Showing papers on "Absorption spectroscopy published in 1991"

Ultraviolet–visible absorption spectra of the colloidal metallic elements

Abstract: The ultraviolet–visible absorption spectra are given for 10 nm diameter colloidal particles of 52 of the metallic elements, calculated from the optical constants of the metals by means of Mie theory. For most of the elements the spectra cover the range 200–900 nm. Well resolved absorption bands are observed for colloidal Sc, Ti, V, Y, Cd, Eu, Yb, Hg and Th as well as for colloids of Cu, Ag, Au and the s-block metals. However, for the majority of the colloidal metallic elements in this size range there is only a continuous absorption in the visible range, rising to broad and poorly resolved absorption bands in the ultraviolet near 200 nm. The difference in the way that the spectra of colloidal particles of different metals change when the particle shape is varied from spherical to spheroidal is investigated systematically in the dipole approximation. This is achieved by means of contour plots of the absorbance cross-section for the particles vs. the real and imaginary parts of the dielectric function for the metals, and this method of investigation is extended also to hollow spherical particles. The results suggest that Ca, Sr, Ba, Eu, Yb, Th and possibly Sc, Ti, V and Y may merit experimental investigation as new metals for exhibiting surface-enhanced Raman scattering.

Photophysical properties of C60

Abstract: A number of important photophysical properties of C{sub 60} have been determined, including its lowest triplet-state energy (near 33 kcal/mol), lifetime, and triplet-triplet absorption spectrum. The triplet state is formed in near quantitative yield and produces a very high yield of singlet oxygen by energy transfer. C{sub 60} does not react with singlet molecular oxygen and quenches it only slowly by an unknown mechanism. These results are discussed in terms of the unusual geometry of this molecule.

A description of the correlated k distribution method for modeling nongray gaseous absorption, thermal emission, and multiple scattering in vertically inhomogeneous atmospheres

Abstract: A radiative transfer method for treating nongray gaseous absorption and thermal emission in vertically inhomogeneous multiple scattering atmospheres is described. Probability density distributions of absorption coefficient strength are derived from line-by-line calculations to construct line-by-line and band model based k distributions. The monotonic ordering of absorption coefficient strengths in these k distributions implicitly preserves the monochromatic structure of the atmosphere at different pressure levels, thus simulating monochromatic spectral integration at a fraction of the line-by-line computing cost. The k distribution approach also permits accurate modeling of overlapping absorption by different atmospheric gases and accurate treatment of nongray absorption in multiple scattering media. It is shown that the correlated k distribution method is capable of achieving numerical accuracy to within 1 percent of cooling rates obtained with line-by-line calculations throughout the troposphere and most of the stratosphere.

The analysis of apparent optical depth profiles for interstellar absorption lines

Abstract: Attention is given to the apparent optical depth method, a procedure for analyzing interstellar absorption lines. Observed absorption-line profiles are converted into profiles of apparent optical depth, and apparent column density per unit velocity. By comparing the latter for a given interstellar species having two or more absorption lines which differ in the product, the presence or absence of unresolved saturated structure in the profiles can be directly inferred. The method is illustrated using absorption-line data from the IUE satellite for the highly ionized gas toward HD 64760. Additional illustrations and a study of the method's accuracy are provided through a series of numerical simulations of multicomponent interstellar absorption situations. The method is compared to the standard growth curve method for deriving interstellar column densities. The principal value of the apparent optical depth method is that the absorption-line data are directly converted into a form that provides for direct scientific interpretations of the physical conditions in the interstellar absorbing medium as a function of velocity.

From femtoseconds to biology: mechanism of bacteriorhodopsin's light-driven proton pump.

Abstract: Bacteriorhodopsin is a retinal-containing protein that functions as a light-driven proton pump. Resonance Raman and femtosecond dynamic absorption spectroscopy are being used to elucidate the molecular mechanism of bacteriorhodopsin. The primary photochemical process is atrans- to-cis isomerization about the C13=C14 bond of the retinal chromophore that has been directly observed using femtosecond dynamic absorption spectroscopy. The excited state isomerization dynamics can be quantitatively analyzed using a new theory for nonstationary state spectroscopy. Resonance Raman vibrational spectroscopy has been used to determine the structure of the chromophore in each of bacteriorhodopsin’s intermediates and to analyze the kinetics of the photocycle. These results are integrated into an explicit molecular model (the C-T Model) for proton pumping in bacteriorhodopsin.

Strong magnetic-x-ray dichroism in 2p absorption-spectra of 3d transition-metal ions

Abstract: From atomic calculations in crystal-field symmetry we find a very strong circular and linear dichroism in the 2p x-ray absorption edges of magnetically ordered 3d transition-metal ions. The spectral shape changes drastically with the character of the ground state, which is determined by the presence of a crystal field, and the relative magnitudes of the exchange and 3d spin-orbit interaction. The difference in integrated intensity of the 2p absorption for left- and right-circular polarization provides a measure for the 3d spin-orbit coupling in spherical and crystal-field symmetry. The anisotropic branching ratio depends on the spin-orbit interaction and the Van Vleck contribution of the exchange interaction. The results of our calculations provide a basis for the study of magnetic x-ray dichroism on materials with localized 3d electrons.

The melanosome: threshold temperature for explosive vaporization and internal absorption coefficient during pulsed laser irradiation.

Abstract: The explosive vaporization of melanosomes in situ in skin during pulsed laser irradiation (pulse duration less than 1 microsecond) is observed as a visible whitening of the superficial epidermal layer due to stratum corneum disruption. In this study, the ruby laser (694 nm) was used to determine the threshold radiant exposure, H0 (J/cm2), required to elicit whitening for in vitro black (Negroid) human skin samples which were pre-equilibrated at an initial temperature, Ti, of 0, 20, or 50 degrees C. A plot of H0 vs Ti yields a straight line whose x-intercept indicates the threshold temperature of explosive vaporization to be 112 +/- 7 degrees C (SD, N = 3). The slope, delta H0/delta Ti, specifies the internal absorption coefficient, mua, within the melanosome: mua = -rho C/(slope(1 + 7.1 Rd)), where rho C is the product of density and specific heat, and Rd is the total diffuse reflectance from the skin. A summary of the absorption spectrum (mua) for the melanosome interior (351-1064 nm) is presented based on H0 data from this study and the literature. The in vivo absorption spectrum (380-820 nm) for human epidermal melanin was measured by an optical fiber spectrophotometer and is compared with the melanosome spectrum.

Polarization Modulation FT-IR Spectroscopy of Surfaces and Ultra-thin Films: Experimental Procedure and Quantitative Analysis

Abstract: Polarization modulation of the incident electromagnetic field is used to increase the sensitivity and the in situ experiment ability of the well-known method of reflexion absorption FT-IR spectroscopy, for the characterization of surfaces and ultra-thin films. The experimental procedure and signal processing of the detected intensity are described and illustrated with the use of results obtained with Langmuir-Blodgett monolayers. The quantitative analysis of the spectra is then developed, and a linear behavior of the band intensities is found for ultra-thin films exhibiting no strong absorptions. This result is checked with the use of organic and inorganic ultra-thin films of increasing thicknesses.

Deposition and characterization of fullerene films

Abstract: Thermal sublimation of pure C60 and C70 has been used for depositing well‐characterized fullerene films on a variety of substrates. Film purity is determined by infrared absorption spectra and the extent of crystallinity of the face‐centered cubic structure by x rays. Thickness‐dependent optical and electrical measurements reveal uniform films over the thickness range 200–1000 A. We obtain optical absorption coefficients having values between those of Si and Ge and a relative permittivity having a value close to that of amorphous SiO2.

Chemical etching of vicinal Si(111): Dependence of the surface structure and the hydrogen termination on the pH of the etching solutions

Abstract: Infrared spectroscopy is used to study the etching process of stepped Si(111)9° surfaces as a function of the pH of the etching HF solutions. This process results in complete H termination of the silicon surface, including terraces, steps, and defects; the surface structure can therefore be well studied using infrared (IR) spectroscopy. Polarized IR absorption spectra of the Si–H stretching vibrations (i.e., in the region 2060–2150 cm−1) vary dramatically as the pH of the etching solutions increases from 2.0 to 7.8. In general, higher pH solutions yield sharper bands and more easily assigned spectra, making it possible to identify the step and terrace species and thus to infer the surface structure and step morphology (i.e., to investigate the etching process). The data are explained by a model involving different etching rates for each individual surface species: The highest rate of removal is for isolated adatom defects located on (111) planes and the lowest is for the ideally H‐terminated (111) planes ...

Direct Observation of Vibrational Coherence in Bacterial Reaction Centers Using Femtosecond Absorption Spectroscopy

Abstract: It is shown that vibrational coherence modulates the femtosecond kinetics of stimulated emission and absorption of reaction centers of purple bacteria. In the DLL mutant of Rhodobacter capsulatus, which lacks the bacteriopheophytin electron acceptor, oscillations with periods of approximately 500 fs and possibly also of approximately 2 ps were observed, which are associated with formation of the excited state. The kinetics, which reflect primary processes in Rhodobacter sphaeroides R-26, were modulated by oscillations with a period of approximately 700 fs at 796 nm and approximately 2 ps at 930 nm. In the latter case, at 930 nm, where the stimulated emission of the excited state, P*, is probed, oscillations could only be resolved when a sufficiently narrow (10 nm) and concomitantly long pump pulse was used. This may indicate that the potential energy surface of the excited state is anharmonic or that low-frequency oscillations are masked when higher frequency modes are also coherently excited, or both. The possibility is discussed that the primary charge separation may be a coherent and adiabatic process coupled to low-frequency vibrational modes.

Time-Resolved FT-IR Absorption Spectroscopy Using a Step-Scan Interferometer

Abstract: The implementation of time-resolved step-scan FT-IR spectroscopy with a commercial interferometer is described. With the use of the photoreaction of the biological system bacteriorhodopsin as an example which exhibits infrared spectral changes smaller than 10−2 absorbance units, the quality of the method is demonstrated. A comparison with conventional flash-photolysis experiments with a monochromatic infrared monitoring beam clearly demonstrates the multiplex advantage. The advantage of covering the total time course of the reaction allows for a variety of data analysis, such as forming difference spectra between intermediates of the reaction and the deduction of time courses of absorbance changes at selected wavenumbers. The mirror stability is better than ±1.5 nm, which is sufficient for the reliable measurement of small absorbance changes.

Techniques for characterization of electrodes and electrochemical processes

Abstract: STRUCTURE AND COMPOSITION OF ELECTRODES AND ELECTROLYTES: Techniques for the Study of Solid Ionic Conductors Ellipsometry as an In Situ Probe for Study of Electrode Processes In Situ Characterization of Electrode Processes by Photothermal Deflection Spectroscopy Surface X-Ray Absorption Spectroscopy, EXAFS and NEXAFS for the In Situ and Ex Situ Study of Electrodes Surface-Enhanced Raman Spectroscopy In Situ Characterization of Electrodes by Neutron Scattering Diffraction Techniques for Determining the Structure of Electrolytes in the Liquid Phase Moessbauer Spectroscopy Laser Interferometry Study of Mass Transport Limitation in Electro-Deposition of Metals/Polymers. ELECTROCHEMICAL MEASUREMENT TECHNIQUES: An Introduction to Electrochemistry in Molten Salts Application of Electrochemical Impedance Spectroscopy in Electrochemistry and Corrosion Science Electrochemical Impedance Spectroscopy (EIS)- Application in Corrosion Science and Technology Numerical Techniques for Modeling Current Distribution at Plane or Flow-By Electrodes Experimental Simulation of Porous Electrodes Techniques for Study of Nucleation during Metal Electrodisposition.

Tracing the conversion of aurichalcite to a copper catalyst by combined X-ray absorption and diffraction

Abstract: EVER since X-ray sources first became available, the merit of deploying diffraction and absorption spectroscopic studies simultaneously has been acknowledged1. Information on oxidation states and local (~6-A radius) atomic environments is now obtained routinely from X-ray absorption measurements using synchrotron sources2–4. Synchrotron radiation is also used commonly for high-resolution powder diffraction crystallography. We report here an instrumental arrangement that has allowed us to extract quantitative short- and long-range structural information on samples undergoing chemical change by measuring X-ray absorption spectra and X-ray diffraction patterns in situ and within a few seconds of one another, using a synchrotron X-ray source. To illustrate the combination of these techniques, we have followed the structural and chemical changes that occur within the layered mineral aurichalchite (Cu5−xZnx(OH)6(CO3)2) when heated in dry air to ~ 450 °C. Despite marked changes in crystallinity, the local environment and electronic state of the Cu2+ ions remain unchanged, even when at ~ 450 °C the material is converted to a mixture of CuO and ZnO. Heating this mixture in H2/N2 produces an active catalyst for the water-gas shift reaction (CO2 + H2 → CO + H2O), which our studies show to consist of small particles of copper metal (with some zinc incorporated) supported on ZnO.

Ligand Field Strengths and Oxidation States from Manganese L-Edge Spectroscopy

Abstract: L,, X-ray absorption spectra have been recorded for Mn(II), Mn(III), and Mn(IV) samples with a variety of ligands. For high-spin Mn(I1) complexes, a systematic variation in spectra is observed as the ligand field is increased. A dramatically different spectrum is observed for Mn(CN)6", consistent with the presence of a low-spin complex. Progressing in oxidation state from Mn(1I) to Mn(II1) through Mn(IV) complexes, the primary peak position shifts first 1.5-2 eV and then 1-2 eV to higher energy, and the ratio of L, to L2 intensity decreases. The spectra have been quantitatively simulated with an atomic multiplet program with an octahedral crystal field superimposed. The high resolution, strong sensitivity to chemical environment and amenability to quantitative spectral shape analysis indicate that L-edges of the first transition series metals are a potentially useful probe for bioinorganic studies.

Electroabsorption (stark effect) spectroscopy of mono- and biruthenium charge-transfer complexes : measurements of changes in dipole moments and other electrooptic properties

Abstract: Electroabsorption (Stark effect) spectra are reported for the charge-transfer transitions of (NH3)SRuL2+ and [(NH3)5R~]2L4+*5+, where L is pyrazine (pz) or 4,4'-bipyridine (4,4'-bpy). The spectra permit experimental estimates of the susceptibility of the transition dipole moment to an electric field, the change in polarizability (TrAu), and the magnitude of the change in permanent electric dipole moment (IApI) associated with many of the metal-to-ligand and metal-to-metal charge-transfer (MLCT and MMCT, respectively) transitions in these complexes. The observed electroabsorption spectra of the MLCT transitions of the monoruthenium complexes are interpreted as arising predominantly from Au and Ap. When L = pz and 4,4'-bpy, the observed values of IApl are (5.3 0.8)lfand (15.8 * 0.2)/fD, respectively, compared with the values of 16.5 and 27.1 D expected for full charge transfer from the metal to the geometric center of the ligand (fis a local electric field correction). Protonation of the monoruthenium complexes has relatively small effects on the observed Au and Ap when L = 4,4'-bpy, but when L = pz, Au appears to change its sign while Ap virtually disappears. A simple electrostatic model qualitatively accounts for the results and indicates that pyrazine allows a much greater degree of delocalization from the ruthenium than 4,4'-bipyridine whose pyridyl rings are probably not coplanar. The electroabsorption spectra of the MLCT region of the biruthenium complexes are very complicated and not quantitatively interpretable on the basis of current information, though interesting and qualitatively suggestive features appear. For the MMCT transitions in the biruthenium mixed-valence complexes where L is pz and 4,4'-bpy, the observed values of lApl are (0.7 * O.l)/fand (28.5 A 1.5)/fD, respectively, compared with the values of 32.7 and 54.3 D expected fur charge transfer in fully localized complexes. These latter results demonstrate that electronic delocalization between the two metals is essentially complete when the bridging ligand is pyrazine, whereas it is significant but incomplete when the bridge is 4,4'-bipyridine. In all complexes, the angle dependence of the electroabsorption demonstrates that Ap and the other field-interactive molecular properties are parallel to the transition dipole moment, as expected if the transition moment lies along the metal-ligand axis. The electroabsorption spectra of all of the monoruthenium and [(NH3)5R~]2L4+ complexes also show evidence for transitions that are weak or obscured in conventional absorption spectra; possible assignments are discussed within the context of a molecular orbital model.

Transient absorption studies of C60 in solution

Abstract: Visible transient absorption spectra of C{sub 60} in solution are presented for time delays of 15 ps to 30 ns following the excitation of the ground-state molecule. These spectra demonstrate the presence of a hitherto unreported strong absorption feature in the triplet spectrum peaking near 740 nm. Kinetics measurements near the peak of this absorption band yield a singlet state lifetime of 650 {plus minus} 100 ps.

Luminescence and Absorption Spectra of C sub 60 Films

Abstract: : Solid films of C60 deposited on CaF2 under ultra-high vacuum show luminescence in the 700-1100nm spectral range at 20K. A 1400 cm-1 progression in the ag soccerball inflation mode is observed. The low-temperature absorption spectrum exhibits similar fine structure. The characterization of vibronic splittings in the ground and emitting state and the transition intensity mechanism are discussed.

A molecular dynamics study of the far infrared spectrum of liquid water

Abstract: The far infrared spectrum of liquid water at room temperature is calculated by molecular dynamics simulation over the spectral range 0.5–1000 cm−1. It is shown that the experimental absorption intensity can be reproduced satisfactorily provided that the dipole induced dipole mechanism is conveniently implemented in the calculation and the classical profile corrected for quantum effects. The contribution due to exchange overlap dipoles between O and H atoms is also investigated but its role in the genesis of the far infrared (FIR) spectrum is negligible. Although the dipole induced dipole (DID) mechanism is found to be responsible for the peculiar band shape near 200 cm−1 by revealing the intermolecular oscillations of the hydrogen bond network, no other translational band is detected in the region 10–60 cm−1, a result in contradistinction with data put forward recently. Moreover, it is shown that the absorption spectrum is the seat of various cancellation effects between permanent and induced dipoles, eff...

Electronic states and phases of KxC60 from photoemission and X-ray absorption spectroscopy

Abstract: HIGH-resolution photoemission and soft X-ray absorption spectroscopies have provided valuable information on the electronic structure near the Fermi energy in the superconducting copper oxide compounds1–4, helping to constrain the possible mechanisms of superconductivity. Here we describe the application of these techniques to KxC60, found recently to be superconducting below 19.3 K for x ≈ 3 (refs 5–7). The photoemission and absorption spectra as a function of x can be fitted by a linear combination of data from just three phases, C60, K3C60 and K6C60 indicating that there is phase separation in our samples. The photoemission spectra clearly show a well defined Fermi edge in the K3C60 phase with a density of states of 5.2 x 10-3 electrons eV-1 A-3 and an occupied-band width of 1.2 eV, suggesting that this phase may be a weakly coupled BCS-like (conventional) superconductor. The Cls absorption spectra show large non-rigid-band shifts between the three phases with half and complete filling, in the K3C60 and K6C60 phases respectively, of the conduction band formed from the lowest unoccupied molecular orbital of C60. These observations clearly demonstrate that the conduction band has C 2p character. The non-rigid-band shift coupled with the anomalous occupied-band width implies that there is significant mixing of the electronic states of K and C60 in the superconducting phase.

Dissociation of small methanol clusters after excitation of the O–H stretch vibration at 2.7 μ

Abstract: Molecular beam depletion spectroscopy has been employed to study the dissociation of small methanol clusters upon excitation of the O–H stretch vibration at 2.7 μ. The tunable infrared radiation has been obtained from a Nd:YAG laser pumped optical parametric oscillator. Pure methanol dimer spectra, without contamination from larger clusters, have been measured using the scattering selection technique or working at reduced temperatures. The dimer spectrum features two absorption peaks at 3574.4 and 3684.1 cm−1. The lower frequency peak is redshifted by 106.6 cm−1 from the monomer value and is assigned to the excitation of the proton donor in the hydrogen bonded complex. The proton acceptor peak is blueshifted by only 3.1 cm−1. Methanol trimer dissociation is observed at 3462 cm−1, whereas larger clusters absorb at still lower frequencies. Absolute photodissociation cross sections were measured for both methanol dimer bands. The integrated dissociation cross sections were 5.59(25)×10−21 cm2 /molecule and 7.1(2.2)×10−22 cm2 /molecule for the proton donor and acceptor bands, respectively. The integrated dissociation cross sections are directly related to the O–H line strengths in the dimer. The increased line strength, for the proton donor band relative to the acceptor band by a factor of ∼8, is in accord with previous observations associated with hydrogen bond formation.

Opacity measurements in a hot dense medium

Abstract: Measurements of the opacity of aluminum in a well characterized, hot, dense, laser produced plasma are reported. Measurements of the absorption of X-rays by 1 to 2 transitions in Al XII through Al VIII have been made in a laser-heated slab plasma at the measured temperature and density of 58 ^ 4e V and 0.020 ^ 0.007 g cm~3. Separate measurements of the temperature and density were made. The con- ditions in the plasma were determined to be reproducible, spatially uniform, and in nearly complete local thermodynamic equilibrium. The absorption spectra and the temperature-density data obtained provide an improved means for comparison with detailed atomic physics and opacity calculations. Subject headings: atomic datamethods: laboratoryplasmasX-rays: general

A novel small molecular luminescent gelling agent for alcohols

Abstract: 2,3-Bis-n-decyloxyanthracene can form thermoreversible gels with alcohols at very low concentrations; electronic absorption spectra and fluorescence emission studies suggest that the network of the gel consists of head-to-tail assemblies of the aromatic component coexisting with the alcohol, the organization of the system essentially relying upon dipolar forces and van der Waals interactions.