bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical models for charge collection efficiency and variance in semiconductor spectrometers
Authors: Ruzin A. , Nemirovsky Y.
Published: July 2002

Charge collection efficiency and the variance in the collected charge of semiconductor spectrometers are modeled. The model is based on a statistical approach and the extended Ramo theorem. The model yields an expression for variance in charge collection efficiency as a function of photon energy, bias voltage, and semiconductor parameters. These calculations as a function of absorption depth are particularly important in semiconductors with high atomic numbers, such as CdZnTe, since in these materials a uniform absorption cannot be assumed for a wide range of energies. Three different spectrometer configurations were considered: resistive, partially depleted Schottky barrier, and fully depleted Schottky barrier. An analytical model for the resistive configuration is presented and the results are compared to numerically obtained results of the Schottky configuration.
Oxidation characteristics and magnetic properties of iron ultrafine particles
Authors: Li X. G. , Chiba A. , Takahashi S. , Sato M.
Published: July 2002

Oxidation of Fe ultrafine particles has been studied in air at temperatures ranging from 300 to 873 K. It is found that γ-Fe2O3 and α-Fe2O3 are formed at different temperatures, which leads to significant changes in the particle features and magnetic properties.
Development of ferroelectric mixed states in a random field of static defects
Authors: Semenovskaya S. , Khachaturyan A. G.
Published: July 2002

A para→ferroelectric stress-free transition in a random field of static defects is studied by the computer simulation method. It is shown that the electrostatic dipole–dipole interaction may produce in the ferroelectric transition the same “martensitelike” effects as the strain-induced interaction in the martensitic transition. These effects are the formation of the mixed two-phase equilibrium between the ferroelectric and paraelectric phases, the thermal hysteresis, and the loss of ergodicity. The origin of the mixed state is discussed. It is shown that the free energy hypersurface forms a multiplicity of local minima corresponding to the metastable and stable states similar to those in a spin-glass system. The hysteresis loop obtained in computer simulations is a locus of projections of the local free energy minima points on the hysteresis loop plane. The pre-transitional mixed states predicted in this simulation can be attributed to the relaxor state observed in many ferroelectric materials.
Photoluminescence and transport studies of boron in 4H SiC
Authors: Sridhara S. G. , Clemen L. L. , Devaty R. P. , Choyke W. J. , Larkin D. J. , Kong H. S. , Troffer T. , Pensl G.
Published: July 2002

Two distinct boron-related centers are known in silicon carbide polytypes, one shallow (ionization energy ∼300 meV) and the other deep (∼650 meV). In this work, 4H SiC homoepitaxial films are intentionally doped with the shallow boron center by controlling the silicon to carbon source gas ratio during chemical vapor deposition, based on site competition epitaxy. The dominance of the shallow boron center for samples grown with a low Si/C ratio, favoring the incorporation of boron onto the silicon sublattice, is verified by the temperature dependent Hall effect, admittance spectroscopy and deep level transient spectroscopy. In these samples a peak near 3838 Å appears in the low temperature photoluminescence spectrum. Further experiments support the identification of this peak with the recombination of a four particle (bound exciton) complex associated with the neutral shallow boron acceptor as follows: (1) The intensity of the 3838 Å peak grows with added boron. (2) Momentum conserving phonon replicas are observed, with energies consistent with other four particle complexes in SiC. (3) With increasing temperature excited states are observed, as for the neutral aluminum and gallium acceptor four particle complexes. However, the intensity of the shallow boron spectrum is quenched at lower temperatures than the corresponding spectra for Al and Ga, and the lineshapes are strongly sample dependent. These results may be related to the unusual configurational and electronic structure of this center inferred from recent spin resonance experiments by other groups. When the Si/C ratio is high, the optical signatures of the deep boron center, nitrogen-boron donor-acceptor pairs and conduction band to neutral acceptor free-to-bound transitions, are observed in the photoluminescence. At T=2 K well resolved, detailed nitrogen-boron pair line spectra are observed in addition to the peak due to distant pairs. As the temperature is raised, the donor-acceptor pair spectrum decreases in intensity while the free-to-bound no-phonon peak appears. Extrapolation of the temperature dependence of the free-to-bound peak to T=0 K, after correction for the temperature dependence of the exciton energy gap, leads to the value EA(B)−EX=628±1 meV, where EA(B) is the ionization energy of the deep boron center and EX is the binding energy of the free exciton which, for 4H SiC, can only be estimated at this time.
Generalized analytic formulae for magneto-optical Kerr effects
Authors: You Chun-Yeol , Shin Sung-Chul
Published: July 2002

We have developed simplified analytic expressions for magneto-optical Kerr effects of both optically thick and ultrathin films in the general case, where a magnetic medium had an arbitrary direction of magnetization and a beam of light was obliquely incident to the medium. It was found that the simplified analytic formulae for the Kerr effects of p and s waves consisted of a product of two factors for both optically thick and ultrathin films: the prefactor dependent only on the optical parameters of the system and the main factor of the polar Kerr effect for a normal incidence case. We have also derived some useful relations among the Kerr effects in the polar and longitudinal configurations. We have demonstrated that the theoretical calculations using the present analytic formulae could well match the experimental polar and longitudinal Kerr rotation angles of magnetic films measured with varying incident angles.
Efficient XeI* excimer ultraviolet sources from a dielectric barrier discharge
Authors: Zhang Jun-Ying , Boyd Ian W.
Published: July 2002

A dielectric barrier discharge in mixtures of xenon and iodine to provide intense narrow band ultraviolet (UV) radiation at λ=253 nm (XeI*) has been investigated. The characteristics of the emission spectra of the excited dimer (excimer) and UV intensity formed from mixtures of xenon and iodine for different total gas pressures are reported. The absolute UV output power of the XeI* lamp has been determined using actinometry based on the photohydrate of uridine. The electrical power dependence of the UV intensity generated as well as the conversion efficiency of the lamps has also been studied. Conversion efficiencies (from input electrical to output optical energy) as high as 22.5% have been achieved for an electrical power input of 70 W, with 15.75 W of optical output being generated.
Photoluminescence study of the interface in type II InAlAs–InP heterostructures
Authors: Vignaud D. , Wallart X. , Mollot F. , Sermage B.
Published: July 2002

Spatially indirect radiative recombinations (type II) have been studied in InAlAs–InP heterostructures grown by gas source molecular beam epitaxy with emphasis on the direct (InAlAs grown on InP) or inverse (InP on InAlAs) interface composition profile. Based on the results of their injection-dependent energy, lifetime and polarization, a new transition scheme is proposed: type II transitions have a low injection limit between 1.27 and 1.28 eV, a long lifetime (τ>1 μs) and strongly shift towards higher energy when increasing the injection. The type II recombination is polarized, the direction of maximum intensity being correlated with the expected interface structure. Lower energy transitions (E⩽1.2 eV) indicate the presence of a well transition material at the interface: they should be better labeled as mixed type I–II. Previously published results are also reconsidered and seem to fit well within this model.
Diamond deposition by chemical vapor deposition process: Study of the bias enhanced nucleation step
Authors: Barrat S. , Saada S. , Dieguez I. , Bauer-Grosse E.
Published: July 2002

In the case of diamond films synthesized by the microwave plasma assisted chemical vapor deposition technique (MPCVD), the bias enhanced nucleation (BEN) step has been developed to avoid the scratching pretreatment and to palliate the low nucleation density of diamond crystals on the most common substrate used: pristine single silicon substrates. This treatment that occurs before the diamond growth step often consists in the application of a negative bias voltage of the substrate/substrate holder system, which is electrically insulated from the reactor wall. In the case of the MPCVD process, this bias induces a complex superposition of two cold discharges: the microwave and the bias discharges. Unfortunately, this complex configuration leads to inhomogeneous deposits in terms of quality, nucleation rate, and thickness. Furthermore, the reproducibility of the BEN step is generally poor in terms of diamond deposits and electrical BEN parameters. In order to better understand and overcome this pretreatment step, we have studied the temporal and spatial evolution of the bias discharge according to diamond propagation (in terms of kinetic and geometrical effects) and its electron emission, the nature and the shape of the substrate holder (in terms of aging and point effects). We have shown that the presence of the bias plasma is necessary. Based on this observation, we propose a phenomenological mechanism to explain the heterogeneous deposit and the poor reproducibility. Our results with a MPCVD reactor confirm the proposed model and some experimental modifications allow us to obtain homogeneous diamond films elaborated with reproducible electrical parameters. This work would permit the synthesis of a large area of highly oriented films obtained by BEN on single silicon substrates.
Growth and composition of dual-plasma polymer-like amorphous carbon films
Authors: Godet C. , Heitz T. , Bourée J. E. , Drévillon B. , Clerc C.
Published: July 2002

Polymer-like hydrogenated amorphous carbon (a-C:H) films have been grown in a dual-plasma [radio frequency (rf)–microwave (MW)] reactor using butane as a carbon precursor and different mixtures (H2–Ar or He–Ar) in the MW plasma in order to vary the atomic hydrogen flux impinging on the growing film. Decreasing the rf power (i.e., the energy of ion bombardment on a-C:H) or increasing the H atom flux both result in a wide band gap H-rich polymer-like a-C:H network. Nuclear techniques have been combined with UV-visible ellipsometry to determine the stoichiometry, density and growth rate of a-C:H films as a function of the plasma parameters. Parametrization of UV-visible optical properties allows to monitor the changes in the optical parameters (optical gap and density of π states) attributed to the formation of structural units containing C=C double bonds. C–H bonds observed by in situ infrared ellipsometry have been used to investigate the role of ions in the growth processes (densification, cross linking) and to interpret the observed changes in optical parameters in terms of a two-phase microscopic description of polymer-like a-C:H. In order to understand the kinetic results, a phenomenological growth model is proposed including the respective roles of ion bombardment and H atom flux in the activation and deactivation of surface sites available for CxHy radical incorporation (chemisorption) to the growth zone. The activation mechanism corresponds to a chemical modification at the growth zone sites. This adlayer model includes the physisorption of both H atoms and CxHy radicals and explains why the temperature dependence of the deposition rate is found to be opposite for the limiting cases of low and high H atom fluxes. Some consequences of the model on the film stoichiometry (H/C ratio) and microstructure (sp2 C/sp3 C ratio) have also been evaluated.
Photoluminescence from SiO2 films containing Si nanocrystals and Er: Effects of nanocrystalline size on the photoluminescence efficiency of Er3+
Authors: Fujii Minoru , Yoshida Masato , Hayashi Shinji , Yamamoto Keiichi
Published: July 2002

SiO 2 films containing Si nanocrystals (nc-Si) and Er were prepared and their photoluminescence (PL) properties were studied. The samples exhibited PL peaks at 0.8 and 1.54 μm, which could be assigned to the electron-hole recombination in nc-Si and the intra-4f transition in Er3+, respectively. Correlation between the intensities of the two PL peaks was studied as functions of the size of nc-Si, Er concentration, excitation power and excitation wavelength. It was found that the 1.54 μm PL of Er3+ is strongly enhanced by incorporating nc-Si in films. Furthermore, the intensity of the 1.54 μm peak was found to depend strongly on the size of the incorporated nc-Si.
Disturbed state constitutive model for thermomechanical behavior of dislocated silicon with impurities
Authors: Desai Chandra S. , Dishongh Terrance J. , Deneke Paul
Published: July 2002

A constitutive modeling approach, called the disturbed state concept (DSC), is developed to characterize the mechanical response of silicon with dislocations and oxygen impurity. Factors such as dislocation density, temperature, strain rate, and oxygen concentration are included in the model. The parameters for a silicon crystal are calibrated based on stress–strain data reported by Dillon et al. [J. Appl. Phys. 60, 1784 (1986)] and those for silicon with oxygen impurity based on laboratory stress–strain data reported by Yonenaga et al. [J. Appl. Phys. 56, 2346 (1984)]. The predictions from the DSC model are compared with those based on the model proposed by Dillon et al. [J. Appl. Phys. 60, 1784 (1986)], and with laboratory test data by Yonenaga et al. [J. Appl. Phys. 56, 2346 (1984)]. The correlation between the DSC predictions and test data is very good. It is believed that the DSC can provide a unified and improved constitutive model, compared to previously available models, for the thermomechanical behavior of silicon and other materials.
Role of space charge in scanned probe oxidation
Authors: Dagata J. A. , Inoue T. , Itoh J. , Matsumoto K. , Yokoyama H.
Published: July 2002

The growth rate and electrical character of nanostructures produced by scanned probe oxidation are investigated by integrating an in situ electrical force characterization technique, scanning Maxwell-stress microscopy, into the fabrication process. Simultaneous topographical, capacitance, and surface potential data are obtained for oxide features patterned on n- and p-type silicon and titanium thin-film substrates. The electric field established by an applied voltage pulse between the probe tip and substrate depends upon reactant and product ion concentrations associated with the water meniscus at the tip-substrate junction and within the growing oxide film. Space-charge effects are consistent with the rapid decline of high initial growth rates, account for observed doping and voltage-pulse dependencies, and provide a basis for understanding local density variations within oxide features. An obvious method for avoiding the buildup of space charge is to employ voltage modulation and other dynamic pulse-shaping techniques during the oxidation pulse. Voltage modulation leads to a significant enhancement of the growth rate and to improvements in the aspect ratio compared with static voltage pulses.
X-ray studies of polymer/gold nanocomposites
Authors: Lin Binhua , Morkved Terry L. , Meron Mati , Huang Zhengqing , Viccaro P. James , Jaeger Heinrich M. , Williams Scott M. , Schlossman Mark L.
Published: July 2002

We demonstrate that x-ray reflection standing wave fluorescence spectroscopy is a highly sensitive technique for the determination of the distribution of metal nanoparticles inside polymer thin films. We investigate both the depth profile and the in-plane spatial correlation of gold nanoparticles in an asymmetric polystyrene-b-poly(2-vinylpyridine) block copolymer film. These copolymer films self-assemble into alternating, nanometer-sized domains that are, upon annealing, selectively decorated by thermally evaporated metal nanoparticles to form metal/polymer composites.
Laser crystallization and structural characterization of hydrogenated amorphous silicon thin films
Authors: Toet D. , Smith P. M. , Sigmon T. W. , Takehara T. , Tsai C. C. , Harshbarger W. R. , Thompson M. O.
Published: July 2002

The influence of hydrogen and film thickness on the excimer laser-induced crystallization of plasma-enhanced chemical-vapor deposition-grown hydrogenated amorphous silicon thin films has been studied. Films with hydrogen concentrations varying from 1 to 10 at. % have been crystallized using short pulses from a XeCl excimer laser, at fluences sufficient to induce melting throughout the thickness of the films. Dehydrogenation prior to laser exposure was necessary only for films having initial hydrogen concentrations larger than 5 at. %. The structural properties of the laser-crystallized materials were studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM measurements revealed that films requiring no dehydrogenation are smoother after laser crystallization than those requiring laser dehydrogenation, and that the roughness of these films increases as a function of film thickness, although it could be reduced by multiple laser exposure. Smoother films (root-mean-square roughness ∼4.1 nm) were obtained by melting only the near-surface region of the film after the initial full melt exposure. TEM revealed that the laser-crystallized films consist of grains with sizes that are comparable to the film thickness.
Field-effect passivation of the SiO2Si interface
Authors: Glunz S. W. , Biro D. , Rein S. , Warta W.
Published: July 2002

The field-effect passivation of the interface of thermal oxides on silicon is experimentally investigated by depositing corona charges on the oxide of solar cells and of lifetime test structures. The open circuit voltage of solar cells with interdigitated rear contacts can be increased by +12 mV or decreased by −34 mV, respectively, by depositing positive or negative corona charges on top of the front oxide. The resulting effective surface recombination velocity, Seff, is determined on carrier lifetime test structures for different injection levels and charge densities using microwave-detected photoconductance decay and a new expression for the Auger-limited bulk lifetime. Seff can be varied between 24 cm/s and 538 cm/s on a 1 Ω cm p-type wafer with a thermal oxide of 105 nm thickness. The measurements are compared with theoretical predictions of an analytical model for the calculation of the surface recombination. Measured values for the capture cross sections and interface trap densities are used for the calculation. The model predicts an optimum passivation for strong positive compared to strong negative charge densities. This is due to the asymmetry of the capture cross sections for electrons and holes. This prediction is in very good agreement with the measured Seff values. However, the predicted Seff values of well below 1 cm/s for 1 Ω cm p-type silicon cannot be achieved in the experiment. This discrepancy can be explained by an inhomogeneous charge distribution resulting in potential fluctuations and additional loss currents. With a new extended analytical model for the calculation of Seff the measured Seff values can be described quantitatively.
A method to predict electromigration failure of metal lines
Authors: Sasagawa Kazuhiko , Naito Kazushi , Saka Masumi , Abé Hiroyuki
Published: July 2002

A new calculation method of atomic flux divergence (AFDgen) due to electromigration has recently been proposed by considering all the factors on void formation, and AFDgen has been identified as a parameter governing void formation by observing agreement of the numerical prediction of the void with experiment. In this article, a method to predict the electromigration failure of metal lines was proposed by using AFDgen. Lifetime and failure site in a polycrystalline line were predicted by numerical simulation of the processes of void initiation, its growth to line failure, where the change in distributions of current density and temperature with void growth was taken into account. The usefulness of this prediction method was verified by the experiment where the angled aluminum line was treated. The failure location was determined by the line shape and the operating condition. The present simulation accurately predicted the lifetime as well as the failure location of the metal line.
Cooperative magnetic behavior in the coordination polymers [Cu3(TMA)2L3] (L=H2O, pyridine)
Authors: Zhang Xi Xiang , Chui Stephen S.-Y. , Williams Ian D.
Published: July 2002

The temperature and magnetic field dependent magnetization of the polymeric solids [Cu3(TMA)2L3], where TMA=benzene-1,3-5-tricarboxylate and L=H2O or pyridine have been measured up to 5 T. These polymers contain dimeric [Cu2(O2CR)4] units. Discrete dimers in molecules such as cupric acetate have strong antiferromagnetic coupling between their two Cu d9 centers and at low T their susceptibility diminishes to nearly zero. While at higher T (300–100 K) the magnetism of the [Cu3(TMA)2L3] polymers resembles that of individual the [Cu2] dimer molecules, at low T a new cooperative behavior is found, with a sharp increase in χ seen at temperature below 75 K. In the case of L=pyridine a plot of 1/χ vs T shows linearity from 65 to 20 K and fits the Curie–Weiss law χ=C/(T− Θ ). Θ is found to be +4.7 K, indicative of a weak ferromagnetic coupling between dimers. This interaction may be facilitated by the planar aromatic bridges between the [Cu2] units.
Kinetic aspects of the growth of hydrogen induced platelets in SiC
Authors: Grisolia J. , Cristiano F. , De Mauduit B. , Assayag G. Ben , Letertre F. , Aspar B. , Di Cioccio L. , Claverie A.
Published: July 2002

Annealing of heavily hydrogen-implanted silicon carbide (SiC) leads to the formation of one specific type of defect: hydrogen induced platelets. These defects may be regarded as two-dimensional precipitates of H atoms stored in a stable configuration. In this article, we have studied the growth kinetics of these platelets upon annealing in the 800–1000 °C range by transmission electron microscopy. We show that the growth of these defects proceeds through the exchange of H atoms with the result that larger ones grow at the expense of the smaller ones during annealing. This process can be described in terms of a conservative Ostwald ripening mechanism. The activation energy for this growth is found to be about 3.4 eV, a value similar to that observed for the “effective” diffusion of H in heavily H-implanted SiC.
Influence of oxygen stoichiometry on the electronic properties of La4Ni3O10±δ
Authors: Carvalho M. D. , Cruz M. M. , Wattiaux A. , Bassat J. M. , Costa F. M. A. , Godinho M.
Published: July 2002

Some different oxygen content La4Ni3O10+δ (δ=0.02,0.12,−0.22) compounds were prepared in order to study the influence of Ni3+ content on the electronic properties. A metal–metal transition was detected at 140 K for the as-prepared (La4Ni3O10.02) and reduced (La4Ni3O9.78) compounds. From the electrical conductivity, Seebeck coefficient and magnetic studies of the different samples, oxygen stoichiometry was found to be a critical parameter in defining the existence of a less conductive low temperature state. The results are interpreted assuming the existence of charge order in the nickel oxide planes of the layered perovskite-type structure.
First observation of the f(0+g) ion-pair state of Cl2 by optical–optical double resonance
Authors: Ishiwata Takashi , Shinzawa Tsutomu , Kusayanagi Tatsuo , Tanaka Ikuzo
Published: July 2002

The two-photon excitation spectrum of Cl2 using two tunable dye lasers has been studied in the 7 eV energy range. The f(0+g) ion-pair state correlating with Cl−(1S)+Cl+(3P0) at the dissociation limit appeared in the spectrum resulting from an optical–optical double resonance transition through the B 3Π(0+u) state. The absolute vibrational numbering of the f(0+g) state was established by direct observation of the f(0+g)–B 3Π(0+u) fluorescence. The molecular constants for the Cl2 f(0+g) state are derived (in cm−1 and 3σ in parentheses): Te=59 356.124(41), ωe=256.981(30), ωexe=1.2032(83), ωeye =0.003 69(47), Be=0.115 03(10), αe=7.58(17)×10−4, and D=1.44(68)×10−7.
VUV-optical oscillator strength distributions of C3H6 and C4H8 isomers
Authors: Koizumi Hitoshi , Yoshimi Tohru , Shinsaka Kyoji , Ukai Masatoshi , Morita Makoto , Hatano Yoshihiko , Yagishita Akira , Ito Kenji
Published: July 2002

The photoabsorption cross sections of C3H6 isomers (cyclopropane and propylene) and C4H8 isomers (1-butene, iso-butene, cis-2-butene, and trans-2-butene) have been measured in the wavelength region from 300 Å to the ionization potential of each molecule (about 1200–1400 Å) using an ionization chamber with four ion-collection plates. The obtained photoabsorption cross sections show a maximum value at about 700–800 Å for each molecule. In the wavelength region which is shorter than about 500 Å the cross sections are almost the same among the isomers and equal to the sum of the cross sections for the constituent atoms. In the longer wavelength region, however, the cross sections have different peaks and shoulders depending on an isomer, i.e., on its molecular structure. The obtained cross section values, together with previously reported ones in the wavelength region outside of the present experiment, satisfy the Thomas–Kuhn–Reiche sum rule within experimental errors.
The structure of mixed alkali halide clusters
Authors: Diefenbach J. , Martin T. P.
Published: July 2002

Mixed clusters composed of two alkali halides have been produced by combining the two vapors at the time of cluster condensation, and also by evaporating a premixed solid solution. Mass spectra show that mixed cluster ions are stable even for compositions that do not form solid solutions in the condensed phase. However, the mass spectra are not consistent with a simple random mixing of the two alkali halides. Total energy calculations indicate that clusters with certain sizes and compositions have somewhat enhanced stability.
Photoionization of ammonia clusters: Detection and distribution of unprotonated cluster ions (NH3)+n, n=2–25
Authors: Shinohara Hisanori , Nishi Nobuyuki , Washida Nobuaki
Published: July 2002

Unprotonated ammonia cluster ions (NH3)+n of various sizes (n=2–25) are detected for the first time, in addition to normally observed protonated ions (NH3)n−1H+, via the molecular-beam VUV photoionization by using Ar (11.83, 11.62 eV), Kr (10.64, 10.03 eV), and Xe (9.57 eV) resonance lamps. The observation of the species indicates that there exist potential minima for the various (NH3)+n cluster ions along the proton-transfer reaction coordinates. Considerations based on intracluster ion–molecule reactions reveal that the unprotonated cluster ions have the proton-transferred structures of (NH3)n−2⋅NH+4  ⋅⋅⋅ NH2 rather than (NH3)+n which usually lead to the dissociated protonated products (NH3)n−1H+. Furthermore, the results also provide evidence that the so-called ‘‘magic numbers’’ found in the intensity distributions of both the protonated (NH3)4⋅NH+4 and unprotonated (NH3)+5, i.e., (NH3)3⋅NH+4⋅NH2 cluster ions suggest the special stability of a certain ion cluster rather than that of the corresponding neutral cluster.
Theoretical study of the spin-orbit autoionization in molecules application to the HI photoionization spectrum
Authors: Lefebvre-Brion H. , Giusti-Suzor A. , Raşeev G.
Published: July 2002

A theoretical study of the spin-orbit autoionization in molecules with 2Π state ion cores is presented. The multichannel quantum defect theory is used with an ab initio calculation of the relevant electronic quantities. In the application to the HI photoionization spectrum the energy positions of the observed resonances between the 2Π1/2 and 2Π3/2 ionization thresholds are well reproduced. The importance of the l mixing especially for the π3σ channels is stressed. An attempt to include the S-uncoupling part of the rotational operator in the calculations gives rise to additional autoionization peaks, associated with a new type of rotational autoionization specific to 2Π-state cores. Theoretical predictions are also made for the angular distribution and spin polarization parameters of the photoelectron in the 1155–1168 Å energy range.
Calculation of NMR chemical shifts. VI. Gauge invariant and Hermitian condition
Authors: Fukui H. , Miura K. , Shinbori H.
Published: July 2002

A clear definition for the division of the magnetic shielding tensor (σ) into two terms of the diamagnetic part (σ d) and paramagnetic part (σ p) is proposed based on the gauge invariant coupled-Hartree–Fock (GI–CHF) theory. In this definition, both σ d and σ p are written individually by gauge invariant and Hermitian operators. The present formulas are applied to calculate the magnetic shielding tensors of diatomic molecules, and lead to better results, even with small basis sets, than gauge dependent results obtained with large basis sets.
Ammonia dimer: A surprising structure
Authors: Nelson D. D. , Fraser G. T. , Klemperer W.
Published: July 2002

High resolution rotational spectra for (14NH3)2, (14ND3)2, 15NH3–14NH3, and 14NH3–15NH3 have been obtained using the molecular beam electric resonance technique. For 14NH3 dimer the spectra are simple and consist only of the J=0–1 and J=1–2 transitions with K=0 for two vibrational states (hereafter designated α and β) of the complex. The spectroscopic constants for (14NH3)2 are: 1/2 (B+C)α=5110.412(2), 1/2 (B+C)β=5110.564(2), DJ=0.0529(5), eqQ1aa =−0.639(13), eqQ2aa =0.904(8) (MHz);  μαa =0.7497(10),  μ βa =0.7376(9) (D). For (ND3)2 the vibrational states are unsplit 1/2 (B+C)=4190.300(50) MHz and  μa=0.567(4) D. These spectroscopic constants are inconsistent with the theoretically predicted structure for NH3 dimer. In the predicted structure one NH3 unit hydrogen bonds to the other with a nearly linear N–H- –N arrangement so that the C3 axis of the basic NH3 is collinear with the hydrogen bond. If θi is the angle between the C3 axis of the ith NH3 unit and the a-inertial axis of the complex, θ1∼0° and θ2∼68° for

the theoretically predicted structure. In contrast, for 14NH3 dimer we measure the average values of θ1 and θ2 as 48.6(1)° and 115.5(1)°, respectively, using the quadrupole hyperfine structure. Furthermore, the component of the dipole moment along the a-inertial axis,  μa, is found to be much smaller (0.75 D) than expected for the theoretical structure (∼2 D). The distance between the centers of mass of the NH3 subunits, RCM, is also determined and is found to be 3.3374(1) Å. No evidence exists for nonrigidity in the ground vibrational state of this complex, i.e., no high frequency tunneling splittings were observed in either the microwave or the infrared spectrum of the complex. The relationship between these structural results and the concept of hydrogen bonding in NH3 systems is discussed. The apparent inability of measured values of heats of formation for NH3 dimer to correctly predict dissociation energies of the complex is also commented upon. The dissociation energy of NH3 dimer has been previously determined as less than 2.8 kcal/mol.
Ionization in K(n
d)–SF6 and K(n
d)–CCl4 collisions at intermediate n
Authors: Zollars B. G. , Walter C. W. , Lu F. , Johnson C. B. , Smith K. A. , Dunning F. B.
Published: July 2002

Rate constants for ion production in collisions between potassium atoms in nd Rydberg states and SF6 and CCl4 have been investigated over a range of principal quantum numbers n between 9 and 45. The data suggest that, at the lower values of n, electrostatic interactions between the product ions become important and lead to a marked decrease in free ion production for both target species. The data also demonstrate that only a small fraction of the energy released by dissociative electron attachment to CCl4 appears as kinetic energy of the Cl− fragment.
Theoretical study of small silicon clusters: Equilibrium geometries and electronic structures of Sin (n=2–7,10)
Authors: Raghavachari Krishnan
Published: July 2002

The geometries and energies of small silicon clusters have been investigated in a systematic manner by means of accurate ab initio calculations. The effects of polarization functions and electron correlation have been included in these calculations. Several geometrical arrangements and electronic states have been considered for each cluster. All the geometries considered have been completely optimized within the given symmetry constraints with several basis sets at the Hartree–Fock level of theory. Single point calculations have been performed at these geometries using complete fourth-order perturbation theory with the polarized 6-31G* basis set. The effects of larger basis sets including multiple sets of polarization functions have been considered for Si2 and Si3. Singlet ground states are found for Si3–Si7 with the associated geometries corresponding to a triangle, a planar rhombus, a trigonal bipyramid, an edge-capped trigonal bipyramid, and a tricapped tetrahedron, respectively. The best calculated structure for Si10 corresponds to a tetracapped octahedral arrangement where alternate faces of the octahedron have been capped to yield a structure with overall tetrahedral symmetry. All the geometries are considerably different from those derived from microcrystal fragments. Binding energies have been computed for all clusters and used to interpret the distribution and fragmentation patterns of small silicon cluster ions observed recently.
Autoionization between the ionization thresholds for the 2Π3/2 and 2Π1/2 states of HI+
Authors: Carlson Thomas A. , Gerard Patrick , Krause Manfred O. , Von Wald Grant , Taylor James W. , Grimm Frederick A.
Published: July 2002

Angle-resolved photoelectron spectroscopy was carried out on hydrogen iodide using synchrotron radiation. Applying the constant ionic state (CIS) method, studies were made on resonances associated with autoionization between the ionization potential for the two spin-orbit split lone pair orbitals, π3/2 and π1/2. Data on both partial cross sections and angular distribution parameters, β, were obtained for the production of the first ionic state 2Π3/2 (v=0). These data indicate the presence of several series of Rydberg states leading to the second ionization potential 2Π1/2 (v=0). Both broad and sharp resonances were observed. A detailed comparison of the data is made with a MQDT calculation in the range of 10.6 to 10.73 eV. Although theory predicts some of the features correctly, there is much room for improvement. Results on the first excited vibrational state [2Π3/2 (v=1)] are also presented and the effects of vibrational states are discussed.
The generalized H theorem in the Hilbert theory
Authors: Delale Can F.
Published: July 2002

The Hilbert expansion of the generalized H theorem for a multiple gas mixture is considered. It is shown that the first approximation of the Hilbert theory results in entropy conservation for ideal flow. The law of increase of entropy is discussed in the second approximation with the entropy flux derived and with the expression for the local rate of entropy production being positive definite.
Photoelectron spectroscopy of the negative ion SeO−
Authors: Coe J. V. , Snodgrass J. T. , Freidhoff C. B. , McHugh K. M. , Bowen K. H.
Published: July 2002

We have recorded the photoelectron spectrum of SeO− using a newly constructed negative ion photoelectron spectrometer. The adiabatic electron affinity of SeO is determined to be 1.456±0.020 eV. Values of ν00(a 1Δ–X 3Σ−0+) and ΔG1/2(a 1Δ) are found to be 5530±200 and 916±35 cm−1, respectively, in substantial accord with previous measurements. The negative ion parameters determined in this work are: B″e(SeO−) =0.4246±0.0050 cm−1 which leads to  r′e(SeO−)=1.726±0.010 Å, ω″e(SeO−)=730±25 cm−1, ω′e x″e(SeO−)=2±4 cm−1, and  D0(SeO−)=3.84±0.09 eV. In addition, the spectroscopic parameters of SeO− are compared with those of the electronically analogous negative ions: O−2, SO−, and S−2.
State selective population of H2(v″=1, J″=1) and D2(v″=1, J″=2) and rotational relaxation in collisions with H2, D2, and He
Authors: Meier W. , Ahlers G. , Zacharias H.
Published: July 2002

Single rotational states were populated in vibrationally excited hydrogen by stimulated Raman pumping. The population in H2 X 1∑+g(v″=0,1) and D2 X 1∑+g(v″=0,1) was probed state selectively by tunable vacuum ultraviolet (VUV) laser radiation around λ=110 nm, and the fluorescence induced when exciting the hydrogen molecules in the (0–0), (1–0), (2–0), (3–1), and (4–1) Lyman bands of the (B 1∑+u←X 1∑+g) transition monitored. From a comparison of line heights, the stimulated Raman pumping efficiency is estimated to be 30%–50% in the focal volume. Rotational transitions in X 1∑+g(v″=1) were induced in collisions with H2, D2, and He. State-to-state rotational relaxation rates were measured for the (J″=1→J″=3) transition in H2(v″=1) and for the (J″=2→J″=0,4) transitions in D2(v″=1). These rates were found to be generally higher than the corresponding previously determined ones in ground state hydrogen, in qualitative accord with recent theoretical calculations. A comparison with available theoretical state-to-state cross sections shows that the rates obtained with these cross sections are generally lower than the relaxation rates directly measured in this work.
A measurement of the average kinetic energy released during the unimolecular decomposition of argon ion clusters
Authors: Stace A. J.
Published: July 2002

The average kinetic energy released during the unimolecular decomposition of argon ion clusters has been measured as a function of cluster size from Ar+5 up to Ar+24. Although there are considerable uncertainties in the experimental data, when compared with an RRKM model the results do suggest that excess energy in the ion clusters is partitioned in a statistical manner.
Structures and energies for C4
Authors: Ritchie James P. , King Harry F. , Young William S.
Published: July 2002

Optimized geometries and relative energies for three states of the C4 molecule have been obtained from single-reference configuration interaction (SRCI) calculations. The 1Σ+g state, which is formed without activation from the dimerization of ground state C2 molecules, is calculated to lie approximately 25 kcal above the 3Σ−g state. At the SRCI level, a rhombic form is calculated to lie 1.2 kcal below the triplet form; consideration of the Davidson correction reduces this difference to 0.4 kcal, inclusion of a second set of diffuse d functions increased the difference only by about 0.2 kcal. Consideration of these effects, the difference in zero-point energy and previous results for methylene leads to a final estimated separation of 4.9 kcal, favoring the rhombus. Electron density distribution analysis for the rhomb is consistent with the existence of a bond between inverted sp2 carbon atoms. To aid the detection of this unusual molecule, preliminary estimates of the lowest optical transitions were obtained from SRCI calculations and vibrational frequencies were obtained from SCF calculations. Comparison of the calculated results with experimentally obtained spectra suggest the possibility that both the linear triplet and the rhombus may have already been observed.
The absolute value of the quantum yield of the fluorescence of the 1B3u 0–0 state of pyrazine as a function of the rotational quantum numbers
Authors: de Lange Pieter J. , van der Meer Barend J. , Drabe Karel E. , Kommandeur Jan , Meerts W. Leo , Majewski W. A.
Published: July 2002

In this paper we fit low-resolution spectra of pyrazine by assuming Coriolis coupling between S1 and {S0}. Evidence for Coriolis coupling is particularly obvious in the rovibronic spectra of pyrazine-d3h1 of which we give high resolution examples. For the lowest rotational temperature we noticed a non-Boltzmann distribution of the J″=0 ground state, which is probably caused by a bottleneck for ΔJ″=−2 transitions. Using the lifetimes of molecular eigenstates belonging to P(1) we can calculate the absolute quantum yield of the vibrationless and rotationless 1B3u state. Using this number and the obtained interstate Coriolis coupling rate constants we calculate the variation of the absolute quantum yield across the rotational contour. For low J we get satisfactory agreement with experiments, but for high J our calculations drop off too fast with J. We explain this by the fact that at high J values triplet decay becomes dominant because of K scrambling in the triplet manifold.
On the theory of concentration self-quenching by statistical traps
Authors: Knoester J. , Van Himbergen J. E.
Published: July 2002

This work is concerned with the theory of concentration self-quenching of luminescence due to incoherent energy transfer between randomly distributed molecules. A simple model of self-quenching is considered, in which luminescent molecules within a critical distance of each other act as perfect traps. Due to the statistical nature of these traps, few practical results can be obtained from systematic or formal theories. We demonstrate, however, that within the framework of hopping theories for energy transfer an exact and a simple intuitive treatment of the donor and trap distributions differ only very little in their results for the quantum yield and emission anisotropy.
The quenching of Na*(3p) in collisions with N2: A classical trajectory study
Authors: Poppe Dirk , Papierowska-Kaminski Daniela , Bonačić-Koutecký Vlasta
Published: July 2002

The quenching reaction Na(2P)+N2(1Σ+g,v,J)→Na(2S) +N2(1Σ+g,v′,J′) has been studied. Scattering calculations have been performed using the surface hopping trajectory method for the two energetically lowest potential energy surfaces of NaN2. The latter have been determined in an ab initio MRD-CI treatment. They exhibit a (avoided) crossing where quenching is likely to occur. Model potentials, which are constructed along the lines given by the ab initio surfaces, are used to investigate the influence of the shape of the potential on the scattering process. Cross sections and final translational energy distributions are compared with experimental data. Alignment and orientation as expressed by the collision induced density matrix have also been considered. Theoretical and experimental results show good agreement. The detailed analysis of the scattering calculations have provided with a better understanding of the quenching process.
Dynamical calculations on the photoisomerization of small polyenes in a nonadiabatic formalism
Authors: Dormans Guido J. M. , Groenenboom Gerrit C. , Buck Henk M.
Published: July 2002

The dynamics of the cis–trans isomerization in 10° monopyramidalized ethylene, butadiene, and hexatriene has been studied in a nonadiabatic treatment including explicitly the twisting motion. The electronic energy curves, (transition) dipole moments, and nonadiabatic couplings have been obtained from ab initio CI calculations. An initially (Franck–Condon) excited wave function is constructed from the final vibrational eigenfunctions and energies. The appearance of the calculated prepared state is discussed in relation to the increasing line broadening vs decreasing chain length of the primary vibrational bands in the 1 1B+u absorption spectra of polyenes. It is found that this feature can be explained from the increasing tendency of shorter polyenes to be nonplanar in the excited state. The time evolution of the molecules is monitored by calculating the radiationless decay, the dipole moment, and the contributions of the electronic states to the total wave function. The role of the nonabsorbing 2 1A−g state is discussed. It is demonstrated that the dynamics of the cis–trans isomerization are directly correlated to the shape of the potential energy curve of the absorbing excited state. Ethylene is found to rotate once about the double bond in 0.075 ps and butadiene in 0.2 ps, whereas hexatriene is excited to an almost stationary wave function. Because of the limitations of the present model, the molecules do not show any substantial (‘‘sudden’’) polarization upon inclusion of the nonadiabatic coupling. For the same reason, the contribution of the electronic ground state to the total wave function is small.
Photoionization dynamics and abundance patterns in laser vaporized tin and lead clusters
Authors: LaiHing K. , Wheeler R. G. , Wilson W. L. , Duncan M. A.
Published: July 2002

Tin and lead clusters are produced by laser vaporization in a pulsed nozzle source and studied with laser photoionization mass spectroscopy. ‘‘Magic numbers’’ are observed in both cluster size distributions under a variety of laser wavelength and power conditions which can be understood in terms of ionization thresholds, relative ionization cross sections, and multiphoton-induced fragmentation. After investigation of the photoionization dynamics, relative abundances of different sized clusters are estimated. Abundance patterns of tin and lead clusters are compared to those reported previously for other group IV elements (C, Si, Ge) to investigate the role of periodicity in cluster growth and bonding properties. Especially abundant 10-atom cluster species are observed for both tin and lead, as has been observed previously for both silicon and germanium. Other features not observed for silicon and germanium, such as abundance patterns characteristic of atom closepacking geometries, are observed to a limited degree for tin clusters and are more prominent in lead clusters.
Near Hartree–Fock quality GTO basis sets for the second-row atoms
Authors: Partridge Harry
Published: July 2002

Energy optimized, near Hartree–Fock quality Gaussian basis sets ranging in size from (17s12p) to (20s15p) are reported for the ground states of the second-row atoms and for Na (2P), Na+, Na−, Mg (3P), P−, S−, and Cl−. In addition, optimized supplementary functions are given for the ground state basis sets to describe the negative ions, and the excited Na (2P) and Mg (3P) atomic states. The ratios of successive orbital exponents describing the inner part of the 1s and 2p orbitals are found to be nearly independent of both nuclear charge and basis set size. This provides a method of obtaining good starting guesses for other basis set optimizations.
Infrared photodissociation of size selected internally excited ethylene clusters
Authors: Buck U. , Huisken F. , Lauenstein Ch. , Meyer H. , Sroka R.
Published: July 2002

Ethylene clusters (C2H4)n are generated in a supersonic expansion with He and size selected by scattering from a helium beam. The clusters are dissociated upon absorption of a photon from a pulsed CO2 laser by exciting the ν7 mode of the monomer. During the collision about 30 meV of internal energy is transferred to the cluster so that the laser photons interact with internally hot clusters. The frequency and fluence dependences of the photodissociation cross sections are measured for (C2H4)n with n=2,3,4,5, and 6. Nearly all spectra exhibit structure which is most pronounced for the dimer. The overall width (FWHM) decreases from 31.2 cm−1 for the dimer to 12.2 cm−1 for the hexamer, while the maximum position is nearly the same (951.6 cm−1). The structure of the dimer spectrum is attributed to hot bands. The large linewidth corresponds to a short lifetime in the ps range.
Isotope enrichment in Ne clusters
Authors: Scheier P. , Märk T. D.
Published: July 2002

Clusters of Ne atoms produced by adiabatic nozzle expansion have been studied with a high resolution double focusing sector field mass spectrometer. Analysis of the isotopic composition of these clusters (up to n=22) showed instead of the expected statistical distribution a pronounced shift to the heavier neon isotope 22Ne. The shift was found to correspond to an increase of the isotope ratio of 22Ne from 8.7% in the monomer to ∼12.5% in the clusters with n>4 (enrichment factor of ∼1.5.)