Showing papers on "Absorption band published in 1994"

The origin of the intrinsic 1.9 eV luminescence band in glassy SiO2

Abstract: The current controversy over the nature of the centers giving rise to the 1.9 eV photoluminescence (PL) band (the R-band), the suggested defect models and the relevant experimental data are briefly reviewed. The luminescence emission, excitation and polarization spectra of neutron-irradiated synthetic silica were studied between 6 and 300 K using site-selective dye-laser and Ar ion laser excitation. Resonant zero-phonon lines (ZPL) were observed below 80 K both in luminescence emission and excitation spectra in the 1.9–2.1 eV region. A vibration line in emission spectra 890 cm−1 below the ZPL energy is attributed to the symmetric stretching vibration of the silicon-non-bridging oxygen bond in the ground electronic state of the non-bridging oxygen hole center. A similar line 860 cm−1 above the ZPL in the excitation spectra corresponds to the same vibration in the excited state. The intensities of the resonant ZPLs are dependent on the excitation energy and show a nearly Gaussian distribution with the peak at 1.935 ± 0.01 eV and halfwidth 82 ± 7 meV. In the zero approximation, this distribution describes the concentration distribution of the PL centers with the respective energies of the excited electronic state. The 4.8 eV excitation band of the 1.9 eV PL is complex, due to different electronic transitions. No ZPLs or vibrational structures are observed under excitation with KrF excimer laser or Xe lamp in this band. The optical absorption in this region is due to overlapping bands of several different defect centers. The low-temperature luminescence bands at 2.05–2.1 eV and 2.35–2.4 eV, excited by the green (2.41 eV) and blue (2.71 eV) Ar ion laser lines, have a nature different from the 1.9 eV band. Several different defects contribute to the 2.0 eV optical absorption band in irradiated glassy SiO2.

Subpicosecond time response of third‐order optical nonlinearity of small copper particles in glass

Abstract: Femtosecond pump‐probe experiments with a time resolution of 100 fs have been performed for copper particles with a radius of 4 nm Differential absorption spectra for a pump centered at 205 eV indicates the broadening of the absorption band due to the surface plasmon The nonlinear response time derived from the recovery time of the nonlinear absorption is dependent on the pumping laser fluences, and is as short as 07 ps for 210 μJ/cm2 The relaxation dynamics of nonequilibrium electrons can be described by the usual electron‐phonon coupling model

Optical and Photophysical Properties of the Oxazole Yellow DNA Probes YO and YOYO

Abstract: The photophysical properties of the optical DNA probe YOYO (homodimeric derivative of oxazole yellow) have been characterized in terms of the monomeric part, the YO chromophore. In aqueous solutions YO is virtually nonfluorescent but upon binding to DNA its fluorescence quantum yield is strongly increased. A similar enhancement of the fluorescence is observed for YO in the viscous solvent glycerol. The high fluorescence quantum yield of YO, when bound to DNA or in a viscous solution, is proposed to be a result of decreased rotational mobility around the internuclear bridge between the two aromatic ring systems. This hypothesis is based on similar values of the activation energies for the temperature-dependent nonradiative decay processes (E(A) = 53 kJ/mol) and viscous flow (E(A) = 63 kJ/mol), suggesting related rate-limiting mechanisms. A single electronic transition is found to be responsible for the intense visible absorption band. This conclusion is based on the observation of an essentially wavelength-independent reduced linear dichroism and similarly wavelength independent fluorescence anisotropy, and the fact that the emission spectrum is very nearly a mirror image of the absorption spectrum. The conclusion is further supported by quantum mechanical calculations (CNDO/S). By combination of measurements of fluorescence anisotropy of YO in glycerol and linear dichroism of YO in a stretched poly(vinyl alcohol) film, the transition moment of the strong visible absorption band was found to be nearly long axis polarized, in agreement with the CNDO/S calculations. The low-energy electronic transition and its polarization direction in the YO chromophore remain essentially unperturbed in the YOYO dye, suggesting that the results obtained for the excited state of the YO chromophore are applicable also to YOYO. One difference, though, is that in aqueous solutions the two YO chromophores of YOYO interact with each other, forming an internal dimer, resulting in a distorted absorption spectrum.

Ultrafast excitation energy transfer and exciton-exciton annihilation processes in isolated light harvesting complexes of photosystem II (LHC II) from spinach

Abstract: Excitation energy transfer and exciton-exciton annihilation in the isolated light-harvesting chlorophyll a/b protein complex of spinach photosystem II (LHC II) has been studied by two-color absorption difference spectroscopy with femtosecond time resolution. After selectively exciting Chl b at 645 nm, the transient absorption changes were monitored at wavelengths where either Chl b (655 nm) or Chl a (680 nm) dominates the absorption of LHC II. From the good correspondence of the lifetimes obtained from a numerical analysis of the very fast relaxation in the Chl b absorption band (160 [+-] 20 fs) and the rise kinetics in the Chl a absorption band (145 [+-] 20 fs), it is suggested that the Chl b [yields] Chl a excitation energy transfer occurs on a time scale of about 150 fs. In addition, at both probe wavelengths (655 and 680 nm) lifetimes of 3-7 ps were observed which likely arise from excitation energy transfer processes connected with spectral shifting. The kinetic curves of the transient absorption changes at 680 nm show a remarkable intensity dependence which is ascribed to exciton-exciton annihilation. Since at a probe wavelength of 655 nm no intensity effect on the kinetics was observed, it is concluded that annihilation processesmore » preferably occur among excited singlet states of Chl a molecules. 28 refs., 6 figs.« less

Photoluminescence of Mn2+‐Activated ZnGa2 O 4

Abstract: The synthesis and photoluminescence mechanism of are presented. Monitored at 450 nm, exhibits an absorption band at 245 nm; monitored at 506 nm exhibits absorption bands at 245 and 283 nm. Under 254 nm excitation exhibits a broad‐band emission extending from 360 to 610 nm, peaking at 450 nm. This emission is gradually quenched, as a new band emerges, peaking at 506 nm, as Mn2+ is incorporated into the lattice, where at maximum intensity. Activator sensitization and host‐to‐activator energy transfer are suggested.

A Theory for the Temperature Dependence of Hole-Burned Spectra

Abstract: A theory is presented for the temperature dependence of the absorption and hole-burned spectra of chromophores imbedded in solids characterized by structural heterogeneity The theory is applicable for arbitrarily strong linear electron-phonon coupling and describes the overall hole profile which consists of the zero-phonon hole and its associated phonon sideband hole structure A novel and convenient form for the thermally averaged Franck-Condon factors is employed Illustrative calculations are presented which pertain to the temperature dependence of the absorption band of the primary electron donor of the photosynthetic bacterial reaction center and the design of high-temperature hole-burning materials for high-density frequency domain optical storage 44 refs, 4 figs, 3 tabs

Low-Energy Exciton Level Structure and Dynamics in LightHarvesting Complex II Trimers from the Chl a/b Antenna Complex of Photosystem II

Abstract: Nonphotochemical hole-burned spectra obtained as a function of burn wavelength at 4.2 K are reported for the isolated LHC II peripheral antenna complex of photosystem II. The lowest-energy state of the trimer complex is shown to lie at 680 nm, 4 nm below the most intense Chl a band at 676 nm. The linear electron-phonon coupling for the 680-nm state is characterized and used to predict that its fluorescence origin should lie at 681 nm, precisely coincident with the observed origin at 4.2 K. The 680-nm band carries the equivalent absorption strength of about one chlorophyll a molecule per C[sub 3] trimer complex, which contains about 27 chlorophyll a molecules. The 680-nm absorption band possesses an inhomogeneous width of [approximately] 120 cm[sup [minus]1], and its zero-phonon line distribution function is largely uncorrelated with those of the higher-energy states. Zero-phonon hole widths are used to determine that the fluorescent 680-nm state dephases in 10 ps at 4.2 K. An interpretation of this dephasing is given in terms of the trimer of subunits structure. Based on the satellite hole structure observed upon hole burning into the 680-nm state, two new states at 674 and 678 nm are identified. 56 refs., 5 figs.

Optical absorption of copper phosphate glasses in the visible spectrum

Abstract: Optical absorption of copper phosphate glasses with batch compositions of 40, 50 and 55 mol% CuO have been examined in the visible and near-infrared range as a function of [Cu2+]/[Cutotal] in the glasses An absorption band centered at about 11 000 cm−1 is produced due to an octahedral coordination of Cu2+ in the glass with strong tetragonal distortion This absorption band is resolved into three-component Gaussian bands around 8500, 12 000 and 13 250 cm−1 These bands are assigned to the energy transitions 2 A 1 g → 2 B 1 g , 2 B 2 g → 2 B 1 g and 2 E g → 2 B 1 g , respectively The degree of tetragonal distortion of Cu2+ coordination is slightly enhanced with increases in the [Cu2+] ratio and CuO content A minimum absorption in the visible spectrum is produced by overlapping of the absorption bands of Cu2+ and Cu1+ The position of the minimum absorption shifts to higher wavenumber with increasing [Cu2+] ratio in the glass regardless of glass composition and is responsible for color changes

Enhanced rates of subpicosecond energy transfer in blue-shifted light-harvesting LH2 mutants of Rhodobacter sphaeroides.

Abstract: Energy transfer within various LH2 antenna complexes of the photosynthetic purple bacteria Rhodobacter sphaeroides and Rhodopseudomonas acidophila has been studied at 77 K using tunable femtosecond and subpicosecond infrared pulses. The complexes examined include the wild-type B800-850 as well as three different specifically mutated complexes. The site-directed mutant strains were altered at positions 44 and 45 near the C-terminus of the alpha-subunit, which introduces a spectral blue-shift of the 850-nm absorption band. In addition to a constant band at 800 nm, the mutations alpha Tyr44,Tyr45-->Phe,Tyr; -->Tyr,Phe; and -->Phe,Leu have absorption peaks at 838, 838, and 826 nm, respectively. As the spectral overlap between the B800 and the variable bands increases, the rate of energy transfer as measured by the lifetime of the B800 excited state also increases from 2.4 +/- 0.2 to 1.8 +/- 0.2, 1.6 +/- 0.2, and 0.8 +/- 0.1 ps. This correlation between energy-transfer rate and spectral blue-shift of the B850 absorption band is in qualitative agreement with the trend predicted from Forster spectral overlap calculations, although the variation of the experimentally determined rate through the series of mutants is somewhat wider than what is predicted by simulations. In addition to the decay time constants relate to the B800-->B850 energy transfer, the B800 excited state is seen to decay with a faster 150-500-fs component due to energy transfer between spectrally inhomogeneous B800 molecules and possibly also vibrational relaxation and cooling in the bacteriochlorophyll excited state.

Exciton Level Structure and Dynamics in the CP47 Antenna Complex of Photosystem II

Abstract: Persistent nonphotochemical and population bottleneck hole-burning results obtained as a function of burn wavelength are reported for the CP47 proximal antenna protein complex of photosystem II. Attention is focused on the lower energy chlorophyll a Q[sub y] states. Results are presented for the CP47 complex from two preparations. The Chl a content per CP47 complex was determined, spectroscopically, to be 14 [+-] 2. On the basis of the analysis of the hole spectra and the 4.2 K static fluorescence spectrum, the lowest energy state of CP47 lies at 690 nm (fluorescence origin at 691 nm). The width of the weak 690-nm absorption band from inhomogeneous broadening is 100 cm[sup [minus]1]. The linear electron-phonon coupling of the 690-nm state is weak with a Huang-Rhys factor (S) of about 0.2 and a mean phonon frequency ([Omega][sub m]) of 120 cm[sup [minus]1], which explains why the Stokes shift (2S[Omega][sub m]) is so small. The 690-nm state is found to be excitonically correlated with a hitherto unobserved state at 687 nm. However, the combined absorption intensity of the 690- and 687-nm states was determined to be equivalent to only 1 Chl a molecule. Results are presented which illustrate that these two states are fragilemore » (i.e., their associated chlorophyll a molecules are disrupted). Thus, it is possible that the correct number of Chl a molecules is 2, not 1. 53 refs., 9 figs., 1 tab.« less

Ultraviolet absorption in modified chemical vapor deposition preforms

Abstract: Using a simple and effective technique, we have systematically studied the UV absorption in a number of fiber preforms made by modified chemical vapor deposition. The 242-nm absorption band due to germanium-related oxygen-deficient centers (GODC’s) is found to be approximately linearly dependent on germania concentration and, after a small initial increase, is almost constant throughout the preform collapse process. However, we found that the band can be increased by 700% when a reducing atmosphere (He + 1%D2) is substituted for O2 Several codopants have been evaluated to determine their effect on the GODC concentration. Phosphorus, boron, aluminum, and fluorine all reduce the 242-nm band; phosphorus causes the largest change.

Optical and electron spin resonance spectroscopy of Ti3+ and Ti4+ in Al2O3

Abstract: The absorption spectrum of Ti3+:Al2O3 crystals grown in a reducing atmosphere consists of the main blue‐green absorption band and a weak infrared band. This infrared absorption decreases in intensity on annealing the as‐grown samples in a reducing atmosphere. The analysis of electron spin resonance spectra of the as‐grown samples, in which the infrared absorption band is observed, indicates that the center associated with the band is a cluster involving Ti3+ and Ti4+ ions with a neighboring, charge compensating Al3+ vacancy. The coexistence of a Ti4+ ion and an Al3+ vacancy in the neighborhood of the Ti3+ ion weakens the crystal field at this ion more than a single Ti4+ ion, giving rise to a red shift of the Ti3+ absorption.

Photoinduced Proton Transfers in Methyl Salicylate and Methyl 2-Hydroxy-3-Naphthoate

Abstract: The effects of solvent and temperature on the dual fluorescence emission of methyl salicylate (MSA) have been reinvestigated, and new insight regarding the photoinduced proton-transfer reactions is reported. The steady-state spectral data obtained in this work are found to be consistent with the spectral assignments proposed by previous investigators. Specifically, the dual emission bands in alcohols are shown to occur from excited states derived from two ground-state rotamers, a and b. The emission band from excited a (the normal band) is in mirror-image relationship with the absorption band and the Stokes shift of this band is ∼5000 cm -1

Raman spectra of mass‐selected cobalt dimers in argon matrices

Abstract: The absorption and Raman spectra for nickel dimers in an argon matrix prepared by the mass‐selected ion deposition technique have been measured. A weak dimer absorption band is centered around 480 nm. Resonance Raman spectra obtained from exciting into the dimer 480 nm band show a vibrational progression for which ωe=259.2±3.0 cm−1 with ωexe=1.9±0.7 cm−1. The dinickel Raman spectra are discussed in terms of isotopic shifts, as well as the changeover from d–d bonding to s–s bonding along the series Fe2→Cu2.

Emission spectroscopy of dissociating H2S: Influence of nonadiabatic coupling

Abstract: The emission spectroscopy of H2S excited in the first absorption band around 195 nm is investigated theoretically using ab initio potential energy surfaces (PES) and transition dipole moment functions. As shown in our previous studies, the photodissociation involves two excited electronic states, one which is binding and another one which is dissociative. The nonadiabatic coupling between these two states is very strong and described in a diabatic representation in which only the binding state is optically excited while the dissociative state is dark. As in the case of H2O excited in the 165 nm band, the emission spectrum shows a long progression of stretching states up to seven HS vibrational quanta. In contrast to water, however, some weak activity in the bending mode is also observed. Most remarkable is a prominent wavelength dependence which is attributed to the strong nonadiabatic coupling between the two excited electronic states. The agreement with experimental data is only fair; the essential feat...

Characteristics of 5‐eV absorption band in sputter deposited GeO2‐SiO2 thin glass films

Abstract: Thin films (4.6 μm thick) of 5 GeO2‐95 SiO2 and 55 GeO2‐45 SiO2 (mol %) glasses were prepared by rf sputtering method in an Ar‐O2 atmosphere. An intense absorption band at around 5 eV was distinctly observed in both films after the as‐deposited films were annealed at 350 °C for 30 min in a vacuum. A part of this 5‐eV band was gradually decreased by UV irradiation. Saturated absorptivity changes (−Δα∞) of the UV bleached component, which is considered to be the origin of Hill gratings [K. O. Hill, Y. Fujii, and B. S. Kawasaki, Appl. Phys. Lett. 32, 647 (1978)] and second‐harmonic generation in SiO2 glass fibers doped with GeO2, after a prolonged irradiation were 50 cm−1 for 5 GeO2‐95 SiO2 films and 400 cm−1 for 55 GeO2‐45 SiO2 films. These values are greater by one or two orders of magnitude than those (∼2 cm−1) of bulk germanosilicate glasses prepared by the vapor axial deposition method.

Electroabsorption study of excited states in hydrogen-bonding solids: epindolidione and linear trans-quinacridone

Abstract: Electroabsorption (EA) spectra within the first absorption band of vacuum-evaporated films of epindolidione (EPI) and linear trans-quinacridone (QAC) have been studied in search of the hydrogen-bonding effect on electronic excited states. Whereas the EPI EA spectra are determined by well-localized molecular excitons, the EA spectra of QAC can be rationalized only by the involvement of the low-energy charge-transfer (CT) excitons. The lowest CT state is found to appear within the first dominant absorption transition at 2.22 ± 0.02 eV. Near resonance between an intramolecular (Frenkel type) excitation and nearest-neighbour CT states is suggested to underlie the strong absorption and electroabsorption features in this spectral range. The EA effect is ascribed to the electric-field-induced mixing of the hybridized Frenkel/symmetric CT excitons with anti-symmetric CT states. Intermolecular hydrogen bonding of the NH⋯O type is suggested to play an important role in creation of the CT states in QAC.

Fluctuations in absorption spectra and final product state distributions following photodissociation processes

Abstract: We present a quantum mechanical wave packet study for the unimolecular dissociation of a triatomic molecule into an atom and a diatom. The 3D potential energy surface used in the dynamics calculations is that of the B state of water corresponding to the second absorption band. Both OH stretching coordinates and the bending angle are included. What is not taken into account is the strong nonadiabatic coupling to the lower‐lying A and X states which in reality drastically shortens the lifetime in the B state. For this reason the present study is not a realistic account of the dissociation dynamics of water in the 122 nm band. It is, however, a representational investigation of a unimolecular reaction evolving on a realistic potential energy surface without barrier. The main focus is the resonance structure of the absorption spectrum and the final rotational state distributions of the OH fragment. The total absorption spectrum as well as the partial dissociation cross sections for individual rotational s...

Preparation and Nonlinear Optical Properties of Ag/SiO2 Glass Composite Thin Films.

Abstract: A multitarget sputtering method was applied to prepare Ag particles embedded in SiO2 glass thin films. In the optical absorption spectra of the films, the absorption peak due to the surface plasmon resonance of Ag particles was clearly observed at the wavelength of 390–406 nm. The full width at half-maximum of the absorption band is decreased with increasing diameter of Ag particles. The third-order nonlinear susceptibility at 400 nm was estimated to be 1.6×10-8 esu for the film with 9.0 at% of Ag by means of a degenerated four-wave mixing.

Characterization of the guanidine-hydrochloride-denatured state of iso-1-cytochrome c by infrared spectroscopy

Abstract: Infrared spectroscopy has been used to monitor residual ordered structure in the denatured state of wild-type and two mutants of iso-1-cytochrome c. The technique used involves a careful digital subtraction procedure that removes spectral contributions from buffer, water vapor, and the denaturant guanidine hydrochloride. Reliable and reproducible spectra can be produced using these methods. The data for iso-1-cytochrome c show upon denaturation a shift of the structure-sensitive amide I infrared band away from the spectral region associated with random structure. Second-derivative resolution enhancement of the amide I absorption band uncovers several bands which can be associated with various residual ordered structures in the denatured state. Gradual changes in the amide I band after denaturation are also observed as the guanidine hydrochloride concentration is increased. Two single-site mutants of iso-1-cytochrome c, which have been shown to have more compact denatured states than the wild-type protein, exhibited denatured-state infrared spectra with significant differences from the wild-type protein spectra. These observations provide new insight into the characteristics of protein denatured states.

A Theoretical Study on the Electronic and Vibrational Structure of the Phenoxyl Radical

Abstract: The electronic structures of the phenoxyl radical in the electronic ground and excited states are studied by ab initio MO calculations. The geometries are optimized by the CAS-MCSCF procedure with the Huzinaga–Dunning DZV basis set. The electronic excitation energies and transition dipole moments are calculated by the CI procedure. According to the result of the calculation the observed absorption bands at about 600, 400, and 300 nm are assigned as , respectively. In order to analyze the vibrational character of the absorption band at about 400 nm the force constants, the potential energy distributions, the normal modes, and the Franck–Condon integrals are calculated for the three states, . The force constants are scaled by referring to the experimental resonance Raman, Trapped Ion Photodissociation (TIP), and the matrix-isolated electronic absorption spectra. The strongest vibrational band of the TIP and the absorption spectra is assigned to the 0–0 band of the transition and the second strongest, at a h...

Solvent Effects on Molecular and Ionic Spectra. 4. Photochemistry of Fe2+(H2O)6 in Water Revisited: Possible Mechanisms for the Primary Absorption Process Leading to Electron Ejection

Abstract: As the simulation of the structure of ions in solution is not completely straightforward, a number of aspects of the simulation procedure are investigated, concentrating on solvent shift sensitivity. Specifically, we investigate the ultraviolet absorption spectrum of aqueous Fe[sup 2+](H[sub 2]O)[sub 6]; for centrosymmetric systems such as this, it is found to be most important to correctly represent the structure of the second coordination shell. We consider the energetics and solvent shift of the first three of these processes, concluding that the MLCT band is too high in energy, the d [yields] s band could participate, and the photodetachment band is at the correct energy and intensity to account for all that is (as yet) observed of the absorption band. In general, a rather complicated picture of this process in inorganic complexes emerges. 144 refs., 9 figs., 1 tab.

Structural origin of the 5.16 eV optical absorption band in silica and Ge‐doped silica

Abstract: The origin of the 5.16 eV absorption band observed in silica and Ge‐doped silica was studied using optical and electron spin resonance (ESR) measurements. The band was observed only in samples containing Ge, suggesting that it is related to the Ge impurity in silica, while a lack of correlation between the ESR intensity of the induced hydrogen‐associated doublet and the absorption coefficient of the 5.16 eV band indicates that it is not related to two‐coordinated Si or Ge. The observation of the absorption coefficient increased as the square root of the Ge concentration demonstrates that the 5.16 eV band is not related to two‐coordinated Ge defects but that it is an oxygen deficiency center of the divacancy type associated with Ge.

Laser‐induced gratings in free jets. III. Saturation‐induced anharmonic gratings

Abstract: We apply two‐color laser‐induced grating spectroscopy (LIGS) to obtain excitation spectra of the S1(A 1B1)–S0(X 1A1) system of jet‐cooled azulene. The ground‐state depletion grating created by excitation with crossed beams is probed via either the S2(B 1A1)–S0(X 1A1) or S4(D 1A1)–S0(X 1A1) transitions. First‐order LIGS spectra of the S1–S0 origin band at several grating‐laser intensities reveal saturation‐induced line broadening and significant saturation dips at line center. We develop a simple model, based on the anharmonic grating created in the saturated regime, and use it to fit the first‐order data. From these fits we determine a saturation parameter and the linewidth of the S1–S0 origin absorption band, which is homogeneously broadened due to rapid internal conversion (τ∼1 ps) in the S1 state. We also observe LIGS spectra of the azulene origin taken at the second Bragg order of the induced grating; the observed second‐order lineshapes are also fit with the anharmonic‐grating model.