Electronic State Transitions Research Articles

Realistic Cluster Modeling of Electron Transport and Trapping in Solvated TiO2 Nanoparticles

We have developed a cluster model of a TiO(2) nanoparticle in the dye-sensitized solar cell and used first-principles quantum chemistry, coupled with a continuum solvation model, to compute structures and energetics of key electronic and structural intermediates and transition states. Our results suggest the existence of shallow surface trapping states induced by small cations and continuum solvent effect as well as the possibility of the existence of a surface band which is 0.3-0.5 eV below the conduction band edge. The results are in uniformly good agreement with experiment and establish the plausibility of an ambipolar model of electron diffusion in which small cations, such as Li(+), diffuse alongside the current carrying electrons in the device, stabilizing shallowing trapping states, facilitating diffusion from one of these states to another, in a fashion that is essential to the functioning of the cell.

On the importance of excited state dynamic response electron correlation in polarizable embedding methods

We investigate the effect of including a dynamic reaction field at the lowest possible ab inito wave function level of theory, namely the Hartree-Fock (HF) self-consistent field level within the polarizable embedding (PE) formalism. We formulate HF based PE within the linear response theory picture leading to the PE-random-phase approximation (PE-RPA) and bridge the expressions to a second-order polarization propagator approximation (SOPPA) frame such that dynamic reaction field contributions are included at the RPA level in addition to the static response described at the SOPPA level but with HF induced dipole moments. We conduct calculations on para-nitro-aniline and para-nitro-phenolate using said model in addition to dynamic PE-RPA and PE-CAM-B3LYP. We compare the results to recently published PE-CCSD data and demonstrate how the cost effective SOPPA-based model successfully recovers a great portion of the inherent PE-RPA error when the observable is the solvatochromic shift. We furthermore demonstrate that whenever the change in density resulting from the ground state-excited state electronic transition in the solute is not associated with a significant change in the electric field, dynamic response contributions formulated at the HF level of theory manage to capture the majority of the system response originating from derivative densities.

Quantum chemical studies of photochromic properties of benzoxazine compound

Molecular electronic structure of ground and excited states of a photochromic indolo[2,1-b][1,3]benzoxazine compound incorporating closed-ring system, which opens upon UV light excitation, was studied using various quantum chemical methods. Three local minima of the ground electronic state potential energy surface and related transition states were identified along the path of rotation of 4-nitrophenol group. Additionally, three local minima of the excited electronic states were located. The evaluated transition energy barriers between local ground-state minima nearest to the initial structure of the investigated molecule are less than 2 kBT, making open structures likely to revert to the initial structure by thermalization. Results obtained using ab initio GMC–QDPT method were explored and compared to the widely used TD-DFT and semi-empiric ZINDO methods.

Atmospheric hydrocarbonactivation by the hydroxyl radical: a simple yet accurate computational protocol for calculating rate coefficients

The overall rate coefficient at standard temperature and pressure for the hydrogen abstraction reaction by the hydroxyl radical (HO˙) from common saturated volatile organic compounds (VOCs) is derived theoretically using electronic structure calculations and transition state theory (TST). The computational approach used is based on relatively efficient methods, and hence is applicable to a large number of compounds with only a modest use of computer resources. The key methods used are density functional theory (for the calculation of barrier heights) and simple transition state theory (TST), including a simple correction for tunnelling. All thermally relevant conformers of the reactant and the abstraction TS are included in the study. For all compounds in a test set of thirty-four, the calculated rate coefficient agrees with the experimental value to within better than an order of magnitude, and to within better than a factor of three for all but six cases, so that the accuracy is of predictive utility.

Theoretical study of the P-branch spectral lines in d1+b1+ electronic state transition of NbN molecule

The accurate P-branch spectral lines of high-lying rotational quantum states of (1,1) band in d1+b1+ electronic state transition of NbN molecule are obtained in this work using the analytical formula proposed in Sun's previous work. The formula not only reproduces all known experimental spectral lines excellently, but also generates the correct values of the unknown spectral lines up to J=80 that are unavailable experimentally for this band.

Cooperative transition of electronic states of antisite As defects in Be-doped low-temperature-grown GaAs layers

Magnetic properties resulting from localized spins associated with antisite arsenic ions AsGa+ in Be-doped low-temperature-grown GaAs (LT-GaAs) layers were studied by measuring the magnetization of lift-off samples. With fast cooling, the magnetization of samples at 1.8 K becomes significantly lower than that expected from Curie-type paramagnetism in the range of the applied field to 7 T, and a transition from low magnetization to the magnetization of paramagnetism occurs upon the heating of samples to 4.5 K. With slow cooling, on the other hand, samples have a paramagnetic temperature dependence throughout the measurement-temperature range. The magnetization was found to decrease monotonically when a sample was kept at a fixed low temperature. These observations are explained by the cooperative transition of electron states of AsGa defects, which is closely related to the normal-metastable state transition of EL2 defects in semi-insulating GaAs. The results of the magnetization measurements in the present study suggest that AsGa+ ions are spontaneously displaced at low temperature without photoexcitation in Be-doped LT-GaAs. The similarity of the transition observed in this system to the normal-metastable state transition of the EL2 defect was also suggested by first-principle calculations of the electron state of an AsGa defect with a doped Be atom.

Theoretical study on contribution of charge transfer effect to surface-enhanced Raman scattering spectra of pyridine adsorbed on Ag n ( n = 2–8) clusters

We investigate surface-enhanced Raman scattering (SERS) spectra of pyridine–Ag n ( n = 2–8) complexes by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. In simulated normal Raman scattering (NRS) spectra, profiles of pyridine–Ag n ( n = 2–8) complexes are analogical with that of isolated pyridine. Nevertheless, calculated pre-SERS spectra are strongly dependent on electronic transition states of new complexes. Wavelengths at 335 nm, 394.8 nm, 316.9 nm and 342.6 nm, which are nearly resonant with pure charge transfer excitation states, are adopted as incident light when simulating pre-SERS spectra for pyridine–Ag n ( n = 2–8) complexes, respectively. We obtain enhancement factors from 10 3 to 10 5 in pre-SERS spectra compared with corresponding NRS spectra. The obvious increase in Raman intensities mainly result from charge transfer resonance Raman enhancement. A charge difference densities (CDDs) methodology is adopted in describing chemical enhancement mechanism. This methodology aims at visualizing charge transfer from Ag n ( n = 2–8) clusters to pyridine on resonant electronic transition, which is one of the most direct evidences for chemical enhancement mechanism.

Challenges of laser-cooling molecular ions

The direct laser cooling of neutral diatomic molecules in molecular beams suggests that trapped molecular ions can also be laser cooled. The long storage time and spatial localization of trapped molecular ions provides an opportunity for multi-step cooling strategies, but also requires careful consideration of rare molecular transitions. We briefly summarize the requirements that a diatomic molecule must meet for laser cooling, and we identify a few potential molecular ion candidates. We then carry out a detailed computational study of the candidates BH+ and AlH+, including improvedab initio calculations of the electronic state potential energy surfaces and transition rates for rare dissociation events. On the basis of an analysis of the population dynamics, we determine which transitions must be addressed for laser cooling, and compare experimental schemes using continuous-wave and pulsed lasers.

Effects of Interface Grading on Electronic States and Optical Transitions in GaSb Type-II Quantum Dots in GaAs

Effects of Interface Grading on Electronic States and Optical Transitions in GaSb Type-II Quantum Dots in GaAs

Effects of Interface Grading on Electronic States and Optical Transitions in GaSb Type-II Quantum Dots in GaAs

Electronic states in type-II GaSb quantum dots (QDs) in GaAs are studied theoretically and compared with experiments to clarify how the spatial overlap of holes in the dot and electrons outside is affected by the interface grading caused by the interdiffusion of Sb and As. Cone-shaped QDs with the initial size of 6 nm height and 20 nm radius are analyzed. The wavefunctions of electrons and holes and their overlap Θ are calculated. In an initial stage of grading, when the grading length Lg is below 1.2 nm, the mixing occurs only near the dot/matrix interface, leading to a decrease in Θ, since holes are squeezed into a smaller volume whereas electrons are more repelled by the dot. In later stages where Lg exceeds 1.2 nm, however, the hole confinement and the electron repulsion by the dot both weaken, leading to an increase in Θ; this accounts for a recent finding that the annealing of GaSb QDs induces a blue shift and an intensity increase in photoluminescence spectra.

BN分子b1∏–a1∑+的P线系发射谱线研究

本文采用能精确计算双原子分子P线系跃迁谱线的新公式，首次研究了BN分子从电子态 向 电子态跃迁中(2, 1)跃迁带的P支发射光谱。获得的理论结果不仅很好地重复了已知的实验结果，同时还正确预测了目前实验上和其它理论方法仍未获得的该跃迁带包含转动量子数J = 80在内的高激发转动态的跃迁谱线数据。从理论上为进一步分析BN分子结构的研究提供了重要的物理信息。 By the analytical formula which is deduced to obtain P-branch transitional spectra of diatomic molecule in Sun’s previous work, the emission spectral lines up to J = 80 those may not be available experimentally or quantum mechanically for (2, 1) band in – electronic state transition of BN molecule were investigated in this work. This study suggests important physical information for those who are working in BN molecular structure.

Effect of the pseudogap on suppressing high energy inelastic neutron scattering in superconductingYBa2Cu3O6.5

We have measured the spin fluctuations in the YBa2Cu3O6.5 (YBCO6.5, Tc=59 K) superconductor at high-energy transfers above ~ 100 meV. Within experimental error, the momentum dependence is isotropic at high-energies, similar to that measured in the insulator for two dimensional spin waves, and the dispersion extrapolates back to the incommensurate wave vector at the elastic position. This result contrasts with previous expectations based on measurements around 50 meV which were suggestive of a softening of the spin-wave velocity with increased hole doping. Unlike the insulator, we observe a significant reduction in the intensity of the spin excitations for energy transfers above ~ 100 meV similar to that observed above ~ 200 meV in the YBCO6.35 (Tc=18 K) superconductor as the spin waves approach the zone boundary. We attribute this high energy scale with a second gap and find agreement with measurements of the pseudogap in the cuprates associated with electronic anomalies along the antinodal positions. In addition, we observe a sharp peak at around 400 meV whose energy softens with increased hole doping. We discuss possible origins of this excitation including a hydrogen related molecular excitation and a transition of electronic states between d levels.

Microwave spectroscopy of the Ne–OH(2Πi) complex and three-dimensional intermolecular potentials

Pure rotational transitions in the ground electronic state for Ne-OH and Ne-OD have been observed by Fourier-transform microwave spectroscopy with resolved fine and hyperfine structures. For Ne-OH, rovibrational transitions between the levels with P = 0.5 and 1.5 have also been observed by double-resonance spectroscopy in the millimetre wave region. A three-dimensional potential energy surface has been determined for the complex by a simultaneous analysis utilizing the data of the two isotopologues, where all the freedom of motions for an atom-diatom system in the Jacobi coordinate, R, theta, and r, were considered. Rovibrational energies calculated using the determined potential surface are compared with those by previous experimental results by Chuang et al. (J. Chem. Phys., 1995, 103, 3418).

Density functional theory study on Herzberg–Teller contribution in Raman scattering from 4-aminothiophenol-metal complex and metal-4-aminothiophenol-metal junction

Density functional theory (DFT) and time-dependent DFT calculations have been performed to investigate the Raman scattering spectra of metal-molecule complex and metal-molecule-metal junction architectures interconnected with 4-aminothiophenol (PATP) molecule. The simulated profiles of normal Raman scattering (NRS) spectra for the two complexes (Ag(2)-PATP and PATP-Au(2)) and the two junctions (Ag(2)-PATP-Au(2) and Au(2)-PATP-Ag(2)) are similar to each other, but exhibit obviously different Raman intensities. Due to the lager static polarizabilities of the two junctions, which directly influence the ground state chemical enhancement in NRS spectra, the calculated normal Raman intensities of them are stronger than those of two complexes by the factor of 10(2). We calculate preresonance Raman scattering (RRS) spectra with incident light at 1064 nm, which is much lower than the S(1) electronic transition energy of complexes and junctions. Ag(2)-PATP-Au(2) and Au(2)-PATP-Ag(2) junctions yield higher Raman intensities than those of Ag(2)-PATP and PATP-Au(2) complexes, especially for b(2) modes. This effect is mainly attributed to charge transfer (CT) between the metal gap and the PAPT molecule which results in the occurrence of CT resonance enhancement. The calculated pre-RRS spectra strongly depend on the electronic transition state produced by new structures. With excitation at 514.5 nm, the calculated pre-RRS spectra of two complexes and two junctions are stronger than those of with excitation at 1064 nm. A charge difference densities methodology has been used to visually describe chemical enhancement mechanism of RRS spectrum. This methodology aims at visualizing intermolecular CT which provides direct evidence of the Herzberg-Teller mechanism.

Theoretical characterization of electronic states in interacting chemical systems

In this article we characterize, by means of the perturbed matrix method, the response of the electronic states of a chemical system to the perturbing environment. In the theory section we describe in detail the basic derivations and implications of the method, extending its theoretical framework to treat possible excitonic effects, and we show how to characterize the perturbed electronic states. Finally, by using a set of chemical systems interacting with complex atomic-molecular environments, we describe the nature and general features of the electronic state mixing and transitions as caused by atomic and molecular interactions.

Giuseppe Franco Bassani

Giuseppe Franco Bassani, a cofounder of the modern Italian school of condensed-matter theory, died in Pisa on 25 September 2008 after a long and tenacious fight against cancer. Born on 29 October 1929 in Milan, Franco studied at the University of Pavia, with Piero Caldirola and Fausto Fumi as his thesis advisers. After finishing his laurea thesis, “The energy levels of the F-center,” in 1952, Franco went to the US to work at the University of Illinois at Urbana-Champaign, where he collaborated with Frederick Seitz and considered himself Seitz’s student.Franco returned to Italy in 1956 to take a position as an assistant professor, first at the University of Palermo and then at Pavia University. In 1960 he moved to Chicago to work at Argonne National Laboratory as a research associate. His research during that time was instrumental in the formulation of the modern electronic theory of solids based on pseudopotentials and in the analysis of the connections between electronic structure and optical properties, particularly of semiconductors. That work was described by David Brust, James Phillips, and Franco in a 1962 article published in Physical Review Letters (volume 9, page 94). His book Electronic States and Optical Transitions in Solids (Pergamon Press, 1975), written with his student Giuseppe Pastori Parravicini, inspired at least two generations of theorists worldwide who were working in that field. (A Russian translation was published in the Soviet Union without any royalties paid to the authors!) The book made extensive use of group theory a subject that Franco mastered, beginning with an advanced course he took in 1955 under Seitz at Illinois.In 1963 Franco returned to Italy and took a job as a professor of theoretical physics at the University of Messina. He moved to the University of Pisa in 1966 and then went to the University of Rome, where he held the chair of solid-state physics from 1969 to 1980. But the vagaries of student politics in Italy’s capital were too much for Franco’s conservative spirit. After spending 1979 as a visiting scholar at the University of Illinois, Franco returned to Pisa in 1980, where he had been offered the chair of solid-state physics at the prestigious Scuola Normale Superiore. He later served as the director of the Scuola Normale from 1995 to 1999.In addition to teaching students and encouraging them to engage in vigorous research, Franco carried out his own studies and organized lively condensed-matter physics groups. He extended his research interests to the theory of excitons, the theory of nonlinear optical properties, and the electronic structure and optical properties of layered compounds, including organic-inorganic heterojunctions. In studying the electron-lattice interactions at F-centers, he revisited the subject of his 1952 thesis.Franco steadfastly supported Italian and European physics in all its manifestations. He was a member of Italy’s Accademia Nazionale dei Lincei, and he served as chairman of the European Physical Society’s condensed-matter division from 1986 to 1992 and as president of the Italian Physical Society from 1999 to 2007. Franco received numerous recognitions and awards, including the 1979 Somaini Prize of the Italian Physical Society, the 1996 Italgas Prize for technology and science of materials, and the Italian Presidential Gold Medal for Science and Culture for 2000.Besides his contributions to science, Franco was a cultivated man with a love for history that rivaled that of professional historians. One morning while driving along the Muro Torto, part of Rome’s Aurelian Walls, he realized that a small statue, usually encased in the wall, was missing and called the superintendent to the monuments of Rome. After a lengthy and in-depth discussion with the superintendent, who at first thought he was talking to the well-known writer Giorgio Bassani, Franco learned that the statue had been removed for restoration.Many of Franco’s friends remember him stopping his car in the middle of traffic to point out some antiquities and explain their history. Other drivers would furiously blow their horns, but their reactions did not seem to deter him from his mission.Franco also was well regarded for his expertise and appreciation of wine. With his brother Giancarlo, he produced excellent wines from a vineyard on his family estate. Once Franco acted as the “defender of red wine” in a fictitious trial in which he brilliantly illustrated the wine’s almost miraculous and therapeutic virtues.An exceedingly warm and gentle human being, Franco was beloved by his many students and was well respected as a teacher, mentor, and colleague. He was a man of exceptional vigor and principles. His personality, humor, and many interests are reflected in numerous other enjoyable anecdotes that make the rounds among his friends, colleagues, and students. They and the physics community will dearly miss him.Giuseppe Franco BassaniPPT|High resolution© 2009 American Institute of Physics.

Impact of Cyanoethy Groups on Structure and Spectroscopy of a Few Aromatic Amines

The impact of introducing cyanoethy groups in aromatic amines on structure,charge transfer,frontier orbital energy and composition,as well as electronic absorption spectra was investigated in this work. Theoretical studies of aniline,p-chloroaniline,p-toluidine,and cyanoethy derivatives were carried out at density functional theory (DFT) B3LYP/6-31G* and B3LYP/6-311+G* levels to obtain optimized geometrical structures. Time-dependent density functional theory was applied to calculate the first excited state electronic transition energy and maximum absorption wavelength λmax. Our results indicate that the introduction of a cyanoethy group has limited effect on the composition of frontier orbitals. The main absorption spectrum originates from the featured π →π* electronic transition. The predicted spectra agree well with the available experimental findings.

Theoretical study on the structure，spectra and thermodynamic property of 2-（toluene-4-sulfonylamino）-benzoic

The structure optimization and frequency calculation have been carried out at B3LYP/6-31++G** theoretical level，and IR spectrum，Raman spectrum and thermodynamic property at different temperatures have been obtained. The results indicate that the carbon and the oxygen in the —COOH group，and the nitron in —（OS）2—NH group come into formation of different large π bonds with the benzene rings，and there is a dihedral angle of 63.2° because of place blocking and conjugate effect. The first vertical excited state electronic transition energy was calculated by time-dependent density functional theory，and the maximal absorption wavelength was obtained at 312.7?nm，belonging to near UV，which is in good agreement with experimental value of 307?nm in dichloromethane solvent.

Electric dipole rovibrational transitions in the HD molecule

The rovibrational electric dipole transitions in the ground electronic state of the HD molecule are studied. A simple, yet rigorous formula is derived for the transition rates in terms of the electric dipole moment function $D(R)$, which is calculated in a wide range of $R$. Our numerical results for transition rates are in moderate agreement with experiments and previous calculations, but are at least an order of magnitude more accurate.

Morphology and Composition of Selected High-k Materials and Their Relevance to Dielectric Properties of Thin Films

We discuss some of the issues associated with the relation between the leakage current and the dielectric constant on the one hand, and the crystallographic structure and the chemical composition of the film on the other. Our focus is on dielectrics containing Pr oxides. Starting with the binary , we investigate electronic properties and formation energies of point defects as revealed by ab initio calculations and we attempt to associate this data with the experimental information on the influence of processing on the dielectric quality of the film. We then consider Pr silicates on Si for metal-oxide-semiconductor field effect transistors and on TiN for metal–insulator–metal front-end applications. In the latter case, annealing above about , needed to noticeably increase the effective dielectric constant, causes an increased leakage. Investigating this effect, in particular we discuss the diffusion mechanism for Ti and the influence of Ti on the leakage current, taking into account the stoichiometry dependence of defect formation energies and the position of electron transition states obtained from ab initio data.