HeI Spectra Research Articles

Study of spatial distributions of highly ionized nonequilibrium helium plasma at atmospheric pressures

Experimental study of helium plasma in the state of quasistationary heating under atmospheric pressure was made. The plasma state is shown to fail to be described by Saha- Boltzmann approximation at high ionization levels αi = 0.5-0.9, temperatures 2.5-4.0 eV and electron concentrations about 1017 cm-3. The deviation from the equilibrium state of the plasma is caused by lack of spatial uniformity due to charged particles loss by ambipolar diffusion. In order to thoroughly study the temporal changes of plasma radiation characteristics, spectroscopic analysis was carried out with DFS-452 spectrometer and high-speed CMOS camera Andor iStar attached to its output. The system yields the spatial resolution of 30-50 μm and temporal resolution of 5-50 μs. Electron concentration ne was measured from the half-width of the local Hei spectrum line contours having dominant quadruple Stark effect with well-known constants. In order to determine the temperature of heavy particles, Doppler component of HeI line triplet at 1083 nm was studied. The temporal evolution of the following important characteristics has been determined for helium plasma during pulsed heating: current power, intensities of a number of HeI and HeII spectral lines, electron temperatures and concentrations.

Surface Interaction of PuO2, UO2+x and UO3 with Water Ice

The interaction of PuO2, UO2+x and UO3 with water ice was studied using ultraviolet photoelectron spectroscopy (UPS). Water was adsorbed at 80–120 K as thick ice multilayers. Surface modification after desorption of the ice around 200 K was investigated. Main information on the surface oxidation state was obtained by highly surface sensitive UPS-HeII spectra, probing primarily the first monolayer. The oxidation state was directly deduced from the intensity of the actinide 5f levels. The surface character of the phenomenon was further confirmed by comparing HeII spectra with the more bulk sensitive HeI spectra. Spectral interpretation was done using the cross-section variations in HeI and HeII spectra and by comparing the spectra with theoretical density of states curves, obtained by LSDA + U calculations. It was shown previously that reduction to Pu2O3 takes place, when the ice covered PuO2 films are warmed up under UV light and ice is desorbed. In this paper, this effect was investigated in further detail. It was shown that only the top surface layer is reduced. Reduction is inhibited by surface diffusion of oxygen trapped in the films during sputter deposition and not incorporated in the lattice. UO2+x and UO3 also undergo reduction, but to a significant lesser extent than PuO2. Reoxidation of surface U by bulk oxygen was much slower than that of surface Pu. It was shown that for all oxides, reduction needs the illumination of an ice overlayer by UV light. Surface reduction by atomic hydrogen was investigated to check for possible influence of ice photolysis products. UO3 was shown to be reduced to UO2 while PuO2 is not further reduced. The observations are explained by photochemical decomposition of water by the UV light (used for UPS) at the oxide-ice interface. It is thought that the oxide acts as photocatalyst, absorbing light and splitting adsorbed water. The thick ice layer traps the reaction products on the surface, thereby enabling them to react and reduce the surface. Why only the reductants (probably H) and not the concomitant oxidants react with the surface is still unknown.

Esters of Feist's Acid: Some Insights into the Electronic Structure and Reactivity of Methylenecyclopropanes Ring‐Substituted with Acceptor Groups

Abstract1,3‐Dipolar cycloaddition of C‐aryl‐N‐aryl‐ or N‐methylnitrones with esters of Feist's acid (3‐methylenecyclopropane‐trans‐1,2‐dicarboxylic acid) occurs with the formation of the corresponding spiro[cyclopropane‐1,4‐isoxazolidine] cycloadducts as single isomer with yields in the range of 17–59 %. The reaction proceeds with inverse regiochemical outcome compared to cycloadditions with unsubstituted methylenecyclopropane. Feist's esters are significantly less active towards cycloaddition with nitrones than methylenecyclopropane and reactions require prolonged heating at elevated temperature. The electronic structures of Feist's esters were investigated by means of molecular photoelectron spectroscopy and theoretical calculations (HF/6‐31G*, B3LYP/6‐31G*). Both, measured photoelectron HeI spectrum and ground state calculations (HF/6‐31G*) of Feist's esters, indicate considerable lowering of the methylenecyclopropane π‐HOMO energies by substitution with an acceptor group. B3LYP calculations of addends and transition states forthe interaction of methylenecyclopropane with C,N‐diphenylnitrone correctly predict regiochemical outcome. However, B3LYP calculations do not account for the observed regiochemical outcome and diminished reactivity in reactions of Feist's esters with C,N‐diphenylnitrone.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Investigating the possibility of a monitoring fast ion diagnostic for ITER

In burning plasma fusion devices, fast ion transport plays a central role in the performances of the machines. Moreover the losses of energetic particles might cause severe damages on plasma facing components. Therefore real time measurements of fast ion transport would provide valuable information for safe and reliable plasma operations. In this paper, we examine the feasibility of a monitoring system based on active charge exchange recombination spectroscopy making use of the 0.5 MeV/amu ITER heating neutral beams for detecting fast (4)He(+2) (alphas) particles in ITER plasmas. There are two time scales relevant to fast ion dynamics: the first is the slowing down time of the distribution function which is of the order of 1 s, and the second is the time scale of burstlike transport events such as collective Alfven mode excitations, which--for typical ITER plasma parameters--can be as low as 0.2-1 ms. To detect such fast events a broadband high-throughput spectrometer is needed, while for the reconstruction of the alpha velocity distribution function a higher resolution spectrometer and longer integration time are necessary. To monitor a spatial redistribution of fast particles due to the propagation of the instability, it is proposed to use a limited number of spatial channels, looking at the charge exchange He II spectra induced by the heating beams, whose energy matches the slowing down energies of fast particles. The proposal is to share the motional stark effect periscope on equatorial port 3 [A. Malaquias et al., Rev. Sci. Instrum. 75, 3393 (2004)] adding additional fibers and suitable instruments. A signal to noise ratio of 5 could be achieved with a spatial resolution of a/15 and a time resolution of 5 ms, in a broad spectral band of 100 A, corresponding to the spectral broadening of the line emitted by alpha particles with energies DeltaE < or = 1.5 MeV. Fast H and D ion populations created by heating neutral beam or ion cyclotron resonance heating are expected to produce significantly lower charge exchange signal levels and can only be monitored on substantially longer time scales as it is expected because of the strong energy difference with respect to the heating neutral beam and the consequently low charge exchange cross sections.

The characterization of SrTiO 3( [formula omitted]) with MIES, UPS(HeI) and first-principles calculations

The metastable impact electron spectra (MIES) and photoelectron spectra (UPS(HeI)) of undoped SrTiO 3(0 0 1) single crystals are presented. The work function is 2.6 eV; the top of the valence band is located 3.1 eV below the Fermi level which coincides with the bottom of the conduction band. An interpretation of the spectra is made on the basis of first-principles electronic structure results, also presented in this paper. We find that the HeI spectra can be well described by the bulk density of states of the SrTiO 3(0 0 1) valence band. The MIES spectra are best understood when assuming that the He 2s electron localizes at surface cations after its resonant transfer to the surface. This He-induced “dynamical” cation reduction greatly affects the He-surface interaction. The MIES spectra are similar to Auger deexcitation spectra, but are shifted by about 1.2 eV towards smaller kinetic energies. The shape of the spectra is best described by the density of states at about 4 a.u. in front of the surface.

Surface analysis of the electropolishing layer on Si(111) in ammonium fluoride solution

The influence of potential and pH on the thickness and composition of the anodic films formed on n-Si(111) in dilute ammonium fluoride solutions is investigated by photoelectron spectroscopy. A combined electrochemistry/surface analysis system with very low contamination levels is used for the investigation of the oxidic layers. XPS data show that the film thicknesses increase with decreasing pH reaching 25 Å at pH 3. A potential dependence measured at lower coverage at a pH of 4.9 reveals changes in thickness and composition by increasing the potential from flatband to +5 V. Fluorine is incorporated into these films in substantial amounts which otherwise consist predominantly of silicon oxide. Silicon fluoride and silicon oxyfluoride are found. We define a current transmissivity, T c, of the layers which shows strong variations with potential and pH. For a pH of 3, T c is about 5×10 2 larger than for pH 4.9. The potential dependence shows a variation of T c by a factor of three and an increase for thicknesses exceeding 5 Å. These observations are explained by a structural transition from a high density pseudomorphic interfacial layer to a more open structure at larger thicknesses (up to 25 Å). The analysis of the UP-spectra shows only little changes of the electron affinity χ with coverage. We obtain χ≅4.15±0.1 eV. A distinct band bending is not detected. From HeII spectra, we determine the valence band offset Δ E v between Si and the oxidic layer to be 5.2 eV.

Study of the electronic structure of TiO2(110) and Cs/TiO2(110) with metastable impact electron spectroscopy and ultraviolet photoemission spectroscopy (HeI)

We report the metastable impact electron spectroscopy (MIES) (He∗1s2s) and ultraviolet photoemission spectroscopy (UPS) (Hei) spectra for clean TiO2(110) and Cs/TiO2(110). The cesium coverage is varied at room temperature between zero and saturation. Simulations of the MIES spectra for clean and cesiated TiO2(110), based on presently available information on the electronic structure of the clean and cesiated surfaces, facilitate the interpretation of the spectra. The MIES results for low Cs coverages are compatible with fully ionic adsorption of Cs whereby the 6s electron of Cs is transferred to the substrate. This causes reduction of Ti4+ cations as detected with UPS. For larger coverages the charge transfer is incomplete as judged from the formation of temporary negative He−∗(1s2s2) ions during the interaction of the metastable He atoms with the cesiated surface. The ionicity, i.e. the charge transfer per adsorbed Cs atom, decreases from practically unity to about 12% at saturation. At saturation a rearrangement of the oxygen in the surface layer takes place with time. The results suggest that the Ti and Cs ions compete for the bonding to the oxygen.

Adsorption and decomposition of formic acid on potassium-modified Cu(1 1 0)

Adsorption and decomposition of HCOOH on potassium-modified Cu(110) surface has been investigated by temperature-programmed desorption (TPD) and photoemission. It has been found that preadsorbed potassium enhances deprotonation of HCOOH at low temperatures. Potassium, in coverages up to one monolayer, stabilizes a formate species at 300–450K characterized by photoemission peaks at 5.8, 9.0–9.3, 10.9 and 14.3eV in the HeII spectrum. The tendency to create potassium carbonate species has been found at high temperatures for potassium coverages greater than one monolayer.

ELECTRONIC STRUCTURE AND CHEMICAL BONDING IN ALKALINE EARTH METAL SUBNITRIDES: PHOTOEMISSION STUDIES AND BAND STRUCTURE CALCULATIONS

The electronic structure of various alkaline earth metal subnitrides has been studied by X-ray and ultraviolet photoelectron spectroscopy and by LMTO band structure calculations. Improved methods for the synthesis of several of the materials are described. The electronic structure of compounds with discrete clusters (Na 22Ba 14SrN 6) or Ba 3N chains (Ba 3N, NaBa 3N, Na 5Ba 3N) in the crystal structure is characterized by a narrow atomic-like N-2p band occurring between 2 and 2.5 eV in the photoemission spectra. The spectra and the results from the band structure calculations support the idea of ionic bonding between N 3− ions and surrounding partly oxidized alkaline earth metal atoms. The N-2p regions in valence band spectra and density-of-states curves of Ca 2N, Sr 2N and Ba 2N, which have layered crystal structures, extend over a broader energy range, suggesting a more pronounced dispersion of the N-2p band in these compounds. Several feature seen in NeI, HeI, NeII and HeII spectra of the series M 2N are assigned to Auger transitions involving a primary hole in a shallow core level (N-2s or M- np) and either electrons from the metal valence band or from N-2p levels.

The study of HeI photoelectron spectroscopy (PES) of the electronic structure for CIONO 2

The HeI photoelectron spectroscopy of chlorine nitrate (CIONO 2) is reported. The assignment of the HeI spectrum has been made using bandshape and vibrational structures of the bands, as well as GAUSSIAN 86 ab initio SCF MO calculations. The vibrational spacings of three bands in the low ionization potential region (< 13.50 eV) are respectively: 1246 ± 60 cm −1 for the first band, 770 ± 50 cm −1 for the second band and 545 ± 50 cm −1 for the third band.

Spectroscopic study of ion-induced photon emission under D + and He + bombardment

In order to investigate the interaction of energetic ions with a solid surface, we have performed a spectroscopic study of the ion-induced photon emission under D + or He + bombardment with incident energies ranging from 10 to 25 keV. The line spectra of D α (656.10 nm) and HeI (578.76 nm) emitted from the reflected particles in excited states were analyzed in terms of incident ion energy, target material and residual gas pressure. With increasing incident ion energy, both the D α and HeI spectra shifted to a shorter wavelength and broadened, while the photon intensities of both the D α and HeI emission decreased. The intensities also grew with the Z number of the target materials showing a very similar relation to that reported between the reflection coefficients and the Z number. Oxygen at the surface significantly enhanced the HeI emission, whereas the D α emission was hardly influenced. Compared with oxygen, the effects of hydrogen and nitrogen on the intensity were very small. The difference could be attributed to a resonance excitation of the electrons of reflected neutrals.

UPS differentiation between molecular NH 3 and partially dissociated NH 2 fragments adsorbed at low temperature on Si(001) surfaces

For the first time physisorbed non dissociated NH 3 molecules and chemisorbed partially dissociated NH 2 fragments, adsorbed at low temperature on Si(001)-2 × 1 surfaces are clearly differentiated by UPS (ultraviolet photoemission spectroscopy) HeII spectra. This result is obtained by a careful scanning of the relevant UPS features as a function of T s between 80 K where the contribution of the condensed NH 3 molecules prevails and 300 K where only that of NH 2 is prominent. Actually, at 80 K, for sufficiently high exposures, condensed molecules mainly overlay a weak coverage of dissociated species capping themselves some of the Si electronic surface states. The passage at 120 K enables us to observe simultaneously the two species, providing an indubitable proof for their differentiation. This assignment is ascertained by correlated binding energy variations of the N 1s core level signals in relation with the two species.

Photoelectron spectroscopy study of substituent effect of halogens on the thiophene ring

The UV photoelectron spectra of 21 halogen (chloro- bromo- and iodo ) -substituted thiophenes have been studied. The assignment was based on band shapes, correlation of band positions of related compounds and CNDO/S calculations. In the case of compounds bearing iodine in the three and four positions the above considerations gave contradictory results and only the relative intensities in the HeI and HeII spectra provided decisive proof for the assignment. The substituent effect in the α positions differs considerably from that in the β position. To explain these effects a simple conjugative model was proposed. Not only does the achieved assignment fit the correlation along the chloro-bromo-iodo series well, but it also provides a simple method for constructing the electronic structure of polyhalogenated systems from simpler building blocks of molecules with a lower degree of substitution (e.g. the band structure of the tetrachloro derivative from those of 2,5-dichloro- and 3,4-dichloro-thiophenes). This proves that a consistent picture was obtained of the electronic structure of halogen thiophenes.

Photoemission valence band spectra and electronic density of states in copper oxides and copper based ceramic super-conductors

Photoemission valence band spectra with three different photon energies (21.2 eV, 40.8 eV, 1253.6 eV) for Cu2O and CuO and a number of copper oxide based superconducting ceramics are investigated, namely La2CuO4 (LCO), Y1Ba2Cu3O7 (YBCO), Bi2Sr2CaCu2O8 (BSCCO) and Tl2Ba2Ca2Cu3O10 (TBCCO). From the Cu2O data it can be infered, that the He-II (40.8 eV) spectra give a fair representation of the density of states (DOS). In addition the agreement between the calculated density of states (DOS) and the He-II photoemission spectrum is almost perfect for Cu2O. This agreement is worse for CuO and the other materials which all have a Cu groundstate close to a 3d 9 configuration indicating a large contribution of thed-d correlation energy to the excitation spectra which is absent in Cu2O because of the filled 3d-shell. In all cases the experimental DOS atE F is very small and only the spectra of BSCCO show a well defined Fermi edge. The relevance of these findings with respect to the theoretical local functional density DOS calculations is discussed.

A study of the electronic structures of n v addition compounds of BH 3 by a combined use of ups and eels

Orbital energies and electronic transition energies of BH 3·H 2S and BH 3·CO obtained from ultraviolet (HeI) photoelectron spectroscopy and electron energy loss spectroscopy are discussed in the light of quantum mechanical calculations. BH 3·H 2O has been characterized, for the first time, by means of the HeI spectrum and the ionization energies assigned to the various orbitals based on calculations.

The behaviour of certain helium lines in the P Cygni spectrum

Sixty-two spectrograms of the supergiant P Cyg, obtained with the Coude spectrograph of the 2 m RCC telescope at NAO (National Astronomical Observatory, Bulgaria) were used in order to study the behaviour (we mean the displacements of the absorption components only) of the Hei spectrum of this star. Twelve lines from four series (23P0−n3S, 23P0−n3D, 21P0−n1D, and 21S−n1P0) were examined. It was established that all observed cases can be reduced to a few (three or four) which are typical. These cases are stated in the form of the comparison between the hydrogen and helium lines. All profiles were rectified and their complex structure was taken into consideration. The fixed peculiarities were explained by an optical depth effect.

A mercaptide intermediate on Cu(111)

Angle resolved ultraviolet photoemission (ARUPS) and sulphur K-edge near edge X-ray absorption fine structure (NEXAFS) have been used to study the interaction of dimethyl disulphide ((CH 3 S) 2 ) and methyl mercaptan (CH 3 SH) on Cu(111) Both species are adsorbed molecularly at low temperature while on warming to, or exposing at, room temperature, a single common species is found which we identify as methyl mercaptide, CH 3 S-. ARUPS from this species shows only three peaks in the HeI spectra which were ascribe to an increase in the symmetry to C 3v . Assinging the first stron gpeak in the NEXAFS spectrum to a σ CS shape resonance, we find that the polarisation dependence of this peak indicates a tilt in the C-S bond away from the surface normal of approximately 30°. This degree of tilt is apparently insufficient to introduce significant loss of energy degeneracy in the valence orbitals probed by ARUPS.

Studies of molecular oxygen adsorbed on Cu surfaces

Molecular oxygen adsorbed on (110) and polycrystalline Cu surfaces has been investigated by UPS, XPS, AES, HREELS and LEED. Molecularly adsorbed O 2 on the (110) surface shows the characteristic three-peak HeII spectrum due to π g, π u and σ g orbitals, accompanied by an O-O stretching frequency at 660 cm -1. On the polycrystalline Cu surface, adsorbed O 2 shows the three peak HeII spectra with a considerably smaller separation between the π u and σ g band and two O-O stretching bands at 610 and 880 cm -1. O 2 adsorbed on the Cu(110) surface gives rise to a (1×1) LEED pattern and characteristic Kπ ∗π ∗ transition in the Auger spectrum.

Semi-empirical pertci ionization spectra of some cumulenes and cumulene-like systems. Low-energy non-koopmans' ionizations

A theoretical approach to the question of satellite structure in the photoelectron spectra of cumulenes and cumulene-like systems is made. PERTCI calculations including all singles and doubles with respect to the Koopmans' and non-Koopmans' configurations were made using semi-empirical MNDO, CNDO/S and LNDO/S wavefunctions. The method was applied to dimethylcarbodiimide, fulvenallene, fulvenone, fulventhione, diazocyclopentadiene, butatriene, tetrafluorobutatriene, pentatetraene, carbon suboxide and carbon subsulfide. Calculated ionization spectra for all these systems are presented. They are compared with the He-I photoelectron spectra. For butatriene, the results are in accord with preceding ab initio findings. Except for dimethylcarbodiimide the calculated ionization spectra exhibit low-energy satellite structure (i.e. more ionizations of reduced intensity than expected on the basis of the Koopmans' configurations alone). From the comparisons of the calculated and measured spectra, hints to possible shake-up structures in the He-I spectra of the molecules considered are given. The results call for refined theoretical and especially experimental investigations in order to substantiate the present theoretical predictions.

Uv photoelectron spectrum of o-xylylene — detection of a low-energy non-koopmans (shake-up) ionization

The HeI spectrum of the transient title compound has been obtained using the variable-temperature photoelectron- spectroscopy technique. Strong experimental evidence for a low-energy shake-up structure in the ionization pattern of the polyene-like system is presented. Agreement between the semi-empirical PERTCI and experimental ion-state energies is very good.