Shape Of Absorption Bands Research Articles

Analysis and prediction of absorption band shapes, fluorescence band shapes, resonance Raman intensities, and excitation profiles using the time-dependent theory of electronic spectroscopy

A general method for the simulation of absorption (ABS) and fluorescence band shapes, resonance-Raman (rR) spectra, and excitation profiles based on the time-dependent theory of Heller is discussed. The following improvements to Heller's theory have been made: (a) derivation of new recurrence relations for the time-dependent wave packet overlap in the case of frequency changes between the ground and electronically excited states, (b) a new series expansion that gives insight into the nature of Savin's preresonance approximation, (c) incorporation of inhomogeneous broadening effects into the formalism at no additional computational cost, and (d) derivation of a new and simple short-time dynamics based equation for the Stokes shift that remains valid in the case of partially resolved vibrational structure. Our implementation of the time-dependent theory for the fitting of experimental spectra and the simulation of model spectra as well as the quantum mechanical calculation of the model parameters is discussed. The implementation covers all electronic structure approaches which are able to deliver ground- and excited-state energies and transition dipole moments. The technique becomes highly efficient if analytic gradients for the excited-state surface are available. In this case, the computational cost for the simultaneous prediction of ABS, fluorescence, and rR spectra is equal to that of a single excited-state geometry optimization step while the limitations of the short-time dynamics approximation are completely avoided. As a test case we discuss the well-known case of the strongly allowed 1 (1)A(g) --> 1 (1)B(u) transition in 1,3,5 trans-hexatriene in detail using method ranging from simple single-reference treatments to elaborate multireference electronic structure approaches. At the highest computational level, the computed spectra show the best agreement that has so far been obtained with quantum chemical methods for this problem.

Effect of Grown-In Defects on the Structure of Oxygen Precipitates in Cz-Si Crystals with Different Diameter

IR-spectroscopy with computer analysis of the shape of the Si-O absorption band, electron microscopy, X-rays diffraction and measurements of unsteady photoconductivity timedecay under band-to band excitation were used to investigate the influence of defects in different diameter (40 – 300 mm) Si ingots on the oxygen precipitation due to two-stage annealing (750 оС + 1050 оС). It is shown that large size Cz-Si ingots have a relatively low concentration of electrically active micro-defects, containing small (0.06 – 0.1 μm) dislocation loops. During thermal treatments this leads to the formation of a low stressed oxide phase (SiO2) with an enhanced thermo-stability. The precipitates in small size ingots, however, contain distorted 4-fold rings of SiO4 tetrahedra.

Probing Excitation Delocalization in Supramolecular Chiral Stacks by Means of Circularly Polarized Light: Experiment and Modeling

Photoexcitations in helical aggregates of a functionalized, chiral oligophenylenevinylene (MOPV) are described going beyond the Born-Oppenheimer approximation, in the form of dressed (polaronic) Frenkel excitons. This allows for accurate modeling of the experimentally observed wavelength dependence of the circular polarization in fluorescence, which directly probes the non-adiabatic nature of the electron-vibration (EV) coupling in this system. The fluorescence photon is emitted from a nuclear geometry in which one MOPV and its two nearest neighbors have a nuclear equilibrium that differs appreciably from the ground state due to the presence of the excited state. The absorption and emission band shape and the circular dichroism are consistent with a coherence range of the emitting excitation of approximately two neighboring molecules. Random fluctuations in the zero-order excited-state energy of the MOPVs (disorder) limit the exciton delocalization and can be described by a Gaussian distribution of energies with a width sigma=0.12 eV and a spatial correlation length l0 approximately 5 molecules. We find that disorder and EV coupling act synergistically in localizing the emitting exciton to a single MOPV in the aggregate with 95% probability.

Vibronic Quasi-Free Rotation Effects in Biphenyl-Like Molecules. TD-DFT Study of Bifluorene.

In this paper, we investigate the vibronic shape of the lowest UV-visible absorption band of biphenyl-like systems, using the bifluorene molecule as a workhorse. The molecule is here regarded as a one-dimensional two-level system, whose ground and excited states are simulated with time-dependent density functional theory and semiempirical methods. The vibrational Schrödinger equation is then numerically solved along the torsional coordinate, and the vibronic shape of the absorption band is modeled. Comparisons with the harmonic approximation, with or without the Franck-Condon approximation, are performed. This study confirms that a vibronic effect is most likely responsible for the strong dissymmetry of the lowest UV-visible absorption band of biphenyl-like systems and that, for such systems, the experimental data should be extracted using the whole absorption band, instead of a Gaussian fit on the first part of the band, as it is often done when a superposition between several electronic transitions is suspected.

MERIS in‐flight spectral calibration

This paper will describe the spectral calibration activities conducted during the MERIS commissioning phase and during operation since orbit 12000. MERIS is a medium resolution (300–1200m) push‐broom imaging spectrometer covering the spectral domain 390–1040nm with 15 bands, programmable in position and width down to steps of 1.25nm. The onboard spectral calibration hardware is based on the use of an Erbium doped‐diffuser panel presenting well‐defined absorption peaks. In the spectral calibration mode, MERIS is configured with narrow bands centred on an Erbium absorption feature (two are used). The first orbit, the instrument is calibrated by viewing the “white” radiometric diffuser plate and the following orbit the “pink” Erbium diffuser plate is deployed. This method allows each of the MERIS detectors involved to be characterized in wavelength. The Fraunhofer absorption lines were used to complement these results by providing additional measurements in the violet and near infrared part of the spectrum. For this method, MERIS was configured both for Earth and diffuser observations and acquired data for only a limited number of orbits. This procedure was repeated for different band settings covering a number of Fraunhofer absorption lines. Finally, using Oxygen (O2A) absorption Earth observation data, two different approaches were developed, one based on the retrieval of surface pressure and one based on the shape of the O2A absorption band. Both methods were developed for clear sky land observations, but their performances are improved over bright land targets. Both methods agree to within an accuracy of 0.02 nm. The results from the different methods are analyzed in order to propose a spectral model for the MERIS instrument. Preliminary results of the spectral variation with time are reported. Except camera 4, the instrument is quite stable with time. Camera 4 needs further investigations to better understand its behaviour. Except for the use of the MERIS oxygen band, the spectral characterization of the other MERIS bands is achieved within the nominal accuracy (1 nm).

Modeling of electron-vibrational 4f n–4f n − 1 5d spectra in LiYF 4:RE 3+ crystals

In the present work, the low temperature absorption band shapes of the impurity Ce 3+ (4f–5d) and Lu 3+ (4f 14–4f 135d) ions in LiYF 4 crystals have been simulated within adiabatic approximation on the basis of a derived microscopic model of electron–phonon interaction and phonon spectrum of the host lattice. Crystal field parameters and electron–phonon coupling constants were treated in the framework of the exchange charge model. Results of simulations of the spectral envelopes agree satisfactorily with the experimental data available from the literature.

Single Particle Spectroscopic Investigation on the Interaction between Exciton Transition of Cyanine Dye J-Aggregates and Localized Surface Plasmon Polarization of Gold Nanoparticles

The interaction between molecular exciton and localized surface plasmon (LSP) polarization has been investigated by measuring light scattering spectra of single gold nanoparticles coated with cyanine dye J-aggregate. The spectral dip reflecting absorption spectral band of the J-aggregate varies with the particle size and the distance between the J-aggregates and the surface of gold nanoparticles. By comparing the experimental results with theoretical calculations based on the Mie theory, we demonstrate that the LSP polarization affects the peak and shape of the J-aggregate absorption band when the exciton resonance peak locates near the LSP resonance peak and when the separation of the J-aggregate from the metal surafce is short. The modification of the J-aggregate exciton band induced by LSP is considered to be due to strong coupling between the molecular exciton transition and the LSP resonance of metallic nanoparticles.

Infrared Absorption Study of the Heme Pocket Dynamics of Carbonmonoxyheme Proteins

The temperature dependencies of the infrared absorption CO bands of carboxy complexes of horseradish peroxidase (HRP(CO)) in glycerol/water mixture at pH 6.0 and 9.3 are interpreted using the theory of optical absorption bandshape. The bands’ anharmonic behavior is explained assuming that there is a higher-energy set of conformational substates (CSS h), which are populated upon heating and correspond to the protein substates with disordered water molecules in the heme pocket. Analysis of the second moments of the CO bands of the carboxy complexes of myoglobin (Mb(CO)) and hemoglobin (Hb(CO)), and of HRP(CO) with benzohydroxamic acid (HRP(CO)+BHA), shows that the low energy CSS h exists also in the open conformation of Mb(CO), where the heme pocket is spacious enough to accommodate a water molecule. In the HRP(CO)+BHA and closed conformations of Mb(CO) and Hb(CO), the heme pocket is packed with BHA and different amino acids, the CSS h has much higher energy and is hardly populated even at the highest temperatures. Therefore only motions of these amino acids contribute to the band broadening. These motions are linked to the protein surface and frozen in the glassy matrix, whereas in the liquid solvent they are harmonic. Thus the second moment of the CO band is temperature-independent in glass and is proportional to the temperature in liquid. The temperature dependence of the second moment of the CO peak of HRP(CO) in the trehalose glass exhibits linear coupling to an oscillator. This oscillator can be a moving water molecule locked in the heme pocket in the whole interval of temperatures or a trehalose molecule located in the heme pocket.

Tunable photoluminescence of polymer doped with PbSe quantum dots

Semiconductor nanoparticle/polymer composites potentially allow the design of photonic materials for optoelectronic devices. The optical characteristics of PbSe quantum dots placed in polymer matrices are investigated for potential applications in electrically controlled absorption modulators and integrated optical circuit components. The photoluminescence yield, shape of absorption band, and effects of size variation of PbSe quantum dots on spectral features are analyzed near the absorption edge of host polymers for the different concentrations of nanocrystals in the composite. It was found that for quantum dots of nominally the same size, there is a strong dependence of position of the absorption peaks in the spectrum depending on the concentration of quantum dots. This results in the emission in the 1500–1600 nm range being tunable with quantum dot concentration. A second emission in the 1200–1300 nm range was also observed and energy transfer from the polymer matrix to the quantum dot appears to mediate the strength of this photoluminescence.

Environment effect on the electronic absorption spectra of crystal violet

The optical characterization and the influence of solvent–solvent interaction on the visible spectra of crystal violet were investigated. The spectroscopic measurements of the solvation of the dye in a polar nematic matrix and some isotropic solvents, mainly alcoholic systems, were correlated with the solvent properties as represented by their dielectric constants, ε, and dipole correlation factors, g. The interaction of the ionic dye with the anisotropic surrounding and with that of the isotropic solvents was investigated and compared. The solvent–solvent interactions can strongly influence the shape and intensity of absorption bands as well as solvent–solute and solute–solute interactions (i.e. dye aggregation, ion pair formation). Dichroic ratios R and degree of anisotropy S of this ionic dye in the liquid crystalline host were investigated by measuring the intensity of the absorption bands in the visible region of parallel aligned samples.

Using Visible Absorption To Analyze Solutions of Kool-Aid and Candy

This article describes a set of materials and a brief description of a technique that may be used to introduce concepts of color and visible absorption to students. Because everyday food items are used, the activity may be presented to high school students, nonscience majors, or may be used with science majors to help them develop good analytical techniques without worry about hazardous waste. A description is given of the use of Kool-Aid solutions to prepare a Beer's law plot, from which an unknown's concentration can be determined. Although a scanning spectrophotometer was used at our institution, a Spectronic 20 would be usable as well. Also, solutions of Kool-Aid, MandMs, and Skittles are compared in terms of their visible absorption band shapes. Students use these candy spectra to determine whether the dyes used to color these food items might be the same.

Theoretical studies of electron-vibrational 4f N–4f N−1 5d spectra in LiYF 4:RE 3+ crystals

The UV and VUV inter-configuration 4f N –4f N−1 5d spectra of rare-earth ions in insulators consist of broad electron-vibrational bands which sometimes have a resolved fine structure. In the present work, the low-temperature absorption band shapes of the impurity Ce 3+, Pr 3+ and Nd 3+ ions in LiYF 4 crystals have been simulated in the framework of the microscopic theory operating with the real phonon spectrum of the host crystal lattice. The energy levels and wave functions of the ground (4f N ) and excited (4f N−1 5d) electronic configurations of rare-earth ions were obtained from the numerical diagonalization of the Hamiltonian containing energies of electrostatic Coulomb and exchange interactions between electrons, spin–orbit interactions and the crystal field interaction. Crystal field parameters and electron–phonon coupling constants were treated in the framework of the exchange charge model. Form-functions of the spectral bands proportional to the calculated integral intensities of electric dipole transitions are obtained as a sum of convolutions of spectral densities of multiphonon correlation functions and form-functions of zero-phonon lines with the widths determined by distributions of random lattice strains and non-radiative transition probabilities. The calculated values of the Huang–Rhys parameters of crystal field states in mixed 4f N−1 5d configurations vary in the range from 0.1 to 15 and correspond to intermediate or strong electron–phonon interactions. Results of simulations of the spectral envelopes agree satisfactorily with the experimental data available from the literature.

Atmospheric correction of hyperspectral data over dark surfaces via simulated annealing

A method [atmospheric correction via simulated annealing (ACSA)] is proposed that enhances the atmospheric correction of hyperspectral images over dark surfaces. It is based on the minimization of a smoothness criterion to avoid the assumption of linear variations of the reflectance within gas absorption bands. We first show that this commonly used approach generally fails over dark surfaces when the signal to noise ratio strongly declines. In this case, important residual features highly correlated with the shape of gas absorption bands are observed in the estimated surface reflectance. We add a geometrical constraint to deal with this correlation. A simulated annealing approach is used to solve this constrained optimization problem. The parameters involved in the implementation of the algorithm (initial temperature, number of iterations, cooling schedule, and correlation threshold) are automatically determined by using a standard simulated annealing theory, reflectance databases, and sensor characteristics. Applied to a HyMap image with available ground truths, we verify that ACSA adequately recovers ground reflectance over clear land surfaces, and that the added spectral shape constraint does not introduce any spurious feature in the spectrum. The analysis of an AVIRIS image of Central Switzerland clearly shows the ability of the method to perform enhanced water vapor estimations over dark surfaces. Over a lake (reflectance equal to 0.02, low signal to noise ratio equal to about 6), ACSA retrieves unbiased water vapor amounts (2.86 cm/spl plusmn/0.36 cm) in agreement with in situ measurements (2.97 cm/spl plusmn/0.30 cm). This corresponds to a reduction of the standard deviation by a factor 3 in comparison with standard unconstrained procedures (1.95 cm/spl plusmn/1.08 cm). Similar results are obtained using a Hyperion image of DoE ARM SGP test site containing a very dark area of the land surface.

Vibrational Markers for the Open-Shell Character of Transition Metal Bis-dithiolenes: An Infrared, Resonance Raman, and Quantum Chemical Study

Transition metal complexes involving the benzene-1,2-dithiol (L(2-)) and Sellmann's 3,5-di-tert-butylbenzene-1,2-dithiol(L(Bu 2-)) ligands have been studied by UV-vis, infrared (IR), and resonance Raman (rR) spectroscopies. Raman spectra were obtained in resonance with the intervalence charge transfer (IVCT) bands in the near-infrared region and ligand-to-metal charge transfer (LMCT) bands in the near-UV region. Geometry optimization and frequency calculations using density functional theory (DFT) have been performed for [M(L)(2)](z) and [M(L(Bu))(2)](z) species (M = Ni, Pd, Pt, Co, Cu, Au, z = -1; M = Au, z = 0). On the basis of frequency calculations and normal-mode analysis, we have assigned the most important totally symmetric vibrations as well as corresponding overtone and combination bands that appear in rR spectra of compounds [Ni(L)(2)](1-), [M(L(Bu))(2)](1-) (M = Ni, Pt, Co, Cu). Experimental values of dimensionless normal coordinate displacements in excited states have been determined by fitting of rR spectra together with the absorption band shape, based on the time-dependent theory of Heller. Time-dependent density functional theory (TD-DFT) and multireference post-Hartree-Fock ab initio calculations, using the difference dedicated configuration interaction (MR-DDCI) method, were carried out to evaluate dimensionless normal coordinate displacements quantum chemically. The calculations show encouraging agreement with the experimental values. The large distortions along several normal modes led to significant vibronic broadening of IVCT and LMCT bands, and the broadening was accounted for in the deconvolution of the absorption spectra. The presence of an intense rR band around approximately 1100 cm(-1) was found to be a reliable marker for the presence of sulfur-based radicals.

Band shapes of the electronic spectra of anisotropic centers in uniaxial crystals recorded in non-polarized light

The impurity absorption band shape (in UV or visible region) in a uniaxial crystal recorded in non-polarized light has been found to depend on the technique of recording the optical spectrum and the parameters of each component recorded in polarized light. With the optical density increase in the maximum of the impurity absorption band the number of points of inflection can change from two to four. With the impurity concentration increase one can observe the shift of the impurity absorption band maximum. The impurity absorption band calculated as the difference between a total spectrum and host matrix absorption spectrum has one or two maxima.

Symmetry-lowering distortion of near-infrared polymethine dyes—a study by first-principles methods

The shape of the longest-wavelength absorption band of near-infrared (NIR) polymethine dyes in polar solvents is not welli-understood. Among different tentative explanations a lowering of the molecular symmetry has been previously discussed. Polymethines of the symmetry point group C 2 v may undergo Peierl's distortion resulting in a structure of lower symmetry. A symmetry collapse was reported based on semiempirical AM1 calculations. The potential change of the molecular symmetry of polymethines was reexamined by first-principle methods. As in the case of AM1 calculations spin-restricted ab initio Hartree–Fock (RHF) calculations of long-chain strepto[ N]methines ( 1) and pyrido[ N]methines ( 2) resulted in symmetry-lowered ground state structures. This occurs with N=19 for 1 and of N=11 with 2 at the RHF/6-31G* level. However, if electron correlation is taken into account by Møller–Plesset perturbution theory of second-order (MP2) or by density functional theory (DFT) the classical symmetric structures are favored. In contrast to spin-restricted DFT calculations with Becke's three-parameter exchange functional (B3LYP), symmetry-breaking occurs at the DFT BHLYP level but at much longer-chain lengths than in the HF approximation. According to polarized continuum model (PCM) calculations, low-symmetry structures are favored in polar solvents. The interpretation of the absorption spectra of the polymethine dyes dissolved in polar solvents is more difficult as earlier assumed and additional experimental studies are necessary to rationalize the nature of the NIR absorptions.

Influence of Temperature and Pressure on Shape and Shift of Impurity Optical Bands in Polymer Glasses

The shape, broadening, and shift of optical absorption spectra of molecular impurity centers in polymer glasses are considered in terms of inhomogeneous energy distributions and coupling of electronic transitions to vibrations. Persistent spectral hole burning was applied for frequency-selective probing of zero-phonon lines. The shift and broadening of spectral holes were studied between 5 and 50 K and by applying a hydrostatic He gas pressure up to 200 bar. Broadband absorption spectra were recorded between 5 and 300 K in poly(methyl methacrylate) and polyethylene. In addition to "normal" thermal broadening, due to the first- and second-order electron phonon coupling, several narrowing components were predicted on the basis of frequency dependent hole behavior. Thermal expansion of the matrix and the relaxation of local strains, previously accumulated on cooling below the glass temperature can lead to shrinking of the inhomogeneous width. A Voigt treatment of absorption band shapes reveals that the Gaussian component can indeed suffer remarkable narrowing. Inhomogeneous band shapes and the frequency-dependent thermal and baric line shifts were rationalized with the aid of a pair of two-body Lennard-Jones potentials. The shift of potential well minima is a crucial factor influencing solvent shifts, inhomogeneous band shapes, pressure shift coefficients, and quadratic electron phonon coupling constants.

Line mixing in the ν 3 and forbidden ν 2 bands of CH4 in gaseous helium

The paper reports on studies of the possibility of the transfer of the adjustable parameters of the line mixing model describing the ν3 absorption band shape of CH4 in helium to the case of the forbidden ν2 band shape. This transfer provides a reasonable overall agreement with the measured spectra, with deviations greater than the experimental uncertainties remaining in some J-manifolds. The model has been improved by taking into account the mixing of spectral lines with the same initial level. It was found that the description of the shapes of certain manifolds can be improved by fitting corresponding matrix elements of the optical rotational relaxation matrix, providing, in some particular cases, more detailed information on the matrix structure.

Density evolution of absorption bandshapes in the water vapor OH-stretching fundamental and overtone: evidence for molecular aggregation

Hydrogen bonding among water molecules is largely responsible for non-ideality in water vapor. Spectroscopic observations in pressurized water vapor using either IR or Raman techniques provide evidence of substantial molecular aggregation up to the critical point and above. The mean size or the molar fractions of the individual aggregates which are formed are, however, very difficult to determine from the analysis of spectra. Present paper aims at the spectral bandshape modeling in sub-critical water vapor making use of available IR absorption spectra in the OH fundamental and overtone. The modeling is supported by quantum-chemical ab initio and anharmonic vibrational calculations for the water dimer, the results of which are compared with previous ab initio and low-temperature laboratory data.

Substituent effects on the electrochemical and spectral characteristics of N, N, N′, N′-tetraaryl- p-phenylenediamine derivatives

Mixed-valence compounds of N, N, N′, N′-tetraaryl- p-phenylenediamine derivatives (PDs) with various electron-donating and -withdrawing substituents have been synthesized and their electrochemical and spectral characteristics have been investigated. According to the molecular symmetry, PDs are grouped as two series. The two series of PDs follow Hammett relationships both in the first and the second half-wave oxidation potentials. The isomeric cation radical/dication redox couples occur at the same potentials and the cation radicals exhibit similar absorption maxima and band shapes in the NIR regions, implying the highly delocalized electronic structures in the singly oxidized states. The stability of the oxidation products was monitored with UV/vis/NIR spectroelectrochemical investigations. PDs with electron-withdrawing substituents are stable after one electron oxidation, while PDs with electron-donating substituents are stable after one and two electron oxidation.