Physics

Recent experiments on isolated Co clusters have shown huge orbital magnetic moments in comparison with their bulk and surface counterparts. These clusters hence provide the unique possibility to study the evolution of the orbital magnetic moment with respect to the cluster size and how competing interactions contribute to the quenching of orbital magnetism. We investigate here different theoretical methods to calculate the spin and orbital moments of Co clusters, and assess the performances of the methods in comparison with experiments. It is shown that density functional theory in conventional local density or generalized gradient approximations, or even with a hybrid functional, severely underestimates the orbital moment. As natural extensions/corrections we considered the orbital polarization correction, the LDA+U approximation as well as the LDA+DMFT method. Our theory shows that of the considered methods, only the LDA+DMFT method provides orbital moments in agreement with experiment, thus emphasizing the importance of dynamic correlations effects for determining fundamental magnetic properties of magnets in the nano-size regime.

To greatly enhance output of nuclear fusion produced neutrons in a laser-initiated Coulomb explosion of Deuterium clusters, we propose to accelerate the resulting ions by a quasi- dc electrical pulse to the energies where the D + +D collision cross-section is the highest. With D + ions bombarding then a Deuterium-rich solid-state cathode, this allows one to solve a few problems simultaneously by (a) completely removing electron cloud hindering the Coulomb explosion of ionic core, (b) utilizing up to 100% of the cluster ions to collide with the high-density packed nuclei, and (c) reaching highly increased cross-section of neutron production in a single D + +D collision, in particular by using a multi-layered target. We also consider the use of E-pulse acceleration for diagnostic purposes.

L-shell ionisation and subsequent Coulomb explosion of fully deuterated methyl iodide, CD 3 I, irradiated with hard x-rays has been examined by a time-of-flight multi-ion coincidence technique. The core vacancies relax efficiently by Auger cascades, leading to charge states up to 16+. The dynamics of the Coulomb explosion process are investigated by calculating the ions' flight times numerically based on a geometric model of the experimental apparatus, for comparison with the experimental data. A parametric model of the explosion, previously introduced for multi-photon induced Coulomb explosion, is applied in numerical simulations, giving good agreement with the experimental results for medium charge states. Deviations for higher charges suggest the need to include nuclear motion in a putatively more complete model. Detection efficiency corrections from the simulations are used to determine the true distributions of molecular charge state produced by initial L1, L2 and L3 ionisation.

The covalent-like characteristics of hydrogen bonds offer a new perspective on intermolecular interactions. Here, using density functional theory and post-Hartree-Fock methods, we reveal that there are two bonding molecular orbitals (MOs) crossing the O and H atoms of the hydrogen-bond in water dimer. Energy decomposition analysis also shows a non-negligible contribution of the induction term. These results illustrate the covalent-like character of the hydrogen bond between water molecules, which contributes to the essential understanding of ice, liquid water, related materials, and life sciences.

We show theoretically that it is possible to create and manipulate a pair of bound states in continuum in ultracold atoms by two lasers in the presence of a magnetically tunable Feshbach resonance. These bound states are formed due to coherent superposition of two electronically excited molecular bound states and a quasi-bound state in ground-state potential. These superposition states are decoupled from the continuum of two-atom collisional states. Hence, in the absence of other damping processes they are non-decaying. We analyze in detail the physical conditions that can lead to the formation of such states in cold collisions between atoms, and discuss the possible experimental signatures of such states. An extremely narrow and asymmetric shape with a distinct minimum of photoassociative absorption spectrum or scattering cross section as a function of collision energy will indicate the occurrence of a bound state in continuum (BIC). We prove that the minimum will occur at an energy at which the BIC is formed. We discuss how a BIC will be useful for efficient creation of Feshbach molecules and manipulation of cold collisions. Experimental realizations of BIC will pave the way for a new kind of bound-bound spectroscopy in ultracold atoms.

The influence of the crystallographic orientation of a typical metal surface, like aluminum, on electron emission spectra produced by grazing incidence of ultrashort laser pulses is investigated by using the band-structure-based-Volkov (BSB-V) approximation. The present version of the BSB-V approach includes not only a realistic description of the surface interaction, accounting for band structure effects, but also effects due to the induced potential that originates from the collective response of valence-band electrons to the external electromagnetic field. The model is applied to evaluate differential electron emission probabilities from the valence band of Al(100) and Al(111). For both crystallographic orientations, the contribution of partially occupied surface electronic states and the influence of the induced potential are separately analyzed as a function of the laser carrier frequency. We found that the induced potential strongly affects photoelectron emission distributions, opening a window to scrutinize band structure effects.

Helium nanodroplets are doped with cesium and molecular hydrogen and subsequently ionized by electrons. Mass spectra reveal H x Cs + ions that contain as many as 130 hydrogen atoms. Two features in the spectra are striking: First, the abundance of ions with an odd number of hydrogen atoms is very low; the abundance of HCs + is only 1 % that of H 2 Cs + . The dominance of even-numbered species is in stark contrast to previous studies of pure or doped hydrogen cluster ions. Second, the abundance of (H 2 ) n Cs + features anomalies at n = 8, 12, 32, 44, and 52. Guided by previous work on ions solvated in hydrogen and helium we assign the anomalies at n = 12, 32, 44 to the formation of three concentric, solid-like solvation shells of icosahedral symmetry around Cs + . Preliminary density functional theory calculations for n ≤ 14 are reported as well.

A dispersion-corrected density functional theory study of the photosensitizer [Ir(ppy) 2 (bpy)] + and its derivatives bound to silver clusters Ag n ( n =2-20, 92) is performed. The goal is to provide a new system-specific set of C 6 interaction parameters for Ag and Ir atoms. To this end a QM:QM scheme is employed using the PBE functional and RPA as well as MP2 calculation as a reference. The obtained C 6 coefficients were applied to calculate dissociation curves of selected IrPS-Ag n complexes and binding energies of derivatives containing oxygen and sulphur as heteroatoms in the ligands. Comparing different C 6 parameters it is concluded that RPA-based dispersion correction produces binding energies close to standard D2 and D3 models, whereas MP2-derived parameters overestimate these energies.

Spontaneous decays of small, hot silver cluster anions Ag n , n=4−7 have been studied using one of the rings of the Double ElectroStatic Ion Ring ExpEriment (DESIREE). Observation of these decays over very long time scales is possible due to the very low residual gas pressure ( ∼ 10 −14 ) and cryogenic (13 K) operation of DESIREE. The yield of neutral particles from stored beams of Ag 6 and Ag 7 anions were measured for 100 milliseconds and were found to follow single power law behaviour with millisecond time scale exponential cut-offs. The Ag 4 and Ag 5 anions were stored for 60 seconds and the observed decays show two-component power law behaviors. We present calculations of the rate constants for electron detachment from, and fragmentation of Ag 4 and Ag 5 . In these calculations, we assume that the internal energy distribution of the clusters are flat and with this we reproduce the early steep parts of the experimentally measured decay curves for Ag 4 and Ag 5 , which extends to tens and hundreds of milliseconds, respectively. The fact that the calculations reproduce the early slopes of Ag 4 and Ag 5 , which differ for the two cases, suggests that it is the changes in fragmentation rates with internal cluster energies of Ag 4 and Ag 5 rather than conditions in the ion source that determines this behavior. Comparisons with the measurements strongly suggest that the neutral particles detected in these time domains originate from Ag 4 → Ag 3 + Ag and Ag 5 → Ag 3 + Ag 2 fragmentation processes.

Recent developments in attosecond spectroscopy yield access to the correlated motion of electrons on their intrinsic time scales. Spin-flip dynamics is usually considered in the context of valence electronic states, where spin-orbit coupling is weak and processes related to the electron spin are usually driven by nuclear motion. However, for core-excited states, where the core hole has a nonzero angular momentum, spin-orbit coupling is strong enough to drive spin-flips on a much shorter time scale. Using density matrix based time-dependent restricted active space configuration interaction including spin-orbit coupling, we address an unprecedentedly short spin-crossover for the example of L-edge (2p → 3d) excited states of a prototypical Fe(II) complex. This process occurs on a time scale, which is faster than that of Auger decay ( ∼ 4\,fs) treated here explicitly. Modest variations of carrier frequency and pulse duration can lead to substantial changes in the spin-state yield, suggesting its control by soft X-ray light.