bluuebunny/crossref_metadata_2025 huggingface dataset:
Neutron scattering investigation of the structure and spin dynamics in La0.85Sr0.15MnO3
Authors: Vasiliu-Doloc L. , Lynn J. W. , Moudden A. H. , de Leon-Guevara A. M. , Revcolevschi A.
Published: July 2002

We have performed diffraction and inelastic neutron scattering experiments on a La0.85Sr0.15MnO3 single crystal. In the ferromagnetic phase of this system (Tc=235 K) the long wavelength dispersion relation along the (0,1,0) direction is well represented by E=E0+Dq2, but with the spin wave spectrum almost gapless (E0<0.1 meV), and with DT=0=83.75±0.36 meV Å2. The spin-wave stiffness constant D(T) exhibits a power law behavior as a function of temperature, and appears to collapse as T→Tc. An orthorhombic–rhombohedral structural phase transition is observed at Ts=360 K, and exhibits a hysteretic character suggesting a first-order transformation. Anomalies in magnetization measurements and Bragg peak intensities indicate the existence of a second phase transition at T=200 K.
Fast, high-resolution atomic force microscopy using a quartz tuning fork as actuator and sensor
Authors: Edwards Hal , Taylor Larry , Duncan Walter , Melmed Allan J.
Published: July 2002

We report a new method of achieving tip–sample distance regulation in an atomic force microscope (AFM). A piezoelectric quartz tuning fork serves as both actuator and sensor of tip–sample interactions, allowing tip–sample distance regulation without the use of a diode laser or dither piezo. Such a tuning fork has a high spring constant so a dither amplitude of only 0.1 nm may be used to perform AFM measurements. Tuning-fork feedback is shown to operate at a noise level as low as that of a cantilever-based AFM. Using phase-locked-loop control to track excursions in the resonant frequency of a 32 kHz tuning fork, images are acquired at scan rates which are fast enough for routine AFM measurements. Magnetic force microscopy using tuning-fork feedback is demonstrated.
Assessment and parameterisation of Coulomb-enhanced Auger recombination coefficients in lowly injected crystalline silicon
Authors: Altermatt Pietro P. , Schmidt Jan , Heiser Gernot , Aberle Armin G.
Published: July 2002

In traditional band-to-band Auger recombination theory, the low-injection carrier lifetime is an inverse quadratic function of the doping density. However, for doping densities below about 3×1018 cm−3, the low-injection Auger lifetimes measured in the past on silicon were significantly smaller than predicted by this theory. Recently, a new theory has been developed [A. Hangleiter and R. Häcker, Phys. Rev. Lett. 65, 215 (1990)] that attributes these deviations to Coulombic interactions between mobile charge carriers. This theory has been supported experimentally to a high degree of accuracy in n-type silicon; however, no satisfactory support for it has been found in p-type silicon for doping densities below 3×1017 cm−3. In this work, we investigate the most recent lifetime measurements of crystalline silicon and support experimentally the Coulomb-enhanced Auger theory in p-type silicon in the doping range down to 1×1016 cm−3. Based on the experimental data, we present an empirical parameterisation of the low-injection Auger lifetime. This parameterisation is valid in n- and p-type silicon with arbitrary doping concentrations and for temperatures between 70 and 400 K. We implement this parameterisation into a numerical device simulator to demonstrate how the new Auger limit influences the open-circuit voltage capability of silicon solar cells. Further, we briefly discuss why the Auger recombination rates are less enhanced under high-injection conditions than under low-injection conditions.
Novel theoretical aspects on photorefractive ultrasonic detection and implementation of a sensor with an optimum sensitivity
Authors: de Montmorillon Louis-Anne , Delaye Philippe , Launay Jean-Claude , Roosen Gérald
Published: July 2002

We here expose theoretical and experimental results on homodyne detection using near-infrared laser sources, at 1.06, 1.32, and 1.55 μm wavelengths. The used photorefractive crystals are two large size CdZnTe:V samples. With speckled beams such as the ones scattered by diffusive objects, we reach a detection limit which, at 1.55 μm, is only 1.6 times above the one obtained with plane waves in a classical interferometer and only 2 and 2.2 times above at 1.32 and 1.06 μm, respectively. It is then demonstrated that the electron–hole competition, which varies enormously between these three wavelengths and gives a nearly zero two-wave-mixing gain at 1.32 μm, does not influence the sensitivity of the system. Moreover, we show that the frequency cutoff of the system is four times higher in the attenuation regime than in the amplification one.
Origins and evolution of stress development in sol-gel derived thin layers and multideposited coatings of lead titanate
Authors: Sengupta S. S. , Park S. M. , Payne D. A. , Allen L. H.
Published: July 2002

Stress development in thin layers of lead titanate prepared by sol-gel processing was monitored by in situ laser reflectance measurements. Layers were spin coated onto silicon substrates and thermally cycled to 500 °C. The shrinkage normal to the rigid substrate was determined by in situ ellipsometry. Changes that occurred on drying and firing, which related to densification and stress development, are reported. The observed changes were explained in terms of evaporation and solvent/polymeric network interactions at lower temperatures, and thermal expansion mismatch between the substrate and the coating after formation of the dense oxide. Crystallization into the perovskite structure occurred only in thicker or multideposited coatings, altering the state of stress from tensile, to progressively more compressive, on cooling. The importance of the choice of substrate material, deposition method and heat treatment conditions, in relation to stress development and dependent electrical properties, are discussed.
Influence of the electrical excitation pulse shape on the electro-optical response in SrS:Ce3+ electroluminescent devices
Authors: Barthou C. , Benoit J. , Benalloul P. , Nykänen E. , Leskelä M. , Niinistö L.
Published: July 2002

The electro-optical properties of SrS:Ce thin film electroluminescent devices prepared by atomic layer epitaxy have been studied by varying the shape of the electric excitation pulse. The different electric and optical responses have been explained by considering two sets of states at each interface. The shallower set of states will be populated by the electrons originating from the deeper states when the electric field is turned on. Relaxation of the carriers from the shallower states towards the deeper ones modifies the relative intensity of the different optical responses according to the shape of the excitation pulse.
Modeling of microwave magnetic envelope solitons in thin ferrite films through the nonlinear Schrödinger equation
Authors: Zhang Hong Yan , Kabos Pavel , Xia Hua , Staudinger Reinhold A. , Kolodin Pavel A. , Patton Carl E.
Published: July 2002

A theoretical analysis of microwave magnetic envelope soliton profiles and the soliton peak power response for high power magnetostatic wave (MSW) excitations in yttrium iron garnet (YIG) thin films has been made. This analysis was based on the standard nonlinear Schrödinger equation with all key parameters based on experiment. The measurements were done for magnetostatic backward volume waves in a 10.2 μm YIG film, with a band edge at 5.06–5.07 GHz and operating point frequencies from 4.80 to 5.00 GHz. The use of accurate dispersion and group velocity parameters and the transmitted power versus frequency response of the MSW signal was critical. It was possible to accurately model both the shapes of the soliton pulses and the peak output versus peak input power response over a wide range of power levels.
Theoretical study of the energetics, strain fields, and semicoherent interface structures in layer-by-layer semiconductor heteroepitaxy
Authors: Zepeda-Ruiz Luis A. , Maroudas Dimitrios , Weinberg W. Henry
Published: July 2002

A theoretical analysis based on continuum elasticity theory and atomistic simulations is presented of the interfacial stability with respect to misfit dislocation formation, the strain fields, and the film surface morphology during layer-by-layer semiconductor heteroepitaxy. The strain in the coherently strained films, the energetics of the transition from a coherent to a semicoherent interface consisting of misfit dislocation arrays or networks, the structure of the corresponding semicoherent interfaces, the strain fields associated with different equilibrium states of strain, and the morphological characteristics of the film surfaces are calculated for InAs/GaAs(110) and InAs/GaAs(111)A. The thickness of the epitaxial film is used as the dynamical variable in the analysis. Critical film thicknesses for transition from one equilibrium state of strain to another are computed. The analysis is presented for the more general case of heteroepitaxy on a finite-thickness compliant substrate, while the common case of epitaxy on an infinitely thick substrate is derived as an asymptotic limit of the general case. Continuum elasticity theory is found to describe the atomistic simulation results very well, down to the monolayer-thickness limit. Our theoretical results are discussed in the context of recent experimental data.
Photoluminescence linewidth of self-organized In0.4Ga0.6As/GaAs quantum dots grown on InGaAlAs stressor dots
Authors: Krishna Sanjay , Linder Kojo , Bhattacharya Pallab
Published: July 2002

The molecular beam epitaxial growth of self-organized In0.4Ga0.6As/GaAs quantum dots on buried InGaAsAs/GaAs stressor dots has been characterized by photoluminescence measurements and cross-sectional transmission electron microscopy. The presence of the stressor dots enhances the growth rate and spatial uniformity of the In0.4Ga0.6As dots. The incorporation of Al in the stressor dots not only provides a strain field, but also inhibits carrier recombination therein. A low photoluminescence linewidth of 21 meV, almost invariant in the temperature range of 7–100 K was measured in a heterostructure with an optimal number of stressor and active dot layers.
Photoluminescence properties of metalorganic vapor phase epitaxial Zn3As2
Authors: Botha J. R. , Scriven G. J. , Engelbrecht J. A. A. , Leitch A. W. R.
Published: July 2002

The photoluminescence (PL) properties of the II3V2 compound semiconductor Zn3As2, grown by metalorganic vapor phase epitaxy, is investigated in the temperature range of 4.2–350 K. Several lines are reported, believed to be due to acceptors and donor-acceptor complexes in the epitaxial material. From variable temperature PL, the band gap is deduced to be indirect in nature (in contrast to most experimental findings to date which have indicated the band gap to be direct in nature) and ∼25–30 meV larger than the indirect band gap. The direct band gap energy has been extracted from the PL spectra in the range of 110–350 K. A linear relationship is observed in this temperature range with slope dEg/dT=−4.00×10−4 eV/K. PL yielded a value of 1.005 eV for the direct gap at room temperature in good agreement with values between 0.989 and 0.999 eV obtained from transmission and reflection measurements.
Modification of hydrogen-free amorphous carbon films by focused-ion-beam milling
Authors: Stanishevsky A. , Khriachtchev L.
Published: July 2002

Amorphous hydrogen-free carbon films with various sp3/sp2 ratios were exposed to a 50 kV Ga+ focused ion beam (FIB) with a dose in the range from 1014 to 2.5×1018 ion/cm2. Atomic force microscopy (AFM), measurements of electric resistivity, and Raman spectroscopy were used to study FIB-induced modification of surface topography and film structure. The Raman spectra indicate an increase of sp2 coordination in sp3-rich films after ion irradiation and growth of sp2-bonded clusters under annealing at 550 °C. Light absorption in the FIB-exposed areas increases by a factor of up to 15 compared to the as-deposited material. For the FIB-irradiated sp3-rich films, the AFM measurements show neither significant roughening of the surface nor further modification of surface topography under annealing and over an extended period of time. The behavior of the films resistivity as a result of FIB irradiation and annealing most probably reflects the effect of gallium impurity. Also, the Raman measurements have documented slow room-temperature structural relaxation in the FIB-exposed areas of one sample.
Buffer free direct growth of GaN on 6H–SiC by metalorganic vapor phase epitaxy
Authors: Lahrèche H. , Leroux M. , Laügt M. , Vaille M. , Beaumont B. , Gibart P.
Published: July 2002

The growth of GaN on 6H–SiC is three dimensional (3D) and results in the formation of large islands presenting hexagonal truncated shape with {1–101} lateral facets and a top {0001} facet. In this work, we present a three steps growth process that enables us to grow high quality mirrorlike GaN layers without using AlN buffer layers. During a first step, a thin 3D GaN layer is deposited at high temperature. This layer is smoothed under ammonia flow for several minutes when the growth is interrupted. The subsequent growth of GaN is two dimensional. 600 nm thick GaN films were grown. They were analyzed by high resolution x-ray diffraction, reflectivity, and photoluminescence. All the layers are under strong tensile biaxial strain. The correlation between residual tensile strain in GaN layers and their optical properties is reported for biaxial deformations εxx ranging up to 0.37%.
Comparison of zinc-blende and wurtzite GaN semiconductors with spontaneous polarization and piezoelectric field effects
Authors: Park Seoung-Hwan , Chuang Shun-Lien
Published: July 2002

The electronic properties of (001)-, (111)-oriented zinc-blende and (0001)-oriented wurtzite crystals are investigated theoretically, where the spontaneous polarization and piezoelectric (PZ) field effects are taken into account. We show that the Luttinger–Kohn 6×6 Hamiltonians for the valence bands of the zinc-blende crystals written in the wurtzite bases for (001) and (111) crystal orientations and the Hamiltonian for the (0001)-orientation of wurtzite crystals can all be block diagonalized to two 3×3 Hamiltonians, which have analytical solutions for eigenvalues and eigenvectors. We then derive analytical expressions for the strain dependent band-edge effective masses and interband optical matrix elements of zinc-blende and wurtzite GaN crystals and compare their numerical results as well as valence band structures. Although the compressively strained zinc-blende quantum wells in materials such as GaAs- and ZnSe-based systems show reduced threshold carrier densities due to the lower in-plane effective mass, we find that for GaN the reduction of the effective mass with the biaxial compressive strain is not significant in both zinc-blende and wurtzite structures. An alternative method is the application of a uniaxial strain to reduce the in-plane effective masses in both structures. It is also found that the valence band structures and the overlap integral of the electron and hole wave functions of GaN/AlGaN quantum-well structures are affected significantly by the PZ field for (111) zinc-blende structures and by both the spontaneous polarization and PZ fields for (0001) wurtzite structures.
Recording properties of Co/Pd multilayer using a magnetoresistive sensor
Authors: Chen Ga-Lane
Published: July 2002

We have fabricated Co/Pd, CoCr/Pd, and CoCrTa/Pd multilayer media. Good recording performance was achieved using a magnetoresistive (MR) head for readback. Co/Pd medium has perpendicular coercivity of 5 kOe and perpendicular squareness ratio of 0.83. The recording performance of a Co/Pd multilayer media was compared to conventional longitudinal media Co86Cr10Ta4 media and perpendicular Co78Cr17Ta5 media. The effects of Cr doping of Co, postdeposition annealing of Co/Pd, annealing the plated NiFe keeper layer, the number of layers in the multilayer, and the oxygen concentration of sputtering gas on the noise spectra of media were studied.
Vacancy effects in transient diffusion of Sb induced by ion implantation of Si+ and As+ ions
Authors: Lulli G. , Bianconi M. , Solmi S. , Napolitani E. , Carnera A.
Published: July 2002

The influence of defects injected by room temperature, high-energy implantation of Si and As ions on the diffusion of Sb marker in Si is investigated. MeV ions induce transient enhanced diffusion (TED) of ion implanted Sb, which increases with increasing the vacancy supersaturation generated in the Sb-doped region by the knock-on recoil mechanism. TED lasts a few minutes for annealing at 800 and 900 °C. The results indicate that at these temperatures the buildup and decay of vacancy supersaturation in the near-surface region occurs on a shorter time scale than the release of interstitials from the buried damage layer. The dominant role of vacancies is also indicated by the very low TED observed in B-doped samples processed under similar conditions. For 1000 °C annealing some effect of the retardation induced on Sb diffusion by interstitials flowing from the deep region is found after 15 min annealing. Preliminary results of defect injection by nonamorphizing medium-energy implants indicate that a smaller, yet nonvanishing, effect of Sb TED persists even under conditions where B diffusivity is strongly enhanced.
The E
F, G
K, and H
H̄ 1Σg+ states of hydrogen. Improved ab initio calculation of vibrational states in the adiabatic approximation
Authors: Wolniewicz L. , Dressler K.
Published: July 2002

Electronic energies and diagonal energy corrections for nuclear motion are calculated for the first three excited 1Σg+ states of H2 at some 70 internuclear separations in the interval R(a.u.) ∈{1,15}. More accurate electronic wave functions comprising, respectively, 129, 118, and 110 terms for the EF, GK, and HH̄ states, and a new method for the evaluation of the relevant integrals are used. The maximum values of the adiabatic energy corrections are found to be larger than the previously published values by 60 cm−1 (EF at R=3.2 a.u.), 55 cm−1 (GK at R=2.85 a.u.), and 38 cm−1 (HH̄ at R=2.95 a.u.), while the electronic energies at the same values of R are now lower by −6, −26, and −7 cm−1. The adiabatic ab initio vibrational energies of the EF, v=0, levels lie 1.9 (H2) and 1.4 cm−1 (HD and D2) above the experimental values. All higher vibrational levels of the three electronic states are appreciably affected by nonadiabatic energy shifts.
Photofragmentation dynamics of formaldehyde: CO(v,J) distributions as a function of initial rovibronic state and isotopic substitution
Authors: Bamford Douglas J. , Filseth Stephen V. , Foltz Mary F. , Hepburn John W. , Moore C. Bradley
Published: July 2002

Complete rotational distributions have been obtained for the CO produced following excitation of H2CO, HDCO, and D2CO near the S1 origin. The CO was detected by vacuum ultraviolet laser-induced fluorescence. The distributions show a remarkable amount of rotational excitation, peaking at J=42, 49, and 53 for H2CO, HDCO, and D2CO, respectively, with widths of 20–25 J units (FWHM). CO(v=1) from H2CO photolysis has nearly the same rotational distribution as CO(v=0). The population of CO(v=1) is 14%±5% as large as the population of CO(v=0), in good agreement with earlier measurements. Increased angular momentum of H2CO is only partially transferred to CO, giving slightly wider rotational distributions without changing the peak value. The rotational distributions are highly nonthermal, showing that energy randomization does not occur during the dissociation event. An approximate range of product impact parameters has been determined. The impact parameters are too large to be accounted for by forces along the directions of the C–H bonds. The hydrogen appears to be most strongly repelled by the charge distribution a fraction of an Å outside the carbon atom of the CO. The distribution of impact parameters and the internal energy of the hydrogen fragment apparently do not change significantly upon isotopic substitution. The absence of population in CO(J<20) confirms the identity of CO(J≳25) as the long-lived intermediate in formaldehyde photodissociation.
A work function change study of oxygen adsorption on Pt(111) and Pt(100)
Authors: Derry G. N. , Ross P. N.
Published: July 2002

Oxygen adsorption on Pt(111) and Pt(100) was investigated using a Kelvin probe to measure adsorbate induced work function changes. The variation of Δφ with θ was linear (p=0.103 D/atom) on Pt(111) even up to θmax∼7×1014 atoms cm−2. On Pt(100), there was an abrupt increase in Δφ at low coverage (<1×1014 atom cm−2) which was presumed to be adsorption at defect sites (p=0.62 D/atom). Sticking coefficients were calculated from the rate of change of the work function with time. For room temperature adsorption on Pt(111), the sticking coefficient followed the functional form S0(1−ϑ)2, ϑ=θ/θmax, and S0=0.038. On Pt(100) the functional dependence was more complex with an initial increase in S (from S0=0.042) to a ϑ of ∼0.1, then an approximately quadratic decrease as the coverage increased to saturation. The maximum coverage observed in room temperatue dosing of either surface was ∼3×1014. However, dosing at 200–300 °C at 10−5–10−4 Torr induced coverages as high as (7–9)×1014 on both surfaces. The isosteric heat of adsorption was measured from equilibrium isotherms and found to be 232±36 kJ mol−1 with a coverage dependence less than the experimental precision. For Pt(111), it is postulated that population of the high coverage state proceeds via the activated process of direct dissociation, whereas the unactivated room temperature process is via a molecularly adsorbed precursor. For Pt(100), population of the high coverage state involves activated transitions in the Pt surface structure which require further study.
Autoionization observed in the CO+ (A 2Π–X 2Σ+) and (B 2Σ+–X 2Σ+) fluorescence excitation spectra
Authors: Ito K. , Tabché-Fouhailé A. , Frohlich H. , Guyon P. M. , Nenner I.
Published: July 2002

The CO+ (A 2Π–X 2Σ+) and (B 2Σ+–X 2Σ+) fluorescence excitation spectra (FES) using VUV synchrotron radiation, have been measured from 16 to 38 eV excitation energy. Total photoionization cross section has also been measured from 17 to 20 eV for comparison. Sharp ionization onsets typical of direct ionization process are observed for both A 2Π and B 2Σ+ thresholds. New autoionization structures have been observed above 20 eV besides the broad shape resonance located at 32 eV in the (B–X) FES. Sharp structures observed above the A 2Π (v=0) threshold are interpreted in terms of electronically induced vibrational autoionization. Branching ratios for electronic autoionization of four Ryberg series converging to B into the CO+ A and X states have been measured. Those states decay preferentially into the CO+ A 2Π state as opposed to X 2Σ+.
Multiplet-specific shape resonant features in photoionization of NO
Authors: Smith Maile E. , McKoy V. , Lucchese Robert R.
Published: July 2002

We report photoionization cross sections and asymmetry parameters for the 5σ and 4σ levels of NO obtained using numerical photoelectron continuum functions. For the 5σ orbital multiplet-specific potentials lead to shape-resonant cross sections which show significant nonstatistical multiplet behavior. This behavior, which reflects the sensitivity of shape resonances to the exchange component of the molecular ion potential, is most prominently characterized by resonance peaks at substantially different photoelectron kinetic energies in the two multiplet channels. Comparison of the calculated cross sections with available experimental data for NO shows somewhat poorer agreement than has been seen in the photoionization of closed-shell systems such as N2 and CO. This difference reflects the more complex nature of photoionization processes in open-shell molecules.
Formation and photodetachment of cold metal cluster negative ions
Authors: Zheng L.-S. , Brucat P. J. , Pettiette C. L. , Yang S. , Smalley R. E.
Published: July 2002

A general method is described for the formation of cold metal cluser negative ion beams which serve as excellent sources for photodetachment experiments. The method involves the pulsed laser vaporization of a metal target at the throat of a pulsed supersonic helium expansion. By the optimization of source conditions, intense beams (greater than 105 ions/pulse) of both positive and negative ions are produced routinely. Ionization of the metal cluster molecules, either during vaporization or by irradiation with 193 nm light, occurs prior to supersonic expansion and produces a cold plasma entrained in the neural flow that is renitent to stray electric and magnetic fields, unlike photoions produced in the collisionless downstream molecular beam. The enhancement of the negative ion flux by 193 nm irradiation is believed to be evidence for efficient electron attachment of low energy photoelectrons generated in the nozzle region. This attachment process, however, is apparently not effective for molecules containing less than ∼4 metal atoms. Laser irradition of mass-selected cluster anions extracted from these cold ion beams reveal that photodetachment of the metal cluster negative ion is always the preferred pathway, even when fragmentation of the ion is possible. This new negative ion production technique should therefore permit measurement of both electron affinities and photoelectron spectra as a function of cluster size and composition.
Theory of band shape formation in Auger and autoionization spectra of molecules. Numerical applications and new high-resolution spectra for CO
Authors: Correia Nestor , Flores-Riveros Antonio , Ågren Hans , Helenelund Kurt , Asplund Lars , Gelius Ulrik
Published: July 2002

Theory of band shape formation in Auger and autoionization decay in molecules is presented and numerically applied to spectra of the CO molecule. In particular, the role of lifetime vibrational interference in the various vibronic decay channels is elucidated. New high resolution Auger and autoionization spectra are presented. The calculated band shapes are compared with the experimental recordings. The vibronic spectra of the intermediate states in the Auger and autoionization decay are analyzed and compared with available ESCA and EELS spectra, respectively. The role of the electron correlation on the formation of the core state potential surfaces is investigated in that connection. The assignment problem of the outer part of the CO Auger spectrum is reinvestigated and the effect of interference due to hole mixing in the wave functions is pointed out. The time dependent Franck–Condon (FC) formalism used to derive the lifetime vibrational interference is shown to have a bearing on the analogy of the sudden approximation for electron ionization with the FC principle for vibronic excitations and makes possible a derivation of the lifetime for the vertical vibrational state in electron ionization.
High precision acoustic spectroscopy by laser excitation of resonator modes
Authors: Karbach A. , Hess P.
Published: July 2002

In a cylindrical cavity the longitudinal mode (002), the radial mode (100), and the combination mode (102) were excited with a chopped 3.39 μm He–Ne laser by vibrational excitation of CH4. The recording of acoustic resonances was completely controlled by a microcomputer. This included repeated registration of the resonator temperature, stabilization and measurement of the modulation frequency, and integration of the acoustic signal detected with an electret microphone. The interaction of different standing waves in the cavity was taken into account. From the corrected line profiles, accurate values of the eigenfrequencies and half-widths were determined as functions of pressure between 1–760 Torr. A quantitative theoretical description of these experimental results is given, which takes into account all important loss processes in the resonator. This analysis yields a value of ( pτ)=1.74±0.07 μs bar for the vibrational relaxation time and ( pτ)=1.18±0.24 ns bar for the rotational relaxation time of CH4 at T=292 K.
Ion association and dipolar dumbbells: Solutions of the HNC and HNC/MS approximations at L=σ/2 and σ/3 for the sticky electrolyte model
Authors: Rasaiah Jayendran C. , Lee Song Hi
Published: July 2002

We extend an earlier analytic study of a sticky electrolyte model (SEM) to the case L=σ/3, where L is the distance at which positive and negative ions bind to each other, using the hypernetted chain (HNC) approximation within the spherical core and the mean spherical approximation (MSA) outside. We also present numerical solutions to the HNC approximation alone for σ/3 ≤L≤σ/2. The average number of bonded pairs is found to be essentially the same for the two approximations but the ion–ion correlation functions are very different except at high concentrations when the shielding is large. Small amounts of tetramers are also observed in the HNC correlation functions for σ/3<L≤σ/2, and trimers are found when L=σ/2. An expression for the excess free energy of the SEM electrolyte is derived by turning on the stickiness between oppositely charged ions. The excess energy of the system of dipolar dumbbells with charges at a distance L=σ/3 is obtained in the MSA and the atom–atom correlation functions are compared with the HNC approximation and with recent Monte Carlo simulations. The asymptotic form of the direct correlation functions defined through the Ornstein–Zernike equation is given for dipolar dumbbells.
Laser-induced phosphorescence spectroscopy in supersonic jets. The lowest triplet states of glyoxal, methylglyoxal, and biacetyl
Authors: Spangler Lee H. , Pratt David W.
Published: July 2002

We report the first structural study of the lowest triplet states of three α-dicarbonyls (glyoxal, methylglyoxal, and biacetyl) using the technique of laser-induced phosphorescence (LIP) spectroscopy in supersonic jets. At the level of vibrational resolution, 3Au glyoxal appears to have a geometry very similar to that of the ground state. But the T1←S0 transitions of methylglyoxal and biacetyl each exhibit strong progressions in the torsional vibrations of the methyl groups, showing that these molecules undergo a conformational change on excitation to the lowest triplet state. A Franck–Condon analysis of the methylglyoxal spectrum, with proper consideration for nuclear spin statistics, yields a methyl barrier of V3=115±5 cm−1 in this state. This value has been confirmed by a direct measurement of the tunneling splitting of A and E torsional levels. The hindering potential in the lowest triplet state of methylglyoxal is substantially different from those in the ground (V3=269 cm−1) and first excited singlet (V3=190 cm−1) states. Possible reasons for these differences are discussed.
A
b i
n
i
t
i
o calculations on electronic states of Be13
Authors: Ermler W. C. , Kern C. W. , Pitzer R. M. , Winter N. W.
Published: July 2002

A cluster of 13 beryllium atoms has been studied in ab initio Hartree–Fock calculations. The chosen geometric configuration has D3h symmetry and corresponds to a central Be with 12 atoms situated at the hexagonal close-packed, nearest-neighbor positions. The lowest energy electronic state among those investigated is 5A″1 and the first excited state is 5E″ lying at 1.12 eV. A total of 14 states were identified below 2.0 eV. The 5A′′1 Hartree–Fock ground state has a binding energy of 12.0 kcal/mol relative to the separated atoms and an ionization potential of 0.54 eV. Total valence energies, orbital energies, binding energies, electric field gradients, diamagnetic shielding constants, nuclear–electron potentials, second moments, and Mulliken populations are reported. In addition, the cluster calculations give a reasonable estimate for the Sternheimer correction to the electric field gradient of the bulk metal.
Shape resonances above the Si 2p threshold in SiF4
Authors: Bancroft G. M. , Aksela S. , Aksela H. , Tan K. H. , Yates B. W. , Coatsworth L. L. , Tse J. S.
Published: July 2002

Using monochromatized synchrotron radiation and angle integrated photoelectron spectroscopy, the Si 2p, Si LVV Auger, and F 2s cross sections have been obtained for SiF4 molecules in the gas phase between 116 and 150 eV photon energies. These cross sections and the Si 2p β value have also been calculated using the MS-Xα method. There is qualitative agreement between theory and experiment. Together with recent experimental and theoretical Si 2p cross sections from SiCl4 and valence band cross sections on SiF4, the two major resonances at 5 and 22 eV kinetic energies are assigned as t2 resonances. The valence band branching ratios are not affected by interchannel coupling above the Si 2p threshold.
Fluorescence excitation spectra of butynal in a supersonic jet. The effect of internal rotation
Authors: Mühlbach J. , Huber J. Robert
Published: July 2002

The fluorescence excitation spectrum of the 000 and 1110 vibronic bands of the S1(nπ*) electronic state of butynal, CH3–C≡CCHO, and that of the 000 vibronic band of α-d-butynal have been measured in a supersonic expansion. The effects of internal rotation are discussed in comparison with the rigid-rotator spectrum of the 000 band of propynal, HC≡CCHO. Rotational constants and barrier heights were determined from contour calculations based on a rigorous semirigid CsF-C3vT model. At beam temperatures of 3 K≤Trot ≤8 K, only the Γ0+ ground state and the Γ1 first excited state of the internal rotation are populated significantly. Already in the latter state the methyl group behaves as an almost free rotator owing to a very low barrier which is V″3 =3.7±1 cm−1 in the electronic ground state and V3 =10±1 cm−1 in the S1 state. By means of quantum-beat spectroscopy assignments of the rovibronic spectra obtained from simulated spectra are verified. A non-Boltzmann population observed in the ground state is shown to be consistent with a conservation of nuclear spin symmetry during the supersonic expansion process.
Sub-Doppler laser-induced fluorescence measurements of the velocity distribution and rotational alignment of NO photofragments
Authors: Dubs M. , Brühlmann U. , Huber J. Robert
Published: July 2002

The photodissociation of dimethylnitrosamine, (CH3)2N–NO, at 363.5 nm produces ro-vibrationally excited NO fragments. With two-photon laser-induced fluorescence the Doppler profiles of single rotational transitions were probed, using various polarization schemes for dissociation and probe lasers. These measurements provided a detailed product state distribution, the three-dimensional recoil velocity distribution of specific fragment states, and the alignment of their angular momentum. We present evidence of the presence of correlations between fragment recoil direction and alignment of fragment angular momentum.
A theoretical study of collision induced desorption spectroscopy from Si(111) surfaces
Authors: Zeiri Yehuda , Low John J. , Goddard William A.
Published: July 2002

A theoretical classical stochastic trajectory study of collision induced desorption spectroscopy (CIDS) is presented in this paper. These theoretical results suggest that CIDS might provide useful experimental information on the binding energy and binding sites for adsorbates on surfaces. This information is obtained from analysis of the kinetic energy and angular distributions of both the collider atom and the desorbed adsorbate. Simulations of CIDS are reported for Xe colliding beams of 1 to 5 eV with monoatomic adsorbates on Si(111).
The microwave spectrum of an unstable molecule, HPO
Authors: Saito Shuji , Endo Yasuki , Hirota Eizi
Published: July 2002

The HPO molecule was identified by its microwave spectrum in the reaction of PH3 with microwave discharge products of an H2/O2 mixture; the a-type R branch transitions of J=1 ← 0 up to 4 ← 3 were observed in the frequency region of 40 to 162 GHz. High- K lines showed small splittings due to the phosphorus nuclear spin-rotation interaction. In addition to the rotational and centrifugal distortion constants, the observed spectrum yielded the nuclear spin-rotation tensor, the aa component which is −1.12 (37) MHz with 2.5 standard errors in parentheses. This large interaction is discussed in terms of second-order perturbation theory.
The microwave spectrum of the K=0 states of Ar–NH3
Authors: Nelson D. D. , Fraser G. T. , Peterson K. I. , Zhao K. , Klemperer W. , Lovas F. J. , Suenram R. D.
Published: July 2002

The microwave spectrum of Ar–NH3 has been obtained using molecular beam electric resonance spectroscopy and pulsed nozzle Fourier transform microwave spectroscopy. The spectrum is complicated by nonrigidity and most of the transitions are not yet assigned. A ΔJ=1, K=0 progression is assigned, however, and from it the following spectroscopic constants are obtained for Ar–14NH3: (B+C)/2=2876.849(2) MHz, DJ =0.0887(2) MHz, eqQaa =0.350(8) MHz, and μa =0.2803(3) D. For Ar–15NH3 we obtain (B+C)/2 =2768.701(1) MHz and DJ =0.0822(1) MHz. The distance between the Ar atom and the 14NH3 center of mass RCM is calculated in the free internal rotor limit and obtained as 3.8358 Å. In the pseudodiatomic approximation, the weak bond stretching force constant is 0.0084 mdyn/Å which corresponds to a weak bond stretching frequency of 35 cm−1. The NH3 orientation in the complex is discussed primarily on the basis of the measured dipole moment projection and the quadrupole coupling constant. It is concluded that the Ar–NH3 intermolecular potential is nearly isotropic and that the NH3 subunit undergoes practically free internal rotation in each of its angular degrees of freedom. Spectroscopic evidence is presented which indicates that the NH3 subunit also inverts within the complex. These conclusions concerning the internal dynamics in the Ar–NH3 complex support the model initially proposed in our previous study of the microwave and infrared spectra of this species.
Linear prediction and resolution enhancement of complex line shapes in two-dimensional electron-spin-echo spectroscopy
Authors: Millhauser G. L. , Freed J. H.
Published: July 2002

A new type of two-dimensional electron-spin-echo (2D-ESE) spectroscopy was recently shown to be useful for studying slow molecular motions in liquids. A recently developed method of spectral enhancement based upon linear prediction with singular value decomposition (LPSVD) is applied in the present work to dramatically improve the signal-to-noise ratio and to correct for finite dead time in the data from this 2D-ESE experiment. This permitted a more accurate comparison between theory and experiment. Good agreement is now obtained with a model of nearly isotropic Brownian motion for tempone in glycerol/water solvent.
Vibrational effects in the spin-orbit autoionization of HBr
Authors: Lefebvre-Brion H. , Dehmer P. M. , Chupka W. A.
Published: July 2002

The high resolution relative photoionization cross section for HBr is reported in the energy region between the 2Π3/2 and 2Π1/2 ionic limits. A theoretical interpretation of the spectrum is given using the multichannel quantum defect theory with an ab initio calculation of the relevant electronic quantities. The vibrational couplings due to the R variation of both the electrostatic and the spin-orbit matrix elements are introduced and are shown to be essential to reproduce the qualitative features of the experimental data in the 1052–1060 Å wavelength region.
A microscopic theory for spherical interfaces: Liquid drops in the canonical ensemble
Authors: Lee D. J. , Telo da Gama M. M. , Gubbins K. E.
Published: July 2002

We have studied the interfacial properties of droplets of (cutoff and shifted) Lennard-Jones fluids in the canonical ensemble, using mean field theory. We have examined the effects of system size, overall density (supersaturation) and temperature on the density profiles, density and pressure at the center of the droplet, and surface tension. The numerical accuracy of the theory was tested by extensive comparisons of the results with the molecular dynamics simulations of Thompson et al. Good agreement was found. We have used the theory to calculate the energy of formation of a droplet and the stability temperature Ts for droplet formation as a function of the system size and overall density. We find Ts to be lower than the coexistence temperature for the planar surface, and to fall as the drop size falls.
Theory of Stark spectroscopy of molecules in 1Π electronic states: Coherence effects and quantum beats
Authors: Derouard Jacques , Alexander Millard H.
Published: July 2002

A density operator formalism is used to describe the fluorescence of a molecule in a 1Π electronic state in a static electric field under both pulsed and cw excitation. Coherences can be created both between M levels as well as between the e and f Λ-doublet levels. Explicit solution of the generalized master equation allows the development of general expressions for the excited state density matrix elements, under conditions where collisions, hyperfine structure, optical pumping, and optical saturation are ignored. Simple expressions are obtained for the fluorescence intensities, valid at high J and whenever the Stark shifts are significantly smaller than zero-field Λ-doublet splitting. A simulation study of the expected quantum beat patterns is reported based on the parameters reported by Mandich, Gaebe, and Gottscho [J. Chem. Phys. 83, 3349 (1985)] in their experimental study of BCl(A1Π). Attention is focused on the extent to which quantum beat effects will be obscured by the finite widths of the laser excitation pulse and detector response functions.
Autoionization and isotope effect in the threshold photoelectron spectrum of 12CO2 and 13CO2
Authors: Baer Tomas , Guyon Paul M.
Published: July 2002

Threshold photoelectron spectra of 12CO2 and 13CO2 have been obtained from the onset at 900 Å (13.75 eV) to 620 Å (20 eV) at a 9 meV resolution using dispersed synchrotron radiation with a 0.3 Å bandpass. A number of forbidden and/or previously unobserved transitions have been assigned for the X̃, Ã, B̃, and C̃ states of CO+2. Photoelectron spectra by electron time of flight at selected excitation wavelengths in the Franck–Condon gap region below the Ã state, reveal the autoionization of neutral valence states as well as Rydberg series converging to the Ã 2Πu and B̃ 2Σ+u states. The photoelectron energy distribution obtained upon excitation of the Rydberg states is consistent with that given by the Bardsley–Smith model for autoionization, while the superexcited valence states decay via a resonance autoionization process proposed previously by Guyon, Baer, and Nenner. These valence states are also responsible for the strong enhancement of vibrationally forbidden transitions, such as (000)→(010). Finally, several threshold electron peaks in the region between the Ã and C̃ states, whose origin can be attributed to autoionization, are shown to be highly sensitive to isotopic substitution.
Thermodynamics of polymer solutions
Authors: Muthukumar M.
Published: July 2002

The free energy of a polmyer solution is derived by a consideration of the monomer density fluctuations and incorporating three-body interactions. Explicit interpolation formulas are obtained for the concentration dependence of the correlation length for arbitrary strengths of two- and three-body interactions within the random phase approximation. When the ternary interactions are important, as is the case under the conditions of phase separation in polymer solutions, the derived free energy leads to new corresponding-states equations for the spinodals. The critical volume fraction φc, and ‖φ−φc‖/φc are found to be proportional to n−1/3 and n1/9, respectively, where n is the degree of polymerization of the polymer and φ is the coexistent polymer volume fraction. A comparison is made between the predictions and the experimental results reported in the literature.
Measurement of rotational energy transfer rates for HD (v=1) in collisions with thermal HD
Authors: Chandler David W. , Farrow Roger L.
Published: July 2002

We report state-to-state rotational energy transfer rates for HD excited to the first excited vibrational level of the ground electronic state. Stimulated Raman scattering is used to produce the rotationally selected, vibrationally excited HD. Subsequent collisional energy transfer from the prepared state, upon collision with a thermal distribution of HD, is monitored by multiphoton ionization through the E,F electronic state. The data are analyzed by solving the rate equations coupling the lowest six rotational states of the first excited vibrational level. In this manner, both the absolute rate constants and the shape of the energy transfer probability density function are determined. The best fit of the data to trial probability density functions indicates that the HD–HD collisions preserve the magnetic sublevel, mj. The total rotational energy transfer rate out of a particular rotational level is compared to high resolution Raman linewidth measurements in order to determine the degree to which the rotational energy transfer rate contributes to the linewidth.
Nearly free internal rotation in Ar–CH3Cl
Authors: Fraser G. T. , Suenram R. D. , Lovas F. J.
Published: July 2002

Rotational spectra of Ar–CH3Cl, for both Cl isotopes, have been observed, at 4 kHz resolution, using a pulsed nozzle Fourier transform microwave spectrometer. The observed spectra are consistent with a T-shaped complex in which the methyl group is undergoing nearly free internal rotation. Analysis of the ground (A) internal rotor state spectrum for Ar–CH335Cl using an asymmetrical top Hamiltonian produces the following spectroscopic constants (in MHz); A=13 633.020(14), B=1593.5683(79), C=1420.4572(52), ΔJ=0.012 16(14), ΔJK =0.1381(48), δJ =0.000 96(17), HJJK=−0.000 58(25), eQqaa =34.895(30), eQqbb =−72.185(25), and eQqcc =37.290(35). A combined analysis of the ground and excited (E) internal rotor states places an upper bound of 20 cm−1 on the threefold barrier to internal rotation. The Coriolis interactions in the E state also allow the determination of ‖eQqab‖ for Ar–CH335Cl as 13.0(3) MHz. The symmetry axis of the CH3Cl subunit is nearly perpendicular (∼82°) to the line joining the centers of mass of the two binding partners. The isotopic data indicate that the Cl end of the methyl chloride is tilted toward the argon. The distance between the centers of mass of the two subunits is 3.7826 Å for Ar–CH335Cl and 3.7839 Å for Ar–CH337Cl implying an Ar–Cl distance of 3.750 Å. Centrifugal distortion analysis yields a weak bond stretching force constant of 0.0157 mdyn/Å and stretching frequency of 34.6 cm−1 for Ar–CH335Cl. The results from this investigation are related to previous lower resolution microwave studies on Ar–CH3Cl.
Inelastic electron scattering from atomic targets. Bethe surface for helium
Authors: Froelich P. , Flores-Riveros A.
Published: July 2002

The energy, momentum, and angle dependent cross section for the inelastic scattering of electrons from atoms has been calculated in the form of the Bethe surface, by means of the complex-coordinate technique which avoids the explicit calculation of the final scattering states by extracting the necessary scattering information from the expectation value of the resolvent with respect to the initial (ground) state. An application to the scattering of electrons from the helium atom is presented. Both the global structure and the spectral properties are obtained, and the method appears to be especially successful in describing the fine structure due to the presence of the doubly excited resonant states. The interaction between the resonant and nonresonant (background continuum) contributions to the cross section is automatically included, and in the optical limit is seen to result in the typical Fano shapes of the resonant profiles. For higher momentum transfers the development of Bethe ridge and the optically forbidden spectral lines is followed.