Individual Electronic Transitions Research Articles

Charge-transfer-to-solvent states provide a sensitive spectroscopic probe of the local solvent structure around anions

ABSTRACT This computational study characterises charge-transfer-to-solvent (CTTS) states of aqueous thiocyanate anion using equation-of-motion coupled-cluster methods combined with electrostatic embedding quantum mechanics/molecular mechanics (QM/MM) scheme. Equilibrium sampling was carried out using classical molecular dynamics (MD) with standard force-fields and QM/MM ab initio molecular dynamics (AIMD) using density functional theory. The two calculations yield significantly different local structure around solvated SCN − . Because of the diffuse character of CTTS states, they are very sensitive to the local structure of solvent around the solute and its dynamic fluctuations. Owing to this sensitivity, the spectra computed using MD and AIMD based snapshots differ considerably. This sensitivity suggests that the spectroscopy exploiting CTTS transitions can provide an experimental handle for assessing the quality of force-fields and density functionals. By combining CTTS-based spectroscopies with reliable theoretical modeling, detailed microscopic information of the solvent structure can be obtained. We present a robust computational protocol for modeling spectra of solvated anions and emphasise the use of an ab initio characterization of individual electronic transitions as CTTS or local excitations.

Hydration of a Binding Site with Restricted Solvent Access: Solvatochromic Shift of the Electronic Spectrum of a Ruthenium Polypyridine Complex, One Molecule at a Time.

We report the electronic spectra of mass selected [(bpy)(tpy)Ru-OH2]2+·(H2O)n clusters (bpy = 2,2'-bipyridine, tpy =2,2':6'2″-terpyridine, n = 0-4) in the spectral region of their metal-to-ligand charge transfer bands. The spectra of the mono- and dihydrate clusters exhibit partially resolved individual electronic transitions. The water network forming at the aqua ligand leads to a rapid solvatochromic shift of the peak of the band envelope: addition of only four solvent water molecules can recover 78% of the solvatochromic shift in bulk solution. The sequential shift of the band shows a clear change in behavior with the closing of the first hydration shell. We compare our experimental data to density function theory (DFT) calculations for the ground and excited states.

Near-edge x-ray absorption and natural circular dichroism spectra of L-alanine: A theoretical study based on the complex polarization propagator approach

The complex polarization propagator method [J. Chem. Phys. 123, 194103 (2005)] has been employed in conjunction with density functional theory and gauge-including atomic orbitals in order to determine the near-edge x-ray absorption and natural circular dichroism spectra of L-alanine in its neutral and zwitterionic forms. Results are presented for the K-edges of carbon, nitrogen, and oxygen. In contrast to traditional methods, the proposed approach enables a direct determination of the spectra at an arbitrary frequency instead of focusing on the rotatory strengths for individual electronic transitions. The propagator includes a complete set of nonredundant electron-transfer operators and allows for full core-hole relaxation. The theoretical spectrum at the nitrogen K-edge of the zwitterion compares well with the experimental spectrum.

Hypochromism in Two Low-Energy Transitions of B2 (B 2u ) Symmetry in the Electronic Spectrum of the Carbazole Crystal

The effect of intermolecular interactions on oscillator strengths of individual electronic transitions of B2 ( B 2u ) symmetry in the carbazole crystal was studied. We have shown that the two lowest-energy transitions of this symmetry exhibit very strong hypochromism—only 22% of the molecular oscillator strength is retained in the crystal spectrum. Polarized transmittance spectra of thin (∼0.1 μ) carbazole monocrystals were obtained in the near UV region at normal and oblique incidence of light on the sample. An expression for transmittance as a function of oscillator strengths of the B2 ( B 2u ) symmetry electronic transitions in the crystal, and of the angle of incidence of light on the ac face of the crystal, was derived. The experimental oscillator strengths were obtained by fitting the measured transmittance spectra with the derived function. Local field theory was employed to calculate the theoretical values of the oscillator strengths of five individual electronic transitions in the carbazole crystal. The calculations are in agreement with the oscillator strengths sum rule. The theoretically predicted and experimentally obtained values of oscillator strengths of the two lowest B2 ( B 2u ) symmetry transitions in the carbazole crystal are in very good agreement.

Transient coherent nonlinear spectroscopy of single quantum dots

We review our recent advances in four-wave mixing spectroscopy of single semiconductorquantum dots using heterodyne spectral interferometry, a novel implementation oftransient nonlinear spectroscopy allowing the study of the transient nonlinear polarizationemitted from individual electronic transitions in both amplitude and phase. We presentexperiments on individual excitonic transitions localized in monolayer islands ofGaAs/AlAs quantum wells and in self-assembled CdTe/ZnTe quantum dots. We investigatethe formation of the photon echo from individual transitions, both with increasing numberof transitions in the ensemble, and in the presence of temporal jitter of the energy ofa single transition. The detection of amplitude and phase of the signal allowsthe implementation of a two-dimensional femtosecond spectroscopy, in whichmutual coherent coupling of single quantum dot states can observed and quantified.

Heterodyne spectral interferometry for multidimensional nonlinear spectroscopy of individual quantum systems

A novel implementation of transient nonlinear spectroscopy is presented that allows the study of the transient nonlinear polarization emitted from individual electronic transitions. Both the amplitude and the phase of the polarization are retrieved, permitting the implementation of multidimensional spectroscopy in the excitation pulse delays. The technique is shown to detect mutual coherent coupling in a group of individual, localized excitonic transitions.

Electronic photodissociation spectroscopy of Aun- x Xe (n = 7-11) versus time-dependent density functional theory prediction.

Electronic (one-photon) photodepletion spectra were recorded for gold cluster anions complexed with one xenon atom over the photon energy range 2.1-3.4 eV. Clusters were generated by pulsed laser vaporization and probed under collisionless molecular beam conditions. The spectra obtained are highly structured with the narrowest features--assigned to individual electronic transitions--having bandwidths of less than 40 meV. Time-dependent density functional theory predictions of optically allowed transitions for the most stable--planar--isomers of the corresponding bare metal cluster anions are generally consistent with the experimental observation.

Luminescence study of tetravalent uranium in aqueous solution.

The luminescence spectrum of U4+ in aqueous solution was observed in the UV-Vis region with the lifetime < 20 ns at room temperature by excitation light corresponding to the 5f-5f electronic transition. All the luminescence peaks were assigned to individual electronic transitions.

Effect of quantized electronic states on the dispersive Raman features in individual single-wall carbon nanotubes

This work reports how resonance Raman experiments are used to study details of the electronic structure of individual single-wall carbon nanotubes ~SWNTs! by measuring the phonon spectra and how the quantized electronic structure affects the dispersive Raman features of SWNTs. We focus our analysis on the dispersive D and G8 bands observed in the Raman spectra of isolated semiconducting nanotubes. By using a laser excitation energy of 2.41 eV, we show that both the D-band and G8-band frequencies are dependent on the wave vector kii where the electrons are confined in the one-dimensional subbandi of the electronic structure of SWNTs. By making use of the (n,m) assignment for each tube, we theoretically correlate the observed frequency dependences for the D- and G8-band modes with the electronic structure predicted for each ( n,m) pair and we determine the dependence of v D and v G8 on the diameter and chirality for individual electronic transitions Eii for nanotube bundles. We use the D- and G8-band dependence on electron wave vector k ii to predict the dominant phonon wave vector q selected by the quantum-confined electronic statekii and to explain the anomalous dispersion observed for v D and v G8 in SWNT bundles as a function of laser excitation energy, yielding excellent agreement between experiment and theory.

Optical properties of zirconia–yttria single crystal compounds by reflection electron energy loss spectroscopy

Optical characterization of zirconia–yttria (YSZ) based single crystal compounds (9–32 mol% of yttria) has been carried out in the 5–60 eV energy range from reflection electron energy loss spectroscopy measurements. The stoichiometry and the chemical state of the elements at the surface of the samples were studied by x-ray photoemission spectroscopy. The procedure of analysis to obtain optical properties relies in a quantitative description of the electron energy losses. Inelastic mean free paths of 23, 13 and 5.8 Å for electrons traveling in a reflected geometry in YSZ are obtained for electrons of 1500, 800 and 300 eV, respectively, independently from its yttria content. It has been found that there is a dependence of optical magnitudes on the Zr/Y ratio. Thus, the energy of the bulk plasmon excitations varies from 14.3 to 14.6 eV when the doping dose increases from 9 to 32 mol% of yttria. Surface plasmon excitations appear at 12.6 and 13.2 eV for the 9 and 32 mol% YSZ samples, respectively. Individual electronic transitions have been identified in the absorption coefficient. Besides, the analysis of the absorption coefficient suggests that for the increasing amount of yttria, the conduction band loses gradually the two peak structure observed in pure ZrO2 due to the crystal field splitting of the 4d orbitals of zirconium.

Electronic transitions and intramolecular hydrogen bonding in anthralin. UV-VIS linear dichroism spectroscopy and quantum chemical calculations

The electronic and molecular structure of the antipsoriatic drug anthralin (1,8-dihydroxy-9(10 H)-anthracenone) is investigated by UV-VIS linear dichroism (LD) spectroscopy in stretched polyethylene and by quantum chemical model calculations. Seven individual electronic transitions are resolved below 47 000 cm −1 and assigned to calculated π– π* transitions. The low-energy region is characterized by a relatively broad band around 28 000 cm −1 that can be assigned to two overlapping, differently polarized electronic transitions involving a considerable degree of intramolecular charge transfer from the phenolic moieties to the carbonyl group. Computed wavenumbers for these transitions depend significantly on the assumed geometrical parameters for the intramolecular H-bonds in anthralin; best agreement with observed data is obtained with a geometry corresponding to strong H-bonding. The calculations also indicate that excited state intramolecular proton transfer (ESIPT) is likely to occur, leading to the prediction of a very large Stoke's shift.

Response to “Comment on ‘Shallow donors in GaN studied by electronic Raman scattering in resonance with yellow luminescence transitions’ ” [Appl. Phys. Lett. 70, 909 (1997)

In their comment, Siegle et al. present Raman and photoluminescence ~PL! data from samples grown either on GaAs by molecular beam epitaxy ~MBE! or on sapphire by metal organic vapor phase epitaxy and hydride vapor phase epitaxy. They observe electronic Raman scattering ~ERS! peaks like those reported in Ref. 2 and claim, however, our assignments to be incorrect. Siegle et al. report peaks at 60 ~a!, 95 ~b!, 102.5 ~g!, 125 ~d!, and 250 cm ~e! in addition to the ERS peaks at 151 ~A*!, 189 (A), 218 ~B*!, and 237 cm (B) discussed in Ref. 2 and assert that all these peaks have the same origin. In fact, we also observe peaks b and d in the spectra of our GaN samples. According to our model, we expect a very similar Raman resonance behavior for different donor species with sufficiently low binding energy. Therefore, the argument based on the resonance behavior is invalid in attributing all the observed peaks to the same origin. If the same origin is assumed for all peaks, their relative intensities should be nearly constant for different samples. This is by far not the case as shown, e.g., in Fig. 1 of Ref. 2 for peaks A and B vs A* and B* as well as for peaks b vs d in Fig. 1 ~compare also Figs. 1 and 2 in Ref. 1!. In contrast to the ERS peaks discussed in Ref. 2, peak d has been observed also in p-type GaN and shows a weak temperature dependence. Therefore, it is not even clear whether peak d is due to ERS. Siegle et al. further claim as a main statement that the ERS peaks are only observable for GaN grown on GaAs. However, we observe the ERS peaks reported in Ref. 2 also in GaN layers after removal of the GaAs substrate @Fig. 1~c!# demonstrating that they do not originate from GaAs. It is furthermore known that N impurities in GaAs introduce a resonant energy level in the conduction band which is not occupied under normal conditions. Moreover, peak B* was observed previously by absorption spectroscopy and, more recently, peaks A* and B* by Raman scattering from hexagonal GaN grown on sapphire. These findings completely rule out any connection with GaAs and therefore refute the statement of Ref. 1. The observability of the ERS peaks rather depends on the donor concentration and the Fermi level in the GaN layer. For too small donor concentrations, the intensity of the corresponding Raman excitations vanishes below the detection limit. At very large concentrations, a pronounced formation of impurity level bands is expected to destroy the observability of individual electronic transitions. In addition, the resonance enhancement should be only observable for sufficiently high concentrations of the involved deep level defect. In this respect, arguments based

Electrochemistry and spectroscopy of magnesium octaethyltetraazaporphyrin and magnesium octakis(methylthio)tetraazaporphyrin

Electrochemical properties, absorption and magnetic circular dichroism (MCD) spectra are reported for two peripherally substituted tetraazaporphyrins (TAP). Data are described for magnesium octaethyltetraazaporphyrin (MgOETAP) and magnesium octakis(methylthio)tetraazaporphyrin (MgOMTTAP) in dichloromethane (DCM) and N,N-dimethylformamide (DMF) solutions. Differential pulse Voltammograms (DPV) and cyclic voltammograms (CV) measurements yielded ϵ 1 2 values of: +0.81 V forming the ring oxidized π cation [MgOETAP(−1)] +, −0.70 V forming the ring reduced π anion [MgOETAP(−3)] − and −1.15 V forming the ring reduced π dianion [MgOETAP(−4)] 2−) and E 1 2 values of 0.95 V forming the oxidized π cation [MgOMTTAP(−1)] +, −0.68 V forming the ring reduced π dianion [MgOMTTAP(−4)] 2− and −1.05 V forming the ring reduced π dianion [MgOMTTAP(−4)] 2− Band deconvolution calculations, that couple the absorption and the MCD spectra, are reported for both compounds. The energies of the fitted bands are compared with experimental results for magnesium phthalocyanine reported by Ough et al. [E.A. Ough, T.N. Nyokong, K.A.M. Creber and M.J. Stillman, Inorg. Chem., 27 (I988) 2725]. Deconvolution analysis isolated an additional degenerate transition in the Q band region (700-500 nm), which has been assigned as π vibrational component of the Q band. Individual electronic transitions (assignments) are located at: 595 (Q 00), 570 (Q 01), 342 (B1), and 327 nm (B2) for MgOETAP and at: 663 (Q 00), 611 (Q 02), 374 (B1), 352 (B2), and 250 nm (L) for Mg0MTTAP

Absolute oscillator strengths for the photoabsorption of silane in the valence and Si 2p and 2s regions (7.5–350 eV)

Abstract Absolute photoabsorption oscillator strengths (cross sections) for the valence and Si 2p discrete regions of silane have been measured using high resolution (∼ 0.05–0.1 eV fwhm) dipole (e, e) spectroscopy. Long range (7.5–350 eV) lower resolution data (1 eV fwhm) have also been obtained, from which the absolute oscillator strength scale has been determined using TRK (i.e. S (0)) sum-rule normalization. The accuracy of the presently reported measurements has been tested using the S (−2) sum-rule: Evaluation of the S (−2) sum using the presently reported data gives a dipole polarizability for silane within 0.2% of the experimental value. The present valence shell measurements were undertaken in order to investigate the differences between earlier low resolution dipole (e, e) work (Cooper et al., Chem. Phys. 140 (1990) 133) and subsequent valence shell synchrotron radiation photoabsorption measurements reported by Kameta et al. (J. Chem. Phys. 95 (1991) 1456). The presently reported high and low resolution absolute phtoabsorption oscillator strengths are much more consistent with the direct photoabsorption measurements than the earlier dipole (e, e) work (within ∼ 15%). The resolution of the present high resolution data is sufficient to allow absolute photoabsorption oscillator strengths to be determined for many of the individual electronic transitions in the Si 2p discrete spectrum.

Resolution of the electronic transitions of cytochrome c oxidase: evidence for two conformational states of ferrous cytochrome alpha.

Second-derivative absorption spectra are reported for a variety of oxidation and ligation states of bovine cytochrome c oxidase (ferrocytochrome-c:oxygen oxidoreductase, EC 1.9.3.1). The high resolving power of the second-derivative method allows us to assign the individual electronic transitions of cytochrome alpha and cytochrome alpha 3 in many of these states. In the fully reduced enzyme, one observes a single electronic transition at 444 nm, corresponding to the Soret transition for both ferrous cytochrome alpha and ferrous cytochrome alpha 3. When the cytochrome alpha 3 site is occupied by an exogenous ligand (CN or CO), one observes two absorption bands assignable to the ferrous cytochrome alpha chromophore, one at ca, 443 nm and the other at ca, 450 nm. The appearance of the 450-nm band is dependent only on ligand occupancy at the cytochrome alpha 3 site and not on the oxidation state of the cytochrome alpha 3 iron. These results can be interpreted either in terms of a heterogeneous mixture of two ferrous cytochrome alpha conformers in the cytochrome alpha 3-ligated enzyme or in terms of a reduction in the effective molecular symmetry of the ferrous cytochrome alpha site that results in a lifting of the degeneracy of the lowest unoccupied molecular orbital associated with the Soret pi,pi* transition of cytochrome alpha. In either case, the present data indicate that ferrous cytochrome alpha can adopt two distinct conformations. One possible structural difference between these two states could be related to differences in the strength of hydrogen bonding between the ferrous cytochrome alpha formyl oxygen and a proton donor from an unidentified amino acid side chain of the enzyme. The implications of such modulation of hydrogen-bond strength are discussed in terms of possible mechanisms of proton translocation and electron transfer in the enzyme.

Exciton interactions in phycoerythrin

Upon assembly of the phycoerythrin trimer into hexamer and the hexamer into dodecamer, marked spectral changes are observed. The absorption and circular dichroism spectra of the various phycoerythrin aggregates were resolved into Gaussian components representing individual electronic transitions of phycoerythrobilin chromophores within these proteins. While the contribution of a broad, sensitizing band (at 525 nm) is constant, with increasing aggregate size, a short-wavelength pair of bands centered at 555 nm decreases concomitantly with a dramatic increase in the intensity of a long-wavelength pair of chromophore transitions centered at 563 nm. The implications of these spectral changes for efficient energy transfer in the phycobilisome are discussed.

Hypochromic effect and electronic excitation energy transfer in Y t(CH 2) nadenine systems

Measurements of the UV absorption spectra of aqueous solutions of the bichromophore Y t(CH 2) n adenine systems ( n = 2, 3, 5 or 6) and the corresponding solutions of 1-methyladenine were carried out. All absorption spectra were subjected to a gaussian component computer analysis and the values of hypochromicity were determined for the individual electronic transitions S 0 → S n in the molecule under investigation. The strongest hypochromic effects were found in systems with n = 3. The relative fluorescence quantum yields of the systems were measured using an equimolar mixture of 1-methyl-Y t and 9-methyladenine as a reference solution for two different wavenumbers of exciting light: ν 1 = 32 000 cm −1 (where only 1-methyl-Y t absorbs) and ν 2 = 38 000 cm −1 (where both chromophores absorb). From these measurements, the efficiencies T of the energy transfer from adenine to the Y t base were determined as 0.41, 0.81, 0.44 and 0.17 for systems with n = 2, 3, 5 or 6 respectively. The results obtained by measuring the hypochromism and the energy transfer are in good agreement.

On the molecular conformation of human haemopexin: II. Analysis of circular dichroic spectra

The circular dichroic (CD) spectra of haemopexin in the far ultraviolet region exhibit an atypical positive maximum at 231 nm, which prevents determination of the secondary structure by the usual methods. We have developed a modified analytical method by which it was possible to calculate the secondary structure of the protein (8% of α-helix, 10% of β-conformation) and the actual wavelength (229 nm) and intensity of the above-mentioned positive band. Measurements and analysis of the CD spectra of haemopexin in an alkaline medium gave information ascribing a major part of intensity of the band at 229 nm to the B Iu transition of Tyr; we believe that some other electronic transitions contribute to this band, too. An assignation of the individual extremes of the CD spectrum in the near ultraviolet region to the individual electronic transitions is propounded. The dichroic bands of Trp in this region and the temperature dependence of the CD spectra in the far ultraviolet region corroborate our idea that the haemopexin molecule contains a very compact hydrophobic peptide core. Similarities of the CD spectra of haemopexin to those of the haem. haemopexin complex throughout the ultraviolet region suggest a conformational likeness of these two molecules.

ChemInform Abstract: EXCITED STATES OF DINAPHTH(DE‐1,2,3/D′E′‐5,6,7)AZULENE, POLARIZED ABSORPTION IN STRETCHED POLYMERS AND MAGNETIC CIRCULAR DICHROISM

Chemischer InformationsdienstVolume 5, Issue 34 Physical Organic Chemistry ChemInform Abstract: EXCITED STATES OF DINAPHTH(DE-1,2,3/D′E′-5,6,7)AZULENE, POLARIZED ABSORPTION IN STRETCHED POLYMERS AND MAGNETIC CIRCULAR DICHROISM JAROSLAV KOLC, JAROSLAV KOLCSearch for more papers by this authorJOSEF MICHL, JOSEF MICHLSearch for more papers by this author JAROSLAV KOLC, JAROSLAV KOLCSearch for more papers by this authorJOSEF MICHL, JOSEF MICHLSearch for more papers by this author First published: August 27, 1974 https://doi.org/10.1002/chin.197434066AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume5, Issue34August 27, 1974 RelatedInformation

Polarization Spectra in Stretched Polymer Sheets. V. Excited States of Anthracene and Transition Moment Directions in Aminoanthracenes

AbstractLinear dichroism in stretched polyethylene has been measured for anthracene and ten substituted anthracenes. The results show mixed polarization in the region of the 1La band which is best ascribed to vibronic mixing with the 1Bb band. A discussion of all available evidence for the often claimed presence of a weak 1Lb band in the region of the 1La band shows that there is no compelling reason to believe that the 1Lb state has ever been observed.A SCF‐CI PPP calculation for anthracene including systematically selected doubly excited configurations is reported and compared with presently available experimental data. Encouraging agreement is found.Orientation factors of the symmetrical molecules are simply related to molecular shape in agreement with the previously proposed orientation model. This permits an estimate of orientation factors for the unsymmetrical members of the series. This can be done in a nicely self‐consistent manner. It is found that methyl and fluoro substitution barely affects the polarization directions, while amino substitution has a large effect.Moment directions for the individual electronic transitions in the three aminoanthracenes obtained from the measurements are compared with SCF‐CI PPP calculations. The latter are used to examine systematically the effect of an electron‐donating mesomeric substituent on the anthracene spectrum as a function of its electronegativity.