolved photo-electron
spectra of free, monodispersed cluster anions using repetition rates up to 100 Hz. Cluster anions are synthesized in an electric
arc and subsequently cooled in a helium carrier gas. A time-of-flight spectrometer is used for mass separation of the negatively
charged clusters. The kinetic energy of the photo-electrons is analyzed by a magnetic-bottle time-of-flight spectrometer,
which guarantees a maximum collection efficiency. Femtosecond laser pulses are generated by a seeded regenerative Ti:Sa amplifier,
which is externally pumped with the second harmonic of a diode-pumped solid-state Nd:YAG laser. A retroreflector mounted on
a computer-controlled translation stage serves as a reproducible time delay of the probe pulse. The high energy output of
the laser pulses (∼3 mJ) in combination with the variable repetition rate and the high stability of the amplified pulses provide
excellent conditions for recording pump-probe photo-electron spectra of mass-separated cluster anions even at the fairly low
ion density of pulsed plasma cluster sources. First results on the electron dynamics of the Pt3
- cluster demonstrate the reliability of the whole system. Further experimental investigations will concentrate on electron-relaxation
processes in transition- and noble-metal clusters as well as on the nuclear and transition-state dynamics of chemically reacted
adsorbate clusters.
Received: 7 January 2000 / Published online: 7 August 2000 相似文献
2.
G. Grégoire M. Mons C. Dedonder-Lardeux C. Jouvet 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,1(1):5-7
clusters (solvents being , or ) have been studied by resonance enhanced two photons ionization, leading to the detection of clusters. When water is the solvent, large clusters up to n>50 can be observed, whereas for and no clusters larger than 10 could be evidenced. Because the first step in the ionization process is the excitation from the
ground solvated () ion pair state to a covalent excited state, the differences in the cluster size distribution for different solvent may be
interpreted as a difference in cluster structures leading to a difference in the charge separation in the ground state.
Received: 30 September 1997 / Revised in final form: 30 October 1997 / Accepted: 30 October 1997 相似文献
3.
4.
Resonance-enhanced two-photon ionization spectra of benzotriazole, benzimidazole and their water clusters have been taken
in a molecular beam. The tautomerism of the benzotriazole and benzimidazole monomers is studied.
In order to obtain an assignment of the vibronic bands of the benzotriazole- and benzimidazole-water clusters, ab initio calculations
have been performed, which allow the assignment of the intermolecular vibrations and the determination of the most stable
cluster structures. In the case of benzotriazole the cyclic cluster of the 2H-tautomer has been found to be the most stable
one, whereas for benzimidazole the most stable cluster could be shown to have a linear arrangement, with the benzimidazole
moiety acting as proton donor.
Received: 2 March 2000 / Revised version: 31 May 2000 / Published online: 5 October 2000 相似文献
5.
M. Bertolus V. Brenner P. Millié 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,1(2):197-205
A theoretical study of clusters with using density functional theory is presented. Tests of various functionals demonstrate that local spin density approximation
(LSDA) is the most adequate functional for the study of these systems. Structures, vibrational frequencies, and IR intensities
of the lowest energy isomer of the studied clusters obtained using LSDA are described, and the unusual properties of the Si-C
clusters are discussed. A quantitative analysis of the obtained structures was carried out, and relations between the coordinations,
interatomic distances, and angles observed in the Si-C clusters were obtained through introduction of the notion of coordination.
This analysis also shows that the carbon atoms mainly exhibit sp and sp2 hybridizations, and that a majority of silicon atoms do not hybridize. This study is the fi
rst step of the implementation of a semi-empirical potential, which would describe the moderately small Si-C clusters.
Received: 20 October 1997 / Received in final form: 16 December 1997 / Accepted: 17 December 1997 相似文献
6.
S.S. Khasanov B.Z. Narymbetov L.V. Zorina L.P. Rozenberg R.P. Shibaeva N.D. Kushch E.B. Yagubskii R. Rousseau E. Canadell 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(4):419-428
The effect of the first-order phase transition occurring around 220 K on the crystal and electronic structure of the molecular
superconductor (BEDO-TTF) has been studied. The crystal structure at 170 K has been determined and it is found that the main structural change resulting
from the phase transition is connected with a rotation of the anions. This rotation leads to changes in the SS and SO intermolecular contacts within the BEDO-TTF donor layers. How these structural changes affect the electronic structure of
the salt is studied by comparing the transfer integrals for the different donordonor intermolecular interactions calculated for the room temperature and 170 K crystal structures. It is shown that Fermi
surfaces for the 170 K and room temperature structures are very similar, do not exhibit nesting properties, and can be described
as resulting from the hybridization of superposing ellipses.
Received: 4 September 1997 / Received in final form: 2 December 1997 / Accepted: 3 December 1997 相似文献
7.
The effects of lattice vibration on the thermodynamics of nanosized coherent clusters in bcc-Fe consisting of vacancies and/or copper are investigated within the harmonic approximation. A combination of on-lattice simulated annealing based on Metropolis Monte Carlo simulations and off-lattice relaxation by molecular dynamics is applied to obtain the most stable cluster configurations at T = 0 K. The most recent interatomic potential built within the framework of the embedded-atom method for the Fe-Cu system is used. The total free energy of pure bcc-Fe and fcc-Cu as well as the total formation free energy and the total binding free energy of the vacancy-copper clusters are determined for finite temperatures. Our results are compared with the available data from previous investigations performed using many-body interatomic potentials and first-principles methods. For further applications in rate theory and object kinetic Monte Carlo simulations, the vibrational effects evaluated in the present study are included in the previously developed analytical fitting formulae. 相似文献
8.
M.R Manaa 《Solid State Communications》2004,129(6):379-383
I report electronic structures and the cohesive energy for face-centered-cubic (fcc) solid C48N12 using generalized-gradient density-functional theory. The full vibrational spectrum of the C48N12 cluster is calculated within the harmonic approximation at the B3LYP/6-31G* level of theory. The results show that fcc is energetically preferred and a more stable crystal form than body-centered-cubic (bcc). C48N12 clusters are found to condense by a weak (0.29 eV) van der Waals force. The band gap of fcc C48N12 is calculated to be 1.3 eV at the GGA-PW91 level, whereas the HOMO-LUMO gap is calculated to be 2.74 eV using B3LYP/6-31G*. 相似文献
9.
10.
A. Miotello G. De Marchi G. Mattei P. Mazzoldi A. Quaranta 《Applied Physics A: Materials Science & Processing》2000,70(4):415-419
The diffusional growth regime of silver clusters in hydrogenated silver–sodium exchanged glasses has been numerically analysed.
The diffusion coefficients of the three elements that contribute to silver cluster formation, namely H, Ag+, and Ag0, have been established. The activation energy of the reactant Ag0 diffusion coefficient is consistent in the three analysed different experimental results: Ag loss through evaporation, near-surface
Ag accumulation, and optical absorption density. The calculated Ag0 diffusion coefficient accounts for the threshold time necessary to obtain a cluster size to give rise to a detectable absorption.
Received: 27 September 1999 / Accepted: 11 October 1999 / Published online: 8 March 2000 相似文献
11.
W. Helfrich 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(4):481-489
The problem of membrane softening by thermal undulations is revisited. In contrast to general belief, fluid membranes are
predicted to be stiffened, not softened, by their undulations. Equal values of the effective bending rigidity are calculated
from the interplay of local mean curvature modes (hats) on the basically flat membrane and from the coupling of spherical
harmonic modes with spherical curvature. In addition, a conjecture is made on the entropy of membrane closure. It relies on
a similarity of membrane closure to periodic boundary conditions.
Received: 10 June 1997 / Revised: 7 October 1997 / Accepted: 2 December 1997 相似文献
12.
Ionization of metal clusters by ions in the Fermi velocity range 总被引:1,自引:0,他引:1
P.G. Reinhard E. Suraud C.A. Ullrich 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,1(3):303-311
We simulate excitation of metal clusters by highly charged, energetic ions, analyzing electron emission in terms of discrete
ionization probabilities. Our test case is the collision of on the cluster at velocities around the electronic Fermi velocity of bulk sodium. The calculations are performed with a density-functional
approach, using the time-dependent local density approximation. We find that ionization takes place on an extremely short
time scale of less than 5 fs. The preferred final charge state depends sensitively on the impact parameter. High ionization
can easily be achieved in sufficiently close collisions. Direct trapping through the by-passing ion is found to be of little
importance at the velocities considered.
Received: 28 July 1997 / Received in final form: 23 December 1997 / Accepted: 8 January 1998 相似文献
13.
Sabín J Prieto G Ruso JM Sarmiento F 《The European physical journal. E, Soft matter》2007,24(2):201-210
We present a study of the fractal dimension of clusters of large unilamellar vesicles (LUVs) formed by egg yolk phosphatidylcholine
(EYPC), dimyristoylphosphocholine (DMPC) and dipalmitoylphosphocholine (DPPC) induced by Ca2+ . Fractal dimensions were calculated by application of two methods, measuring the angular dependency of the light scattered
by the clusters and following the evolution of the cluster size. In all cases, the fractal dimensions fell in the range from
2.1 to 1.8, corresponding to two regimes: diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation
(RLCA). Whereas DMPC clusters showed a typical transition from the RLCA to the DLCA aggregation, EYPC exhibited an unusual
behaviour, since the aggregation was limited for a higher concentration than the critical aggregation concentration. The behaviour
of DPPC was intermediate, with a transition from the RLCA to the DLCA regimes with cluster sizes depending on Ca2+ concentration. Studies on the reversibility of the aggregates show that EYPC and DPPC clusters can be re-dispersed by dilution
with water. DMPC does not present reversibility. Reversibility is evidence of the existence of secondary minima in the DLVO
potential between two liposomes. To predict these secondary minima, a correction of the DLVO model was necessary taking into
account a repulsive force of hydration. 相似文献
14.
U. Heiz 《Applied Physics A: Materials Science & Processing》1998,67(6):621-626
We report on the size-dependent chemical reactivity of nickel clusters with up to 30 atoms. Monodispersed Ni30 clusters show a higher reactivity for CO dissociation than Ni11 and Ni20. Under our experimental conditions the smallest nickel clusters (Nix, x<4) produce nickelcarbonyl complexes. These results demonstrate that such small clusters are unique for catalytic reactions
not only due to their high surface-to-volume ratio but also essentially because of the distinctive properties of different
cluster sizes. In addition thermal desorption spectroscopy of CO shows that on average four molecules are weakly adsorbed
per Ni11 at saturation coverage. Using an isotopic mixture of 12CO and 13CO, infrared spectroscopy reveals the existence of a vibrational coupling interaction between the four COs. A semi-classical
model of interacting dipoles is applied to correlate the observed vibrational frequency shifts with the arrangement of the
COs on the cluster. This simple analysis favors a three-dimensional structure for the deposited clusters.
Received: 23 March 1998/Accepted: 25 August 1998 相似文献
15.
R. S. Berry B. M. Smyrnov A. Yu. Strizhev 《Journal of Experimental and Theoretical Physics》1997,85(3):588-592
The binding energy of atoms in icosahedral and face-centered-cubic clusters is calculated numerically for pairwise Morse-potential
interactions between atoms and for clusters containing from 561 to 923 atoms, which corresponds to gradual filling of the
sixth layer of the icosahedral cluster. Perturbation theory is used to calculate the cluster binding energy, in which the
small parameter is the ratio of the interaction energy between non-nearest neighbor atoms to the interaction energy between
nearest neighbors. Values of the Morse interaction potential parameter are found for which the energies of clusters with different
structures coincide. Under the conditions used in these computations, the strain energy of a cluster can be neglected. Although
the contribution of the interaction energy between non-nearest neighbors to the total cluster energy is small, it turns out
to be important in finding the level crossing parameter.
Zh. éksp. Teor. Fiz. 112, 1082–1090 (September 1997) 相似文献
16.
用遗传算法结合Gupta紧束缚模型势研究了Irn(n=2-25)团簇的基态结构.分析了Irn(n=2-25)团簇的基态结构随团簇尺寸的变化规律.计算结果表明,Irn(n=2-25)团簇的每个原子的平均束缚能和平均第一近邻随团簇尺寸的增加而增大,以总束缚能的二阶差分为判据,Irn(n=2-25)团簇的幻数是4、7、9、13、15、19、23. 相似文献
17.
M.-H. Schaffner F. Patthey U. Heiz W.-D. Schneider O. Kuffer H.-V. Roy P. Fayet J.K. Gimzewski R. Berndt 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,2(1):79-82
The emission of photons in the visible wavelength range from mass-selected Ag+
n, Cu+
n, Pt+
n and Pd+
n () clusters is observed. Photons are detected 10-4 s after the cluster generation in a sputter source. The emission intensities display distinct variations with cluster size
and material. The observations are interpreted in terms of the decay of metastable states which are excited during the high-energy
sputtering process used for the generation of these clusters.
Received: 28 October 1997 / Revised: 5 January 1998 / Accepted: 30 January 1998 相似文献
18.
F. Balembois M. Gaignet F. Louradour V. Couderc A. Barthelemy P. Georges A. Brun 《Applied physics. B, Lasers and optics》1997,65(2):255-258
4 laser. The amplified pulses were frequency-converted in the blue and in the UV by harmonic generation (doubling and tripling)
in two lithium triborate (LBO) crystals.
Received: 11 March 1997/Accepted: 24 April 1997 相似文献
19.
Tatiana E. Itina Karine Gouriet Sylvie Noël Marc Sentis 《Applied Surface Science》2007,253(19):7656-7661
The mechanisms involved into the formation of clusters by pulsed laser ablation are studied both numerically and experimentally. To facilitate the model validation by comparison with experimental results, the time and length scales of the simulation are considerably increased. This increase is achieved by using a combination of molecular dynamics (MD) and the direct simulation Monte Carlo (DSMC) methods. The combined MD-DSMC model is then used to compare the relative contribution of the two channels of the cluster production by laser ablation: (i) direct cluster ejection upon the laser-material interaction, and (ii) collisional sticking and aggregation in the ablated gas flow. Calculation results demonstrate that both of these mechanisms play a role. The initial cluster ejection provides cluster precursors thus eliminating the three-body collision bottleneck in the cluster growth process. The presence of clusters thus facilitates the following collisional condensation and evaporation processes. The rates of these processes become considerable, leading to the modification of not only the plume cluster composition, but also the dynamics of the plume expansion. Calculation results explain several recent experimental findings. 相似文献
20.
W. Vervisch C. Mottet J. Goniakowski 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,24(1-3):311-314
Morphology and atomic structure of supported Pd clusters
on MgO(100) substrate are investigated theoretically using a
mixed approach: a semi-empirical potential for the metal bonding
within the cluster and a potential fitted to
ab initio calculations for
the metal-oxide interaction. We find that the clusters adopt a
truncated pyramidal morphology in agreement with experimental
results. The detailed study of the epitaxial relation as a
function of cluster size shows the existence of a critical size
around 3 nm where elastic strain due to the misfit between the
substrate and the deposit is released by the introduction of
interfacial dislocations. 相似文献