Calculations Of Optical Spectra Research Articles

Time‐dependent and time‐independent approaches for the computation of absorption spectra of Uracil derivatives in solution

AbstractIn this contribution we discuss the most significant benefits and drawbacks of the alternative time‐independent (TI) and time‐dependent (TD) approaches to the calculation of absorption spectra of molecules in solutions. Eigenstate‐free TD methods are in principle the most suitable route to face with the calculation of optical spectra in nonadiabatic systems, most of all in presence of conical intersections easily accessible from the Franck–Condon region. However, here we show that, in selected cases, a combined strategy that adopts TD methods to assess the impact of nonadiabatic couplings on the spectrum and subsequently applies TI methods to include all the degrees of freedom can reveal very convenient. Such a combined TD/TI strategy has been applied to the calculation of the spectrum of Uracil and 5Fluoro‐Uracil in acetonitrile. TD studies on reduced dimensionality diabatic models indicate that nonadiabatic effects are moderate and are not the main origin of the diffuse spectral shapes observed in experiments. Subsequent full‐coordinate TI calculations allow assigning this feature to intrinsic characteristics of a ππ* excitation of a small molecular ring structure. This latter introduces remarkable deformation in all the ring structure thus inducing a FC activity in many molecular normal modes both due to displacements of the equilibrium structures and to Duschinsky mixings of the normal coordinates. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Evolution of the electronic structure and physical properties ofFe2MeAl (Me = Ti,V, Cr) Heusler alloys

We present the results of experiments on the optical, electrical and magnetic propertiesand electronic structure and optical spectrum calculations of the Heusler alloysFe2TiAl,Fe2VAl and Fe2CrAl. We find that the drastic transformation of the band spectrum,especially near the Fermi level, when replacing the Me element(Me = Ti, V, Cr), is accompanied by a significant change in the electricaland optical properties. The electrical and optical properties ofFe2TiAl are typical for metals. The abnormal behavior of the electricalresistivity and the optical properties in the infrared range forFe2VAl and Fe2CrAl are determined by electronic states at the Fermi level. Both the optical spectroscopicmeasurements and the theoretical calculations demonstrate the presence of low-energy gaps inthe band spectrum of the Heusler alloys. In addition, we demonstrate that the formation ofFe clusters may be responsible for the large enhancement of the total magnetic moment inFe2CrAl.

Effects of mode-mixing and non-Condon interaction in the vibronic spectra of multimode systems

A method of calculation of optical spectra, which allows one to take into account the mixing of an arbitrary number of modes in the transition, is developed. Thus the dependence of electronic transition matrix elements on nuclear coordinates is also taken into consideration. In this method, the calculation of the Fourier transform of the spectrum is reduced to the calculation of the determinant of the 2 N×2 N matrix and its reciprocal matrix; the elements of the matrix are given by time-dependent pair-correlation functions.

Analysis of the Vignale-Kohn current functional in the calculation of the optical spectra of semiconductors

In this work, we investigate the Vignale-Kohn current functional when applied to the calculation of optical spectra of semiconductors. We discuss our results for silicon. We found qualitatively similar results for other semiconductors. These results show that there are serious limitations to the general applicability of the Vignale-Kohn functional. We show that the constraints on the degree of nonuniformity of the ground-state density and on the degree of the spatial variation of the external potential under which the Vignale-Kohn functional was derived are almost all violated. We argue that the Vignale-Kohn functional is not suited to use in the calculation of optical spectra of semiconductors since the functional was derived for a weakly inhomogeneous electron gas in the region above the particle-hole continuum, whereas the systems we study are strongly inhomogeneous and the absorption spectrum is closely related to the particle-hole continuum.

Exchange and Correlation Effects in Electronic Excitations ofCu2O

State-of-the-art theoretical methods fail in describing the optical absorption spectrum, band gap, and optical onset of Cu(2)O. We have extended a recently proposed self-consistent quasiparticle approach, based on the GW approximation, to the calculation of optical spectra, including excitonic effects. The band structure compares favorably with our present angle-resolved photoemission measurements. The excitonic effects based on these realistic band structure and screening provide a reliable optical absorption spectrum, which allows for a revised interpretation of its main structures.

Gold and Silver Nanoparticles in Sensing and Imaging: Sensitivity of Plasmon Response to Size, Shape, and Metal Composition

Plasmonic metal nanoparticles have great potential for chemical and biological sensor applications, due to their sensitive spectral response to the local environment of the nanoparticle surface and ease of monitoring the light signal due to their strong scattering or absorption. In this work, we investigated the dependence of the sensitivity of the surface plasmon resonance (frequency and bandwidth) response to changes in their surrounding environment and the relative contribution of optical scattering to the total extinction, on the size and shape of nanorods and the type of metal, that is, Au vs Ag. Theoretical consideration on the surface plasmon resonance condition revealed that the spectral sensitivity, defined as the relative shift in resonance wavelength with respect to the refractive index change of surrounding materials, has two controlling factors: first the bulk plasma wavelength, a property dependent on the metal type, and second on the aspect ratio of the nanorods which is a geometrical parameter. It is found that the sensitivity is linearly proportional to both these factors. To quantitatively examine the dependence of the spectral sensitivity on the nanorod metal composition and the aspect ratio, the discrete dipole approximation method was used for the calculation of optical spectra of Ag-Au alloy metal nanorods as a function of Ag concentration. It is observed that the sensitivity does not depend on the type of the metal but depends largely on the aspect ratio of nanorods. The direct dependence of the sensitivity on the aspect ratio becomes more prominent as the size of nanorods becomes larger. However, the use of larger nanoparticles may induce an excessive broadening of the resonance spectrum due to an increase in the contribution of multipolar excitations. This restricts the sensing resolution. The insensitivity of the plasmon response to the metal composition is attributable to the fact that the bulk plasma frequency of the metal, which determines the spectral dispersion of the real dielectric function of metals and the surface plasmon resonance condition, has a similar value for the noble metals. On the other hand, nanorods with higher Ag concentration show a great enhancement in magnitude and sharpness of the plasmon resonance band, which gives better sensing resolution despite similar plasmon response. Furthermore, Ag nanorods have an additional advantage as better scatterers compared with Au nanorods of the same size.

Tight-binding model for semiconductor quantum dots with a wurtzite crystal structure: From one-particle properties to Coulomb correlations and optical spectra

In this work we investigate the electronic and optical properties of self-assembled quantum dots by means of a tight-binding model. Coulomb and dipole matrix elements are calculated from the one-particle wave functions which fully include the atomistic wurtzite structure of the low-dimensional heterostructures and serve as an input for the calculation of optical spectra. For the investigated $\mathrm{In}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ material system, the optical selection rules are found to be strongly affected by band-mixing effects for the localized valence band states. Within this framework, excitonic absorption and emission spectra are analyzed for different sizes of the investigated lens-shaped quantum dots, including the influence of the intrinsic and strain-induced electrostatic field of the wurtzite structure. A dark exciton ground state for small quantum dots is found.

Efficient Approach to Time-Dependent Density-Functional Perturbation Theory for Optical Spectroscopy

Using a superoperator formulation of linearized time-dependent density-functional theory, the dynamical polarizability of a system of interacting electrons is represented by a matrix continued fraction whose coefficients can be obtained from the nonsymmetric block-Lanczos method. The resulting algorithm, which is particularly convenient when large basis sets are used, allows for the calculation of the full spectrum of a system with a computational workload only a few times larger than needed for static polarizabilities within time-independent density-functional perturbation theory. The method is demonstrated with calculation of the spectrum of benzene, and prospects for its application to the large-scale calculation of optical spectra are discussed.

Many-body effects in the electronic spectra of cubic boron nitride

We present state of the art first-principles calculations of optical spectra and the loss function of bulk cubic boron nitride $(c\text{\ensuremath{-}}\mathrm{B}\mathrm{N})$, starting from a density functional Kohn-Sham band structure. We investigate the influence of many-body effects beyond the random phase approximation (RPA) on the optical spectra through the inclusion of self-energy and excitonic effects by a GW calculation and the solution of the Bethe-Salpeter equation. For the loss function we only perform RPA calculations, since Bethe-Salpeter results are already available in the literature. We show to which extent, and in which kind of spectra, the description of many-body effects is important for a meaningful comparison with experiment, and when they can be neglected due to mutual cancellation. We also present results obtained within time-dependent density functional theory, both in the adiabatic local density approximation (TDLDA) and using a recently proposed long-range approximation for the exchange-correlation kernel. Our results show that the latter corrects a big part of the error with respect to RPA or TDLDA; however, the corrections are not sufficient to qualify the method for further quantitative predictions, in particular for the study of the optical gap. In fact, since experiments often quote a relatively low (around $6.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) band gap, whereas the calculated optical absorption spectrum already in the random-phase approximation appears blueshifted by more than $2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with respect to the available experimental curve, we study in particular the question of the optical gap in this material. It turns out that, although there is evidence for a weakly bound exciton in $c\text{\ensuremath{-}}\mathrm{B}\mathrm{N}$, the optical gap of pure monocrystalline cubic BN should be around $11\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, hence significantly bigger than has sometimes been quoted from experiments.

A T-matrix many-particle theory for coherently coupled superlattice optics

The high-order Coulomb correlations described by T-matrix diagrams in both carrier occupation and polarization functions are studied here with the Keldysh-Green's Functions formalism. Numerical applications for low dimensional semiconductor systems illustrate the relevance of the effects and their importance for realistic nonlinear optical spectra calculations. A frequency and momentum resolved numerical demonstration of the Mott transition at the T-matrix level is presented.

On the relation of protein dynamics and exciton relaxation in pigment–protein complexes: An estimation of the spectral density and a theory for the calculation of optical spectra

A theory for calculating time– and frequency–domain optical spectra of pigment–protein complexes is presented using a density matrix approach. Non-Markovian effects in the exciton–vibrational coupling are included. A correlation function is deduced from the simulation of 1.6 K fluorescence line narrowing spectra of a monomer pigment–protein complex (B777), and then used to calculate fluorescence line narrowing spectra of a dimer complex (B820). A vibrational sideband of an excitonic transition is obtained, a distinct non-Markovian feature, and agrees well with experiment on B820 complexes. The theory and the above correlation function are used elsewhere to make predictions and compare with data on time–domain pump–probe spectra and frequency–domain linear absorption, circular dichroism and fluorescence spectra of Photosystem II reaction centers.

First-principles density-functional calculations for optical spectra of clusters and nanocrystals

Electronic and structural calculations for atomic clusters present many challenges for traditional theoretical methods. While the computational framework for ground-state properties of clusters is relatively well established, calculations for excited states remain difficult. In this paper we implement a linear-response theory within the time-dependent local density approximation (TDLDA) and apply this technique to calculate excitation energies and optical absorption spectra for a variety of systems ranging from single atoms to semiconductor quantum dots up to several hundred atoms in size. The TDLDA formalism represents a fully ab initio formalism for excited states that avoids many of the drawbacks associated with empirical or semiempirical methods. Compared to other ab initio techniques for excited states, the TDLDA method requires considerably less computational effort and can be applied to much larger systems. We find the computed excitation energies, photoabsorption spectra, and optical absorption gaps to be in good agreement with available experimental data. Our calculations show that the accuracy of the TDLDA method in the range of lower transition energies is often comparable to that of more computationally intensive techniques, such as methods based on the exact exchange, optimized effective potential, or on solving the Bethe-Salpeter equation within the GW approximation.

Theoretical investigation of the optical spectrum and gyromagnetic g factor for GaAs : Co 2+

A covalence crystal field model based on a cluster approach is presented. It enables the calculation of optical spectra and the ground state g-factor for a 3d 7 ion in the T d system. In this model, not only the effect of the difference between the t 2g orbit and e g orbit but also two spin–orbit coupling parameters are included. The model is applied for GaAs : Co 2+. The calculated values of the energy levels and the g-factor agree well with observed values.

Calculation of optical spectra in liquid methanol using molecular dynamics and the chemical potential equalization method

We apply the chemical potential equalization (CPE) method to the calculation of the optical spectra in liquid methanol at 298 K and normal pressure. The configurations of the liquid are obtained by conventional molecular dynamics (MD) using a completely flexible all-atoms model. The infrared and Raman spectra are computed a posteriori using a CPE parametrization of methanol calibrated to reproduce the electronic properties of the isolated molecule evaluated with accurate ab initio calculations. The MD/CPE method reproduces correctly the optical spectra in the region of the intermolecular motions. The spectra are discussed and interpreted on the basis of hydrogen bonding structure and dynamics.

Accurate mesh truncation for Schrödinger equations by a perfectly matched layer absorber: Application to the calculation of optical spectra

Quasibound and continuum states are of particular importance for the numerical investigation of coherence properties and are sensitive with respect to the boundary condition chosen at the edge of the computational window. An open boundary condition will be derived which is particularly suitable for dynamical problems described by Schr\odinger-type equations. With this approach, bound states as well as unbound states can be described adequately. The boundary condition is derived from a perfectly matched layer (PML) formalism commonly used in the field of electrodynamics. Consequently, the calculation domain is reduced leading to a calculation time reduction by orders of magnitude. From the physical point of view this formulation allows an adequate analysis of transport phenomena or absorption spectra, e.g., the results obtained by the PML formalism are compared with accurate numerical results calculated using a large mesh and show an excellent performance. For example, the Coulomb enhanced Franz-Keldysh effect is investigated, which cannot be analyzed adequately without using proper open boundary conditions.

First-Principles Calculation of Optical Spectra for Transition-Metal Impurities Doped in Zno and Zns

ABSTRACTThe multiplet structures of Co2+ doped in ZnO, Co2+ doped in ZnS and Ni2+ doped in ZnS are calculated from first principles using the recently developed discrete variational-multielectron (DV-ME) method, in which the matrix elements of electron-electron repulsion are calculated numerically using the molecular orbitals obtained by cluster calculations. The transition probabilities between the multiplet states are also calculated from first principles using the many-electron wave functions obtained by the DV-ME calculations. The optical spectra of these materials are well reproduced, indicating that the effects of covalency and configuration interactions are properly taken into account in the present calculations.

Calculation of optical spectra in the fundamental absorption range for crystals with the inversion of birefringence sign

Calculation of optical spectra in the fundamental absorption range for crystals with the inversion of birefringence sign

Engineering calculations of optical spectra of large molecules and polymers

The sense of the term “engineering” is discussed with respect to calculating the optical spectra of complex compounds (large molecules, polymers and molecular crystals). The conditions which have to be satisfied when choosing the most expedient strategies for developing methods of the engineering calculations at the molecular level are introduced. The relationship between the ab initio and semiempirical methods is analyzed. The features of solving inverse spectral problems, the occurrence of subjective factors in the course of the solution, and the effect of the factors on the compilation of the respective knowledge banks are discussed. The engineering calculations of IR spectra are exemplified. The ways of developing the methods for the engineering calculations of selected spectra of complex compounds are analyzed. The potential of the current theory is illustrated.

Dithiocarbamate radicals in laser flash photolysis of thiuram disulphide and dithiocarbamate anion: calculation of optical spectra

The optical spectrum of the dithiocarbamate radical (dtc ) R 2NCS 2 (RCH 3, C 2H 5, n- Bu; Bubutyl) and its kinetic parameters have been detected by means of laser flash photolysis of thiuram disulphide (tds) [R 2NC(S)S] 2 and dithiocarbamate anion (dtc −) in different solvents. The dtc absorption band has a maximum at about 600 nm, the extinction coefficient of the band being about 3100 1 mol −1 cm −1. In all the solvents studied, the radicals decay owing to recombination at almost the diffusion rate. Quantum-chemical calculation of the geometry and spectra of dtc radicals has been performed in a many- configuration approximation. A good fit of the calculation results to experimental spectral data has been obtained.

Inelastic tunneling excitation of tip-induced plasmon modes on noble-metal surfaces.

Light emission characteristics from the tunnel gap of a scanning tunneling microscope are used to elucidate the interaction of tunneling electrons with tip-induced plasmon modes on Ag, Au, and Cu surfaces. Enhanced redshifted spectra are observed in the tunneling regime. Model calculations of optical spectra in this range agree well with the experimental data. Isochromat spectra are used to demonstrate that the principal excitation process occurs via inelastic tunneling.