Energies Of Intermediate States Research Articles

Spin-vibronic coherence drives singlet-triplet conversion.

Design-specific control over the transitions between excited electronic states with different spin multiplicities is of the utmost importance in molecular and materials chemistry1-3. Previous studies have indicated that the combination of spin-orbit and vibronic effects, collectively termed the spin-vibronic effect, can accelerate quantum-mechanically forbidden transitions at non-adiabatic crossings4,5. However, it has been difficult to identify precise experimental manifestations of the spin-vibronic mechanism. Here we present coherence spectroscopy experiments that reveal the interplay between the spin, electronic and vibrational degrees of freedom that drive efficient singlet-triplet conversion in four structurally related dinuclear Pt(II) metal-metal-to-ligand charge-transfer (MMLCT) complexes. Photoexcitation activates the formation of a Pt-Pt bond, launching a stretching vibrational wavepacket. The molecular-structure-dependent decoherence and recoherence dynamics of this wavepacket resolve the spin-vibronic mechanism. We find that vectorial motion along the Pt-Pt stretching coordinates tunes the singlet and intermediate-state energy gap irreversibly towards the conical intersection and subsequently drives formation of the lowest stable triplet state in a ratcheting fashion. This work demonstrates the viability of using vibronic coherences as probes6-9 to clarify the interplay among spin, electronic and nuclear dynamics in spin-conversion processes, and this could inspire new modular designs to tailor the properties of excited states.

Dirac Bands in the Topological Insulator Bi2Se3 Mapped by Time‐Resolved Momentum Microscopy

AbstractThe energy dispersion of the unoccupied Dirac bands of the topological insulator Bi2Se3 has been studied up to large parallel momenta and intermediate state energies using a setup for laser‐based time‐resolved momentum microscopy with 6 eV probe‐photons. A strongly momentum‐dependent evolution of the topologically protected Dirac states into a conduction band resonance is observed, highlighting the anisotropy dictated by the symmetry of the surface. The results are in remarkable agreement with the theoretical surface spectrum obtained from a GW‐corrected tight‐binding model, suggesting the validity of the approach in the prediction of the quasiparticle excitation spectrum of large systems with non‐trivial topology. After photoexcitation with 0.97 eV photons, assigned to a bulk valence band‐conduction band transition, the out‐of‐equilibrium population of the surface state evolves on a multi‐picosecond time scale, in agreement with a simple thermodynamical model with a fixed number of particles, suggesting a significant decoupling between bulk and surface states.

Novel C-dots/titanate nanotubular hybrid materials with enhanced optical and photocatalytic properties

Advanced nanomaterials with enhanced optical and photocatalytic properties for the photodegradation of organic pollutants, in particular pharmaceuticals and personal care products (PPCPs), were successfully prepared by a swift one-pot synthesis. Nanostructured materials were synthesized through an integrated hydrothermal procedure which generates titanate nanotubes (TNTs) with different carbon dots (C-dots) contents, from an amorphous titanium oxide-based source and cork industry wastewaters (CIWW) as carbon source. Their structural, microstructural, morphological, and optical properties were studied by XRD, TEM, XPS, UV-Vis diffuse reflectance and photoluminescence spectroscopies. As aimed, the hybrid C-dots/TNT nanomaterials extend their light absorption towards the red, in comparison to pristine TNT, prompting them for a more efficient use of light in photocatalysis by widening the TNT energy uptake range. The decrease of bandgap energy with increasing sample’s C-dots content seems to be originated from energy intermediate states formed within the TNTs’ forbidden band resulting from Ti−O−C bonds established between the TNTs and the C-dots that form tails of states.The as-synthesized C-dots/TNT samples were tested in the photodegradation of caffeine as a pollutant model for PPCPs. Rewarding results were obtained, with the hybrid C-dots/TNT nanomaterials showing significant enhanced photocatalytic ability toward caffeine degradation in comparison to pristine TNTs. Photocatalysis assays in the presence of scavengers and/or in the absence of oxygen were also performed aiming to characterize the most reactive species formed during the semiconductor photo-activation process and thus assessing to possible reactive pathways underpinning the photocatalytic activity of the hybrid C-dots/TNT nanomaterials.

La-RuO2 nanocrystals with efficient electrocatalytic activity for overall water splitting in acidic media: Synergistic effect of La doping and oxygen vacancy

The development of high-performance and durable Ru-based electrocatalysts in proton exchange membrane (PEM) water electrolyzers for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) remains a huge challenge due to the severe catalyst’s corrosion and high reaction overpotential in acid media. Herein, we reported a synergistic strategy of effectively combining hetero-atom doping and oxygen vacancy in the La-doped RuO2 nanocrystals, successfully realized by a hydrothermal and followed annealing process aiming for efficiently overall water splitting. The as-obtained La-doped RuO2 nanocrystals demonstrate superior electrocatalytic performances with ultra-low overpotential of 208 mV for OER and 71 mV for HER at the current density of 10 mA/cm2 together with robust long-term stability in 0.5 M H2SO4 electrolyte, which are 222 and 15 mV lower in overpotentials than that of bare RuO2 electrocatalyst. Moreover, the La-doped RuO2 nanocrystals only require a cell voltage of 1.53 V to drive the overall water splitting reaction and exhibit a remarkable stability. Density functional theory (DFT) calculations further reveal that the synergistic effect of La doping and oxygen vacancy engineering could tune the d-band center of Ru active sites and optimize the Gibbs free energy of intermediate state, thus enhancing the activity of RuO2. Such synergistic strategy of aliovalent doping and vacancy engineering for much enhanced acid electrocatalysis may offer important referential values for exploring efficient electrocatalysts towards overall water splitting in acidic media.

Modification of Metal (Fe, Al) Doping on Reaction Properties of a NiO Oxygen Carrier with CO during Chemical Looping Combustion

Oxygen carriers can significantly enhance the performance of chemical looping combustion at low energy-cost CO2 capture. Based on the density functional theory, a microscopic model of the metal Fe, Al-doped NiO oxygen carrier was established. The results indicate that the intermediate state energy and the reaction energy reduce due to electronic interaction of the Al-doped surface. With the progress of the reaction, the NiO–Al surface promotes the oxidation process of CO, indicating that the activity of the NiO surface enhanced, which is attributed to the electronic and steric effects of the Al–O structure. For the decomposition of CO on the OC surface, doping with other atoms is beneficial to suppress the carbon deposition, which is related to the steric hindrance caused by doping with other atoms. Besides, doping with iron and aluminum atoms is more conducive to the movement of OC bulk crystal lattice oxygen to the surface, thereby promoting subsequent reactions. Therefore, it is feasible to improve the reactivity of the Ni-based OC by doping metal Al, and its modification effect is closely related to the characteristics of the components.

Resonant control of photoelectron directionality by interfering one- and two-photon pathways

Coherent control of interfering one- and two-photon processes has for decades been the subject of research to achieve the redirection of photocurrent. The present study develops two-pathway coherent control of ground state helium atom above-threshold photoionization for energies up to the $N=2$ threshold, based on a multichannel quantum defect and R-matrix calculation. Three parameters are controlled in our treatment: the optical interference phase $\Delta\Phi$, the reduced electric field strength $\chi=\mathcal{E}_{\omega}^2/{\mathcal{E}_{2\omega}}$, and the final state energy $\epsilon$. A small energy change near a resonance is shown to flip the emission direction of photoelectrons with high efficiency, through an example where $90\%$ of photoelectrons whose energy is near the $2p^2\ ^1S^e$ resonance flip their emission direction. However, the large fraction of photoelectrons ionized at the intermediate state energy, which are not influenced by the optical control, make this control scheme challenging to realize experimentally.

Polymer-Like Self-Assembled Structures from Particles with Isotropic Interactions: Dependence upon the Range of the Attraction.

We conduct Metropolis Monte Carlo simulations on models of dilute colloidal dispersions, where the particles interact via isotropic potentials of mean force (PMFs) that display a long-ranged repulsion, combined with a short-ranged and narrow attraction. Such systems are known to form anisotropic clusters. There are two main conclusions from this work. First, we demonstrate that the width of the attractive region has a significant impact on the type of structures that are formed. A narrow attractive well tends to produce clusters in which particles possess fewer neighbors than in systems where the attraction is wider. Second, metastable clusters appear to persist in the absence of specific simulation moves designed to overcome large energy barriers to particle accumulation. The so-called “Aggregation-Volume Bias Monte Carlo” moves were previously developed by Chen and Siepmann, and they facilitate particle exchanges between clusters via unphysical moves that bypass high energy intermediate states. These facilitate the progression of metastable clusters to equilibrium clusters. Metastable clusters are generally large with significant branching of thin filaments of aggregated particles, while stable clusters have thicker backbones and tend to be more compact with significantly fewer particles. This general behavior is observed in both two- and three-dimensional systems. In two dimensions, less anisotropic clusters with backbones possessing lattice structures will occur, particularly for systems where the particles interact with a PMF that has a relatively wide attractive region. We compare our results with PMF calculations established from a more specific model, namely weakly charged polystyrene particles, which carry a thin surface layer of grafted polyethylene oxide polymers in aqueous solution. We hope that our investigations can serve as crude guidelines for experimental research, aiming to construct linear or branched polymers in aqueous solution built up by colloidal monomers that are large enough to be studied by confocal microscopy. We suggest that metastable clusters are more relevant to experimental scenarios where the energetic barriers are too large to be surmounted over typical timescales.

Effects of support and oxygen vacancies on the energetics of NiO reduction with H2 for the chemical looping combustion (CLC) reaction; a DFT study.

Chemical looping combustion (CLC) technology is an innovative energy conversion technology that employs oxygen carriers (OC), typically metal oxides, to burn fossil fuels with a minimal carbon footprint. The performance of OCs can be enhanced by the support on which they are deposited through two mechanisms acting at different scales, viz., microstructural and synergetic effects. In this work, the synergetic effect of NiO supported on TiO2 in reaction with hydrogen as a fuel is studied using density functional theory (DFT). Changes in the energetics of the NiO-hydrogen reaction are explained as a consequence of the interaction between the TiO2 support and NiO. The results indicate that the electronic interaction of the TiO2 support with NiO lowers the energy of intermediate states and the energy of the reaction. The effect of TiO2 increases with the creation of more O vacancies as the reaction proceeded. This enhanced reactivity of the NiO-hydrogen reaction is attributed to both an electronic effect of TiO2 and a geometric effect due to O vacancy creation. The synergetic effect of the support on the OC reactions at the atomic level reported here can pave the path to differentiate the electronic and geometric effects and establish the knowledge for the rational design of OC and support systems.

Phonon-assisted optical absorption in germanium

A comprehensive experimental and theoretical study of indirect gap optical absorption in bulk Ge is presented. While this topic has been the subject of intense studies from the early days of semiconductor physics, the resonant aspect of the absorption received very little attention until now. This is a unique property of Ge related to the proximity of the direct and indirect gaps. The absorption coefficient was measured over the entire spectral range between the two gaps for comparison with theory. It is shown that the standard textbook expressions, obtained by assuming intermediate states with constant energy, are in very poor agreement with experiment. A theory first proposed by Hartman [R. L. Hartman, Phys. Rev. 127, 765 (1962)], which takes into account the energy dependence of the intermediate states, provides a much better account of the photon-energy dependence of the absorption, but the prediction of the experimental absorption strength requires the incorporation of excitonic effects. The latter, however, have only been considered by Elliott [R. J. Elliott, Phys. Rev. 108, 1384 (1957)] in the limit of constant intermediate state energy. A generalization to the case of energy-dependent intermediate states, consistent with Hartman's theory, is presented here. The basic qualitative difference with the classical Elliott theory is that the excitonic character of the intermediate states affects the computed absorption, generating an additional resonant enhancement that is confirmed by the experimental data. The generalized theory presented here agrees very well with the experimental absorption using independently determined band structure parameters.

Examination of the consistency of the quasiparticle random-phase approximation approach to double- β decay of Ca48

The nuclear matrix elements (NMEs) of the neutrinoless and two-neutrino double-$\beta$ decays of $^{48}$Ca are calculated by the quasiparticle random-phase approximation (QRPA) with emphasis on the consistency examinations of this calculation method. The main new examination points are the consistency of two ways to treat the intermediate-state energies in the two-neutrino double-$\beta$ NME and comparison with the experimental charge-exchange strength functions obtained from $^{48}$Ca$(p,n)$ and $^{48}$Ti$(n,p)$ reactions. No decisive problem preventing the QRPA approach is found. The obtained neutrinoless double-$\beta$ NME adjusted by the ratio of the effective and bare axial-vector current couplings is lowest in those calculated by several groups and close to one of the QRPA values obtained by another group.

A paramedic treatment for modeling explicitly solvated chemical reaction mechanisms.

We report a static quantum chemistry modeling treatment to study how solvent molecules affect chemical reaction mechanisms without dynamics simulations. This modeling scheme uses a global optimization procedure to identify low energy intermediate states with different numbers of explicit solvent molecules and then the growing string method to locate sequential transition states along a reaction pathway. Testing this approach on the acid-catalyzed Morita-Baylis-Hillman (MBH) reaction in methanol, we found a reaction mechanism that is consistent with both recent experiments and computationally intensive dynamics simulations with explicit solvation. In doing so, we explain unphysical pitfalls that obfuscate computational modeling that uses microsolvated reaction intermediates. This new paramedic approach can promisingly capture essential physical chemistry of the complicated and multistep MBH reaction mechanism, and the energy profiles found with this model appear reasonably insensitive to the level of theory used for energy calculations. Thus, it should be a useful and computationally cost-effective approach for modeling solvent mediated reaction mechanisms when dynamics simulations are not possible.

Adsorbate-mediated relaxation dynamics of hot electrons at metal/organic interfaces

Hot-electron dynamics at metal surfaces are crucial for charge transport properties and chemical reactions at metal/organic interfaces. In order to study the influence of electron-donating adsorbates on hot-electron lifetimes we performed femtosecond time-resolved two-photon photoemission measurements of tetrathiafulvalene (TTF)-covered Au(111) surfaces. The electron-donating nature of TTF provides density of states near the Fermi energy (${E}_{F}$) increasing the possibility for electron-electron scattering events. This leads to an additional fast electron-electron scattering assisted hot-electron relaxation channel for low intermediate state energies (${E}_{i}\ensuremath{-}{E}_{F}$) at the TTF/Au(111) interface with hot-electron lifetimes up to $340\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$. However, suppressing the electron donation via intercalation of electron-accepting tetracyanoquinodimethane molecules causes quenching of this fast electron relaxation channel. This results in an asymptotic increase of the hot-electron lifetimes for infinitesimal small intermediate state energies up to about $1000\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$ as predicted by the pure Landau theory of Fermi liquids.

Electron and donor-impurity-related Raman scattering and Raman gain in triangular quantum dots under an applied electric field

The differential cross-section of electron Raman scattering and the Raman gain are calculated and analysed in the case of prismatic quantum dots with equilateral triangle base shape. The study takes into account their dependencies on the size of the triangle, the influence of externally applied electric field as well as the presence of an ionized donor center located at the triangle’s orthocenter. The calculations are made within the effective mass and parabolic band approximations, with a diagonalization scheme being applied to obtain the eigenfunctions and eigenvalues of the x-y Hamiltonian. The incident and secondary (scattered) radiation have been considered linearly-polarized along the y-direction, coinciding with the direction of the applied electric field. For the case with an impurity center, Raman scattering with the intermediate state energy below the initial state one has been found to show maximum differential cross-section more than by an order of magnitude bigger than that resulting from the scheme with lower intermediate state energy. The Raman gain has maximum magnitude around 35 nm dot size and electric field of 40 kV/cm for the case without impurity and at maximum considered values of the input parameters for the case with impurity. Values of Raman gain of the order of up to 104cm-1 are predicted in both cases.

Elastic softening behavior of Ti–Nb single crystal near martensitic transformation temperature

The elastic behavior of Ti–40 wt % Nb single crystal near martensite start (Ms) temperature was studied using a resonant ultrasound spectroscopy measurements and the first principle calculation. The experimental results showed that both c′ and cs shear moduli tended to soften as the temperature decreased from room temperature down to and across the Ms temperature of 267 K, suggesting that both the {112}⟨111⟩ and {110}⟨110⟩ shears were operating during martensitic transformations of disordered bcc to orthorhombic structure. The consideration of detailed atomic mechanisms involved in the shear sequences and the calculation of the energy of intermediate state suggested that the {110}⟨110⟩ shear was operating as primary shear and the {112}⟨111⟩ shear operating as a secondary shear to complete the transformation. The measurement further showed that the anisotropy constant, A, was rather increasing at the transition temperature approaching Ms, similarly to the transformation of ordered bcc (B2) to orthorhombic (B19) martensite.

Density-based energy decomposition analysis for intermolecular interactions with variationally determined intermediate state energies

The first purely density-based energy decomposition analysis (EDA) for intermolecular binding is developed within the density functional theory. The most important feature of this scheme is to variationally determine the frozen density energy, based on a constrained search formalism and implemented with the Wu-Yang algorithm [Q. Wu and W. Yang, J. Chem. Phys. 118, 2498 (2003)]. This variational process dispenses with the Heitler-London antisymmetrization of wave functions used in most previous methods and calculates the electrostatic and Pauli repulsion energies together without any distortion of the frozen density, an important fact that enables a clean separation of these two terms from the relaxation (i.e., polarization and charge transfer) terms. The new EDA also employs the constrained density functional theory approach [Q. Wu and T. Van Voorhis, Phys. Rev. A 72, 24502 (2005)] to separate out charge transfer effects. Because the charge transfer energy is based on the density flow in real space, it has a small basis set dependence. Applications of this decomposition to hydrogen bonding in the water dimer and the formamide dimer show that the frozen density energy dominates the binding in these systems, consistent with the noncovalent nature of the interactions. A more detailed examination reveals how the interplay of electrostatics and the Pauli repulsion determines the distance and angular dependence of these hydrogen bonds.

Mechanoenergetics of Actomyosin Interaction Analyzed by Cross-Bridge Model

We present a mathematical model of actomyosin interaction, as a further development of actomyosin model that links mechanical contraction with energetics (Vendelin et al, Annals of Biomedical Engineering: 28, 2000). The new model is a three-state Huxley-type model, with two strong binding states and one weak binding state, for cross-bridge interaction and a model of calcium induced activation. The force produced by the attached cross-bridge in strong binding state is assumed to be elastic and depends linearly on the axial distance z along the myosin and actin filaments between the equilibrium position of the myosin head and the nearest actin binding site. The model is self-consistent and is based on T. Hill formalism linking free energy profile of reactions and mechanical force.In several experimental studies it has been shown that the dependency between oxygen consumption and stress-strain area is linear. Additionally, the relation between stress-strain area and oxygen consumption is the same for isometric and shortening contractions. In this work, we analyzed free energy profiles of actomyosin interaction by changing free energies of intermediate states and free energies of activation of different reactions.In our simulations we replicated the linear dependence between oxygen consumption and stress-strain area together with other important mechanical properties of cardiac muscle such as developed stress dependence on the sarcomere length and force-velocity relationship.

Separating the Vibrationally Resolved Auger Decay Channels for a CO Core Hole State

The K-VV Auger spectrum of carbon monoxide (CO) excited by C 1s photoionization has been investigated with a novel electron-electron coincidence setup. The energy resolution is sufficiently high to resolve the vibrational energy levels of the core-ionized intermediate state and of most dicationic final states in the two-dimensional electron energy map. We demonstrate how the influence of vibrational states on a molecular Auger spectrum can be accessed experimentally without the constraint of averaging over all intermediate state energies.

Competition between Polysaccharide/Polynucleotide Complexation vs. Polynucleotide Hybridization; Salt Concentration Dependence of the Reaction Direction

Schizophyllan is a natural β-(1→3)-D-glucan existing as a triple helix in water and as a single chain in dimethylsulfoxide (DMSO), respectively. As we already reported, when a homo-polynucleotide [ex. poly(A) or poly(dA)] was added to the schizophyllan/DMSO solution and subsequently DMSO was exchanged for water, the single chain of schizophyllan formed a complex with the polynucleotide. In this paper, using an adenine-thymine (or uracil) double strand as a model system, we explored whether s-SPG can unzip the polynucleotide duplexes, examining the competition between the schizophyllan/polynucleotide complexation and the polynucleotide hybridization with circular dichroism and fluorescence spectroscopy at various NaCl concentrations. The unzipping reaction happened for the hetero-duplexes (i.e., RNA-DNA duplexes) at low salt concentrations, on the other hand, it was not observed for the homo-duplexes at all the salt concentrations. When we compared dissociation temperatures, the unzipping reaction of the homo-duplexes was expected at low salt concentrations. This discrepancy between the experimental results and the expectation, as well as the different behavior between the hetero- and homo- duplexes, can be ascribed to the difference in the activation energy of intermediate states. [DOI: 10.1380/ejssnt.2005.38]

AC Stark shift of the ground state of atomic hydrogen

An analytical expression for the second-order AC Stark shift of the ground state of atomic hydrogen is derived, which is convergent for negative as well as for positive energies of intermediate states except for the resonances. To clarify the applicability of the second-order perturbation theory, we compared results with those which are obtained by us and other authors using nonperturbative methods. It appears that values obtained for the AC Stark shift using our simple formula agree on average with Floquet-method calculations up to the field strength F=0.12 (a.u.), which corresponds to I=1015 W/cm2.

Time-Resolved Two-Photon Photoemission Study of Dodecanethiolate-Passivated Ag Nanoparticles

Metallic nanoparticles are attracting much interest from the viewpoints of both fundamental and device physics, since they show the distinctive optical, physical, and chemical properties found in neither bulk nor molecular/ atomic systems. Among the nanoparticle systems, the surface-passivated nanoparticles are very stable and exhibit the closed-packed nanoparticle self-assemblies on the singlecrystalline substrates. Therefore, it is considered that they could be important constituents of future nanostructured devices, such as single electron device, catalyst, and ultra highly density memory. On the other hand, the transient dynamics of hot-electrons created by photoexcitation or inelastic scattering of other energenic particles play an important role in the various physical and chemical phenomena. The excited hot-electrons relax through a number of decay processes, such as electron–electron scattering, electron-phonon scattering, electron-impurity/ defect scattering, and scattering with other elementary excitations. The detailed understanding of hot-electron dynamics will provide not only information about the fundamental interactions in the many-body systems but also knowledge of the various optical, physical and chemical processes. In order to elucidate the intriguing properties of metallic nanoparticles and to develop the future devices, it is indispensable to understand the excited-electron dynamics as well as their electronic structures. In this work, we have carried out a femtosecond time-resolved two-photon photoemission (TR-2PPE) study of dodecanethiolate (DT)-passivated Ag nanoparticles on the highly oriented pyrolytic graphite (HOPG) substrates. Among the time-resolved spectroscopic methods, TR-2PPE spectroscopy is a unique method to observe directly the temporal evolution of excited-electrons that may not be easily accessible through optical techniques. The DT-passivated Ag nanoparticles were synthesized by the two-phase (water–toluene) reduction method. The detailed procedure is described elsewhere. The size and shape of the synthesized nanoparticles were characterized by ex-situ observations with transmission electron microscope (TEM). Figure 1 shows the TEM micrograph and the size distribution in diameter of DT-passivated Ag nanoparticles used in this work. As shown in Fig. 1, the mean diameter and standard deviation are dm 1⁄4 5:3 nm and 1⁄4 0:47 nm, respectively. The present samples have an extremely narrow size distribution in diameter ( 3:0 eV). The total energy resolution was about 150meV, and time resolution was about 10 fs. All measurements were performed at room temperature. Figure 2 shows the two-photon photoemission (2PPE) spectrum of the DT-passivated Ag nanoparticles on the HOPG substrate with dm 1⁄4 5:3 nm measured with photon energy of 3.3 eV at zero pump-probe delay, compared with that of HOPG substrate. These spectra are plotted as a function of intermediate-state energy relative to the Fermilevel. In Fig. 2, the intermediate-state energy of 3.3 eV corresponds to the highest intermediate-state, with onephoton energy (h 1⁄4 3:3 eV) above the Fermi-level. As shown in Fig. 2, the 2PPE spectral features are definitely different among the HOPG substrate and DT-passivated Ag nanoparticles on HOPG substrate. The 2PPE spectrum of Fig. 1. TEM micrograph and size distribution in diameter of dodecanethiolate-passivated Ag nanoparticles with mean diameter of dm 1⁄4 5:3 nm and standard deviation of 1⁄4 0:47 nm.