Potential-energy surfaces for asymmetric heavy-ion reactions

We calculate the macroscopic potential energy of deformation as a function of mass asymmetry and distance between mass centers for shape configurations of interest in heavy-ion reactions. For the system³⁰⁰120 we also study the effect of adding microscopic shell and pairing corrections to the macroscopic potential energy. The shape configurations are generated by bringing together two separated spheres of unequal size. After the spheres touch the shapes are constructed by filling in the neck while keeping constant the radii of the end spheres, the nuclear density and the total nuclear volume. The macroscopic energy is calculated as the sum of a Coulomb energy and a nuclear macroscopic energy that takes into account the finite range of the nuclear force. For systems throughout the periodic table we display the calculated energy as a function of distance between mass centers and mass asymmetry in the form of contour maps. Some important features of the contour maps are the stationary points of the potential energy and how they change in character and location as functions of the nuclear system considered. For example, for light systems there is a maximum in the potential energy for symmetric shapes. As we move to heavier systems this peak in the potential-energy surface splits into two asymmetric peaks that are separated by a symmetric saddle point. This occurs when Z²/A ≈ 30 for the total system. As the systems become still heavier the peaks become more and more asymmetric. In heavy-ion reactions for which the asymmetry of the system is smaller than that corresponding to the peak, the smaller nucleus tends to suck up the larger one. For larger asymmetries the larger nucleus tends to suck up the smaller one. For heavy systems the binary fission saddle point is lower than the maximum in the one-dimensional interaction barrier. The penetrability calculated for the multidimensional potential-energy surface is therefore increased relative to that for the one-dimensional barrier. The microscopic shell and pairing corrections lower the potential energy for configurations in which the target and/or projectile are magic or nearly magic. This effect persists to somewhat inside the point of touching. These corrections also lower the energy near the ground state.     

Angle and speed distributions of hydrogen desorbing thermally from metal surfaces

A quantum theoretical treatment of the angle and speed distributions of recombinatively desorbing hydrogen from metal surfaces is proposed. The desorption rate is discussed in the framework of the transition state theory. The recombinative reaction process of hydrogen due to thermal activation leads to the formation of an activated complex in the transition state. In the vicinity of a saddle point on a three-dimensional potential energy surface, the translational motion of the activated complex in the direction perpendicular to the metal surface is accompanied by its center-of-mass vibrational motion parallel to the metal surface. In order to carry out the quantum mechanical calculation, the potential surface is replaced by a simplified model potential, which provides a square potential barrier along the surface normal. It is shown that, on leaving the potential barrier, the activated complex is reflected by the boundary of the potential barrier with a certain probability and, at the same time, the center-of-mass modes of vibration with frequencies v ₁ and v ₂ are coupled with the translational motion along the surface normal. Vibrational wave functions in the momentum representation are used to calculate the transmission coefficient, which is incorporated into the conventional rate formula. The angle-dependent speed distributions of desorbing molecules are derived from the rate formula.     

Comparative analysis of purely classical and semiclassical approaches to collision line broadening of polyatomic molecules: I. C2H2-Ar case

Semiclassical approaches to the computation of spectral line parameters stay up to nowadays one of the working tools complementary to refined but costly quantum-mechanical methods. Using of the trajectory concept together with quantum treatment of internal molecular motions imposes however the hypothesis of rotation-translation decoupling and translational motion governed by the isotropic potential. When a posteori justified for small heavy colliders, this hypothesis appears as doubtful for long polyatomic molecules. At the same time, purely classical methods, even requiring the artificial procedure of the correspondence principle with quantum mechanics, easily take into account the rototranslational energy transfer through the trajectory governed by the full anisotropic potential. The infrared line broadening of a typically classical C₂H₂-Ar system at various temperatures is analyzed here from these two different points of view. When a refined ab initio potential is chosen to represent the interaction energy, the semiclassical approach leads to a visible overestimation of the line broadening for all values of the rotational quantum number and for all temperatures studied whereas the fully classical treatment gives a quite satisfactory prediction. These fully classical computations show that even for C₂H₂-Ar the rototranslational coupling is quite important, and variations of the translational motion parameters during collisions produce detectable changes in rotation. When, for the sake of a meaningful comparison with the semiclassical approach, the isotropic trajectories are imposed within the classical method, this leads to smaller line widths; the effect strongly depends, however, on the peculiarities of potential energy surface, temperature, and rotational quantum number value.     

Allowance for the shell structure of colliding nuclei in the fusion-fission process

The motion of two nuclei toward each other in fusion-fission reactions is considered. The state of the system of interacting nuclei is specified in terms of three collective coordinates (parameters). These are the distance between the centers of mass of the nuclei and the deformation parameter for each of them (the nose-to-nose orientation of the nuclei is assumed). The evolution of collective degrees of freedom of the system is described by Langevin equations. The energies of the Coulomb and nuclear (Gross-Kalinovsky potential) interactions of nuclei are taken into account in the potential energy of the system along with the deformation energy of each nucleus with allowance for shell effects. The motion of nuclei toward each other are calculated for two reaction types: reactions involving nuclei that are deformed (₄₂¹⁰⁰Mo + ₄₂¹⁰⁰Mo → ₈₄²⁰⁰Po) and those that are spherical (₈₂²⁰⁸Pb + ₈¹⁸O → ₉₀²²⁶Th) in the ground state. It is shown that the shell structure of interacting nuclei affects not only the fusion process as a whole (fusionbarrier height and initial-reaction-energy dependence of the probability that the nuclei involved touch each other) but also the processes occurring in each nucleus individually (shape of the nuclei and their excitation energies at the point of touching).     

多维Kramers公式的研究

本文研究了鞍点附近多维的位能曲面及鞍点所在位置;并用Werner Wheeler及无旋液体等两种方法计算了多维的质量系数与粘滞系数,然后用多维Kramers公式计算了裂变速率.发现裂变速率随着维数的增加而适当增大.不同的形变参量以及不同的计算质量和粘滞系数方法对计算核裂变速率影响不大.从结果看,采用三维计算裂变几率已足够准确.     

Nonuniform character of the population of spin projections K for a fissile nucleus at the scission point and anisotropies in the angular distributions of fragments originating from the induced fission of nuclei

It is shown that the emergence of anisotropies in the angular distributions of fragments originating from the spontaneous and induced fission of oriented actinide nuclei is possible only if nonuniformities in the population of the projectionsM (K) of the fissile-nucleus spin onto the z axis of the laboratory frame (fissile-nucleus symmetry axis) appear simultaneously in the vicinity of the scission point but not in the vicinity of the outer saddle point of the deformation potential. The possibilities for creating the orientation of fissile nuclei for spontaneous and induced fission and the effect of these orientations on the anisotropies under analysis are considered. The role of Coriolis interaction as a unique source of the mixing of different-K fissile-nucleus states at all stages of the fission process is studied with allowance for the dynamical enhancement of this interaction for excited thermalized states of the nucleus involved that is characterized by a high energy density. It is shown that the absence of thermalization of excited states of the fissile nucleus that appear because of the effect of nonadiabaticity of its collective deformation motion in the vicinity of the scission point is a condition of conservation of the influence that transition fission states formed at the inner and outer fission barriers exerts on the distribution of the spin projections K for lowenergy spontaneous nuclear fission. It is confirmed that anisotropies observed in the angular distributions of fragments originating from the fission of nuclei that is induced by fast light particles (multiply charged ions) are due to the appearance of strongly excited equilibrium(nonequilibrium) states of the fissile nucleus in the vicinity of its scission point that have a Gibbs (non-Gibbs) distribution of projections K.     

Model studies of the interaction of h atoms with bcc iron

Ab initio/effective core potential cluster studies are reported for the interaction of H atoms with bcc iron. The calculations use a one-electron ECP based on the 4s¹3d⁷ state of the Fe atom. Two-fold and four-fold sites on the (100) surface as well as octahedral, tetrahedral, and trigonal interior sites were studied. Four-fold surface sites are found to be bound by ~1.5 eV with the H atom ~ 0.5a₀ above the surface. Penetration of the surface at a four-fold site involves movement toward a second layer atom and is expected to be unfavorable. Two-fold surface sites have small binding energies ～ 0.25 eV. Penetration of the surface at this site involves movement toward a tetrahedral interior site and is downhill in energy. Tetrahedral interior sites are found to be bound by ～1.3 eV and are a minimum on the potential energy surface. Octahedral sites are a maximum on the potential energy surface and are estimated to be ～ 0.2 eV higher (including lattice relaxation effects). Trigonal sites are found to be a saddle point connecting adjacent tetrahedral sites and this pathway leads to an estimated barrier to diffusion of ～ 0.1 eV (including lattice relaxation effects). The volume expansion for a H atom in a tetrahedral site is calculated to be 21%.     

高电荷态离子与Si(110)晶面碰撞的沟道效应研究

不同电荷态低速离子(Ar^q+,Pb^q+)轰击Si(110)晶面,测量不同入射角情况下的次级粒子的产额. 通过比较溅射产额与入射角的关系,证实沟道效应的存在. 高电荷态离子与Si相互作用产生的沟道效应说明溅射产额主要是由动能碰撞引起的. 在小角入射条件下,高电荷态离子能够增大溅射产额. 当高电荷态离子以40°—50°入射时,存在势能越高溅射产额越大的势能效应. 关键词： 高电荷态离子 溅射 沟道效应     

Reaction and inelastic processes in the collision O () + O ()

A reduced dimensionality model is used to study the reaction OO O ₃ ( X1A1 ) + O( ³ P ) by means of time-dependent and time-independent quantum-mechanical methods. State-selected probabilities and rate constants are obtained for the reactive process as well as for the inelastic collision in which the vibrationally excited oxygen loses one or more quanta. It is found that the experimentally observed jump in depletion rates above a critical value of v could be partially explained by the vibrational relaxation rather than reaction. Reaction only becomes important for relatively high translational energies and therefore the calculated rates are too small at the temperatures of interest. It is concluded, however, that the reaction saddle point region in the potential energy surface plays a crucial role in the enhancement of vibrational relaxation. Received: 3 February 1998 / Revised: 27 March 1998 / Accepted: 15 May 1998     

Quasi‐particle peak due to magnetic order in strongly correlated electron systems

We study the electron spectral function of the antiferromagnetically ordered phase of the three dimensional Hubbard model, using recently formulated low‐energy theory based on the 2D half‐filled Hubbard model which describes both collective spin and charge fluctuations for arbitrary value of the Coulomb repulsion U. The model then is solved by a saddle‐point approximation within the CP¹ representation for the Neel field. The single‐particle properties are obtained by writing the fermion field in terms of a U(1) phase, Schwinger boson SU(2) fields and a pseudofermion variables. We demonstrate that the appearance of a sharp peak in the electron spectral function in the antiferromagnetic state points to the emergence of the bosonic mode, which is associated with spin ordering.     

Substrate dependence of ion motion in femtosecond laser ablation cloud observed by planar laser-induced fluorescence

We have observed the motion of Sm⁺ ions as well as Sm atoms produced by femtosecond laser ablation of a solidified samarium solution sample on substrates by using a planar laser-induced fluorescence method. Kinetic energies of both Sm⁺ ions and Sm atoms increase as the electrical conductivity of the substrate decreases, which suggests the effect of surface charging. The kinetic energy of Sm⁺ ions is larger than that of Sm atoms for a variety of substrates due to the further electrical acceleration by the surface charge. The knowledge of ion motion will be the key information for the optimization of femtosecond laser simultaneous atomization and ionization of organic and inorganic samples on substrates.     

Single-particle enhancement of heavy-element production

Fusion barriers are calculated in a macroscopic-microscopic model for several cold-fusion heavy-ion reactions leading to heavy and superheavy elements. The results obtained in such a picture are very different from those obtained in a purely macroscopic model. For reactions on ²⁰⁸Pb targets, shell effects in the entrance channel result in fusion-barrier energies at the touching point that are only a few MeV higher than the ground state for compound systems near Z = 110. The entrance-channel fragment-shell effects remain far inside the touching point, almost to configurations only slightly more elongated than the ground-state configuration, where the fusion barrier has risen to about 10 MeV above the ground-state energy. Calculated single-particle level diagrams show that few level crossings occur until the peak in the fusion barrier very close to the ground-state shape is reached, which indicates that dissipation is negligible until very late in the fusion process. Whereas the fission valley in a macroscopic picture is several tens of MeV lower in energy than is the fusion valley, we find in the macroscopic-microscopic picture that the fission valley is only about 5 MeV lower than the fusion valley for cold-fusion reactions leading to compound systems near Z = 110. These results show that no significant “extra-extrapush” energy is needed to bring the system inside the fission saddle point and that the typical reaction energies for maximum cross section in heavy-element synthesis correspond to only a few MeV above the maximum in the fusion barrier.     

Hydration states and metallic intercalation effects in dry DNA

Temperature dependence of the complex dielectric constant at 16.3 GHz has been measured on dry poly(dA)-poly(dT) DNA with Na⁺ counterions under different relative humidity (RH). The rotational motion of sole water dipoles hydrated in PO₂⁻ is detected at low RH, and the collective motion due to water network in major groove is successfully observed at high RH. From the infrared (IR) spectroscopy in dry DNA introducing metallic ions (M-DNA), the symmetric and antisymmetric stretching bands of PO₂⁻ distinctly depend on RH and type of metallic ions. The IR spectral change indicates that the monovalent ions (Li and Na) are arranged in PO₂⁻ though the divalent and trivalent ions connect to the base molecules.     

Microscopic calculation of the collective motion in76Kr

A microscopic calculation of Bohr's collective Hamiltonian is used to describe the collective motion in the⁷⁶Kr isotope. A single-particle basis calculated in a deformed Woods-Saxon potential leads to the potential energy surface obtained by the Strutinsky renormalization procedure, and to the inertial functions determined in the cranking model approximation. The collective Schrödinger equation is solved numerically. The low-energy, even parity states in⁷⁶Kr are analyzed in the frame of this model. The theoretical results involve the potential energy and the inertial parameters as functions of intrinsic quadrupole deformations, the collective levels and wave functions including their transitions and electromagnetic moments. A good agreement between experiment and theory is obtained without adjusting specifically for this nucleus any parameter in the model. Some results regarding statical and dynamical characteristics of even-even^{74, 78, 80}Kr isotopes are also presented.     

The dynamics of direct three-body recombination of ions

The trajectory simulation of the direct recombination of the Cs⁺ and Br^? heavy ions in the presence of third-body atoms R = Kr, Xe, Hg was performed on the potential energy surface that controlled the dissociation of CsBr induced by collisions with the same R. The results showed that the probability of recombination decreased as the energy of the approach of ions to each other and the energy of a third body with respect to the center of mass of the ion pair increased. Direct recombination proceeds according to at least two mechanisms of the stabilization of the molecule formed. The first mechanism involves the collision of the R atom with both ions at low impact parameters with respect to the center of gravity of the ion pair. In the second mechanism, impact parameters are large, and energy is removed through a collision with one of the ions of the pair. The products formed have a strongly nonequilibrium vibrational distribution and almost equilibrium rotational energy distribution.     

LEVEL DENSITY AND FINITE-TEMPERATURE SPECIFIC HEAT OF NUCLEUS 104Pd UNDER MICROSCOPIC IBM

By using the microscopic sdgIBM-F_max approach, procedure of canonical ensemble average and saddle point approximation, thermodynamics of nucleus is established under microscopic IBM. Calculations of spectrum, level density and finite-temperature specific heat for the nucleus ¹⁰⁴Pd are carried out. The calculated values are coincident with the experimental data reported recently. The results predict that the shape phase transition in ground state band appears at about T≈0.230MeV and the phase transition of thermal excitation mode takes place at T≈0.630MeV for nucleus ¹⁰⁴Pd.     

A theoretical spectroscopic study of single ammonia molecules adsorbed on the surface of fcc-type argon clusters. Surface effects on the ν 2 vibration-inversion mode

Ammonia monomers have been adsorbed on argon clusters at low temperature K by Rohmund and Huisken [#!frfh97!#] using the pick-up technique. They measured the spectrum of the NH₃ molecules in the region of the umbrella mode. Two broad bands centered around 970 and 1000 cm^-1 with finer details were observed. The authors attempted to interpret the obtained spectrum on the basis of the free rotation motions of the molecules. In this paper semi-empirical atom-atom potential energy calculations are performed for the ammonia monomer adsorbed on a rigid face-centered-cubic (fcc)-type surface of the argon cluster. In the equilibrium position of the rigid molecule on the cluster surface the orientational potential energy surface exhibits two quasi-equivalent minima separated by a potential barrier of about 100 cm^-1. The symmetry of the molecular vibration-inversion double-well potential is destroyed; the inversion motion is then forbidden in the ground state. On the basis of the two adsorption orientations, the vibrational frequency shifts are calculated and the obtained infrared bar-spectrum agrees with the experimental one. Received 6 April 1999 and Received in final form 19 July 1999