Study of electron states of solids by techniques of electron spectroscopy

Studies of valence bands and core levels of solids by photoelectron spectroscopy are described at length. Satellite phenomena in the core level spectra have been discussed in some detail and it has been pointed out that the intensity of satellites appearing next to metal and ligand core levels critically depends on the metal-ligand overlap. Use of photoelectron spectroscopy in investigating metal-insulator transitions and spin-state transitions in solids is examined. It is shown that relative intensities of metal Auger lines in transition metal oxides and other systems provide valuable information on the valence bands. Occurrence of interatomic Auger transitions in competition with intraatomic transitions is discussed. Applications of electron energy loss spectroscopy and other techniques of electron spectroscopy in the study of gas-solid interactions are briefly presented.     

Triplet states of ketene by trapped electron spectroscopy

The threshold electron impact excitation spectrum of ketene is reported. The spectrum is interpreted in comparison with results of an ab initio frozen core calculation. Triplet states (³A₂, ³A₁, ³B₁) are observed at 3.8, 5.0 and 5.8 eV excitation energy.     

Triplet states of fluorocarbonyl compounds determined by trapped electron spectroscopy

Ion cyclotron resonance trapped electron spectra for carbonylfluoride, trifluoroacetyl fluoride, hexafluoroacetone, acetyl fluoride and I,I.I-trifluoroacetone are reported. Comparisons of data obtained for ³(π → π^*) vertical excitation energies are used in conjunction with photoelectron spectroscopic data to deduce the effects of fluorine and trifluoromethyl groups on the energies of σ, π and π^* orbitals associated with the carbonyl group.     

Identification of oxidative processes during simulated mastication of uncooked foods using electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy is used to measure directly the generation of free radicals during a simulation of the mastication process. This involves the gentle grinding of the food product in the presence of a spin trap, a molecule which reacts selectively with unstable free radicals to generate (more) stable radical adducts, which can then be characterised. With mushrooms of the Agaricus family, adducts consistent with a carbon-centred radical are seen with a wide range of spin traps and this radical has been confirmed as 4-(hydroxymethyl)phenyl. In plant tissues that are rich in ascorbic acid, this molecule competes successfully with spin traps for the free radicals and the (monodehydro)ascorbate radical, formed by the 1-electron oxidation of ascorbic acid, is seen in the EPR spectra. However, with >50% of the plant tissue samples studied in the present experiment, free radicals resulting from oxidation of the spin traps were observed. The formation of such molecules, for which oxygen was found to be necessary, requires the existence of powerful oxidation processes as the plant tissue is broken down. Such pro-oxidant behaviour is contrary to the popular assumption that the beneficial effects of uncooked plant tissues are the result of their high levels of anti-oxidant molecules.     

Excited electronic states of the fluorobenzenes by variable angle electron impact spectroscopy

Electron-impact excitation spectra of benzene, fluorobenzene, o-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3,4-tetrafluorobenzene, pentafluorobenzene, and hexafluorobenzene have been measured at impact energies of 50 eV and either 25 eV or 30 eV, and scattering angles from 5° to 80°. Each molecule shows an absorption maximum at about 3.9 eV corresponding to a singlet → triplet, π → π^*, transition. In benzene, fluorobenzene, o-difluorobenzene, and 1,3,5-trifluorobenzene, an additional singlet → triplet transition was detected at about 5.6 eV. Three singlet → singlet transitions analogous to the 4.90, 6.20, and 6.95 eV transitions in benzene are seen in each of the fluorine-substituted molecules. The more highly substituted compounds exhibit an additional singlet → singlet transition that is most clearly observed in the hexafluorobenzene spectrum with a peak at 5.32 eV.     

Triplet states in isotopically mixed anthracene crystals: High resolution optical spectroscopy

The triplet O,O transitions of guest and host in isotopically mixed anthracene crystals of various compositions (A-h₁₀, ¹³C-monosubstituted A-h₁₀, A-d₁h₉, A-d₂h_g in A-d₁₀ and A-d₁₀ in A-h₁₀) have been investigated using high resolution laser excitation spectroscopy. The guest aggregate spectra have been studied in polarized light as a function of guest concentration up to 15%. The analyses allow us to identify the monomer, dimer and trimer lines. From the dimer splittings the dominant resonance pair interactions are dedu The comparison of different mixed crystal systems with guest levels below and above the host exciton band reveals that quasiresonance and superexchange corrections are of minor importance. The experimental resonance pair interactions are used to calculate the triplet exciton band structure of anthracen and the observed guest polarization behaviour is interpreted quantitatively by the Rashba effect. Finally, the lower Davydov component of the host is s and broadened with increasing guest concentration. The shift is discussed using a theoretical model of Lifshitz.     

Identification of counterfeit drugs using near-infrared spectroscopy

This work was aimed at the investigation of the use of near-infrared spectroscopy (NIRS) for the identification of counterfeit drugs. The identification is based on the comparison of the NIR spectrum of a sample with typical spectra of the authentic drug using multivariate modelling and classification algorithms (PCA/SIMCA). Initially, NIRS was evaluated for spectrum acquisition of various drugs, selected in order to observe the diversity of physico-chemical characteristics found among commercial products. The parameters which could affect the spectra of a given drug (especially if presented in solid form) were investigated and the results showed that the first derivative can minimise spectral changes associated with tablet geometry, physical differences in their faces and position in relation to the probe beam. The power of NIRS in distinguishing among similar pharmaceuticals was demonstrated and a protocol is proposed to construct a multivariate model and to include it in a library allowing testing for drug authenticity. The methodology was evaluated with real samples of counterfeit drugs and was able to recognise all those presenting changes in composition as false. The results show unequivocally the potential of NIRS for rapid, on-site and non-destructive identification of counterfeit pharmaceuticals.     

Identification of archaeological ivories using FT-Raman spectroscopy

FT-Raman spectroscopy has been used successfully for the non-destructive identification of modern ivories and is evaluated here for the identification of osseous materials from archaeological sites. Results on archaeological ivories are reported and the problems faced in matching FT-Raman spectra to standards for the validation of these materials are highlighted. Archaeological specimens of different dates, provenance and taphonomic history, including both conserved and unconserved items, have been analysed.     

A study of the negative ion states of selected cyclodienes by electron transmission spectroscopy

Electron transmission spectroscopy is employed to locate sharp variations in the total cross sections for electrons scattered from several cyclic compounds containing two carboncarbon bonds. For each molecule, structure is observed which we associate with the temporary occupation of the two low-lying, normally unfilled, π^* orbitals by impacting electrons. Electron affinities are reported for 1,5-cyclooctadiene, 1,4-and1,3-cyclohexadiene, norbornadiene and also cyclohexene, propene, and cis-butene.     

Identification of defect sites on oxide surfaces by metastable impact electron spectroscopy

In this review, thin films of SiO₂ on Mo(1 1 2) and MgO(1 0 0) on Mo(1 0 0) have been characterized using metastable impact electron and ultraviolet photoelectron spectroscopies (metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy). The electronic and chemical properties of the thin films are identical to those of the corresponding bulk oxides. For different prepared defective SiO₂ surfaces, additional features are observed in the band-gap region. These features arise from vacancies or excess oxygen and are consistent with theoretical predictions of additional occupied states in the band-gap due to point defects. Extended defect sites on SiO₂ and MgO are identified using MIES by a narrowing of the O(2p) features with a reduction in the density of extended defect sites. MIES of adsorbed Xe (MAX) is also used to estimate the density of extended defect sites. Furthermore, it is shown that CO is an appropriate probe molecule for estimating the defect density of MgO surfaces. Upon Ag exposure, the change in the work function of a low defect MgO(1 0 0) versus a high defect surface is markedly different. For a sputter-damaged MgO(1 0 0) surface, an initial decrease of the work function was found, implying that small Ag clusters on this surface are electron deficient. In contrast, for SiO₂ no significant change of the work function upon Ag exposure with increasing defect density was observed. On MgO(1 0 0), the presence of defect sites markedly alter the electronic and chemical properties of supported Ag clusters. Such a strong influence of defect sites was not found for Ag clusters on SiO₂.     

Studies in mixed solvents: Comparison between solvated electron reactions and quenching of excited states

The rate constants for the reaction of electron pulse produced solvated electrons and a number of solutes in water-isopropanol mixtures have been measured. The quenching of the singlet excited state of naphthalene has also been studied in the same mixtures, using triethylamine and acrylamide as quenchers. The variation of the bimolecular solvated electron reaction rate constants with the composition of the solvent has been compared with the variation in the quenching constants with the composition of the solvent. Both these variations are surprisingly similar, with acrylamide behaving in a reverse manner (to the other solutes) in both the cases. It has been possible to quantitatively correlate both sets of data using dielectric constant (?) as a measure of polarity and the viscosity (η) as an index of the microstructure. The curves obtained provide insights with respect to the nature of charge transfer processes involved. © 1995 John Wiley & Sons, Inc.     

Automatic analysis of mixed spectra : Generalised spectral subtraction applied to electron spin resonance spectroscopy

A new technique is described for estimating the pure component spectra from a set of linearly independent spectra. The process is one of generalised spectral subtraction in which an iterative combination of multivariate linear least-squares analysis and matrix transformation is applied to the input data to give estimates of the number of independent components in the original mixed spectra. This technique is applicable to bipolar data (e.g. from e.p.r. spectra) as well as absorption spectra determined by any spectroscopic technique, provided that the spectra may be reasonably assumed to be an additive mixture of unknown pure components. Numerical model examples are given together with an experimental application to electron spin resonance.     

Vibrational spectroscopy of nitroalkane chains using electron autodetachment and ar predissociation

If the binding energy of an excess electron is lower than some of the vibrational levels of its host anion, vibrational excitation can lead to autodetachment. We use excitation of CH stretching modes in nitroalkane anions (2700-3000 cm(-1)), where the excess electron is localized predominantly on the NO2 group. We present data on nitroalkane anions of various chain lengths, showing that this technique is a valid approach to the vibrational spectroscopy of such systems extending to nitroalkane anions at least the size of nitropentane. We compare spectra taken by using vibrational autodetachment with spectra obtained by monitoring Ar evaporation from Ar solvated nitroalkane anions. The spectra of nitromethane and nitroethane are assigned on the basis of ab initio calculations with a detailed analysis of Fermi resonances of CH stretching fundamentals with overtones and combination bands of HCH bending modes.     

Electronic states of F2CO as studied by electron energy-loss spectroscopy and ab initio calculations

This paper reports on the first measurements of the electron impact electronic excitation cross-sections for carbonyl fluoride, F(2)CO, measured at 30 eV, 10° and 100 eV, 5° scattering angle, while sweeping the energy loss over the range 5.0-18.0 eV. The electronic-state spectroscopy has been investigated and the assignments are supported by quantum chemical calculations. The energy bands above 9.0 eV and the vibrational progressions superimposed upon it have been observed for the first time. Vibronic coupling has been shown to play an important role dictating the nature of the observed excited states, especially for the low-lying energy region (6.0-8.0 eV). New experimental evidence for the 6(1)B(2) state proposed to have its maximum at 12.75 eV according to the vibrational excitation reported in this energy region (11.6-14.0 eV). The n = 3 members of the Rydberg series have been assigned converging to the lowest ionization energy limits, 13.02 eV ((2)B(2)), 14.09 eV ((2)B(1)), 16.10 ((2)B(2)), and 19.15 eV ((2)A(1)) reported for the first time and classified according to the magnitude of the quantum defects (δ).     

Identification of transgenic foods using NIR spectroscopy: A review

The utilization of chemometric methods in the quantitative and qualitative analysis of feeds, foods, medicine and so on has been accompanied with the great evolution in the progress and in the near infrared spectroscopy (NIRS). Hence, recently the application of NIR spectroscopy has extended on the context of genetics and transgenic products. The aim of this review was to investigate the application of NIR spectroscopy to identificate transgenic products and to compare it with the traditional methods. The results of copious researches showed that the application of NIRS technology was successful to distinguish transgenic foods and it has advantages such as fast, avoiding time-consuming, non-destructive and low cost in relation to the antecedent methods such as PCR and ELISA.     

On the carrier properties of perfluoropolyether-betaine mixed vesicles: the contribution of electron spin resonance spectroscopy

The carrier properties of mixed unilamellar vesicles of fluorocarbon-hydrocarbon surfactants built up with the ammonium salt of a perfluoropolyether and n-dodecylbetaine were investigated by electron spin resonance spectroscopy (ESR). The ESR-active lipophilic nitroxide 5-doxylstearic acid (5-DSA) was used as a model of a lipophilic drug to be carried and delivered into cell membranes. Healthy and malignant colorectal tissues were used as the target cells. Cell suspensions of living tissues were studied in physiological conditions. For the maintenance of the surgically removed tissues, McCoy’s 5A culture medium was used. 5-DSA probe was rapidly delivered from perfluoropolyether (PFPE)/betaine mixed vesicles to the membranes of both healthy and malignant colorectal cells. The analysis of the computed 5-DSA ESR line shapes ensured that no spectral differences occurred when 5-DSA was either introduced directly into the cells or through the intervention of vesicles. This well agreed for an unmodified physical status of the membranes where the probe was mainly localized.     

Kinetic spectroscopy of triplet states of benzenoid hydrocarbons using modulated Hg resonance radiation

Triplet—triplet absorption spectra in the 220–250 nm region, and rate constants for unimolecular and bimolecular decay of the lowest triplet states of simply substituted benzenes in the gas phase have been obtained using a phase modulation technique. For deuterium and methyl substitution, first order decay constants are of the same order as in benzene of 10³ s^?1, with second order decay constants of the order of 10⁷ ? mol^?1 s^?1, also as in benzene. For trifluoromethyl substitution, these rate constants increase dramatically to of the order of 10^?5 s^?1 and 10⁸ ? mol^?1 s^?1 respectively, and fluorine substitution is even more dramatic, with values in the 10⁶ s^?1 and 10⁹ ? mol^?1 s^?1 respectively.