Valence and core excitations in rare gas mono- and multilayers: Production,decay, and desorption of neutrals and ions

The aim of this survey is the understanding of the dynamics of medium to high energy excitations in simple condensed systems on very short time scales. For this purpose we examine the modifications of the electronic excitations and their evolution in rare gases (mainly Ar and Kr) due to a nearby metal surface (monolayer case) or by embedding into a rare gas condensate (multilayers). Ionic excitations are shifted to lower energies compared to the gas phase by polarization of the surroundings, while neutral excitations stay constant or are shifted to somewhat higher energies. This decreases the spacing between excitonic and ionic states from both sides. Deexcitation events can be analysed by linewidths for valence excitations, and by comparison of autoionisation and Auger spectra for core excitations. For monolayers, we conclude that excitonic states are unstable relative to ionic states but nevertheless are quite long-lived. For multilayers, only minor modifications relative to the gas phase are usually found. All electronic excitations in Ar and Kr mono- or multilayers lead to desorption of neutrals; core excitations in multilayers also lead to ions and cluster ions. The probable mechanisms in all cases are discussed, and open questions are pointed out.     

Differences between dynamical theories of giant resonances

It is the objective of dynamical theories of collective excitations to describe the influence of particle-vibration coupling on the excitation energies of giant resonances. This yields dynamical corrections to the energies calculated in the random-phase approximation (RPA). The existing dynamical theories can be characterized by the effective particle-hole gap which they prescribe for RPA-type calculations of collective excitations. We investigate three dynamical theories in the framework of a schematic model for the nucleon self-energy. In the case of the giant dipole resonance in ²⁰⁸Pb, the microscopic dynamical model prescribes an effective p-h gap which is smaller than the experimental value; in contradistinction, the effective p-h gap is larger than the experimental value in the case of the isoscalar octupole surface vibration. These dynamical corrections are opposite to the corrections predicted by two other models which have been proposed. The origin of these differences is discussed.     

Topological magnetic insulators with corundum structure

Topological insulators are new states of quantum matter in which surface states residing in the bulk insulating gap are protected by time-reversal symmetry. When a proper kind of antiferromagnetic long-range order is established in a topological insulator, the system supports axionic excitations. In this Letter, we study theoretically the electronic states in a transition metal oxide of corundum structure, in which both spin-orbit interaction and electron-electron interaction play crucial roles. A tight-binding model analysis predicts that materials with this structure can be strong topological insulators. Because of the electron correlation, an antiferromagnetic order may develop, giving rise to a topological magnetic insulator phase with axionic excitations.     

Ion-stimulated desorption from a semiconductor surface containing metal nanodots

A model of the mechanism of ion-stimulated desorption of atoms and molecules pre-adsorbed from the surface of wide-band gap samples with a system of metal nanodots is developed. The relaxation processes of vibration-exciting states in adsorbed layer via electronic channel are considered. It is found in the model that the presence of nanosized metal clusters can affect the relaxation rate of the vibration excitations on the surface and the velocity of ion-stimulated desorption.     

Single-particle excitations and the order parameter for a trapped superfluid Fermi gas

It is found that the character of single-particle excitations of a trapped neutral-atom Fermi gas is strongly influenced by a superfluid phase transition. Below the transition temperature the presence of a spatially inhomogeneous order parameter (gap) shifts the excitation eigenenergies upward and leads to the appearance of in-gap excitations localized in the outer part of the gas sample. The eigenenergies become sensitive to the gas temperature and are no longer multiples of the trap frequencies. These features should manifest themselves in a strong change of the density oscillations induced by modulations of the trap frequencies and can be used for identifying the superfluid phase transition. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 6, 392–397 (25 September 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Coherence during scattering of fast H atoms from a LiF(001) surface

The coherence for diffraction effects during grazing scattering of fast hydrogen and helium atoms from a LiF(001) surface with energies up to some keV is investigated via the coincident detection of two-dimensional angular distributions for scattered projectiles with their energy loss. For keV H atoms, we identify electronic excitations of the target surface as the dominant mechanism for decoherence, whereas for He atoms this contribution is small. The suppression of electronic excitations owing to the band gap of insulators plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as a surface analytical tool.     

Collective excitations in molecular complexes

Green's functions are used to derive an equation for the energies of collective excitations in molecular complexes. It is found that the collective branch contains a solution of exciton type. An expression is derived for the energies of the exciton excitations.     

Evidence for pairing above the transition temperature of cuprate superconductors from the electronic dispersion in the pseudogap phase

In the underdoped high temperature superconductors, instead of a complete Fermi surface above Tc, only disconnected Fermi arcs appear, separated by regions that still exhibit an energy gap. We show that in this pseudogap phase, the energy-momentum relation of electronic excitations near EF behaves like the dispersion of a normal metal on the Fermi arcs, but like that of a superconductor in the gapped regions. We argue that this dichotomy in the dispersion is difficult to reconcile with a competing order parameter, but is consistent with pairing without condensation.     

Low-lying excitations in the S = 1 antiferromagnetic Heisenberg chain

In order to confirm the picture of domain-wall excitations in the hidden antiferromagnetic order of the Haldane phase, the structure of the low-lying excitations in the S = 1 antiferromagnetic Heisenberg chain is studied by a quantum Monte Carlo method. It is confirmed that there exists a finite energy gap between the first- and the second-excited states at k = π as well as between the ground state and the first-excited state at k = π. In the thermodynamic limit, the second-excited state at k = π is separated from the ground state by the gap which is three times as large as the Haldane gap. From the size dependences of the low-lying-excitation energies, the interactions between the elementary excitations in the excited states are concluded to be repulsive.     

Grazing incidence diffraction of keV helium atoms on a Ag(110) surface

Diffraction of fast atoms at grazing incidence has been recently demonstrated on the surface of alkali halides and wide band gap semiconductors, opening applications for the online monitoring of surface processes such as growth of ultrathin layers. This Letter reports energy resolved diffraction of helium on Ag(110) metal surface showing that a band gap is not mandatory to restrict the decoherence due to electron-hole pair excitations by the keV projectile. Measurement of the energy loss, which is in the eV range, sheds light on the scattering process.     

Quantum chemical and NMR investigations on structure of surface complexes

The semiempirical quantum chemical CNDO/2 method is used to calculate models of specific interaction between benzene, toluene, and butene molecules, respectively, and ions or hydroxyls representing active sites of adsorption on zeolitic surfaces. From energy minima of full potential curves the stabilization energies of the surface complexes have been obtained. On the basis of proposed complexes theoretical carbon-13 NMR chemical shifts of adsorbed molecules are calculated. The theoretical results are in rather good agreement with the experimental ones, confirming the conception of surface complexes. Moreover, experimental paramagnetic shifts of surface complexes containing Co²⁺ ions are tried to interprete in a quite similar way.     

Optical absorption of isolated silver cluster-tryptophan: A joint experimental and theoretical study

We present a joint experimental and theoretical gas phase study of photoabsorption and photofragmentation of silver cluster-biomolecule complexes. We demonstrate on the example of [ Trp.Ag₃] ⁺ that binding of the metal cluster to a biomolecule leads to a significant enhancement of the photoabsorption in comparison with [Trp.H]⁺ and [Trp.Ag]⁺. This enhancement arises due to the coupling between the excitations in the metallic subunit with charge transfer excitations between silver cluster and tryptophan. Our experimental studies show that silver clusters up to eleven atoms can be bound to tryptophan and we present first results on the photofragmentation of the Trp.Ag₁₁ ⁺ complex cation, in which properties of cluster subunit remain preserved.     

Solvent effects on guanidinium‐anion interactions and the problem of guanidinium Y‐aromaticity

We have calculated the complexes formed by guanidine/guanidinium and HCl/Cl^?, HNO₃/NO₃^? and H₂SO₄/HSO₄^? both in the gas and aqueous Polarizable Continuum Model (PCM) phase to understand the effect that solvation has on their interaction energies. In the gas phase, the cation–anion complexes are much more stable than the rest; however, when PCM‐water is considered, this energetic difference is not as large due to the extra stabilization that the ions suffer when in aqueous solution. All the complexes were analyzed in terms of their AIM and NBO properties. In all cases, water solvation seems to “dampen” those properties observed in the gas phase. The values of Nucleus Independent Chemical Shift (NICS)(1) and NICS(2) indicate a huge influence of the proximity of the carbon atom for short distances; thus, the 3D NICS values on the van der Waal isosurfaces have been used to evaluate the possible Y‐aromaticity of the guanidinium system. The isosurface in this system is more similar to cyclohexane than to benzene as indication of poor aromaticity. Copyright © 2013 John Wiley & Sons, Ltd.     

Variation of Excitation Spectra in Mixed-Stack Charge-Transfer Complexes

Local excitation spectra in different spin and charge channels are calculated in the one-dimensional extended Hubbard model with alternating energy levels at half filling for mixed-stack charge-transfer complexes. Near the boundary between the neutral and ionic phases, the electronic system is easily distorted by an additional term that reduces the symmetry and opens a gap. Alternating transfer integrals produce a nonmagnetic spin-Peierls phase; while staggered magnetic fields produce an antiferromagnetic phase. Both of them enhance the ionicity when they are introduced into the neutral phase near the boundary. Accordingly, these additional terms enhance low-energy spin excitations, although these excitations are suppressed when compared with those in the regular ionic phase. The regular ionic phase has a larger spectral weight in the local current channel than the neutral phase. This would imply that, in one dimension and if the lattice effect is negligible, the ionic phase has smaller activation energy in the electric conductivity near the boundary than the neutral phase.     

Cation sitting in aromatic cages: ab initio computational studies on tetramethylammonium–(benzene)n (n=3–4) complexes

Quantum chemistry study was performed on interaction between tetramethylammonium (TMA) and aromatic cages by means of the MP2 method to show how TMA sits in an aromatic cage that is composed of benzenes. The MP2 calculations on TMA–(benzene)_n complexes demonstrate that the more the benzene molecules in the aromatic cage, the stronger the binding strength between the cage and TMA. In details, the structure of TMA–(benzene)_n (n = 1–4) complexes can be easily constructed by superimposing n TMA‐benzene complexes via TMA, and the binding energies of the TMA–(benzene)_n complexes are the sum of the n corresponding TMA‐benzene systems. For instance, the distances between the N of TMA and the plane of the benzene ring are 4.238, 4.252, 4.264 ,and 4.276 Å, respectively, for TMA–(benzene)_n (n = 1–4) complexes, and the BSSE corrected binding energies at MP2/6‐311++G** level are ?8.8, ?17.3, ?25.8 and ?34.3 kcal/mol, respectively, for TMA– (benzene)_n (n = 1–4) complexes. Thus, this study provides us useful information on how a cation interacts with an aromatic cage in terms of complex geometry and binding strength. Copyright © 2007 John Wiley & Sons, Ltd.     

Nonlocal screening of plasmons in graphene by semiconducting and metallic substrates: first-principles calculations

We investigate the role of substrates on the collective excitations of graphene by using a first-principles implementation of the density response function within the random-phase approximation. Specifically, we consider graphene adsorbed on SiC(0001) and Al(111) as representative examples of a semiconducting and metallic substrate. On SiC(0001), the long wavelength π plasmons are significantly damped although their energies remain almost unaltered. On Al(111), the long wavelength π plasmons are completely quenched due to the coupling to the metal surface plasmon. The strong damping of the plasmon excitations occurs despite the fact that the single-particle band structure of graphene is completely unaffected by the substrates illustrating the nonlocal nature of the effect.     

Excited states and absorption spectra of β-diketonate complexes of boron difluoride with aromatic substituents

In the approximation of the time-dependent electron density functional theory, we have studied using the quantum-chemical method the nature of excited states of boron difluoride acetylacetonate F₂BAA and its substituted derivatives that contain aromatic groups with one or two benzene cycles in the β-position. Optimization of the geometry of complexes show coplanar positions of cycles for all compounds, except for that with the substituent C₆H₃(CH₃)₂. Based on the calculated transition energies and oscillator strengths, we have simulated the absorption spectra in the prevacuum range. The calculated absorption spectra have been compared with the experimental spectra in the gas phase or in solutions. We show that, in the absorption spectra of complexes that contain substituents with one benzene cycle, the first three bands are caused by the transition of π electrons of the substituent to the LUMO of the chelate cycle. In complexes with two cycles in the substituent, the number of these transitions increases to five. As the π system becomes more extended, a bathochromic shift of the first absorption band and an increase in the transition probability are observed.     

Resonances of thermally stimulated electromagnetic excitations of thin films on a substrate in a near field

The spectral features of thermally stimulated electromagnetic excitations generated by films of wide band gap semiconductors on metals in vacuum at different distances from a sample surface were investigated. Special attention was given to the interaction of characteristic resonances in the excitation spectrum of these fields with local vibrations of Cd impurity atoms inside ZnTe films on metal substrates.