Showing papers on "Absorption spectroscopy published in 1988"

Cavity ring‐down optical spectrometer for absorption measurements using pulsed laser sources

Abstract: We have developed a technique which allows optical absorption measurements to be made using a pulsed light source and offers a sensitivity significantly greater than that attained using stabilized continuous light sources. The technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined within a closed optical cavity. The decay of the light intensity within the cavity is a simple exponential with loss components due to mirror loss, broadband scatter (Rayleigh, Mie), and molecular absorption. Narrowband absorption spectra are recorded by scanning the output of a pulsed laser (which is injected into the optical cavity) through an absorption resonance. We have demonstrated the sensitivity of this technique by measuring several bands in the very weak forbidden b1Σg−X3Σg transition in gaseous molecular oxygen. Absorption signals of less than 1 part in 106 can be detected.

The photodissociation and chemistry of interstellar CO

Abstract: Recent work on the vacuum UV absorption spectrum of CO to the description of the photodissociation of interstellar CO and its principal isotopic varieties is discussed. The effects of line broadening, self-shielding, shielding by H and H2, and isotope-selective shielding are examined as functions of depth into interstellar clouds. The photodissociation rates of the isotopic species are larger than that of (C-12)O inside the clouds by up to one to two orders of magnitude. A simple approximation to the attenuation by line absorption is given in tabular form. Computed abundances of CO and related species C and C+ are presented for a variety of interstellar clouds ranging from diffuse clouds to dense photodissociation regions. Several series of models of translucent clouds are presented which illustrate how the CO abundance increases rapidly with total cloud thickness. The variations of the isotopic abundances with depth and their sensitivity to temperature and total cloud thickness are explored in detail.

Preparation and characterization of quantum-size titanium dioxide

Abstract: The syntheses of transparent colloidal solutions of extremely small titanium dioxide particles (d < 3 nm) in water, ethanol, 2-propanol, and acetonitrile are presented. Quantum-size effects are observed during particle growth and at the final stages of synthesis. They are quantitatively interpreted by using a quantum mechanical model developed by Brus. The particles prepared in aqueous solution possess the anatase structure and consist of about 200 TiO_2 molecules at their final growth stage. The colloidal particles can be isolated from solution as white powders that are soluble in water and ethanol with no apparent change in their properties. In organic solvents the quantum-size TiO_2 particles appear to form with rutile structure. Excess negative charge on the particles resulting either from deprotonated surface hydroxyl groups or from photogenerated or externally injected charge carriers causes a blue shift in the electronic absorption spectrum, which is explained by an electrostatic model. Electrons can be trapped in the solid as a Ti^(3+) species, which has a characteristic visible absorption spectrum. As much as 10% of the available Ti^(4+) ions can be reduced photochemically in the solid with a quantum yield of 3%. Molecular oxygen reoxidizes the Ti^(3+) centers, leading to detectable amounts of surface-bound peroxides. The pH of zero point of charge (pH_(zpc)) of the aqueous colloidal suspension has been determined to be 5.1 ± 0.2. The acid-catalyzed dissolution of the aqueous colloid yielding Ti(IV) oligomers has been studied, and an activation energy E_a = 58 ± 4 kJ/mol has been measured for this reaction. The photocatalytic activity of the small TiO_2 particles is demonstrated.

Direct observation of the femtosecond excited-state cis-trans isomerization in bacteriorhodopsin

Abstract: Femtosecond optical measurement techniques have been used to study the primary photoprocesses in the light-driven transmembrane proton pump bacteriorhodopsin. Light-adapted bacteriorhodopsin was excited with a 60-femtosecond pump pulse at 618 nanometers, and the transient absorption spectra from 560 to 710 nanometers were recorded from -50 to 1000 femtoseconds by means of 6-femtosecond probe pulses. By 60 femtoseconds, a broad transient hole appeared in the absorption spectrum whose amplitude remained constant for about 200 femtoseconds. Stimulated emission in the 660- to 710-nanometer region and excited-state absorption in the 560- to 580-nanometer region appeared promptly and then shifted and decayed from 0 to approximately 150 femtoseconds. These spectral features provide a direct observation of the 13-trans to 13-cis torsional isomerization of the retinal chromophore on the excited-state potential surface. Absorption due to the primary ground-state photoproduct J appears with a time constant of approximately 500 femtoseconds.

Branching ratio in x-ray absorption-spectroscopy

Abstract: The origin of nonstatistical branching ratios in spin-orbit-split x-ray absorption spectra is explained. Atomic calculations for transition metals show a systematic change which is due to initial-state spin-orbit splitting and electrostatic interactions between core hole and valence electrons. We have formulated the results of these atomic calculations in general rules, which are also applicable to solids. In the free atom the branching ratio reaches a maximum for the Hund's-rule ground state and its value decreases gradually for S, L, and J levels of higher energy. The presence of a crystal field results in a lower branching ratio when it produces a low-spin ground state. The rules can be used to assess the spin state and the spin-orbit splitting from the experimental branching ratio in transition-metal and rare-earth compounds. A specific example is given for the influence of second-order spin-orbit interactions in high-spin Ni compounds.

Fourier transform infrared spectroscopy of 13C = O-labeled phospholipids hydrogen bonding to carbonyl groups.

Abstract: Fourier transform infrared spectroscopy has been used to characterize the carbonyl stretching vibration of DMPC, DMPE, DMPG, and DMPA, all labeled with 13C at the carbonyl group of the sn-2 chain. Due to the vibrational isotope effect, the 13C = O and the 12C = O vibrational bands are separated by ca. 40-43 cm-1. This frequency difference does not change when the labeling is reversed with the 13C = O group at the sn-1 chain. For lipids in organic solvents possible conformational differences between the sn-1 and sn-2 ester groups have no effect on the vibrational frequency of the C = O groups. In aqueous dispersion unlabeled phospholipids always show a superposition of two bands for the C = O vibration located at ca. 1740 and 1727 cm-1. These two bands have previously been assigned to the sn-1 and sn-2 C = O groups. FT-IR spectra of 13C-labeled phospholipids show that the vibrational bands of both, the sn-1 as well as the sn-2 C = O group, are clearly superpositions of at least two underlying components of different frequency and intensity. Band frequencies were determined by Fourier self-deconvolution and second-derivative spectroscopy. The difference between the component bands is ca. 11-17 cm-1. Again, the conformational effect as shown by reversed labeling is negligible with only 1-2 cm-1. The splitting of the C = O vibrational bands in H2O and D2O is caused by hydrogen bonding of water molecules to both C = O groups as shown by a comparison with spectra of model ester compounds in different solvents. To extract quantitative information about changes in hydration, band profiles were stimulated with Gaussian-Lorentzian functions. The chemical nature of the head group and its electronic charge have distinctive effects on the extent of hydration of the carbonyl groups. In the gel and liquid-crystalline phase of DMPC the sn-2 C = O group is more hydrated than the sn-1 C = O. This is accord with the conformation determined by X-ray analysis. In DMPG the sn-1 C = O group seems to be more accessible to water, indicating a different conformation of the glycerol backbone.

Photochemistry and radiation chemistry of colloidal semiconductors. 23. Electron storage on zinc oxide particles and size quantization

Abstract: Improved methods for the preparation of colloidal ZnO solutions of different particle size are described, and the relation between absorption threshold and particle size is reported. CH/sub 2/OH radicals, radiolytically generated, transfer electrons to ZnO particles. The electrons are long-lived and cause a substantial blue shift of the absorption spectrum of ZnO in a wavelength range of 60 nm below the threshold. The wavelength of maximum bleaching is shifted to shorter wavelengths with decreasing particle size (size quantization effect). Maximum bleaching occurs with a negative absorption coefficient of 1.1 x 10/sup 5/ M/sup -1/ cm/sup -1/. Electrons are also stored upon UV illumination of colloidal ZnO. The stored electrons react rather slowly with oxygen, the rate constant becoming lower with increasing particle size, and more rapidly with peroxy radicals.

Spectroscopic evidence for pressure-induced coordination changes in silicate glasses and melts.

Abstract: Infrared spectra demonstrate that at pressures above 20 gigapascals and room temperature the regular tetrahedral coordination of oxygen around both silicon and aluminum ions is severely disrupted in SiO2, CaMgSi2O6, and CaAlSi2O8 composition glasses. The spectra are consistent with gradual, pressure-induced increases in the coordination numbers of silicon and aluminum. A variety of coordination environments, from sixfold to fourfold, appears to be present at pressures as high as about 40 gigapascals. This apparent change in coordination is not quenchable at room temperature: on decompression, the glasses return to tetrahedral coordination. This continuous and reversible coordination change in amorphous silicates explains the lack of observation of coordination changes in silicate glasses quenched from high pressure, the shallow melting slopes observed for mantle silicates at high pressures, and the possible presence of neutrally buoyant magmas deep within the terrestrial planets.

Visible spectroscopy of methylene blue on hectorite, laponite B, and barasym in aqueous suspension

Abstract: The absorption spectra of methylene blue ion exchanged on hectorite, Laponite B, Barasym, or sepiolite in dilute aqueous suspensions show the presence of the monomer, the protonated monomer, the dimer, and the trimer. Due to conformational differences, the absorption band maximum of the monomer with respect to its maximum in aqueous solution is red shifted when it is adsorbed on the external surface and blue shifted when it is adsorbed on the interlamellar surface. The availability of the interlamellar surface for methylene blue as a function of the type of clay and/or the counterion present thereby can be probed. The results indicate that 0.6-0.7% of the cation-exchange capacity of Barasym consists of acid sites capable of protonating methylene blue. Counterions of low hydration energy were found to induce a small number of similar sites in hectorite and Laponite B; hence, these sites must be situated on the external surface. Dimers formed on external surfaces show one absorption band. Dimers formed on the interlamellar surface of hectofite yield spectra having two absorption bands. The trimer was formed only at the external surface. With increasing loading of the clays, the spectra of methylene blue showed metachromasy. The metachromatic behavior can be fully explained by dye aggregation, which is the result of its concentration on the surface. No 7r-electron interaction with the surface oxygens need be invoked.

Electronic states of semiconductor clusters: Homogeneous and inhomogeneous broadening of the optical spectrum

Abstract: The homogeneous (single‐cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35–50 A diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm−1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron–vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm−1, or seven times the homogeneous width, in the most monodisperse sample.

Quantum electronic properties of the Na/sub 3/Ga/sub 2/Li/sub 3/F/sub 12/:Cr/sup 3+/ laser

Abstract: Few of the existing Cr/sup 3+/ vibronic lasers have achieved the slope efficiency and tuning range expected based on their known spectroscopic properties. To discover the cause of this behavior, the performance of chromium-doped gallium fluoride garnet, Na/sub 3/Ga/sub 2/Li/sub 3/F/sub 12/:Cr/sup 3+/, as a laser material has been investigated experimentally. The data reported include absorption and emission spectra, emission rates, quantum efficiency, laser wavelength tuning range, laser output slope efficiencies, and excited-state absorption spectra. Similar properties of the alexandrite laser material were studied for comparison. The results indicate that the performance of the gallium fluoride garnet laser is severely limited by Cr/sup 3+/ excited-state absorption (ESA). A model is presented to account for the unexpected nature of the ESA, which appears to be a common problem for all Cr/sup 3+/ vibronic lasers. Criteria are suggested for choosing Cr/sup 3+/ hosts for which the effects of ESA will be minimized. >

Laser action in chromium‐activated forsterite for near‐infrared excitation: Is Cr4+ the lasing ion?

Abstract: Room‐temperature pulsed laser action has been obtained in chromium‐activated forsterite (Cr:Mg2SiO4) for excitation of the near‐infrared absorption band of the system by the 1064 nm radiation from a Nd:YAG laser. The characteristics of laser emission are similar to those observed for 532 nm pumping. It is suggested that the laser action is due to a ‘‘center’’ other than the trivalent chromium (Cr3+), presumably the tetravalent chromium (Cr4+).

Raman spectra of diamondlike amorphous carbon films

Abstract: Raman spectra of diamondlike amorphous carbon (a‐C) films prepared under atmosphere with various hydrogen gas content have been measured as a function of excitation wavelength. The Raman spectral profiles vary with excitation wavelength depending on electronic absorption spectra associated with π‐π* electronic transitions. Dependence of Raman spectra on excitation wavelength is interpreted in terms of π‐π* resonant Raman scattering from aromatic rings with various sizes rather than polyene chains. The relative intensity of a 1400 cm−1 band against a 1530 cm−1 band is found to decrease with an increase of sp3 content in a‐C films. It is shown that the relative intensity can be used as a parameter for sp3 content.

Thermochromism in poly(3‐hexylthiophene) in the solid state: A spectroscopic study of temperature‐dependent conformational defects

Abstract: Reversible thermochromism in thin solid films of poly(3‐hexylthiophene), or P3HT, has been studied using ultraviolet and x‐ray photoelectron spectroscopies (UPS and XPS, respectively). The UPS and XPS spectra, as well as previously published optical absorption spectra, are analyzed using the results of valence effective Hamiltonian (VEH) quantum chemical calculations of the electronic structure of isolated polymer chains. The analysis of the spectra indicates that at elevated temperatures thermally induced electronic localization occurs as a consequence of thermally induced conformational disorder.

Infrared detection of gaseous species during the filament‐assisted growth of diamond

Abstract: Infrared diode laser absorption spectroscopy is employed as an in situ method to examine gas phase species present during filament‐assisted deposition of diamond films. From a reactant mixture of 0.5% methane in hydrogen, methyl radical (CH3 ), acetylene (C2H2), and ethylene (C2H4 ) are detected above the growing surface, while ethane (C2H6 ), various C3 hydrocarbons, and methylene (CH2) radicals are below our sensitivity levels. The growth of polycrystalline diamond films on Si wafers and polycrystalline Ni is confirmed with x‐ray and Raman scattering, scanning electron microscopy, and Auger electron spectroscopy.

A picosecond bleaching study of quantum‐confined cadmium sulfide microcrystallites in a polymer film

Abstract: We report a picosecond pump–probe study of 55 A cadmium sulfide microcrystallites embedded in polymer films. Large negative absorbance changes at wavelengths corresponding to energies near the band gap are observed. This absorption bleaching and the associated changes in refractive index are mainly responsible for the large nonlinearity observed in degenerate four‐wave mixing experiments. Based on photoluminescence data, the known electron‐trapping cross section of defects, and these pump–probe experiments, we show that the conventional carrier density‐dependent band‐filling mechanism cannot account for the data, and the absorption bleaching is due to the saturation of the excitonic transition. We further show that the phase‐space filling and exchange effects from exciton–exciton and exciton‐free carrier interactions fail to account for the observed data. Instead, we propose that the exciton‐trapped carrier interaction is mainly responsible for the observed bleaching of the excitonic absorption. This interaction is unique for small semiconductor clusters since the presence of a high density of defects (most likely on the surfaces) causes the extremely rapid trapping of free carriers. According to this model, the recovery time of the absorption bleaching is determined by the trapped‐carrier relaxation time, which is sensitive to the fabrication methods and can be controlled by surface chemistry. Our study also demonstrates that one needs to understand the effects of surfaces and control the surface chemistry before the important question of size effects on the nonlinear optical properties can be addressed.

Dynamics in low temperature glasses: Theory and experiments on optical dephasing, spectral diffusion, and hydrogen tunneling

Abstract: Temperature dependent photon echo (PE) and nonphotochemical hole burning (NPHB) measurements are reported on resorufin in three organic glasses: ethanol (1.5–11 K), glycerol (1.1–25 K), and d‐ethanol (1.5–11 K). In all cases, the NPHB results are broadened considerably from the PE results at low temperatures, but the two measurements coalesce at high temperatures. The temperature dependences are found to deviate from the power law dependence expected for two‐level system dephasing, and the deviation is attributed to dephasing by a pseudolocal mode. The appropriate correlation functions for PE and hole burning experiments are shown to be different from each other. They also differ from the correlation function for the optical absorption (OA) experiment, which has been the basis for most calculations of optical dephasing in glasses. The broadening of hole widths beyond the PE result is shown to be a measure of the slow spectral diffusion processes in the glass. Other types of dephasing measurement are also ...

Proton transfer in benzoic acid crystals: A chemical spin–boson problem. Theoretical analysis of nuclear magnetic resonance, neutron scattering, and optical experiments

Abstract: The double proton transfer in benzoic acid crystals can be described by a double‐minimum potential. At low temperatures one need consider only the two lowest energy eigenstates, which must be coupled to the crystalline phonons in order to obtain relaxation. Thus the benzoic acid system provides a well‐defined chemical example of the spin–boson Hamiltonian. Within this model the tunneling relaxation between localized states occurs by one‐phonon emission or absorption. Alternatively, at high temperatures the proton transfer is thermally activated. With this simple picture in mind we analyze NMR T1 relaxation experiments. The temperature‐dependent proton transfer rate that emerges from the NMR analysis is in good agreement with inelastic neutron scattering experiments. Optical transitions of a dye probe have also been used to determine proton transfer rates in crystalline benzoic acid. Our model allows us to discuss both doped and pure crystal experiments within a unified framework. Thus, we find that all th...

QSO absorption lines : probing the universe : proceedings of the QSO Absorption Line Meeting, Baltimore, 1987 May 19-21

Abstract: Preface 1. QSO absorption lines: introduction W. L. W. Sargent 2. BAL QSOs: observations, models and implications for narrow absorption line systems David A. Turnshek 3. QSO absorption systems with zabs zem C. B. Foltz, F. H. Chaffee, R. J. Weymann and S. F. Anderson 4. The Ly-a forest: observational evidence for line evolution Richard W. Hunstead 5. Properties of the Ly-a clouds R. F. Carswell 6. Absorption lines and galaxy formation Martin J. Rees 7. Properties of the heavy-element absorption systems Jacqueline Bergeron 8. High resolution observations of heavy-element absorption J. Chris Blades 9. In search of a unified description of the narrow absorption line systems David Tytler 10. The properties of the gaseous galactic corona Blair D. Savage 11. Analogies at low redshift for QSO absorption line regions D. G. York 12. The extent of galaxies in 21 cm Renzo Sancisi 13. Evening panel discussion 14. 21 centimeter absorption line F. H. Briggs 15. Damped Ly-a absorption systems Arthur M. Wolfe 16. QSO absorption lines and the universe at redshifts between 1 and 4 J. P. Ostriker.

A priori calculation of the optical absorption spectrum of the hydrated electron.

Abstract: The optical-absorption spectrum of an excess electron solvated in a molecular sample of liquid water at 300 K has been calculated with use of solvent configurations generated via path-integral simulation and subsequent solution of the excess-electronic eigenvalue problem. Electronic transitions from an s-like ground state to three bound-localized, p-like excited states dominate the broad asymmetric spectrum with excitations into an apparent continuum following at higher energy. Asymmetric distortions and radial fluctuations of the solvent cavities contribute comparably to the spectral broadening.

Low-temperature femtosecond spectroscopy of the initial step of electron transfer in reaction centers from photosynthetic purple bacteria

Abstract: The initial step of charge separation at 10 K has been monitored with 100-fs time resolution in reaction centers from Rhodopseudomonas viridis and Rhodobacter sphaeroides as well as in reaction centers from the latter species in which one of the two monomeric bacteriochlorophyll (B) molecules has been removed by treatment with borohydride. Upon excitation at 870 nm, the absorbance changes measured at several wavelengths in the near-infrared absorption bands of the pigments, and notably at the absorption maximum of the B molecule(s), give no indication of a detectable concentration of B-. Instead, the appearance of the cation radical of the dimeric primary electron donor (P) and of the bacteriopheophytin anion develops in concert with the decay of P*. An initial bleaching of the 850-nm band in reaction centers from Rho- dopseudomonas uiridis is consistent with an assignment of at least a large fraction of this band to the high-energy exciton component of P. Upon excitation of the B molecule(s) around 600 nm in the three types of reaction centers investigated, ultrafast energy transfer leads to the formation of P* in less than 100 fs. Under these conditions, a fast transient bleaching decaying with a 400-fs time constant is observed within the absorption band of B. This transient is also present upon preferential excitation of the bac- teriopheophytins in the reaction center of Rhodopseudomonas uiridis.

Free carrier and many-body effects in absorption spectra of modulation-doped quantum wells

Abstract: The temperature-dependent optical absorption and luminescence spectra of GaAs/AlGaAs and InGaAs/InAlAs n-doped modulation-doped quantum wells is discussed with emphasis on the peak seen at the edge of the absorption spectra of these samples A many-body calculation of the electron-hole correlation enhancement is presented, which identifies this peak with the Mahan exciton-the result of the Coulomb interaction between the photoexcited hole in the valence band and the sea of electrons in the conduction band This calculation accounts for the strong dependence of the absorption edge peak on both the temperature and carrier concentration, in good qualitative agreement with experimental data and with previously published results The changes induced by the carriers on the subband structure through self-consistent calculations are also analyzed, and it is concluded that in these symmetric structures, the changes are small for achievable carrier densities >

Photodissociation dynamics of H2O and D2O in the first absorption band: A complete abinitio treatment

Abstract: We report a detailed theortical study of the photodissociation of H2O and D2O in the first absorption band (λ∼165 nm). The calculations are three dimensional and purely quantum mechanical. They include an ab initio potential energy surface for the A state and a calculated SCF dipole moment function for the X→A transition. The dynamical calculations are performed within the infinite‐order‐sudden approximation for the rotational degree of freedom of OH and the LHL approximation for the masses. The resulting vibrational–translational motion is then treated exactly in two dimensions using hyperspherical coordinates. This study does not include any adjustable parameters. The thermally averaged total absorption spectra for H2O and D2O agree perfectly with the experimental spectra. Even finer details such as the progression of ‘‘vibrational’’ structures are well reproduced. They are not induced by any selective absorption but can be explained on the basis of the A state potential energy surface and details o...

Calculations of magnetic x-ray dichroism in the 3d absorption spectra of rare-earth compounds

Abstract: We present atomic calculations for the recently discovered magnetic x-ray dichroism (MXD) displayed by the 3d x-ray-absorption spectra of rare-earth compounds. The spectral shapes expected at T=0 K for linear polarization parallel and normal to the local magnetic field is given, together with the temperature and field dependencies of the intensities of the spectral lines. It is concluded that strong MXD effects can be expected for all rare-earth ions except those having a ground state with either J=0 or L=0. Calculations of magnetic x-ray dichroism in the 3d absorption spectra of rare-earth compounds. - ResearchGate. Available from: http://www.researchgate.net/publication/13344010_Calculations_of_magnetic_x-ray_dichroism_in_the_3d_absorption_spectra_of_rare-earth_compounds [accessed Jun 2, 2015].

Pump excited-state absorption in erbium-doped fibers.

Abstract: Ground-state and excited-state absorption spectra covering the wavelength range of 450–1050 nm are presented for erbium-doped silica fibers with four different core codopants: GeO2, GeO2/B2O3, GeO2/P2O5, and Al2O3. It is shown that the host glass influences the excited-state absorption spectra and that P2O5- or Al2O3-codoped fibers are the preferred choice for 514.5-, 655-, or 807-nm pump wavelengths owing to reduced pump excited-state absorption. However, excited-state absorption is still significant at the 807-nm wavelength. Pump wavelengths of 524, 532, and 980 nm appear ideal because of the strong ground-state absorption and lack of excited-state absorption.

Local probe for spin-orbit interaction.

Abstract: The branching ratio of core-valence transitions in x-ray absorption spectroscopy is linearly related to the expectation value of the spin-orbit operator in the valence states. This offers a direct method to determine the spin-orbit interaction in the local electronic structure of metal compounds and alloys. The method is complementary to susceptibility and paramagnetic resonance measurements because it measures a different operator and is element specific.

Electronic Energy Levels of Small Polyatomic Transient Molecules

Abstract: The experimentally determined eletronic energy levels of approximately 500 neutral and ionic transient molecules possessing from 3 to 6 atoms are tabulated together with the associated vibrational structure, the radiative lifetime, the principal rotational constants, and references to the pertinent literature. Vibrational and molecular beams, and in rare‐gas and nitrogen matrices are included. The types of measurement surveyed include conventional and laser‐ based absorption and emission techniques, laser absorption with mass analysis, and ultraviolet photoelectron spectroscopy.