Cluster–surface interaction studied by time-resolved two-photon photoemission

n ⁺ clusters (n=2-9)deposited onto highly oriented pyrolytic graphite (HOPG) substrates at liquid nitrogen temperatures. The deposition was carried out with variable kinetic energies of the clusters. Clusters deposited with high kinetic energy (up to 60 eV/cluster) become fragmented upon impact. For low deposition energies (1–4 eV/cluster) the size dependence of the photoelectron spectra reveals a pronounced odd/even effect, which is well known for gas phase silver clusters. This indicates that the soft deposited clusters retain their size and identity on the sample. The phase of the odd/even effect suggests that transient negatively charged cluster ions serve as an intermediate step in the two-photon photoemission process. The lifetime of the anions rises with cluster size. This is attributed to an increasing electronic density of states for larger clusters. Received: 26 October 1998 / Revised version: 16 December 1998     

SU(3) coupling schemes for odd-odd nuclei in the interacting boson – fermion – fermion model with both odd proton and odd neutron in natural parity orbits

The SU^BFF(3) dynamical symmetry limits of interacting boson – fermion – fermion model are identified and they are appropriate for heavy deformed odd – odd nuclei for configurations with both the odd proton and odd neutron occupying all the natural parity orbits in the corresponding valence shells. There are three symmetry limits and their correspondence with two quasi-particle (proton-neutron) Nilsson configurations is established; one of the limits mixes both Nilsson n_z's and Λ's and other two limits mix only Nilsson Λ's. The ¹⁹¹Ir (d,t) ¹⁹⁰Ir single nucleon transfer spectroscopic strengths are well described by one of the symmetry limits that mixes only Nilsson Λ's. Received: 22 June 1998     

Steering one-dimensional odd dark beams of finite length

This paper presents arguments for the existence and dynamics of one-dimensional odd dark beams of finite length in bulk Kerr nonlinear media. Their characteristic mixed edge–screw phase dislocations force them to steer in space. The modulational stability of these dark beams and one possible interaction scheme are discussed. Received: 26 October 1998 / Revised version: 19 January 1999 / Published online: 12 May 1999     

Demagnetization following optical excitation in nickel and permalloy films

Time-resolved pump–probe second-harmonic (SH) response following optical excitation has been studied in nickel and permalloy films. Experimental data indicate that the odd SH field represents the transient magnetization, i.e. the behavior of the spin subsystem, while the electron subsystem generates the even SH field. Delays between odd and even SH fields of about 200 fs and 650 fs were detected for nickel and permalloy films, respectively. We found that the dynamics of the odd SH field is influenced by the polarization of the pump pulse. This dependence on excitation conditions suggests a difference in surface and bulk magnetization dynamics. Received: 25 November 2001 / Revised version: 27 March 2002 / Published online: 11 June 2002     

Superheavy nuclei in deformed mean--field calculations

The ground–state properties of superheavy nuclei are investigated within various parametrisations of relativistic and nonrelativistic nuclear mean–field models. The heaviest known even–even nuclei starting with Z = 98 are used as a benchmark to estimate the predictive power of the models and forces. From that starting point, deformed doubly magic nuclei are searched in the region 100 ≤ Z ≤ 130 and 142 ≤ N ≤ 190. Received: 6 April 1998 / Revised version: 16 June 1998     

Orientational photorefractive effects observed in poly(vinyl alcohol)/liquid crystal composites

60 ) as a photoconductive sensitizer under an applied dc field. Orientational photorefractivity was demonstrated by observing Raman–Nath diffraction beams with an external dc field. The photorefractive gratings were partially memorized even in the absence of the applied dc field. Received: 12 January 1998/Revised version: 2 February 1998     

Wet bone ablation with mechanically Q-switched high-repetition-rate CO2 laser

2 laser using a miniature water spray is demonstrated. An ablation threshold of 1.4 J/cm², an optimal energy density of 9–10 J/cm², and a corresponding specific ablation energy of 25–30 J/mm³ are found for pig thighbone compacta at λ=9.57 μm and a beam waist diameter of 0.5 mm. The water spray alleviates tissue carbonization even at high laser pulse repetition rates and increases ablation efficiency. Received: 9 March 1998/Revised version: 6 July 1998     

Mean field methods for atomic and nuclear reactions: The link between time–dependent and time–independent approaches

Abstact: Three variants of mean field methods for atomic and nuclear reactions are compared with respect to both conception and applicability: The time–dependent Hartree–Fock method solves the equation of motion for a Hermitian density operator as initial value problem, with the colliding fragments in a continuum state of relative motion. With no specification of the final state, the method is restricted to inclusive reactions. The time–dependent mean field method, as developed by Kerman, Levit and Negele as well as by Reinhardt, calculates the density for specific transitions and thus applies to exclusive reactions. It uses the Hubbard–Stratonovich transformation to express the full time-development operator with two–body interactions as functional integral over one–body densities. In stationary phase approximation and with Slater determinants as initial and final states, it defines non–Hermitian, time–dependent mean field equations to be solved self–consistently as boundary value problem in time. The time–independent mean field method of Giraud and Nagarajan is based on a Schwinger–type variational principle for the resolvent. It leads to a set of inhomogeneous, non-Hermitian equations of Hartree–Fock type to be solved for given total energy. All information about initial and final channels is contained in the inhomogeneities, hence the method is designed for exclusive reactions. A direct link is established between the time–dependent and time–independent versions. Their relation is non–trivial due to the non–linear nature of mean field methods. Received: 7 January 1998 / Revised version: 20 April 1998     

Radiative damping of a relativistic electron in classical electrodynamics

An exact solution of the problem of the reaction of the field generated by a relativistic classical electron is derived. It is found that the solution differs dramatically from the known formulas by the presence of a component that is even under time reversal. It is also shown that the component of the generalized radiative damping force that is odd under time reversal coincides with the well-known relativistic damping force obtained from the approximate nonrelativistic formula via a Lorentz transformation. Zh. éksp. Teor. Fiz. 114, 1661–1671 (November 1998)     

Calculation of X-ray natural circular dichroism

A method is presented to calculate the natural circular dichroism recently discovered in the X-ray range (XNCD). The basic formula represents XNCD as an odd second-rank tensor and leads to a sum rule that relates XNCD to the mixing of odd and even orbitals in the ground state. A multiple-scattering theory of XNCD is presented, and calculated spectra for the L-edges of iodine in LiIO₃ compare favorably with the experiments. Received: 20 October 1997 / Revised: 18 February 1998 / Accepted: 31 March 1998     

Free-electron laser for infrared SEW characterization surfaces of conducting and dielectric solids and nm films on them

2 , BaF₂, MgO, LiNbO₃ were determined. The nonlinear spectroscopy applications of SEW–FEL techniques to studies of a second-harmonic generation (SHG) on crystal surfaces in the regime of counterpropagating SEWs – the frequency dependence of efficiency and the influence of a thin film deposition on a quartz surface – are described. Received: 15 May 1998/Accepted: 25 August 1998     

Distribution of current, surface impedance, and damping in superconductor coupled microstriplines

The nonuniform current distribution in the transverse cross section of the coupled conductors of a microstripline made with superconductor films leads to a substantial dependence of the losses in the line on the relative position of the coupled strips. Analytical expressions are obtained in the static approximation for the current distribution in the coupled conductors for even and odd modes, and the current distribution in the ground plane is found. The influence of the relative position of the conductors on the impedance per unit length and on the damping of electromagnetic waves in a structure with coupled superconductor lines are investigated for excitation of even and odd modes. Pis’ma Zh. Tekh. Fiz. 67, 83–88 (February 1997)     

Ultra-fast dynamics of coherent lattice and spin excitations at the Gd(0001) surface

The Gd(0001) surface is investigated by pump–probe experiments using femtosecond laser pulses at 740–860 nm wavelength. Employing optical second-harmonic generation, spin and lattice dynamics are separated through the symmetry of optical field contributions that are even and odd with respect to magnetization reversal. A coherent phonon–magnon mode at a frequency of 3 THz that is excited through the exchange-split surface state is observed in the time domain. A magneto-elastic phonon–magnon interaction based on spin–orbit coupling is weak for Gd and a modulation of the exchange interaction mediated by the lattice vibration is proposed as a microscopic interaction mechanism of this coupled mode. In parallel, electron–electron and electron–phonon interactions and their magnetic counterparts lead to incoherent dynamics of the electron, lattice, and spin subsystems. Variation of the optical wavelength shows that for longer wavelengths up to 860 nm the coherent mode dominates, while for shorter ones (≥740 nm) incoherent contributions prevail. This dependence indicates that selective depopulation of the occupied surface state component drives the coherent excitation. However, temperature-dependent studies show that the oscillation amplitude of even and odd contributions scales with the spin polarization of the surface state, suggesting that the spin dependence of the ion potentials contributes as well. Furthermore, the frequency of the coherent mode presents a blue shift with a delay of 0.17 THz/ps that saturates at the static frequency of the respective bulk phonon. This behavior is a consequence of equilibration of the screened ion potential at the surface subsequent to the intense laser excitation. PACS 78.47.+p; 63.22.+m; 63.20.Ls; 75.30.Ds     

Extended analysis of the six-times ionized xenon, Xe VII

The spectrum of six-times ionized xenon, Xe VII, has been studied in the 270–6,500 ? using a pulsed discharge. 110 new classified lines are reported. Ten levels belonging to the odd parity configurations and four belonging to the even configurations have been determined. To obtain the energy parameters Hartree-Fock relativistic calculations were used. Least-squares parametric calculation has been carried out to study the fit between experimental and theoretical values. Received 11 December 2000     

Laser ablation of polysulfone films: a laser ionization TOF mass spectrometric study

2 . Major peaks were assigned to direct fragments and recombination products ejected from the PS surface. The arrival profiles of these ablation products varied from product to product and were fitted by using a shifted Maxwell–Boltzmann distribution with a center-of-mass flight velocity [(1.4-5.6) ×10⁵ cm/s] and a Knudsen layer temperature (350–3810 K). Two types of ablation products, whose velocities and temperatures showed different dependences on laser fluence, were found to exist. Dynamical aspects in the decomposition of the polymer chain, the ejection of various fragments, and their expansion are discussed on the basis of a photothermal ablation model, where a heated surface layer with a temperature gradient along its depth plays an important role. Received: 11 August 1998/Accepted: 12 August 1998     

Organisation of carbon and boron nitride layers in mixed nanoparticles and nanotubes synthesised by arc discharge

2 bonds attests for the presence of ordered BN domains and of carbon domains; (ii) the elemental profiles show that BN layers and carbon layers are immiscible with a radial organisation into two to five domains; and (iii) the sets of layers at free surfaces – including the inner surfaces of tubes – are always made of carbon. The origin of this chemical organisation, which is most likely obtained during the growth, is discussed. For the hafnium-boride metallic particles coated by C/BN envelopes, a model based on the solidification from the outside to the inside of isolated liquid-like droplets is proposed: the carbon phase solidifies first according to theoretical phase diagrams, and forms the outer shells. For the tubes, a directional eutectic solidification process is shown to account for the observed C/BN/C sequence, in a vapour–liquid–solid scheme, with an hafnium-rich liquid-like particle at the tip of the tube. Received: 26 November 1998 / Accepted: 14 January 1999     

Calculation of the Critical Distances in a System of Two Colliding Nuclei Beyond the Monopole Approximation

A method for calculating electronic levels in compact superheavy nuclear quasimolecules based on solving the Dirac equation in spherical coordinates using a multipole expansion of a two-center potential is developed. It is shown that, for internuclear distances up to ∼100 fm, such a technique enables fast convergence, which allows one to calculate the electronic energy levels with an accuracy better than 10−6. Moreover, all the multipole moments can be calculated analytically. The critical distances between the similar colliding nuclei have been calculated in the range Z ∼ 87–100 for bottom electronic levels: even 1σg and odd 1σu, respectively.     

Free expansion of a Bose–Einstein condensate from an Ioffe–Pritchard magnetic trap

5 rubidium-87 atoms released from an Ioffe–Pritchard magnetic trap is investigated experimentally. The expansion dynamics depend only on the trap frequencies, which are determined independently. The data are in good agreement with the expected expansion of a condensate, and are clearly distinct from the behaviour of a classical gas in the hydrodynamic regime. Received: 3 June 1998/Final version: 25 September 1998     

Global Existence for the Einstein–Boltzmann Equation in the Flat Robertson–Walker Spacetime

The initial value problem for the Einstein–Boltzmann equation in the spatially homogeneous and isotropic case is considered. The global in time mild solution is obtained. Received: 18 May 1998 / Accepted: 23 November 1998