Photofield emission spectroscopy and surface density of states

A novel procedure for the investigation of optical excitations on the metal surface by the photofield emission method is reviewed. A separation procedure of thermocurrent, accompanying the measured photocurrent, induced by the chopped light beam, is described. A differentiation of the true photocurrent curve allows one to obtain an energy distribution of photoelectrons. For the (0 0 0 1) face of Ti, it exhibits very small and densely spaced maxima, confirmed by theory as surface resonances. The dependence of the density on the thickness of the Ti crystal, grown by evaporation of Ti on the W field emitter surface, was verified experimentally. The possibility of observing surface resonances on a microcrystal of a homogeneous field emitter is discussed.     

Field emission spectroscopy

The first experimental evidence of the existence of electron states localised on a clean metal surface was available to us since 1967 in the form of the well known Swanson-Crouser hump in the energy distribution of electrons field-emitted from the W(100) plane, although it took some time before its interpretation as such, in 1976, was established beyond any doubt. Subsequently, angle-resolved photoemission proved to be a more competitive method for the investigation of surface states of metals and semiconductors. However, field emission remains a useful tool in surface physics research.

After a brief introduction to field emission spectroscopy, we review recent work which shows the potentiality of this technique in the investigation of the surface properties of semiconductors, ferromagnetic metals, thermal superconductors, and of the electronic structure of supported clusters.     

Photoelectron emission spectroscopy of liquid water

The threshold energy E_t = 10.06 eV (0.002 eV standard deviation) is determined for photoelectron emission by liquid water and is correlated with E_t = 8.45 eV for OH^? (aq). Free energy changes and standard reduction potentials are calculated for both emission processes. Reorganization free energies are correlated to solvation free energies for H₂O⁺(aq) and OH^?(aq).     

X-ray emission spectroscopy of nitrogen-rich compounds

Nonresonant X-ray emission spectroscopy was used to compare the nitrogen-rich compounds ammonium nitrate, trinitrotoluene, and cyclotrimethylene-trinitramine. They are representative of crystalline and molecular structures of special importance in industrial and military applications. The spectral signature of each substance was analyzed and correlated with features in the electronic structure of the systems. This analysis was accomplished by means of theoretical simulations of the emission spectra and a detailed examination of the molecular orbitals and densities of states. We find that the two theoretical methods used (frozen-orbital density functional theory and real-space Green's function simulations) account semiquantitatively for the observed spectra and are able to predict features arising from distinct chemical complexes. A comparison of the calculations and the data provides insight into the electronic contributions of specific molecular orbitals, as well as the features due to bandlike behavior. With some additional refinements, these methods could be used as an alternative to reference compounds.     

Multisignal calibration in optical emission spectroscopy

Summary The possibility of using more than one line for calibration in optical emission spectroscopy has been investigated. By means of principal component regression, sensitivity can be increased and part of noise can be removed. Thus, lower limits of detection can be obtained. The extent of improvement of sensitivity is considered theoretically and investigated by simulation. In the ideal case the sensitivity of the principal component regression for more than one line is calculated from the square root of the sum of squares of the sensitivities of the single lines. Practical applications in arcoptical emission spectroscopy and ICP-OES show that the theoretically expected improvements can almost be reached. The reduction in noise depends on the nature and source of noise.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

Microwaves spark emission spectroscopy for the analysis of cations:A simple form of atomic emission spectroscopy

A novel method for the cation analysis was investigated.The analysis is based on the sparking of the salts of metals in a microwave oven after placing in a graphite cell.The graphite cell absorbs microwaves and produces high temperature which converts the salt into light emitting species.The colour of light was found to dependent on the nature of cation,however,the intensity of the emitted light was found to be depending upon the form and shape of the graphite assembly in addition to the concentration of the salt.This communication presents explanation for all these observations and for the systematic and quantitative analysis using microwave spark emission technique.     

High-resolution X-ray emission spectroscopy of molybdenum compounds

High-resolution molybdenum K-edge X-ray emission spectroscopy (XES) was used to characterize the K beta(4) and K beta' ' valence-to-core transition bands in the oxo-Mo compounds K(2)MoO(4), MoO(S(2)CNEt(2))(2), and MoO(2)(S(2)CNEt(2))(2). The K beta(4) and K beta' ' emission bands are attributed to transitions to the Mo 1s core hole from molecular orbitals possessing primarily molybdenum 4d and oxygen 2s character, respectively. This communication describes the first assignment of the K beta' ' interatomic band in the emission spectra of molybdenum complexes. Additionally, the K beta(4) and K beta' 'transitions are shown to be sensitive to the chemical and electronic environment of the metal, suggesting that high-resolution XES might be an effective method for elucidating the nature of the molybdenum centers in biological systems.     

Evaluation of detection limits in emission spectroscopy

Summary The didactical method for defining the practical noise level considered in Part I is supplemented by a treatment that avoids the disruption of the statistical combination of the error due to the graininess of the emulsion and the error originating from intensity fluctuations in the source. The relationships between the basic standard deviations occurring in the alternative treatments are discussed and the inherent detection limits are compared. The procedure finally adopted is demonstrated with examples.

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Zusammenfassung Anlehnend an die didaktische Methode zur Erfassung der praktischen Nachweisgrenze [I. Teil, diese Z. 220, 241 (1966)] bringt die vorliegende Arbeit eine ergänzende Theorie, welche das Unterbrechen des statistischen Zusammenwirkens von Körnigkeitsstreuung und Intensitätsschwankungen in der Lichtquelle umgeht. Die Beziehungen zwischen den Standardabweichungen, welche den beiden Betrachtungsweisen zugrunde liegen, werden diskutiert, und die zugehörigen Nachweisgrenzen werden mit einander verglichen. Das endlich gewählte Verfahren wird an Hand von Beispielen demonstriert.

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Part I: see this journal220, 241 (1966).

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Dedicated to Prof. Dr. H.Kaiser on his 60th birthday.     

Procedure for correcting interferences in emission spectroscopy

Summary A procedure for correcting interferences in emission spectroscopy is suggested. It is based on the use of error curves, which are a measure of the extent of the enhancing (or suppressing) effect of the interfering element upon the sought-for one. The flame-photometric determination of potassium in the presence of large amounts of sodium is described as an example.

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Verfahren zur Korrektur von Störeinflüssen in der Emissionsspektrographie

Zusammenfassung Das beschriebene Verfahren zur Korrektur des Einflusses störender Begleitionen beruht auf dem Gebrauch von Fehlerkurven, die das Ausmaß der verstärkenden oder schwächenden Wirkung von Fremdionen angeben. Die flammenphotometrische Kaliumbestimmung in Gegenwart großer Mengen Natrium dient als Beispiel.

     

Evaluation of detection limits in emission spectroscopy

Summary The paper considers the evaluation of detection limits in photographic emission spectroscopy in case of the analytical line being superimposed on a blank line, caused by either a molecular band or a residual impurity in the electrodes, the admixture, or the atmosphere. The theory developed is a generalization of Kaiser's treatment of detection limits for a smooth background. The theory permits to compare the ideal detection limit, attainable in the absence of a blank with the practical detection limit, reached when a given blank interferes, all other conditions being equal. The relation between the evaluation of the practical detection limit and the determination of the blank concentration (addition method) is discussed.

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Zusammenfassung Die Auswertung von Nachweisgrenzen in der photographischen Emissionsspektroskopie wird für den Fall betrachtet, daß die Analysenlinie einer Blindlinie überlagert ist, die durch eine Molekülbande oder eine Verunreinigung in den Elektroden, im Zusatz oder in der Atmosphäre verursacht wird. Die aufgestellte Theorie ist eine Verallgemeinerung der Kaiserschen Berechnung von Nachweisgrenzen für glatten Untergrund. Sie ermöglicht es, die praktische Nachweisgrenze, erreicht mit einer gegebenen Blindwertstörung, und die ideale Nachweisgrenze, erzielbar ohne Blindwert, aber unter sonst gleichen Bedingungen, miteinander zu vergleichen. Die Beziehung zwischen der Auswertung der praktischen Nachweisgrenze und der Bestimmung der Blindkonzentration (Restgehaltsbestimmung nach dem Zugabeverfahren) wird diskutiert.

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Nomenclature G nominal concentration (or absolute amount) of an analysis element in a sample or standard - G detection limit - ¯G _B average concentration (or absolute amount) of the analysis element in the blank - I photographic intensity (= exposure) - I _L intensity of an analytical line corrected for the background and without a contribution from the blank - I _L antilogarithm of Y _L - I _U intensity of the background at the wavelength of an analytical line - ¯I _U antilogarithm of ¯Y _U - I _LU sum of intensities of analytical line and background - I _B deintensity emitted by a blank at the wavelength of an analytical line - ¯I _B antilogarithm of ¯Y _B - I _B antilogarithm of Y _B - I _BU sum of intensities contributed by blank and background at the wavelength of an analytical line - I _LB intensity at the wavelength of an analytical line resulting from the contributions from the sample and the blank, but corrected for the background - I _LBU sum of intensities I _LB and I _U - Y logarithmic intensity - Y _L logarithm of I _L - Y _L smallest significant value evaluated for log I _L in a sample - Y _U logarithm of I _U - ¯Y _U mean value of log I _U for a series of spectra taken under standard conditions characteristic for the analysis method - Y _LU logarithm of I _LU - Y _LU smallest significant reading of log I _LU in case there is a smooth background - Y _B logarithm of I _B - ¯Y _B mean value of log I _B calculated from measurements in at least twenty blank spectra taken under standards conditions - ¯Y _B minimum value of Y _B defined by equation (20) - Y _BU logarithm of I _BU - Y _LB logarithm of I _LB - Y _LBU logarithm of I _LBU - Y _LBU smallest significant reading of log I _LBU - Y _U Y _L-¯Y_U, noise level in case there is a smooth background, to be connected with a calibration curve normalized to the background [equation (11)] - (Y _U ) _B Y _L-¯Y_U, noise level in case there is a blank, to be connected with a calibration curve normalized to the background [equation (11)] - Y _B Y _L-¯Y_B, noise level in case there is a blank, to be connected with a calibration curve normalized to the blank [equation (36)] - Y _L,U Y _L-¯Y_U, logarithm of intensity of analytical line normalized to the background, i.e. logarithm of intensity ratio I _L/¯I_U - ¯Y _B-¯Y_U, logarithm of ratio ¯I _B/¯I_U - Y_L,B Y_L-¯Y_B logarithm of intensity of analytical line normalized to the blank, i.e. logarithm of intensity ratio I _L/¯I_B - _U standard deviation in logarithmic intensity due to the graininess - _B standard deviation of Y _B - slope of working curve log G vs Y _L [equations (9) and (31)] - K constant in the working curve log G vs Y _L [equations (9) and (31)] - n number of independent readings - P 32 _U - q 3 _B - r ¯I _b/¯I_U The authors acknowledge Prof. Dr. H. Kaiser and his co-workers for their readiness to discuss the subject and for their critical remarks.     

A modified glow-discharge source for emission spectroscopy

A glow-discharge emission source of the type described by Grimm for spectrochemical analysis has been modified to incorporate a secondary discharge of     

时间分辨发射光谱的应用研究进展

时间分辨荧光技术是研究超分子组装微环境变化的主要手段之一。荧光体与蛋白质、内膜及胶束等相互作用引起最大荧光强度和量子效率急剧变化反映了激发态反应情况,激发态过程通常用时间分辨发射光谱(TRES)测定。国内在这一领域的研究虽有成果,但与国外相比仍有一定差距。本文对时间分辨荧光光谱的应用研究进展进行了综述,阐述了其历史、原理、检测方法及近几年国内外的应用研究进展情况。     

X-ray emission depth-resolved spectroscopy for investigation of nanolayers

The spatial and electronic structures of a series of substituted germylene molecules are calculated by DFT and HF methods using the PC GAMESS-Firefly program package. Molecular structures are optimized at the DFT (B3LYP/6-31G(2d,p)) level; wave functions and topological characteristics of the electron density distribution are calculated in the HF/acc-pvtz approximation. By means of NBO and AIM methods, the main characteristics of Ge-X bonds in these molecules are determined. The Ge-X bonds are shown to be the intermediate type bonds. The energies of these bonds are estimated.     

Mid-infrared and infrared emission spectroscopy of Cu-exchanged montmorillonite

Middle Infrared Spectroscopy (Mid-IR) and Infrared Emission Spectroscopy (IES) were employed to characterise Cu-exchanged montmorillonites, which were derived from two different types of montmorillonite clays, Ca-exchanged montmorillonite (Cheto clay) and Na-exchanged montmorillonite (Miles clay). Copper was exchanged under both acidic and basic conditions at different Cu/clay ratios. All Cu-exchanged montmorillonites experienced a shift in most of non-lattice bands, with hydroxyl bands playing a major role in the characterisation of the clays. Furthermore, a relationship between the ratio of bands at 3630 and 3500 cm(-1) and the Cu concentration of the starting solutions was indicated and used to compare the degree of cation exchange between two preparation methods. Two dehydration stages were observed in the IES experiments. Additional bands were observed in all Cu-exchanged montmorillonites prepared with the 'basic conditions method,' and these bands were assigned to ammonia molecules trapped within the clay structure or absorbed on the surface of the clay.     

Light emitting diode excitation emission matrix fluorescence spectroscopy

An excitation emission matrix (EEM) fluorescence instrument has been developed using a linear array of light emitting diodes (LED). The wavelengths covered extend from the upper UV through the visible spectrum: 370-640 nm. Using an LED array to excite fluorescence emission at multiple excitation wavelengths is a low-cost alternative to an expensive high power lamp and imaging spectrograph. The LED-EEM system is a departure from other EEM spectroscopy systems in that LEDs often have broad excitation ranges which may overlap with neighboring channels. The LED array can be considered a hybrid between a spectroscopic and sensor system, as the broad LED excitation range produces a partially selective optical measurement. The instrument has been tested and characterized using fluorescent dyes: limits of detection (LOD) for 9,10-bis(phenylethynyl)-anthracene and rhodamine B were in the mid parts-per-trillion range; detection limits for the other compounds were in the low parts-per-billion range (< 5 ppb). The LED-EEMs were analyzed using parallel factor analysis (PARAFAC), which allowed the mathematical resolution of the individual contributions of the mono- and dianion fluorescein tautomers a priori. Correct identification and quantitation of six fluorescent dyes in two to six component mixtures (concentrations between 12.5 and 500 ppb) has been achieved with root mean squared errors of prediction (RMSEP) of less than 4.0 ppb for all components.     

Optical emission spectroscopy of electron-cyclotron-resonance-heated helium mirror plasmas

In this experiment emission spectroscopy in the 3000–5000 Å range has been utilized to determine the electron temperature (15–60 eV) and ion density (2–5 x 10¹¹ cm^–3) of helium plasmas produced by the Michigan mirror machine⁽¹⁾ (MIMI). The plasma is generated and heated by whistler-mode electron-cyclotron resonance (ECR) waves at 7.43 GHz with 400–900 W power in 80-ms-long pulses. Gas fueling is provided at the midplane region by a leak valve with a range in pressure of 3 x 10 to 2 x 10⁴ Torr. Emission line intensities are interpreted using a model of the important collisional and radiative processes occurring in the plasma. The model examines secondary processes such as radiation trapping, excitation transfer between levels of the carne principle quantum number, and excitation front metastable states for plasmas in the parameter range of MIMI (n _c = 1–6 x 10¹¹ cm^–3). Front the analysis of line intensity ratios for neutral helium, the electron temperature is measured and its dependence upon the gas pressure and microwave power is determined. These temperatures agree with those obtained by Langmuir probe measurements. Art analysis of the line intensity ratio between singly ionized helium and neutral helium yields a measurement of the ion density which is in good agreement with electron density measurements made by a microwave interferometer.