Showing papers on "Absorption spectroscopy published in 1984"

Reflectance spectroscopy: Quantitative analysis techniques for remote sensing applications

Abstract: The empirical methods and scattering theories that are important for solving remote sensing problems are among the methods for remotely sensed reflectance data analysis presently compared. In the case of the photon mean optical path length concept's implications for reflectance spectra modeling, it is shown that the mean optical path length in a particulate surface is in roughly inverse proportion to the square root of the absorption coefficient. Absorption bands, which are Gaussian in shape when plotted as true absorptance vs photon energy, are also Gaussians in apparent absorptance, although they have a smaller intensity. An apparent continuum in a reflectance spectrum is modeled as a mathematical function that is used to isolate a particular absorption feature for analysis, and it is noted that this continuum should be removed by dividing it into the reflectance spectrum.

Limiting efficiency of silicon solar cells

Abstract: The detailed balance method for calculating the radiative recombination limit to the performance of solar cells has been extended to include free carrier absorption and Auger recombination in addition to radiative losses. This method has been applied to crystalline silicon solar cells where the limiting efficiency is found to be 29.8 percent under AM1.5, based on the measured optical absorption spectrum and published values of the Auger and free carrier absorption coefficients. The silicon is assumed to be textured for maximum benefit from light-trapping effects.

Charge storage in doped poly(thiophene): Optical and electrochemical studies

Abstract: We present a new method of electrochemical polymerization of poly(thiophene) using dithiophene as the starting material, from which we obtain a high-quality film with a sharp interband absorption edge. An in situ study of the absorption spectrum during the electrochemical doping process has been carried out. In the dilute regime, the results are in detailed agreement with charge storage via bipolarons; weakly confined soliton pairs with confinement parameter $\ensuremath{\gamma}\ensuremath{\cong}0.1\ensuremath{-}0.2$. At the highest doping levels, the data are characteristic of the free-carrier absorption expected for a metal. From a parallel electrochemical voltage spectroscopy study, we find evidence of charge injection near the band edge and charge removal from the bipolaron gap states. In the dilute regime, the position of the chemical potential is consistent with charge storage in weakly confined bipolarons. The high Coulombic recovery over a charge-discharge cycle indicates that poly (thiophene) may be an excellent cathode-active material in battery applications.

Photochemical Hole Burning: A Spectroscopic Study of Relaxation Processes in Polymers and Glasses

Abstract: Photochemical hole burning is a special type of saturation spectroscopy in the optical domain having many analogies with NMR methods. The holes, which are burnt with laser irradiation, appear as small indentations in the absorption spectra of dye molecules which are doped into a polymer or glass in minute concentrations. Based on their narrow line width, photochemical holes can be regarded as highly sensitive spectroscopic probes. They can be used to detect small perturbations of the system by external parameters, giving rise to line-shifts and broadenings. Besides the many well documented, spectroscopic applications of hole burning, it may offer interesting future developments for the spectroscopy of biomolecules and for high-density data storage.

Vacuum-Field Rabi Splittings in Microwave Absorption by Rydberg Atoms in a Cavity

Abstract: The absorption spectrum of a system of $N$ atoms interacting with a single mode of the quantized radiation field is exactly calculated. Such a spectrum shows vacuum-field Rabi splittings, and thus microwave absorption by Rydberg atoms in a cavity should be a useful way to observe these splittings.

Measurement of soft X-ray absorption spectra with a fluorescent ion chamber detector

Abstract: The problem of absorption of soft X-rays by thick Be windows in hard X-ray beam lines is well known. Although the signal at 2.4 keV was reduced by ∼ 103 we have routinely measured the absorption spectra of S (2472 eV) and elements at higher energies including Cl, Ar and K. These spectra were obtained on hard X-ray beam lines at Stanford Synchrotron Radiation Laboratory (SSRL) with Si(111) monochromator crystals and a fluorescent ion chamber detector [1]. Higher energy harmonics were minimized by detuning and the end station was enclosed in a helium bag to prevent absorption by air. Although the diminished X-ray flux and decreasing fluorescent yield were serious negative factors at these low X-ray energies the spectra from thick samples were of excellent quality with sufficient sensitivity to characterize 1% S in coal. Representative spectra are shown comparing data from focused and unfocused beam lines and with S data from JUMBO [2]. Comparison of Ar and KCl data to excellent data found in the older literature [3–5] allow a confirmation of the resolution function (energy bandpass) of the monochromator. A simple new beam line is suggested which would allow a substantial increase in low energy X-ray flux (measurements down to Al and Si) with the sample and detector in a He atmosphere.

Photo-Chemistry of Colloidal Metal Sulfides 8. Photo-Physics of Extremely Small CdS Particles: Q-State CdS and Magic Agglomeration Numbers

Abstract: Extremely small CdS particles were prepared in propanol−2 solution at − 78°C and in aqueous solution in the presence of sodium hexametaphosphate at room temperature. These colloids are colorless. Their UV absorption spectra exhibit several maxima. Aging of the colloids is accompanied by intensity variations in the absorption maxima and by a shift of the onset of absorption to longer wavelengths. These small CdS particles hardly possess semiconductor properties (Q-state CdS). A semi-classical treatment of the energies of an electron-hole pair in these particles yielded the wavelengths of their absorption spectra. At the small particle sizes used, the first excited state was reached by photon-absorption in the UV, and the second excited state was generally not reached at all. The various maxima in the absorption spectra are explained in terms of a size distribution of the colloids with preferential agglomeration numbers. Reasons for the formation of such a structured size distribution are given. The fluorescence and fluorescence excitation spectra of the small particles were also investigated. Particles below a certain size have only one broad fluorescence band at a much longer wavelengths than the onset of absorption. As the particle size increases, this band is shifted towards longer wavelengths, and finally an additional rather sharp band appears at the threshold of absorption. CdS colloids in the Q-state can be made which fluoresce as desired anywhere between the red and the blue. Also reported are the first experiments in which the preparation of Q-type CdS in the solid state is achieved by evaporating the solvent from the colloidal solutions.

4.6 Micron absorption features due to solid phase CO and Cyano group molecules toward compact infrared sources

Abstract: Spectra obtained at a resolving power of 840, for seven protostellar sources in the region of the 4.67-micron fundamental vibrational band of CO, indicate that the deep absorption feature in W33A near 4.61 microns consists of three features which are seen in other sources, but with varying relative strength. UV-irradiation laboratory experiments with 'dirty ice' temperature cycling allow the identification of two of the features cited with solid CO and CO complexed to other molecules. Cyano group-containing molecules have a lower vapor pressure than CO, and can therefore survive in much warmer environments. The formation and location of the CO- and CN-bearing grain mantles and sources of UV irradiation in cold molecular clouds are discussed. Plausible UV light sources can produce the observed cyano group features, but only under conditions in which local heat sources do not cause evaporation of the CO molecules prior to their photoprocessing.

Dimers of aromatic molecules: (Benzene)2, (toluene)2, and benzene–toluene

Abstract: The optical absorption spectra of the first excited singlet states of the benzene, toluene, and toluene–benzene dimers, created in a supersonic molecular jet, are reported. The absorption spectra are detected through two‐color time of flight mass spectroscopy; this method eliminates fragmentation of dimers and higher clusters and the dimer spectra are uniquely observed. The benzene dimer observed in this experiment is suggested to have a parallel stacked and displaced configuration of C2h symmetry. Both the toluene and toluene–benzene dimers have two configurations: parallel stacked and displaced [based on (benzene)2] and perpendicular. (Benzene)2, (toluene)2, and toluene–benzene form excimers in the excited state for the parallel stacked displaced configurations. The transformation of (benzene)2 to the excimer takes place at the 00 with a ∼0 cm−1 barrier while the excimer is formed for toluene–benzene with a ∼900 cm−1 barrier. An exciton analysis of the (benzene)2 000 and 610 yields M12, the excitation e...

Chemistry Experiments for Instrumental Methods

Abstract: ELECTROCHEMICAL METHODS. Potentiometric Methods. Conductometric Methods. Controlled Potential Methods (Voltammetry). Electrolytic Methods and Controlled Current Methods. METHODS BASED ON ELECTROMAGNETIC RADIATION. Analytical Ultraviolet-Visible Absorption Spectroscopy. Absorption Spectroscopy of Electronic Transitions. Infrared Spectroscopy. Atomic Absorption and Atomic Emission Spectroscopy. Fluorescence Spectroscopy. Nuclear Magnetic Resonance Spectroscopy. SEPARATION METHODS. Gas Chromatography. High Performance Liquid Chromatography (HPLC). Other Separation Methods. Appendix.

Size quantization of the electron energy spectrum in a microscopic semiconductor crystal

Abstract: The interband absorption spectrum of microscopic CdS crystals ranging in size from ∼ 30 to 800 A and dispersed in a transparent insulating matrix has been studied. There is a significant (∼0.8-eV) shift of the fundamental absorption edge in the short-wavelength direction, and there are oscillations in the interband absorption spectrum caused by quantum size effect.

Photochemistry of Colloidal Metal Sulfides. 7. Absorption and Fluorescence of Extremely Small ZnS Particles (The World of the Neglected Dimensions)

Abstract: Extremely small colloidal ZnS particles (diameter ∼ 1.7 nm) were made by either photo-degradation of 3 nm particles or rapid precipitation in phosphate containing solution at pH = 7-−8. The absorption spectra of these particles are different from that of macrocrystalline ZnS, and the changes are regarded as an indication for the transition from semiconductor ZnS to polymolecular ZnS with decreasing particle size. – The dependence of the intensity of the fluorescence on particle size, temperature, photoanodic corrosion and the quenching of fluorescence were investigated for colloids on a silicon dioxide carrier, in phosphate solution, and without a stabilizer. Photo-anodic corrosion strongly improves the fluorescence properties. – One adsorbed Cd2+ ion per colloidal particle is sufficient for efficient quenching of the fluorescence. However, a new fluorescence band appears which is explained by the formation of a layer of 1:1 co-colloid at the surface of the ZnS particles. – Methylviologen was also found to be a very efficient quencher. The decay of the fluorescence is wavelength dependent, i.e. the fraction of long-lived fluorescence is greater at longer wavelengths. – A mechanism is discussed, where the fluorescence centers are anion vacancies, and fluorescence is emitted when electrons trapped in states of different energies and exhibiting different life-times tunnel to the localized positive holes.

SALT AND pH‐DEPENDENT CHANGES OF THE PURPLE MEMBRANE ABSORPTION SPECTRUM

Abstract: —Purple membrane suspensions change their color to blue and the absorption maximum shifts to 608 nm when the membrane is deionized on a cation exchange column or when it is washed first with < 2N NaCl followed by deionized water. The deionized chromophore is essentially identical with the chromophore produced by lowering the pH of the native membrane to < 4.0 (pK < 3.0). However, the deionized membrane does not aggregate and can be obtained in the pure state. The original purple color of the membrane is restored by addition of around 1 mM Na+, K+ or 10 μM Mg2+, Ca2+, Sr2+, Mn2+, Pb2+ or La2+ when the protein concentration is 5μM. The required salt concentrations decrease with decreasing pH. Direct measurement of bound Ca2+ by atomic absorption spectroscopy yields a ratio of Ca2+ to protein of <2 and a binding constant of 1.4 × 106. Titration of the spectral change with salts at different pH values shows a linear relation between the pH and the logarithm of the salt concentration, with a 1:1 ratio for Na+ and 1:2 ratio for Ca2+. These relations are well predicted by Gouy-Chapman theory; however, the accompanying release of protons, changes of the CD spectrum, the complex kinetics of the spectral change during reconstitution with salt and preliminary X-ray diffraction results all suggest that conformational changes may be occurring in the protein.

Catalysis of photochemical reactions by colloidal semiconductors

Abstract: Illumination of semiconductor colloids such as ZnS , CdS and Ti02, generates electrons and positive holes which may react with the aqueous solvent or dissolved substances In ZnS and CdS , recombination of the charge carriers is often accompanied by emission of light The absorption spectra of very small CdS or ZnS particles (1 3 nm) show changes which indicate a transition from semiconductor to polymolecular material The conduction band energy is shifted, and the exciton transition band becomes more pronounced The fluorescence spectra of small CdS particles contain a band at the onset of absorption and two bands at longer wavelengths They are attributed to emission from the short-lived exciton state and from longer-lived states of trapped electrons and holes The chemical reactions described include the photo-anodic and the photocathodic dissolution of colloidal sulfides, the reduction of water and carbon dioxide , and the oxidation of alcohols and sulfite anions Two-electron (two-hole) and one-electron (one-hole) transfer mechanisms are discussed Flash photolysis studies of colloidal Ti02 gave the absorption spectra of long-lived electrons and positive holes which are trapped in surface states of different energies Chemical reactions of these charge carriers are also described, including the reduction of tetranitromethane, the oxidation of SCN and OHanions, and of various organic compounds The electrochemical nature of all these photo-reactions is emphasized

Direct absorption spectroscopy of jet‐cooled polyenes. II. The 1 1B+u←1 1A−g transitions of butadienes and hexatrienes

Abstract: In the present paper, we report the direct absorption spectra of the 1 1B+u←1 1A−g transitions of gas phase butadiene, deuterated and methylated butadienes, and the cis and trans isomers of hexatriene cooled to low rotational and vibrational temperatures in supersonic molecular jets. These jet absorption spectra allow the more accurate determinations of Franck–Condon factors, upper state vibrational intervals and vibronic band homogeneous widths. We discuss the experimental constraints that the measurements reported here and in the previous paper of this series impose on theoretical models of the equilibrium structures and relaxation dynamics of the 1 1B+u excited states of the small linear polyenes.

Ground‐State Vibrational Energy Levels of Polyatomic Transient Molecules

Abstract: The experimentally determined ground‐state vibrational energy levels of approximately 480 covalently bonded transient molecules possessing from 3 to 16 atoms are tabulated, together with references to the pertinent literature. The types of measurement surveyed include laser‐based high resolution gas phase infrared absorption and visible‐ultraviolet emission techniques, ultraviolet photoelectron spectroscopy, and matrix isolation spectroscopy. An assessment of the magnitude of the uncertainty of observations in neon, argon, and nitrogen matrices is given.

Mixed Valence of Iron in Minerals with Cation Clusters

Abstract: The valence and distribution of iron in vivianite, lazulite, babingtonite, rockbridgeite, acmite, aegirine-augite, hedenbergite, and ilvaite were studied with optical and Mossbauer spectroscopy. Optically activated intervalence charge transfer between Fe2+ and Fe3+ in neighboring sites through common edges or faces is observed in all these minerals irrespective of the polymerization of the iron-oxygen polyhedra ranging from finite clusters to infinite structural units. However, a distinct decrease occurs in the energy of the corresponding optical absorption band with increasing number of Fe2+ and Fe3+ ions involved in the charge transfer process. Thermally activated electron delocalization between Fe2+ and Fe3+ occurs only if Fe2+ and Fe3+ occupy crystallographically equivalent or geometrically very similar neighboring sites which share common edges to form extended structural units such as the ribbon in ilvaite. If the Fe-O polyhedra form finite clusters of two, three, or four polyhedra (e.g., in vivianite, lazulite, and babingtonite, respectively) no thermally-activated mixed-valence states of iron are observed. In aegirine, extended regions of the M1 chain are statistically occupied by Fe2+ and Fe3+ giving rise to thermally-activated electron delocalization in addition to the intervalence band in the optical absorption spectrum. The intensity of the optical intervalence absorption has been measured in a number of systems: ɛ values range from 60 to 210.

Electronic spectra from molecular dynamics: A simple approach

Abstract: : A method is illustrated for computing the contours of electronic absorption bands from classical equilibrium or nonequilibrium molecular dynamics (or equally for equilibrium systems from Monte Carlo or explicit integration over coordinates). The inputs to the calculations are the potential energy curves for the different electronic states and the electronic transition dipole moments between the states as functions of nuclear coordinates. A simple quantum correction by temperature scaling is demonstrated for the thermal equilibrium case. A test is carried out for the I2 visible absorption spectrum involving transitions from the ground X 0+ sub G(1 sigma) to the excited A 1 sub u(3 Pi) 0+ sub u (3 Pi) and B u (II) states, for thermal equilibrium gas phase I2. The electronic band contours are computed and shown to be remarkably similar to the measured contours.

Anderson transition and intermediate polaron formation in WO3-xTransport properties and optical absorption

Abstract: The temperature dependence of electrical conductivities In WO3-x (O ≤ x ≤ 0·28) has been investigated. These compounds show metallic behaviour for x < 0·10 and non-metallic behaviour for x < 0·10. An Anderson transition takes place for the composition WO2·90 with complete polaronic transport in the non-metallic phase. Optical and infrared absorption spectra were recorded for all compounds and they showed that polarons and quasi-free carriers are present simultaneously in the metallic phase. A tentative band model is derived from the experimental absorption spectra and the transport properties, and the influence of metallic dopants is discussed. It is found that the formation of intermediate polarons is independent of the polaron concentration, confirming theoretical predictions of Wegner and Entin-Wohlmann (1983).

The resonance Raman spectrum of phenoxyl radical

Abstract: The Raman spectra of phenoxyl, phenoxyl‐d5, and phenoxyl‐2,4,6‐d3 radicals produced pulse radiolytically in aqueous solutions and observed by time resolved resonance Raman methods are reported. Excitation was mainly in the narrow and moderately intense (0,0) phenoxyl absorption band at 400 nm. These Raman spectra are superimposed on a broad fluorescence in the region of 410–440 nm which is also ascribed to the phenoxyl radical. A very intense Raman band, which is assigned to a mode principally involving the CO stretch (Wilson 7a), is observed at 1505 cm−1. In the fully and partially deuterated radicals this band is at 1489 and 1487 cm−1, respectively, indicating that in the latter instance the vibration is shifted toward a lower frequency, probably by Fermi resonance with an underlying weak vibration. In the protonated radical moderately intense bands are also observed at 990 and 528 cm−1. The higher of these, by virtue of its considerably lower frequency in the deuterated radicals, is assigned to a CH bending mode (Wilson 18a). The other is affected very little by the substitution and is assigned to a CCC bending mode (Wilson 6a). Weak bands at 1157, 1056, and 865 cm−1 in phenoxyl, phenoxyl‐2,4,6‐d3, and phenoxyl‐d5 radicals are assigned to a second CH bending mode (Wilson 9a). Two weak bands are also reported at 1398 and 1331 cm−1 and are attributed to the nontotally symmetric 19b and 14 modes. No Raman band is, however, observed in the 1550–1650 cm−1 region where the Wilson 8a ring stretching motion prominent in semiquinone radicals is expected. The excitation profile of the 1505 cm−1 emission largely follows the narrow absorption spectrum of the radical in the region of 400 nm. Below 390 nm, where the electronic excitation in resonance is to an upper vibrational level, resonance enhancement of the Raman signals is reduced by more than an order of magnitude. The decay of the radical at 10−4 M, as monitored by its Raman signal, corresponds to its loss mainly in second order processes having a rate constant of (2.6±0.3)×109 M−1 s−1, in agreement with measurements by absorption methods at a 50‐fold lower concentration.

Infrared absorption enhancement of monolayer species on thin evaporated Ag films by use of a Kretschmann configuration: evidence for two types of enhanced surface electric fields

Abstract: Infrared absorption enhancement of m- and p-nitrobenzoic acid deposited on thin-evaporated silver films has been investigated using the Kretschmann's ATR coupling method. The absorption spectra provide direct evidence that enhancement is prominent only for vibrations of the first monolayer adsorbed on the Ag surface. It is shown that all of the vibrational modes observed obey the normal dipole selection rule. Moreover, it was found that there exist two types of absorption enhancement; the first is enhanced (∼300) by both p- and s-polarized radiation, and the second is enhanced (∼60) by p-polarized radiation alone. The Ag film thickness optimum is about 50 A in the former case and about 200 A in the latter. The enhancement insensitive to the polarization state of radiation can well be explained by the excitation of the transverse collective electron resonance of the Ag islands, whereas that obtained by p-polarized radiation may be due to the excitation of delocalized surface plasmons modified by surface roughness. The image-dipole effect may also be significant in the mechanisms.

Extracting physical structure information from thin organic films with reflection absorption infrared spectroscopy

Abstract: A RATIO method has been developed to extract physical structure information from anisotropic thin organic films on metal substrates using reflection absorption infrared spectroscopy (RAIR). The physical structure information referred to are the Euler angles, θ and ψ, specifying the molecular orientation relative to the surface normal and f, the fraction of material oriented in that manner. Such films are viewed as consisting of polycrystalline domains which have a natural tendency to orient preferentially with respect to the substrate normal direction. Two equations are developed involving these three physical structure parameters and two measured quantities obtained from the thin‐film RAIR spectra. These measured quantities are derived from the ratios of band absorbances, the transition moments of which belong to different symmetry groups. The method is tested by applying it to vacuum‐deposited metal‐free phthalocyanine (H2Pc) films on air‐oxidized aluminum. The molecular orientation derived in the α‐H2P...

On the thermal broadening of zero‐phonon impurity lines in absorption and fluorescence spectra

Abstract: We examine the problem of the absorption zero‐phonon line shape for dilute impurities in crystals. We consider the usual two level electronic model, where both the ground and excited state Born–Oppenheimer surfaces are harmonic in the phonon coordinates. The difference between the two surfaces (the electron–phonon interaction) has terms which are both linear and quadratic in the phonon coordinates. In contrast to the usual perturbative theories, we calculate the zero–phonon line broadening and shift to all orders in the electron–phonon interaction. We find that only the quadratic term is responsible for line broadening, and that at T=0 K this contribution vanishes. Our results are presented as integrals, which can be performed analytically or numerically, involving the weighted phonon density of states. We also show that within the model, the zero‐phonon lines in the absorption and fluorescence spectra coincide exactly for all temperatures. Our results resolve the theoretical controversy produced by the t...

Infrared birefringence of liquid crystals

Abstract: The first measurements of infrared birefringence of liquid crystals are reported. Continuous birefringence spectral data were calculated from measurements of phase differences which occur when monochromatic polarized light propagates through a medium with an anisotropic refractive index. It was found that molecular absorption bands can provide significant resonant enhancement of the refractive indices of liquid crystals, and this effect causes the birefringence of these materials to be relatively large throughout the spectral region from 2 to 16 μm. The birefringence and absorption spectra of two particular liquid crystal mixtures indicate that liquid crystals will be useful for electro‐optic applications in the infrared region.

Frequency stabilization of 1.5‐μm InGaAsP distributed feedback laser to NH3 absorption lines

Abstract: NH3 absorption lines due to vibration‐rotation transitions are observed at 1.50–1.54 μm by using an InGaAsP superluminescent diode. A 1.5‐μm InGaAsP distributed feedback (DFB) laser is frequency stabilized to an NH3 linear absorption line at 15196 A. Frequency stability of σ(2,τ)=8×10−11τ−1 is achieved for an averaging time range of 10 ms≤τ≤1 s. Such an absolute frequency‐stabilized DFB laser is useful for coherent optical system applications, since it is free from the longitudinal mode jumping which results from a wide range of temperature changes and long‐term device degradation.