Showing papers by "Paul-Gerhard Reinhard published in 2018"

From Calcium to Cadmium: Testing the Pairing Functional through Charge Radii Measurements of Cd 100 − 130

Abstract: Differences in mean-square nuclear charge radii of $^{100--130}\mathrm{Cd}$ are extracted from high-resolution collinear laser spectroscopy of the $5s\text{ }{^{2}S}_{1/2}\ensuremath{\rightarrow}5p\text{ }{^{2}P}_{3/2}$ transition of the ion and from the $5s5p\text{ }{^{3}P}_{2}\ensuremath{\rightarrow}5s6s\text{ }{^{3}S}_{1}$ transition in atomic Cd. The radii show a smooth parabolic behavior on top of a linear trend and a regular odd-even staggering across the almost complete $sdgh$ shell. They serve as a first test for a recently established new Fayans functional and show a remarkably good agreement in the trend as well as in the total nuclear charge radius.

Individual Low-Energy Toroidal Dipole State in ^{24}Mg.

Abstract: The low-energy dipole excitations in ^{24}Mg are investigated within the Skyrme quasiparticle random phase approximation for axial nuclei. The calculations with the force SLy6 reveal a remarkable feature: the lowest I^{π}K=1^{-}1 excitation (E=7.92 MeV) in ^{24}Mg is a vortical toroidal state (TS) representing a specific vortex-antivortex realization of the well-known spherical Hill's vortex in a strongly deformed axial confinement. This is a striking example of an individual TS which can be much more easily discriminated in experiment than the toroidal dipole resonance embracing many states. The TS acquires the lowest energy due to the huge prolate axial deformation in ^{24}Mg. The result persists for different Skyrme parametrizations (SLy6, SVbas, SkM*). We analyze spectroscopic properties of the TS and its relation with the cluster structure of ^{24}Mg. Similar TSs could exist in other highly prolate light nuclei. They could serve as promising tests for various reactions to probe a vortical (toroidal) nuclear flow.

Self-consistency in the phonon space of the particle-phonon coupling model

Abstract: In the paper the nonlinear generalization of the time blocking approximation (TBA) is presented. The TBA is one of the versions of the extended random-phase approximation (RPA) developed within the Green-function method and the particle-phonon coupling model. In the generalized version of the TBA the self-consistency principle is extended onto the phonon space of the model. The numerical examples show that this nonlinear version of the TBA leads to the convergence of results with respect to enlarging the phonon space of the model.

On the inclusion of dissipation on top of mean-field approaches

Abstract: We discuss extensions of time-dependent mean-field theories such as time-dependent local density approximation (TDLDA) in order to include incoherent dynamical correlations, which are known to play a key role in far-off equilibrium dynamics. We focus here on the case of irradiation dynamics in clusters and molecules. The field, still largely unexplored, requires quantum approaches which represents a major formal and computational effort. We present several approaches we have investigated to address such an issue. We start with time-dependent current-density functional theory (TDCDFT), known to provide damping in the linear regime and explore its capability far-off equilibrium. We observe difficulties with the scaling of relaxation times with deposited energy. We next briefly discuss semi-classical approaches which deliver kinetic equations applicable at sufficiently large excitation energies. We then consider a first quantum kinetic equation at the level of a simplified, though rather elaborate in its content, relaxation time approximation (RTA). Thanks to its sophistication, the method allows us to address numerous realistic irradiation scenarios beyond the usual domain of reliability of such theories. We demonstrate in particular the key role played by dense spectral regions in the impact of dissipation in the response of the irradiated system. RTA nevertheless remains a phenomenological approach which calls for more fundamental descriptions. This is achieved by a stochastic extension of mean field theory, coined stochastic time dependent Hartree–Fock (STDHF), which provides an ensemble description of far-off equilibrium dynamics. The method is equivalent to a quantum kinetic equation complemented by a stochastic collision term. STDHF clearly leads to proper thermalization behaviors in 1D test systems considered here. It remains limited by its ensemble nature which requires possibly huge ensembles to properly sample small transition rates. An alternative approach, coined average STDHF (ASTDHF), consists in overlooking mean field fluctuations of STDHF. ASTDHF provides a robust tool, properly matching STDHF when possible and allowing extension to realistic dynamical scenarios in full 3D. It can also be used in open systems to explore, as done in RTA, the competition between ionization and dissipation.

Excitation spectra of exotic nuclei in a self-consistent phonon-coupling model

Abstract: We explore giant resonance spectra and low-lying dipole strength in the Ni and Sn chains from proton-rich to very neutron-rich isotopes, relevant in astrophysical reaction chains For the theoretical description we employ the random-phase approximation plus many-body effects through a phonon-coupling model with optimized selection of phonons The nuclear force is based on the Skyrme-Hartree-Fock energy functional carried consistently through all steps of modeling The main effect of phonon coupling is a broadening of the spectral distributions (collisional width) This broadening is particularly dramatic for low-lying dipole strength in very neutron-rich nuclei delivering there a qualitative change of the spectra

Individual low-energy E1 toroidal and compression states in light nuclei: deformation effect, spectroscopy and interpretation

Abstract: The existence of individual low-energy E1 toroidal and compression states (TS and CS) in 24 Mg was predicted recently in the framework of quasiparticle random phase approximation (QRPA) model with Skyrme forces. It was shown that the strong axial deformation of 24 Mg is crucial to downshift the toroidal strength to the low-energy region and thus make the TS the lowest E1(K=1) dipole state. In this study, we explain this result by simple mean-field arguments. Comparing TS in two strongly axial nuclei, 24 Mg and 20 Ne, we show that the lowest TS is not a universal phenomenon but rather a peculiarity of 24 Mg. The spectroscopy of TS and CS is analyzed and some additional interpretation of these states is suggested.

Nuclear density-functional theory and fission of super-heavy elements

Abstract: We review the prediction of fission properties of super-heavy elements (SHE) by self-consistent mean-field models thereby concentrating on the widely used Skyrme-Hartree-Fock (SHF) approach. We explain briefly the theoretical tools: the SHF model, the calibration of model parameters together with statistical analysis of uncertainties and correlations, and the involved computation of fission lifetimes. We present an overview of fission stability in comparison to other decay channels over the whole landscape of SHE reaching deep into the r-process domain. The main emphasis lies on a detailed discussion of the various ingredients determining eventually the fission properties. The main result is that fission is an involved process which explores many different influences with almost equal share, basic bulk properties (also known as liquid-drop model parameters), pairing strengths, and shell effects.

The impact of the carrier envelope phase-dependence on system and laser parameters

Abstract: We investigate, from a theoretical perspective, photo-emission of electrons induced by ultra-short infrared pulses covering only a few photon cycles. In particular, we investigate the impact of the Carrier-Envelope Phase (CEP) of the laser pulse which plays an increasingly large role for decreasing pulse length. As key observable we look at the asymmetry of the angular distribution as function of kinetic energy of the emitted electrons. The focus of the present study lies on the system dependence of the reaction. To this end, we study two very different systems in comparison, an Ar atom and the Na9+ cluster. The study employs a fully quantum-mechanical description of electron dynamics at the level of Time-Dependent Density Functional Theory (TDDFT). We find a sensitive dependence on the system which can be related to the different spectral response properties. Results can be understood from an interplay of the ponderomotive motion driven by the external photon field and dynamical polarization of the system.

Enhanced energy absorption through dissipation in metal clusters

Abstract: The paper investigates the dissipation in the electronic dynamics of metallic clusters using a theoretical description in terms of time-dependent mean-field theory with an extension to include electron-electron collisions beyond mean field in terms of a relaxation-time approximation (RTA). The topic is demonstrated on two examples: first, the damping of the all-dominant surface plasmon mode in metallic clusters becoming manifest in the spectral width of the mode and in its lifetime (when viewed in the time domain), and second, the influence of dissipation on the energy absorption of the cluster from an external laser field. The example of laser excitation reveals a dramatic enhancement of energy absorption because dissipation suppresses significantly induced photon emission which is else-wise a process limiting energy intake. The enhancement effect becomes the more important the longer the duration of the laser pulse.

The impact of dissipation on plasmonic versus non-collective excitation

Abstract: We explore the impact of dissipation on the response of clusters and molecules to a laser field whereby dissipation is treated at the level of a quantum Relaxation Time Approach (RTA). Test cases are the metal cluster Na40 with pronounced plasmon response and, for comparison, the H2O molecule with strong spectral fragmentation. Laser parameters are selected according to the spectral properties of the considered systems. We consider both on and off resonance laser irradiation and compare dynamical response in terms of net ionization, absorbed energy, and dipole response. The impact of dissipation is tested by comparison of results from RTA with mere mean-field dynamics at the level of time-dependent density functional theory. Only little differences between dissipative and mean-field dynamics are observed for off resonance irradiations. The situation is totally different for laser frequencies matching a resonance where we observe qualitative differences between the RTA and mean-field evolutions.

Individual low-energy E1 toroidal and compression states in light nuclei: deformation effect, spectroscopy and interpretation

Abstract: The existence of individual low-energy E1 toroidal and compression states (TS and CS) in $^{24}$Mg was predicted recently in the framework of quasiparticle random-phase-approximation (QRPA) model with Skyrme forces It was shown that the strong axial deformation of $^{24}$Mg is crucial to downshift the toroidal strength to the low-energy region and thus make the TS the lowest E1(K=1) dipole state In this study, we explain this result by simple mean-field arguments Comparing TS in two strongly axial nuclei, $^{24}$Mg and $^{20}$Ne, we show that the lowest TS is not not a universal phenomenon but rather a peculiarity of $^{24}$Mg The spectroscopy of TS and CS is analyzed and some additional interpretation of these states is suggested