共查询到20条相似文献,搜索用时 31 毫秒
1.
Jesús?álvarez-Ruiz Marien?López-Arias Rebeca?de?Nalda Margarita?Martín Andrés?Arregui Luis?Ba?ares 《Applied Physics A: Materials Science & Processing》2009,95(3):681-687
The formation of cationic clusters in the laser ablation of CdS targets has been investigated as a function of wavelength
and fluence by mass spectrometric analysis of the plume. Ablation was carried out at the laser wavelengths of 1064, 532, 355,
and 266 nm in order to scan the interaction regimes below and above the energy band gap of the material. In all cases, the
mass spectra showed stoichiometric Cd
n
S
n
+ and nonstoichiometric Cd
n
S
n−1+, Cd
n
S
n+1+, and Cd
n
S
n+2+ clusters up to 4900 amu. Cluster size distributions were well represented by a log-normal function, although larger relative
abundance for clusters with n=13, 16, 19, 34 was observed (magic numbers). The laser threshold fluence for cluster observation was strongly dependent on
wavelength, ranging from around 16 mJ/cm2 at 266 nm to more than 300 mJ/cm2 at 532 and 1064 nm. According to the behavior of the detected species as a function of fluence, two distinct families were
identified: the “light” family containing S2+ and Cd+ and the “heavy” clusterized family grouping Cd2+ and Cd
n
S
m
+. In terms of fluence, it has been determined that the best ratio for clusterization is achieved close to the threshold of
appearance of clusters at all wavelengths. At 1064, 532, and 355 nm, the production of “heavy” cations as a function of fluence
showed a maximum, indicating the participation of competitive effects, whereas saturation is observed at 266 nm. In terms
of relative production, the contribution of the “heavy” family to the total cation signal was significantly lower for 266 nm
than for the longer wavelengths. Irradiation at 355 nm in the fluence region of 200 mJ/cm2 has been identified as the optimum for the generation of large clusters in CdS. 相似文献
2.
The question of whether the linear absorption spectra of metal clusters can be interpreted as density oscillations (collective
“plasmons”) or can only be understood as transitions between distinct molecular states is still a matter of debate for clusters
with only a few electrons. We calculate the photo-absorption spectra of Na2 and Na5
+ comparing two different methods: quantum fluid dynamics and time-dependent density functional theory. The changes in the
electronic structure associated with particular excitations are visualized in “snapshots” via transition densities. Our analysis
shows that even for the smallest clusters, the observed excitations can be interpreted as intuitively understandable density
oscillations. For Na5
+, the importance of self-interaction corrections to the adiabatic local density approximation is demonstrated.
Received: 1 July 2001 / Published online: 10 October 2001 相似文献
3.
A. I. Belousova Yu. E. Lozovikb 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,8(2):251-264
A two dimensional (2D) classical system of dipole particles confined by a quadratic potential is studied. This system can
be used as a model for rare electrons in semiconductor structures near a metal electrode, indirect excitons in coupled quantum
dots etc. For clusters of N ≤ 80 particles ground state configurations and appropriate eigenfrequencies and eigenvectors for the normal modes are found.
Monte-Carlo and molecular dynamic methods are used to study the order-disorder transition (the “melting” of clusters). In
mesoscopic clusters (N < 37) there is a hierarchy of transitions: at lower temperatures an intershell orientational disordering of pairs of shells
takes place; at higher temperatures the intershell diffusion sets in and the shell structure disappears. In “macroscopic”
clusters (N > 37) an orientational “melting” of only the outer shell is possible. The most stable clusters (having both maximal lowest
nonzero eigenfrequencies and maximal temperatures of total melting) are those of completed crystal shells which are concentric
groups of nodes of 2D hexagonal lattice with a number of nodes placed in the center of them. The picture of disordering in
clusters is compared with that in an infinite 2D dipole system. The study of the radial diffusion constant, the structure
factor, the local minima distribution and other quantities shows that the melting temperature is a nonmonotonic function of
the number of particles in the system. The dynamical equilibrium between “solid-like” and “orientationally disordered” forms
of clusters is considered. 相似文献
4.
V. Bonačić-Koutecký M. Hartmann J. Pittner 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):133-138
We present the ultrafast multistate nuclear dynamics involving adiabatic and nonadiabatic excited states of non-stoichiometric
halide deficient clusters (NanFn-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 相似文献
5.
Hellmut Baumgärtel 《International Journal of Theoretical Physics》2011,50(7):2002-2008
To asymptotic complete scattering systems {M
++V,M
+} on H+:=L2(R+,K{\mathcal{H}}_{+}:=L^{2}(\mathbf{R}_{+},{\mathcal{K}}, d
λ), where M
+ is the multiplication operator on H+{\mathcal{H}}_{+} and V is a trace class operator with analyticity conditions, a decay semigroup is associated such that the spectrum of the generator
of this semigroup coincides with the set of all resonances (poles of the analytic continuation of the scattering matrix into
the lower half plane across the positive half line), i.e. the decay semigroup yields a “time-dependent” characterization of
the resonances. As a counterpart a “spectral characterization” is mentioned which is due to the “eigenvalue-like” properties
of resonances. 相似文献
6.
A. Frank R. Lemus R. Bijker F. Pérez-Bernal J. M. Arias 《Czechoslovak Journal of Physics》1998,48(6-7):782-788
An algebraic method especially suited to describe the strongly anharmonic vibrational spectra in molecules may be an appropriate
framework to study the vibrational spectra of Na
n
+
clusters, where nearly flat potential energy surfaces and the appearance of close lying isomers have been reported. As an
illustration we describe the model and apply it to the Be4, H
3
+
, Be3 and Na
3
+
clusters.
Presented by A. Frank at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997.
This work was supported in part by the European Community under contract No. CI1*-CT94-0072, DGAPA-UNAM under project IN101997
and Spanish DGCYT under project PB92-0663. 相似文献
7.
J. H. Brewer D. R. Harshman R. Keitel S. R. Kreitzman G. M. Luke D. R. Noakes R. E. Turner E. J. Ansaldo 《Hyperfine Interactions》1986,32(1-4):677-682
Strongly hydrogen-bonded diamagnetic “FμF” centres are formed by theμ
+ in a wide variety of fluoride crystals. Hydrogen atoms are expected to form similar “FHF” complexes. Through the “motional narrowing” of the zero-field muon relaxation function in NaF, we have observed an Arrhenius
temperature dependence of the dissociation rate of theFμF complex, yielding a binding energy of 1700 (200) K for theμ
+ in theFμF centre. 相似文献
8.
V. V. Bolotov V. E. Kan P. M. Korusenko S. N. Nesov S. N. Povoroznyuk I. V. Ponomareva V. E. Roslikov Yu. A. Sten’kin R. V. Shelyagin E. V. Knyazev 《Physics of the Solid State》2012,54(1):166-173
Nanocomposite layers based on multiwalled carbon nanotubes (MWCNTs) and non-stoichiometric tin oxide (SnO
x
) have been grown by magnetron deposition and CVD methods. In the case of the CVD method, the study of the structure and phase
composition of obtained nanocomposite layers has shown that a tin oxide “superlattice” is formed in the MWCNT layer volume,
fixed by SnO
x
islands on the MWCNT surface. During magnetron deposition, the MWCNT surface is uniformly coated with tin oxide islands,
which causes a change in properties of individual nanotubes. Electrical measurements have revealed the sensitivity of nanocomposite
layers to (NO2)− molecule adsorption, which is qualitatively explained by a change in the conductivity of the semiconductor fraction of p-type MWCNTs. 相似文献
9.
T. Ritschel P. J. Kuntz L. Zülicke 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,44(1):93-107
ArnHCl+ van-der-Waals clusters for n = 1–13 are investigated
with the “minimal diatomics-in-molecules (DIM) model”
using ab-initio input data obtained from multi-reference
configuration-interaction calculations plus subsequent projection onto
valence-bond wavefunctions.
The results for the complexes with n = 1–3 are checked against
ab-initio calculations at the
coupled-cluster (CCSD) level with the same one-electron atomic basis set as
for the input data generation (aug-cc-pVTZ from Dunning).
In addition to the electronic ground state,
the first excited
2A¢^2\!A^{\prime}
state for the triatomic
complex (n = 1) is also studied.
The results
from the DIM model are shown to be in fair agreement with those from
advanced conventional ab-initio calculations, although there are
differences in detail. The comparison
justifies the extension of the DIM approach to n > 3.
Systematic analysis of the local minima of the multi-dimensional
potential-energy surfaces (PESs), carried out with the combined
method
described in part I (Monte-Carlo sampling plus subsequent steepest-descent
optimization), reveals simple building-up regularities for the most
stable structures (i.e. those corresponding to the global PES minimum)
at each n: apart from always having a nearly linear
(Ar–H–Cl)+ fragment as core, the aggregates show little
or no symmetry. Secondary local minima are also determined and their
structures interpreted.
The PESs for the low-lying excited states reveal a much more complicated
topography compared to the ArnH+ clusters allowing a variety
of photo-processes.
The energy level sequence of the first five excited electronic states
and the stability of the clusters in these states is studied
as a function of the cluster size n. 相似文献
10.
G. E. Astrakharchik A. I. Belousov Yu. E. Lozovik 《Journal of Experimental and Theoretical Physics》1999,89(4):696-703
A two-dimensional mesoscopic cluster of “dusty plasma” particles, which can be interpreted as a system of microparticles in
an rf gas discharge, is investigated. The ground-state configurations and corresponding eigenfrequencies and eigenvectors
are found for clusters of N=22–40 particles in a harmonic confining potential. It is shown that a change in the Debye screening length R of the particle charge in the plasma can cause structural transformations of the ground state of the system, manifested as
first-order or second-order phase transitions with respect to the parameter R. The disorder (“melting”) of the clusters is analyzed in detail by Monte Carlo simulation and molecular dynamics. By varying
the characteristic range of particle interaction in a cluster, it is possible to modulate its thermodynamic properties and
the character of the phase transitions, thereby causing a controlled transition of the system into the fully ordered, orientationally
disordered, or fully disordered state. The possibility of dusty plasma clusters coexisting in different states is discussed.
Zh. éksp. Teor. Fiz. 116, 1300–1312 (October 1999) 相似文献
11.
S. Okuma H. Miyatake N. Nishida Y. Ueda S. Katsuyama K. Kosuge J. H. Brewer 《Hyperfine Interactions》1991,63(1-4):265-270b
Spin-glass like magnetic ordering of iron moments was observed in both orthorhombic and tetragonal YBa2(Cu1−xFex)3Oy (x=0.08) by μ+SR measurements. In a “Tetra” sample, all the muons sense the superconducting transition at 60 K and magnetic ordering at
around 15 K, while in an “Ortho” sample they reveal that two magnetically different parts exist in the sample: about 40% of
the sample is superconducting withT
c
≈90K and the remaining part is magnetic withT
M≈33K. These phenomena can be explained in terms of clustering of the Fe atoms in the “Ortho” sample. 相似文献
12.
G. D. Alkhazov A. E. Barzakh V. P. Denisov V. S. Ivanov I. Y. Chubukov V. N. Fedoseev V. S. Letokhov V. I. Mishin S. K. Sekatsky 《Hyperfine Interactions》1990,61(1-4):1335-1338
New results on spins, electromagnetic moments and isotopic changes of nuclear charge radii for
63
157–159
Eu,
64
146, 148, 150
Cd and
65
147–155, 157, 159
Tb have been obtained from hyperfine structure and isotope shifte measured with laser resonance photoionization technique.
These results complement two-dimensional picture (with respect to H and Z) of the changes of nuclear ground state properties
near the “critical” neutron numbers N=88–90 in the vicinity of the “magric” proten number Z=64. 相似文献
13.
L. Schimmele A. Seeger W. Staiger W. Templ C. Baines A. Fritzsche M. Hampele D. Herlach K. Maier J. Major 《Hyperfine Interactions》1991,64(1-4):671-677
The longitudinalμ
+-spin relaxation rate has been measured on a high-purity spherical α-iron single crystal at temperaturesT down to 20 mK and in applied magnetic fieldsB
appl parallel to 〈111〉 up to 3 T. Only above 1 K can the data be satisfactorily described by one rate constantГ. At 1 T≤B
appl≤2 T and 50 mK≤T≤300 mK, oscillations (“wiggles”) were in addition superimposed on the longitudinal relaxation. A qualitative understanding
of the measurements may be achieved in terms of the increasing influence of internal stresses onμ
+ diffusion as the temperature is lowered. 相似文献
14.
N. A. Babushkina A. N. Taldenkov A. V. Kalinov L. M. Fisher O. Yu. Gorbenko J. Rohrkamp T. Lorenz D. I. Khomskii K. I. Kugel 《Bulletin of the Russian Academy of Sciences: Physics》2010,74(10):1345-1347
The oxygen isotope effect was investigated in cobalt oxides (Pr1 − y
Eu
y
)0.7Ca0.3CoO3 (0.12 < y < 0.26). The measurements of magnetization, electric resistivity, differential magnetic susceptibility, thermal expansion,
and specific heat show that as the Eu concentration increases, a transition from “ferromagnetic metal” to “weakly magnetic
insulator” is observed at y ≈ 0.18. In the insulating ground state, the transition occurs with a Co spin-state change that is suppressed in the ferromagnetic
(FM) metallic phase. The spin-state transition at y > 0.18 is accompanied by a substantial oxygen isotope effect that is virtually absent in the FM phase (y < 0.18). 相似文献
15.
Relative energies of muon probe sites in the chain region of RBa2Cu3O7 (RBCO) are investigated using a molecular quantum chemistry calculation for (Oμ)− embedded in a cluster of point charges to simulate local charge distributions in RBCO. Partial Cu−O chain covalency results
in a O-μ...O muon site between the chain and bridging oxygens. However, Cu-μ “hydride”-like sites are suggested by results
for nominally ionic clusters. 相似文献
16.
S. Rabinovich 《Journal of Experimental and Theoretical Physics》2011,112(4):637-641
A simple assumption of the emergence in gas of small atomic clusters consisting of c particles each leads to a phase separation (first-order transition). It reveals itself by the emergence of a “forbidden”
density range starting at a certain temperature. Defining this latter value as the critical temperature predicts the existence
of an interval with the anomalous heat capacity behavior c
p
∝ ΔT
−1/c
. The value c = 13 suggested in the literature yields the heat capacity exponent α = 0.077. 相似文献
17.
H. Satz 《The European physical journal. Special topics》2008,155(1):167-175
Multihadron production in high energy collisions, from e+e-
annihilation to heavy ion interactions, shows remarkable thermal
behaviour, specified by a universal “Hagedorn” temperature.
We argue that this hadronic radiation is formed by tunneling
through the event horizon of colour confinement, i.e., that it
is the QCD counterpart of Hawking-Unruh radiation from black holes.
It is shown to be emitted at a universal temperature TH ≃
(σ/2 π)1/2, where σ denotes the string tension.
Since the event horizon does not allow information transfer,
the radiation is thermal “at birth”. 相似文献
18.
Giant magnetoresistance in Hg
1−x−y
Mn
x
Fe
y
Te crystals is caused by clusters with “antiferromagnetic” (Mn-Te-Mn-Te, Mn-Te-Fe-Te) and “ferromagnetic” (Fe-Fe-Fe) ordering. The effect is due to the fact that the charge carriers taking part in electric current interact with the “ferromagnetic”
cluster subsystem (Fe-Fe-Fe) magnetized to saturation and become spin-polarized. These spin-polarized charge carriers are strongly scattered by the “antiferromagnetic”
Mn-Te-Mn-Te and Mn-Te-Fe-Te clusters, because the magnetic moments inside the clusters and resultant moments of clusters have chaotic orientations. Investigations
of kinetic coefficients of Hg
1−x−y
Mn
x
Fe
y
Te crystals before and after thermal treatment show that there is no marked correlation between the giant magnetoresistance
and charge-carrier concentration, mobility, and band parameters of crystals.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 28–33, October, 2007. 相似文献
19.
T. Schlathölter M.W. Newman T.R. Niedermayr G.A. Machicoane J.W. McDonald T. Schenkel R. Hoekstra A.V. Hamza 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,12(2):323-327
The emission of small (hydrogenated) carbon cluster ions CnHm
+
(n
=2-22) upon highly charged Xeq+ (q
=20-44) impact on C84 surfaces is studied by means of time-of-flight secondary ion mass spectrometry. The respective stage of hydrogenation/protonation
of a certain carbon cluster ion Cn
+
is a strong indication for its geometrical structure. From the cluster ion yield as a function of cluster size it can be
concluded, that the hydrogenation takes place after the initial carbon cluster formation. The carbon clusters seem to be emitted
as an entity in agreement with “equilibrium” and “shock wave” models.
Received 4 February 2000 相似文献
20.
E. Ya. Paryev 《Physics of Atomic Nuclei》2006,69(2):332-354
Within the spectral function approach, we study the direct production and decay via the dikaon (dimuon) channel of ϕ mesons
in the interactions of 2.4-and 2.7-GeV protons with light and medium target nuclei. It is shown that the K
+
K
− (μ+ μ−) invariant-mass distribution consists of the two components, which correspond to the ϕ decay “outside” and “inside” the target nucleus. The first (narrow) component has the free ϕ width, while the second (broad) component is distorted by the nuclear matter owing to resonance-nucleon scattering and a
possible in-medium modification of the kaons and ρ meson at finite baryon density. The relative strength of the “inside” and “outside” components is analyzed in different scenarios
for the ϕ width and momentum cut. It is demonstrated that the width of the resulting dimuon invariant-mass distribution on medium nuclei
is larger than the free ϕ width by a factor of about 2 if the total ϕ in-medium width is used and the respective cutoff for the ϕ three-momentum is applied, whereas the resulting dikaon invariant-mass distribution has an insignificant sensitivity to the
ϕ in-medium properties owing to the strong absorption of the K
− in the surrounding nuclear matter. On the other hand, because of the distortion of the K
+ and K
− on their way out of the target nucleus mainly due to the hadronic kaon potentials, the latter distribution is broadened and
shifted to higher invariant masses, which means that the measurement of such broadening would give additional evidence for
the modification of the kaon and antikaon properties in the nuclear medium.
The text was submitted by the author in English. 相似文献