Femtosecond ionization of magnesium clusters grown in ultracold helium droplets

Magnesium clusters grown in helium droplets and ionized with femtosecond laser pulses have been studied by high resolution mass spectrometry. For moderate laser intensities the abundance spectra show characteristic features indicating electronic shell effects. Compared to clusters of s¹-electron metals additional shell closures appear resulting from an electron rearrangement. Irradiation with higher laser intensities leads to a decomposition of the magnesium clusters into atomic ions. Due to charge exchange with the surrounding helium matrix mainly singly and doubly charged magnesium ions remain. In addition, the occurrence of MgHe_N ⁺-complexes is observed. Their abundance depends on the shape of the laser field, i.e. the laser width and the optical delay when applying the pump-probe technique. Received 2 January 2001     

Pre-equilibrium dipole excitation and γ-ray fragment angular correlation in the 32S +74Ge reaction

We report on the results obtained from the study of the ³²S +⁷⁴Ge deep inelastic reaction at incident energy E= 320 MeV. High-energy γ-rays were detected in an array of 6 seven-pack BaF₂ clusters. Coincidence with complex fragments detected in 12 three-stage telescopes ensured the selection of peripheral reaction events. In order to investigate the pre-equilibrium dipole strength excitation two independent analyses were performed. In the first analysis the energy spectra of the γ-rays were evaluated in the statistical model framework while in the second one the γ-ray fragment angular correlation with respect to the nuclear spin vector of the composite system was studied. Both methods indicate the excitation of dipole strength in the highly deformed dinucleus and provide dipole resonance parameter sets that are in good agreement with each other. Received: 24 March 1999 / Revised version: 17 May 1999     

Pre-equilibrium dipole strength in charge asymmetric peripheral heavy-ion reactions

We report on the results obtained from the study of the ³²S + ⁶⁴Ni and ³²S + ⁵⁸Ni peripheral reactions at incident energies E _lab = 288 MeV and E _lab = 320 MeV, respectively. High-energy γ-rays were detected in an array of 8 seven-pack BaF₂ clusters. Coincidence with complex fragments detected in 12 three-stage telescopes ensured the selection of peripheral reaction events. All of the relevant reaction parameters were kept constant with the exception of the different initial dipole moment caused by the different entrance channel charge asymmetry. While for quasi-elastic events no N/Z effect was observed in the differential γ-ray multiplicities of the two reactions, for deep-inelastic events a larger dipole γ-ray emission occurs during the more N/Z asymmetric reaction. A theoretical interpretation based on a collective Bremsstrahlung analysis of the reaction dynamics is presented. Received: 26 September 2002 / Accepted: 13 November 2002 / Published online: 6 March 2003 RID="a" ID="a"e-mail: pierroutsakou@na.infn.it RID="b" ID="b"Present address: INFN, Laboratori Nazionali di Legnaro, Padova, Italy. Communicated by C. Signorini     

Effective nonlinear optical properties of shape distributed composite media

The effective linear and nonlinear optical properties of metal/dielectric composite media, in which ellipsoidal metal inclusions are distributed in shape, are investigated. The shape distribution function P(L _x, L _y) is assumed to be 2Δ^-2θ(L _x - 1/3 + Δ/3)θ(L _y - 1/3 + Δ/3)θ(2/3 + Δ/3 - L _x - L _y), where θ( ^{. . .} ) is the Heaviside function, Δ is the shape variance and L_i are the depolarization factors of the ellipsoidal inclusions along i-symmetric axes (i = x, y). Within the spectral representation, we adopt Maxwell-Garnett type approximation to study the effect of shape variance Δ on the effective nonlinear optical properties. Numerical results show that both the effective linear optical absorption α ∼ ωIm() and the modulus of the effective third-order optical nonlinearity enhancement |χ⁽³⁾ _e|/χ⁽³⁾ ₁ exhibit the nonmonotonic behavior with Δ. Moreover, with increasing Δ, the optical absorption and the nonlinearity enhancement bands become broad, accompanied with the decrease of their peaks. The adjustment of Δ from 0 to 1 allows us to examine the crossover behavior from no separation to large separation between optical absorption and nonlinearity enhancement peaks. As Δ → 0, i.e., the ellipsoidal shape deviates slightly from the spherical one, the dependence of |χ⁽³⁾ _e|/χ⁽³⁾ ₁ on Δ becomes strong first and then weak with increasing the imaginary part of inclusions' dielectric constant. In the dilute limit, the exact formula for the effective optical nonlinearity is derived, and the present approximation characterizes the exact results better than old mean field one does. Received 10 December 2002 Published online 4 June 2003 RID="a" ID="a"e-mail: lgaophys@pub.sz.jsinfo.net     

Melting behavior of large disordered sodium clusters

The melting-like transition in disordered sodium clusters Na₉₂ and Na₁₄₂ is studied by performing density functional constant-energy molecular dynamics simulations. The orbital-free version of the density functional formalism is used. In Na₁₄₂ the atoms are distributed in two distinct shells (surface and inner shells) and this cluster melts in two steps: the first one, at ≈130 K, is characterized by the development of a high intrashell atomic mobility, and the second, homogeneous melting at ≈270 K, involves diffusive motion of all the atoms across the whole cluster volume. On the contrary, the melting of Na₉₂ proceeds smoothly over a very broad temperature interval, without any abrupt change in the thermal or structural indicators. The occurrence of two steps in the melting transition is suggested to be related to the existence of a grouping of the atoms in radial shells, even if those shells present a large degree of internal disorder. It then appears that a cluster can be considered fully amorphous (totally disordered) only when there are no radial regions of low atomic density separating shells. The isomer of Na₉₂ studied here fulfills this criterion and its thermal behavior can be considered as representative of that expected for fully amorphous clusters. Disordered Na₁₄₂, on the other hand, that has a discernible structure of an inner and a surface shell, should be considered as not fully disordered. The thermal behavior of these two clusters is also compared to that of icosahedral (totally ordered) sodium clusters of the same sizes. Received 5 February 2001 and Received in final form 21 May 2001     

Directed spiral site percolation on the square lattice

A new site percolation model, directed spiral percolation (DSP), under both directional and rotational (spiral) constraints is studied numerically on the square lattice. The critical percolation threshold p _c ≈ 0.655 is found between the directed and spiral percolation thresholds. Infinite percolation clusters are fractals of dimension d _f ≈ 1.733. The clusters generated are anisotropic. Due to the rotational constraint, the cluster growth is deviated from that expected due to the directional constraint. Connectivity lengths, one along the elongation of the cluster and the other perpendicular to it, diverge as p → p _c with different critical exponents. The clusters are less anisotropic than the directed percolation clusters. Different moments of the cluster size distribution P _s(p) show power law behaviour with | p - p _c| in the critical regime with appropriate critical exponents. The values of the critical exponents are estimated and found to be very different from those obtained in other percolation models. The proposed DSP model thus belongs to a new universality class. A scaling theory has been developed for the cluster related quantities. The critical exponents satisfy the scaling relations including the hyperscaling which is violated in directed percolation. A reasonable data collapse is observed in favour of the assumed scaling function form of P _s(p). The results obtained are in good agreement with other model calculations. Received 10 November 2002 / Received in final form 20 February 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: santra@iitg.ernet.in     

Prompt dipole γ-ray emission in fusionlike heavy-ion reactions

The ³²S + ¹⁰⁰Mo and ³⁶S + ⁹⁶Mo fusionlike reactions were studied at incident energy of E _lab = 298 MeV and 320 MeV, respectively, with the aim of probing the influence of the entrance channel charge asymmetry on the dipole γ-ray emission. The excitation energy and spin distribution of the compound nucleus created in these reactions were identical, the only difference being associated with the unequal charge asymmetry of the two entrance channels. High-energy γ-rays were detected in an array of 9 seven-pack BaF₂ clusters. Coincidence with fusionlike residues detected in four PPAC ensured the selection of central reaction events. By studying the differential γ-ray multiplicity associated with the two reactions it was shown that the dipole strength excited in the compound nucleus increases with the entrance channel charge asymmetry. From the linearized spectra, the increase of the GDR γ-ray intensity was found to be ∼ 25% for the more charge asymmetric system. The results are discussed and compared with those of previous data obtained at different incident energies. Received: 21 October 2002 / Accepted: 23 December 2002 / Published online: 1 April 2003     

Size and concentration effects in the optical properties of alloyed ( Au x Ag 1 - x ) n and core-shell ( Ni x Ag 1 - x ) n embedded clusters

Optical properties of mixed clusters (Au_xAg _{1 - x} ) _n and (Ni_xAg _{1 - x} ) _n , produced by laser vaporization and embedded in an alumina matrix, are reported. The size effects are investigated for different concentrations (x = 0.25, 0.5 and 0.75) in the diameter range 2-4 nm. For alloyed clusters (Au_xAg _{1 - x} ) _n of a given size an almost linear evolution of the surface plasmon frequency ω _s with the concentration is observed (between those of pure gold and pure silver clusters). Moreover the blue-shift and the damping of the resonance with decreasing size is all the more important as the gold concentration in the particles increases. Such results are in agreement with theoretical calculations carried out in the frame of the time-dependent local-density-approximation (TDLDA) including an inner skin of ineffective screening and the porosity of the matrix. The optical response of (Ni_xAg _{1 - x} ) _n clusters exhibits a surface plasmon resonance in the same spectral range as the one observed for pure silver clusters, but considerably damped and broadened. For a given mean cluster size 3.0 nm, a blue-shift of the resonance is observed when increasing the nickel concentration (between x = 0.25 and x = 0.75). The results are in good qualitative agreement with classical predictions in the dipolar approximation, assuming a core-shell geometry. Received 21 November 2000     

Multiple Coulomb excitation of a 70Ge beam and the interpretation of the 02 + state as a deformed intruder

Electromagnetic properties of the low-lying states in a ⁷⁰Ge nucleus were studied through the multiple Coulomb excitation of a ⁷⁰Ge beam with a ^natPb target. Relative γ-ray intensities were measured as a function of emission angle relative to the scattered projectile. Sixteen E2 matrix elements, including diagonal ones, for 6 low-lying states have been determined using the least-squares search code GOSIA. The expectation values 〈Q ²〉 of 0₁ ⁺ and 0₂ ⁺ states in ⁷⁰Ge are compared with those in ^{72, 74, 76}Ge. Simple mixing calculations indicate that the 0₂ ⁺ states in ⁷⁰Ge and ⁷²Se can be treated as deformed intruder states. It is shown that the deformed intruder becomes the ground state in ⁷⁴Kr. These interpretations of the 0₂ ⁺ states in this region are compared with the potential-energy surface calculations by the Nilsson-Strutinsky model, which allow to interpret the experimental results in a qualitative way from the theoretical point of view. Received: 2 September 2002 / Accepted: 5 November 2002 / Published online: 25 February 2003 RID="a" ID="a"e-mail: sugawara@pf.it-chiba.ac.jp Communicated by D. Schwalm     

Magnetic, electronic, dielectric and optical properties of Pr(Ca:Sr)MnO 3

The charge-ordered perovskite Pr_0.65Ca_0.28Sr_0.07MnO₃ was investigated by means of magnetic susceptibility, specific heat, dielectric and optical spectroscopy and electron-spin resonance techniques. Under moderate magnetic fields, the charge order melts yielding colossal magnetoresistance effects with changes of the resistivity over eleven orders of magnitude. The optical conductivity is studied from audio frequencies far into the visible spectral regime. Below the phonon modes hopping conductivity is detected. Beyond the phonon modes the optical conductivity is explained by polaronic excitations out of a bound state. ESR techniques yield detailed informations on the (H,T ) phase diagram and reveal a broadening of the linewidth which can be modeled in terms of activated polaron hopping. Received 9 August 2000     

Coalescence process of monodispersed Co cluster assemblies

Cluster-cluster coalescence process of monodispersed Co clusters with mean diameter d = 8.5 and 13 nm deposited a plasma-gas-condensation-type cluster beam deposition system was investigated by in situ electrical conductivity measurements and ex situ scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and analyzed by percolation concept. The electrical conductivity measurement and TEM observation indicated that, below temperature T≈ 100^°C, the Co clusters in the assemblies maintain their original structure as deposited at room temperature, while that the inter-cluster coalescence takes place at T > 100^°C, although the size distribution and the interface morphology of the clusters showed no marked change at substrate temperatures T _s≤200^°C. Received 29 November 2000     

Photoexcitation of rare-gas neon and argon clusters doped with <Emphasis Type="Bold">H</Emphasis><Subscript><Emphasis Type="Bold">2</Emphasis></Subscript><Emphasis Type="Bold">O</Emphasis>

We have studied the fluorescence of electronically excited OH^*, H^* and H₂O^+* dissociation fragments after VUV excitation ( h ν≥11.6 eV) of rare-gas clusters (Rg = Ne, Ar) doped with H₂O molecules. In contrast to a free molecule, where Balmer H-series dominate the UV-visible spectra, only the OH ^* ( A ² Σ ⁺ ↦ X ² Π) emission band is observed in neon clusters. No emission of excited water ions has been observed. We find that while higher excitation energies (Ne vs. Ar) induce higher vibrational excitation of the OH^* ( A ) fragment, the rotational temperature is lower. This effect is attributed to the difference in the geometric position of the H₂O molecule on the surface or inside the Rg-cluster. The rotational relaxation in neon clusters is rapid while the vibrational relaxation is slow because of the coupling with the low energy matrix phonons. Received 7 March 2002 / Received in final form 27 May 2002 Published online 19 July 2002     

Hyperfine structure of the ground and first excited states in light hydrogen-like atoms and high-precision tests of QED

We consider hyperfine splitting of 1s and, in part, of 2s levels in light hydrogen-like atoms: hydrogen, deuterium, tritium, helium-3 ion, muonium and positronium. We discuss present status of precision theory and experiment for the hfs intervals. We pay a special attention to a specific difference, D ₂₁ = 8E _hfs(2s) - E _hfs(1s), which is known experimentally for hydrogen, deuterium and ³ He ⁺ ion. The difference is weakly affected by the effects of the nuclear structure and thus may be calculated with a high accuracy. We complete a calculation of the fourth order QED contributions to this difference and present here new results on corrections due to the nuclear effects. Our theoretical predictions appear to be in a fair agreement with available experimental data. Comparison of the experimental data with our examination of D₂₁ allows to test the state-dependent sector of theory of the hfs separation of the 1s and 2s levels in the light hydrogen-like atoms up to 10^-8. Received 22 September 2001     

Observation of multiply charged silver-cluster anions

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 Ag_n ^- 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     

Interplay of structural, magnetic and transport properties in thelayered Co-based perovskite LnBaCo 2 O 5 (Ln = Tb, Dy, Ho)

The oxygen deficient cobaltites LnBaCo₂O₅ (Ln = Tb, Dy, Ho) exhibit two successive crystallographic transitions at T _N ∼340 K and at T _CO ∼210 K. Whereas the first transition (P4/mmm to Pmmm) is related to the long-range antiferromagnetic ordering of the Co ions (spin ordering), the second transition (Pmmm to Pmmb) corresponds to the long-range ordering of the Co²⁺ and Co³⁺ species (charge ordering) occurring in 1:1 ratio in the structure. The charge ordered (CO) state was directly evidenced by the observation of additional superstructure peaks using neutron and electron diffraction techniques. The CO state was also confirmed indirectly from refinement of high resolution neutron diffraction data as well as from resistivity and DSC measurements. From the refined saturated magnetic moment values only, ∼3.7 and ∼2.7 , the electronic configuration of the Co ions in LnBaCo₂O₅ remains conjectural. Two pictures, with Co³⁺ ions either in intermediate spin state ( t ⁵ _2g e ¹ _g ) or in high spin state ( t ⁴ _2g e ² _g ), describe equally well our experimental data. In both cases, the observed magnetic structure can be explained using the qualitative Goodenough-Kanamori rules for superexchange. Finally, in contrast to the parent Ln = Y compound [Vogt et al. , Phys. Rev. Lett. 84, 2969 (2000)], we do not report any spin transition in LnBaCo₂O₅ (Ln = Tb, Dy, Ho). Received 13 December 2000     

The electrostatic persistence length of polymers beyond the OSF limit

We use large-scale Monte Carlo simulations to test scaling theories for the electrostatic persistence length l _e of isolated, uniformly charged polymers with Debye-Hückel intrachain interactions in the limit where the screening length κ^-1 exceeds the intrinsic persistence length of the chains. Our simulations cover a significantly larger part of the parameter space than previous studies. We observe no significant deviations from the prediction l _e∝κ^-2 by Khokhlov and Khachaturian which is based on applying the Odijk-Skolnick-Fixman theories of electrostatic bending rigidity and electrostatically excluded volume to the stretched de Gennes-Pincus-Velasco-Brochard polyelectrolyte blob chain. A linear or sublinear dependence of the persistence length on the screening length can be ruled out. We show that previous results pointing into this direction are due to a combination of excluded-volume and finite chain length effects. The paper emphasizes the role of scaling arguments in the development of useful representations for experimental and simulation data. Received 12 February 2002     

Coulomb excitation of 74Ge beam

⁷⁴Ge beam was Coulomb-excited on a ^natPb target. Ten E2 matrix elements including diagonal matrix elements for 5 low-lying states have been determined using the least-squares search code GOSIA. The expectation values of the rotational invariants 〈Q ²〉 and 〈cos3δ〉 show the small and triaxial deformation of the two lowest members of the ground-state band , while the 0₂ ⁺ and 2₂ ⁺ states are found to be almost spherical. Received: 31 August 2000 / Accepted: 4 December 2000     

Glassy effects in the swelling/collapse dynamics of homogeneous polymers

We investigate, using numerical simulations and analytical arguments, a simple one-dimensional model for the swelling or the collapse of a closed polymer chain of size N, representing the dynamical evolution of a polymer in a Θ-solvent that is rapidly changed into a good solvent (swelling) or a bad solvent (collapse). In the case of swelling, the density profile for intermediate times is parabolic and expands in space as t ^1/3, as predicted by a Flory-like continuum theory. The dynamics slows down after a time ∝N ² when the chain becomes stretched, and the polymer gets stuck in metastable “zig-zag” configurations, from which it escapes through thermal activation. The size of the polymer in the final stages is found to grow as . In the case of collapse, the chain very quickly (after a time of order unity) breaks up into clusters of monomers (“pearls”). The evolution of the chain then proceeds through a slow growth of the size of these metastable clusters, again evolving as the logarithm of time. We enumerate the total number of metastable states as a function of the extension of the chain, and deduce from this computation that the radius of the chain should decrease as 1/ln(ln t). We compute the total number of metastable states with a given value of the energy, and find that the complexity is non-zero for arbitrary low energies. We also obtain the distribution of cluster sizes, that we compare to simple “cut-in-two” coalescence models. Finally, we determine the aging properties of the dynamical structure. The subaging behaviour that we find is attributed to the tail of the distribution at small cluster sizes, corresponding to anomalously “fast” clusters (as compared to the average). We argue that this mechanism for subaging might hold in other slowly coarsening systems. Received 23 October 2000     

Terminating high-spin bands in 101Rh

High spin states of ¹⁰¹Rh have been populated using the reaction ⁷⁰Zn+³⁶S at 130 MeV. γ-rays were detected with the EUROGAM2 array. New high-spin bands have been observed in this nucleus up to 45/2⁻, 57/2⁻ and 65/2⁺ states. They have been interpreted using the Nilsson-Strutinsky cranking formalism as terminating configurations. Two of the bands were observed up to the predicted terminating states which are built up from g_9/2 protons as well as d_5/2, g_7/2 and h_11/2 neutrons relative to a ⁹⁰Zr core. Received: 30 October 1998     

Isovector deformation and its link to the neutron shell closure

DWBA analysis of the inelastic ^30-40S(p,p') and ^18-22O(p,p') scattering data measured in the inverse kinematics has been performed to determine the isoscalar (δ₀) and isovector (δ₁) deformation lengths of the 2⁺₁ excitations in the Sulfur and Oxygen isotopes using a compact folding approach. A systematic N-dependence of δ₀ and δ₁ has been established which shows a link between δ₁ and the neutron-shell closure. Strong isovector deformations were found in several cases, e.g., the 2⁺₁ state in ²⁰O where δ₁ is nearly three times larger than δ₀. These results confirm the relation δ₁>δ₀ anticipated from the core polarization by the valence neutrons in the open-shell (neutron rich) nuclei. The effect of neutron shell closure at N=14 or 16 has been discussed based on the folding model analysis of the inelastic ²²O+p scattering data at 46.6 MeV/u measured recently at GANIL.