共查询到20条相似文献,搜索用时 46 毫秒
1.
Semiclassical expansions derived in the framework of the Extended Thomas-Fermi approach for the kinetic energy density τ(
r) and the spin-orbit density
J(
r) as functions of the local density ρ(
r) are used to determine the central nuclear potentials V
n(
r) and V
p(
r) of the neutron and proton distribution for effective interactions of the Skyrme type. We demonstrate that the convergence
of the resulting semiclassical expansions for these potentials is fast and that they reproduce quite accurately the corresponding
Hartree-Fock average fields.
Received: 12 February 2000 / Accepted: 14 March 2002 相似文献
2.
W. Christen U. Even 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):87-90
This contribution addresses the inelastic interaction of positively charged molecular cluster ions with a solid surface at
kinetic energies up to 30 eV/molecule. We report experimental results on the scattering of mass-selected, protonated methanol
cluster cations (CH3OH)nH+, n = 4-32, off a diamond-coated silicon surface. In particular we provide fragment size distributions of methanol cluster ions
following their impact on the target, as well as surface-induced neutralization probabilities of methanol cluster ions as
a function of the size and the kinetic energy of the parent clusters.
Received 30 November 2000 相似文献
3.
C. Bobbert S. Schütte C. Steinbach U. Buck 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,19(2):183-192
The interaction of large ammonia and water clusters in the size range from <n> = 10 to 3 400 with electrons is investigated in a reflectron time-of-flight mass spectrometer. The clusters are generated
in adiabatic expansions through conical nozzles and are nearly fragmentation free detected by single photon ionization after
they have been doped by one sodium atom. For ammonia also the (1+1) resonance enhanced two photon ionization through the state with v = 6 operates similarly. In this way reliable size distributions of the neutral clusters are obtained which are analyzed in
terms of a modified scaling law of the Hagena type [Surf. Sci. 106, 101 (1981)]. In contrast, using electron impact ionization, the clusters are strongly fragmented when varying the electron
energy between 150 and 1 500 eV. The number of evaporated molecules depends on the cluster size and the energy dependence
follows that of the stopping power of the solid material. Therefore we attribute the operating mechanism to that which is
also responsible for the electronic sputtering of solid matter. The yields, however, are orders of magnitude larger for clusters
than for the solid. This result is a consequence of the finite dimensions of the clusters which cannot accommodate the released
energy.
Received 21 November 2001 相似文献
4.
C. Bobbert C.P. Schulz 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):95-97
Nam(H2O)n Clusters ( n = 1...200, m = 1...50) are formed in a recently build pick-up arrangement. Preformed water clusters traverse a sodium oven, where sodium
atoms are picked up. At low sodium vapour pressure ( < 1×10-4 mbar) pure Na(H2O)n clusters are observed in the mass spectra. At high sodium vapour pressure ( > 1×10-3 mbar) the water cluster pick up more than 50 Na atoms and reaction products Na(NaOH)n ( n = 2, 4...50) dominate the mass spectra. The even number of NaOH units in the products indicate that also in a finite cluster
the reaction occurs in pairs as in the macroscopic reaction.
Received 4 December 2000 相似文献
5.
R.-J. Tarento P. Joyes J. van de Walle M. Nakamura 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):111-114
We investigate the electron capture occurring in the collision between an ion A+ and a cluster An (n = 5). The process has been modelled within the Hubbard Hamiltonian,which takes into account the intrasite U electron correlation. An exact procedure has been numerically applied which involves all the excited states to examine the
time evolution of the system during the collision. We have applied the model to the sodium case. We have investigated the
time evolution of the electron population during the collision on the projectile versus the kinetic energy of the projectile. It displays some oscillations which means that the electron exchanges between the ion
and the cluster occurs alternatively in one direction and the other. We also vary U and examine its influence on the dynamics of the oscillation of the average population. Finally the cross section is derived
versus the energy and U.
Received 29 November 2000 相似文献
6.
R. Cherrabi A. Saout-Elhak M. Benhamou 《The European physical journal. E, Soft matter》2000,2(1):91-101
In this paper, we consider a mixture of two polymers A and B of different chemical nature, dissolved in a common good solvent,
in contact with an interacting surface. We start from a mixture of two incompatible homopolymers A and B in the molten state,
and assume that the surface adsorbs strongly one or both polymer species at high temperature. It is assumed that this is a
strong adsorption, so that chains cannot desorb once they are linked to the surface. This constrains the system to a quenched
composition on the surface. Once the adsorption process is finished, a quantity of a good solvent is added to get a semi-dilute
solution. We assume that demixing transition in the presence of solvent occurs at lower temperature. The purpose is to discuss
the influence of the quenched surface fluctuations on the critical properties of the mixture. Within the framework of the
so-called blob model, we determine the exact shape of the composition profile as a function of the distance z to the surface, for any value of the relevant parameters, namely, the temperature T, the molecular weight M, the monomer concentration c and the surface composition x0. Our analysis reveals a universal character of the composition profile for , where the characteristic size D is some known length depending on the relevant parameters of the problem, and not on temperature, and is the thermal correlation length. Near surface, for (a is the monomer size), the profile is no longer universal, and in particular, it is sensitive to the boundary condition. Far
from the surface, that is , the profile tends exponentially to its bulk value. We show that the length Dapproaches its lowest value as the surface composition reaches its saturated value l. In this limit, we find that the profile shape is a characteristic of critical adsorption in simple binary fluid mixtures.
Finally, this work must be regarded as a natural extension of a previous one, which was concerned with the same problem, but
in the absence of solvent.
Received 24 June 1999 and Received in final form 5 November 1999 相似文献
7.
B. Ketzer T. von Egidy F.J. Hartmann C. Maierl R. Pohl J. Eades E. Widmann T. Yamazaki M. Kumakura N. Morita R.S. Hayano M. Hori T. Ishikawa H.A. Torii I. Sugai D. Horváth 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,13(3):305-316
The quenching of metastable states of antiprotonic helium by collisions with hydrogen and deuterium molecules was studied.
A systematic investigation of the delayed annihilation time spectra at various H2 and D2 admixture ratios at the ppm level revealed characteristic changes of their shape, which indicated a strong principal and
orbital quantum number dependent quenching of levels in both cases. Applying a laser spectroscopy technique to measure the
lifetimes of individual states and cascades we deduced H2 and D2 quenching cross-sections for the states (n, l )= (39, 35) and (37, 34). These cross-sections establish for D2 molecules the strong increase of the quenching efficiency with increasing principal quantum number n of the state under investigation previously reported for the case of H2 admixtures. Our experiments indicate that the low-n state (37, 34) is somewhat less affected by D2 than by H2, while the high-n state (39, 35) is equally quenched by both isotopes.
Received 2 October 2000 相似文献
8.
Ş. Erkoç H. Kökten Z. Güvenç 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,13(3):361-365
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 相似文献
9.
10.
J.N. Fuchs M. Holzmann F. Laloë 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,25(4):463-478
The purpose of this article is to discuss cluster expansions in dense quantum systems, as well as their interconnection with
exchange cycles. We show in general how the Ursell operators of order l≥ 3 contribute to an exponential which corresponds to a mean-field energy involving the second operator U2, instead of the potential itself as usual - in other words, the mean-field correction is expressed in terms of a modification
of a local Boltzmann equilibrium. In a first part, we consider classical statistical mechanics and recall the relation between
the reducible part of the classical cluster integrals and the mean-field; we introduce an alternative method to obtain the
linear density contribution to the mean-field, which is based on the notion of tree-diagrams and provides a preview of the
subsequent quantum calculations. We then proceed to study quantum particles with Boltzmann statistics (distinguishable particles)
and show that each Ursell operator Un with n≥ 3 contains a “tree-reducible part”, which groups naturally with U2 through a linear chain of binary interactions; this part contributes to the associated mean-field experienced by particles
in the fluid. The irreducible part, on the other hand, corresponds to the effects associated with three (or more) particles
interacting all together at the same time. We then show that the same algebra holds in the case of Fermi or Bose particles,
and discuss physically the role of the exchange cycles, combined with interactions. Bose condensed systems are not considered
at this stage. The similarities and differences between Boltzmann and quantum statistics are illustrated by this approach,
in contrast with field theoretical or Green's functions methods, which do not allow a separate study of the role of quantum
statistics and dynamics.
Received 18 October 2001 相似文献
11.
G. Chen Z.F. Liu X.G. Gong 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):33-36
We have studied the atomic structure and the electronic properties of Ban clusters by the ab initio molecular dynamics method. We find that a structural transition to the bulk-like structure begins at Ba9 cluster, and the structures of the clusters are transferred to be icosahedral-like around n
= 13. The relatively high stability for Ba4, Ba10 and Ba13 clusters are observed.
Received 1st December 2000 相似文献
12.
W. Jeżewski 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(1):133-138
Scaling properties of the Gibbs distribution of a finite-size one-dimensional Ising model are investigated as the thermodynamic
limit is approached. It is shown that, for each nonzero temperature, coarse-grained probabilities of the appearance of particular
energy levels display multiscaling with the scaling length ℓ = 1/M
n, where n denotes the number of spins and Mn is the total number of energy levels. Using the multifractal formalism, the probabilities are argued to reveal also multifractal
properties.
Received 10 July 2000 and Received in final form 6 November 2000 相似文献
13.
The lattice QCD studies indicate that the critical temperature T
c ≃ 260-280 MeV of the deconfinement phase transition in quenched QCD is considerably smaller than the lowest-lying glueball
mass m
G ≃ 1500-1700 MeV, i.e., T
c ≪ m
G. As a consequence of this large difference, the thermal excitation of the glueball in the confinement phase is strongly suppressed
by the statistical factor e
-mG/Tc ≃ 0.00207 even near T ≃ T
c. We consider its physical implication, and argue the abnormal feature of the deconfinement phase transition in quenched QCD
from the statistical viewpoint. To appreciate this, we demonstrate a statistical argument of the QCD phase transition using
the recent lattice QCD data. From the phenomenological relation between T
c and the glueball mass, the deconfinement transition is found to take place in quenched QCD before a reasonable amount of
glueballs is thermally excited. In this way, quenched QCD reveals a question “what is the trigger of the deconfinement phase
transition ?”
Received: 18 November 2002 / Accepted: 4 February 2003 / Published online: 29 April 2003 相似文献
14.
A. Herlert L. Schweikhard M. Vogel 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):65-68
Singly charged silver-cluster anions are produced in a laser vaporization source and transferred into a Penning trap. After
size selection the clusters are subjected to an electron bath in the trap, which results in the attachment of further electrons.
The relative abundance of dianions or trianions as a function of the clusters' size is analyzed by time-of-flight mass spectrometry.
Silver-cluster dianions are observed for sizes n≥ 24 and trianions for n > 100. In addition, a detailed study of the cluster sizes 24 ?n? 60 shows a pronounced resistance to electron attachment for singly charged anions Agn
- with a closed electronic shell, in particular Ag29
-, Ag33
-, and Ag39
-. Both the threshold size for the observation of dianionic silver clusters and the shell effects in the production yield correlate
favorably with previous theoretical investigations of the respective electron affinities.
Received 24 November 2000 相似文献
15.
K. Ohshimo H. Tsunoyama F. Misaizu K. Ohno 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):107-110
Intracluster electron transfer and oligomerization reaction were investigated by mass spectrometry of clusters of alkali metal
atom (M) with acrylonitrile (AN; CH2=CHCN). In the photoionization mass spectra of M(AN)n, magic numbers were clearly observed at n = 3k (k = 1-4 for M = Na and K, k = 1 for M = Li). The results of photodissociation of neutral K(AN)n indicate that the n = 3 cluster has an anomalous stability relative to other sizes of clusters. The C=C bond in vinyl molecules is also found
to be necessary to form the magic numbers by measuring the photoionization mass spectrum of K atom with propionitrile. These
results strongly support the intracluster anionic oligomerization reaction initiated by electron transfer from the alkali
atom. The quantum chemical calculations have revealed that the evaporation induced by excess energy generated by intracluster
oligomerization is important to form the magic numbers in the present clusters.
Received 29 November 2000 相似文献
16.
A model for scalar quarks and gluons that successfully gives rise to a ln s behavior in high-energy qq scattering and which contains a non-trivial three-gluon vertex is used to study collision theory with the following aspects:
i) A three-body interaction simulating QCD is present and ii) particle production and annihilation occur naturally. In this
paper, the collision term in the model is examined in detail in the quasiparticle approximation. The construction of cross-sections
in which self-energy terms are ordered according to a coupling constant expansion is undertaken. It is shown explicitly which
terms of second order are required to obtain the scattering amplitudes that are two body in nature. Additional ordering in
the number of colors shows that quark loop diagrams are suppressed and gluon production or scattering processes dominate.
It is also shown that a consistent calculation of the scattering graphs at the two-loop level also simultaneously yields terms
that renormalize one-loop level graphs. This can then be extended to arbitrary m↦n processes. We examine the constraint equation briefly, discussing the appearance of a width. The issue of pinch singularities
is also addressed, and examples of the elimination of such singularities in equilibrium are given explicitly.
Received: 7 November 2001 / Accepted: 12 June 2001 相似文献
17.
V.D. Efros H. Oberhummer A. Pushkin I.J. Thompson 《The European Physical Journal A - Hadrons and Nuclei》1998,1(4):447-453
A semi–microscopic model for the low–energy photodisintegration of the 9Be nucleus is constructed, and the experimental data are analyzed with its help. The older radioactive isotope data are supported
by this analysis. The theoretical photodisintegration cross section is derived. The astrophysical rates for the reaction α+α+n→9Be+γ and the reverse photodisintegration of 9Be are calculated. The new reaction rate for α+α+n→9Be+γ is compared with previous estimations.
Received: 19 January 1998 相似文献
18.
M. Vogel K. Hansen A. Herlert L. Schweikhard 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):73-76
The decay pathway competition between monomer and dimer evaporation of photoexcited cluster ions Au
+
n, n = 2-27, has been investigated by photodissociation of size-selected gold clusters stored in a Penning trap. For n > 6 the two decay pathways are distinguished by their experimental signature in time-resolved measurements of the dissociation.
For the smaller clusters, simple fragment spectra were used. As in the case of the other copper-group elements, even-numbered
gold cluster ions decay exclusively by monomer evaporation, irrespective of their size. For small odd-size gold clusters,
dimer evaporation is a competitive alternative, and the smaller the odd-sized clusters, the more likely they decay by dimer
evaporation. In this respect, Au
+
9 shows an anomalous behavior, as it is less likely to evaporate dimers than its two odd-numbered neighbors, Au
+
7 and Au
+
11. This nonamer anomaly is typical for copper-group cluster ions M
+
9 (M = Cu, Ag, Au) and a similar behavior is found in the anionic heptamers M
-
7. It is discussed in terms of the well-known electronic shell closing at n
e = 8 atomic valence electrons.
Received 2 November 2000 相似文献
19.
P. Farmanara H.-H. Ritze V. Stert W. Radloff I.V. Hertel 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,19(2):193-209
Neutral ammonia clusters (NH3)m are photo-excited to the electronic state by a deep UV femtosecond laser pump pulse. Within a few hundred femtoseconds a significant fraction of the clusters
rearrange to form an H-transfer state (NH3)m-2NH4(3s)NH2 with the subunit NH4 in its 3s electronic ground state. This state is then electronically excited by a time-delayed infrared control pulse of variable wavelength.
Finally, a third (probe) pulse in the UV ionizes the clusters for detection. The lifetime of the excited (NH3)m-2NH4(3p)NH2 states is found to vary between 2.7 and 0.13 ps depending on cluster size and excitation energy. It increases drastically
upon deuteration. The corresponding cluster size-dependent photoelectron spectra allow us to disentangle the underlying energetics
of the excitation and ionization process and reveal additional processes, such as nonresonant ionization or dissociative ionization.
The experimental findings suggest that the excited H-transfer ammonia complexes with m > 2 are deactivated by an internal conversion process back to the electronically lowest H-transfer state followed by fast
dissociation.
Received 22 September 2001 and Received in final form 31 January 2002 相似文献
20.
R. Leone G. Odriozola L. Mussio A. Schmitt R. Hidalgo-Álvarez 《The European physical journal. E, Soft matter》2002,7(2):153-161
Coupled aggregation and sedimentation processes were studied by means of three-dimensional computer simulations. For this
purpose, a large prism with no periodic boundary conditions for the sedimentation direction was considered. Furthermore, three
equally sized and mutually excluded regions were defined inside the prism, a top, a middle and a bottom region. This allows
to study the time evolution of the cluster size distribution and the cluster structure separately for each region. The mass
distribution profile and the center-of-mass position were also accessed as a function of time. For the bottom region, the
effects of the sediment formation on the kinetics of growth and on the cluster structure were clearly observed. The obtained
results not only agree with the experimental data obtained by Allain et al. (C. Allain, M. Cloitre, M. Wafra, Phys. Rev. Lett. 74, 1478 (1995)) and with the simulations made by Gonzalez (A.E. Gonzalez, Phys. Rev. Lett. 86, 1243 (2001)), but also allow to gain further insight into the details.
Received 7 November 2001 相似文献