bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of substrate conditions on the plasma beam deposition of amorphous hydrogenated carbon
Authors: Gielen J. W. A. M. , Kessels W. M. M. , van de Sanden M. C. M. , Schram D. C.
Published: July 2002

A study on the effect of substrate conditions was performed for the plasma beam deposition of amorphous hydrogenated carbon ( a-C:H) from an expanding thermal argon/acetylene plasma on glass and crystalline silicon. A new substrate holder was designed, which allows the control of the substrate temperature independent of the plasma settings with an accuracy of 2 K. This is obtained via a combination of a good control of the holder’s yoke temperature and the injection of helium gas between thermally ill connected parts of the substrate holder system. It is demonstrated that the substrate temperature influences both the a-C:H material quality and the deposition rate. The deposition rate and substrate temperature are presented as the two parameters which determine the material quality. In situ studies prove that the deposition process is constant in time and that thermally activated etching processes are unlikely to contribute significantly during deposition. Preliminary experiments with an additional substrate bias reveal that an energetic ion bombardment of the growing film surface does not influence the deposition process. A tentative deposition model is proposed based on the creation and destruction of active sites, which depend on the particle fluxes towards the substrate and the substrate temperature. This model allows the qualitative explanation of the observed deposition results.
Properties and density of states of the interface between silicon and carbon films rich in sp3 bonds
Authors: Logothetidis S. , Evangelou E. , Konofaos N.
Published: July 2002

The interface states between n-type Si and amorphous carbon films rich in sp3 bonds grown by rf magnetron sputtering at room temperature have been examined. The investigation aimed to examine the effects of the low substrate temperature and the absence of hydrogen during the growth process on the density of interface states. Thus, comparing the values of the interface states to those reported for devices grown by other techniques, the best possible interface required for electronic applications is suggested. The conductance technique was used to measure the density of the interface states. This method revealed a value of the traps for the n-Si(100)-carbon interface of the order of 1010 cm−2 eV−1, nearly one order of magnitude lower than any other previously reported for the same configuration.
Atomic hydrogen concentration profile measurements in stagnation-flow diamond-forming flames using three-photon excitation laser-induced fluorescence
Authors: Bertagnolli Ken E. , Lucht Robert P. , Bui-Pham Mary N.
Published: July 2002

Gas-phase atomic hydrogen concentration profiles were measured near the deposition substrate in atmospheric-pressure, stagnation-flow diamond-forming flames. In these flames, an acetylene-oxygen-hydrogen mixture accelerates through a nozzle and impinges on a water-cooled molybdenum substrate, stabilizing a flat-flame approximately 1 mm below the substrate. A thin, polycrystalline diamond film is deposited on the substrate under appropriate conditions of flame stoichiometry and substrate temperature. Three-photon-excitation laser-induced fluorescence (LIF) was used to determine the H-atom concentration at various points between the substrate and the incoming premixed jet. The estimated H-atom LIF accuracy is ±40–50% in the diamond-forming flames, and the estimated spatial resolution is ±100 μm perpendicular to the deposition surface. The LIF measurements show that the peak atomic hydrogen mole fraction is approximately 5%, significantly less than the calculated adiabatic equilibrium concentration. This subequilibrium mole fraction results from the slow rate of acetylene dissociation for the fuel-rich conditions in the post-flame gases and the creation of superequilibrium concentrations of CO2 and H2O in the reaction zone. The measured subequilibrium H-atom concentrations are consistent with our previous measurements of superequilibrium temperatures in the post-flame region of these flames. We present numerical calculations of species and temperature profiles in our diamond-forming flames. The measured subequilibrium H-atom concentration profiles are in good agreement with theoretical calculations of the profile. Measured peak flame temperatures are in general 100–200 K lower than calculated peak temperatures, but the calculated and measured profiles are in excellent agreement in the high-gradient region near the deposition substrate.
Mixed electron emission from doped Pb(Zr,Ti)O3 ceramics: Microstructural aspects
Authors: Zhang Weiming , Huebner Wayne
Published: July 2002

A mixed type electron emission, i.e., simultaneous ferroelectric and plasma emission, was observed with a negative driving pulse applied to doped Pb(Zr,Ti)O3 ceramics in the absence of any external potential on the electron collector. During these emission studies, significant microstructural changes on the emission surface were observed, and corresponded to the different emission modes. Erosion craters at the edge of the electrode and small particles near these craters reflected the formation of a dense plasma there. Comparatively, cavities, i.e., grain pullouts, accumulated on the bare ferroelectric surface, the frequency of which depended upon its distance from the grid. This phenomenon is proposed to be a result of fringing fields and the associated strain energy due to 90° domain switching, which could be seen as an evidence that ferroelectric emission occurred in these areas.
Analysis of deep traps in hexagonal molecular beam epitaxy-grown GaN by admittance spectroscopy
Authors: Krtschil A. , Witte H. , Lisker M. , Christen J. , Birkle U. , Einfeldt S. , Hommel D.
Published: July 2002

Nominally undoped GaN layers grown by molecular beam epitaxy (MBE) and having resistivities between 105 and 107 Ω were investigated with temperature- and frequency-dependent admittance spectroscopy. The advantage of these measurement methods is shown in terms of the formation of Schottky contacts on high-resistivity GaN layers. The space-charge region, which is needed for detection of deep defects exists at low frequencies only and, therefore, deep level transient spectroscopy (DLTS) measurements fail for this material. Two deep defect levels were identified in MBE-grown GaN layers. The thermal activation energies are (0.45±0.04) and (0.63±0.04) eV, respectively. These deep traps are well known from DLTS and thermal stimulated conductivity measurements in metalorganic vapor phase epitaxy and hydride vapor phase epitaxy-grown GaN.
Determination of hot-carrier induced interface state density in polycrystalline silicon thin-film transistors
Authors: Mariucci L. , Fortunato G. , Carluccio R. , Pecora A. , Giovannini S. , Massussi F. , Colalongo L. , Valdinoci M.
Published: July 2002

Polysilicon thin-film transistors are of great interest for their application in large area microelectronics and especially for their circuit applications. A successful circuit design requires a proper understanding of the electrical characteristics and in the present work some specific aspects related to the hot-carrier induced electrical instabilities are presented. In particular, generation of interface states near the drain junction occurs when the devices are operated for a prolonged time in the so-called kink regime. In the present work we show both experimentally and by numerical simulations how the presence of such interface states affects the electrical characteristics. Furthermore, a novel simple method is proposed to extract, from the analysis of the sheet conductances, the interface state density. The hot-carrier induced interface state density relative to the present devices shows a featureless continuous distribution. Reduction of the generated interface states is observed if trapped holes are annihilated by electron capture. These results suggest that the interface states are induced by the presence of trapped holes, in agreement with similar data reported for c-Si metal–oxide–semiconductor structures.
Step structure and ordering in Te-doped GaInP
Authors: Lee S. H. , Hsu T. C. , Stringfellow G. B.
Published: July 2002

The step structure and CuPt ordering in GaInP layers grown by organometallic vapor phase epitaxy on singular GaAs substrates have been investigated as a function of Te (DETe) doping using atomic force microscopy, and electrical and optical properties measurements. The degree of order decreases for Te concentrations of >1018 cm−3. It is estimated from the photoluminescence peak energy to be approximately 0.5 for undoped layers and the layers are completely disordered at sufficiently high Te doping levels. The bandgap energy is changed by 110 meV as the Te doping level increases from 1017 to 1018 cm−3. The step structure also changes markedly over the range of doping that produces disordering, from a mixture of monolayer and bilayer steps for undoped layers to solely monolayer steps for electron concentrations exceeding 1018 cm−3. For growth at 670 °C, the spacing between [1̄10] steps increased by over an order of magnitude as the doping level was changed over the range investigated, while the step spacing between [110] steps increased only slightly. In general, Te doping significantly improves the surface morphology viewed using atomic force microscopy. The degree of order and surface structure are changed at exactly the same doping concentration. This suggests that the disordering may be controlled by the fast propagation of [1̄10] steps due to kinetic effects at the step edges. A qualitative model is presented to explain these effects.
Enhanced formation of negative ions by electron attachment to highly excited molecules in a flowing afterglow plasma
Authors: Ding Weixing , McCorkle Dennis L. , Pinnaduwage Lal A.
Published: July 2002

Preliminary evidence for efficient negative-ion formation using a plasma mixing scheme was reported in a recent letter [L. A. Pinnaduwage, W. Ding, and D. L. McCorkle, Appl. Phys. Lett. 71, 3634 (1997)]. In the present article we confirm the negative ion formation using a probe-assisted photodetachment technique and estimate rate constants for electron attachment to electronically excited CH4 and NO in a flowing afterglow plasma. It is shown that enhanced electron attachment to molecules in highly excited states populated via excitation transfer from rare gas metastables is responsible for the observed negative ion formation. Implications for plasma processing and plasma remediation discharges are also discussed.
Propagation characteristics of a thin-film superconducting parallel-plate transmission line in the mixed state
Authors: Wu Chien-Jang
Published: July 2002

The microwave propagation properties of a high Tc superconducting transmission line in the mixed state are theoretically investigated by using the electrodynamic field equations for a type-II superconductor developed by Coffey [J. Low Temp. Phys. 108, 331 (1997)]. We have numerically examined the attenuation constant and phase velocity as a function of temperature, frequency, and static magnetic field. The role played by the vortices in the superconducting transmission line is illustrated and discussed.
Spin-dependent Auger effect on shallow donors in the CdF2:Mn2+ luminescence
Authors: Kamińska A. , Suchocki A.
Published: July 2002

It is shown that the Auger effect on shallow donors in the luminescence of manganese ions in semiconducting CdF2:Mn, Y crystals is suppressed by a magnetic field. The Auger effect, which is a spin-dependent energy transfer from the manganese ions to the electrons occupying shallow donors, is sensitive to the orientation of the magnetic moments of the manganese ions and electrons occupying shallow donors. An increase of the magnetic field results in the increase of the luminescence decay times of the Mn2+ ions.
Effect of Ti, V, Cr, and Mn additions on the magnetic properties of a nanocrystalline soft magnetic Fe–Zr–B alloy with high magnetic flux density
Authors: Bitoh T. , Nakazawa M. , Makino A. , Inoue A. , Masumoto T.
Published: July 2002

The effect of the addition of Ti, V, Cr, and Mn on the magnetic properties of a nanocrystalline soft magnetic Fe–Zr–B alloy has been investigated. The addition of the elements increases both the crystallization temperature and the grain size of α-Fe. After crystallization, these elements are observed in both the α-Fe grains and the residual amorphous matrix. It has been found that V is a useful element to control magnetostriction by keeping the saturation magnetic flux density (Bs) high. The simultaneous addition of V and Mn increases Bs. The alloys with high Bs, above 1.75 T, show good soft magnetic properties as well; the Fe90V1Zr6B3 alloy exhibits high Bs of 1.75 T and high permeability (μe) of 31 000, and the Fe89.5V0.5Mn1Zr6B3 alloy exhibits high Bs of 1.78 T and high μe of 23 000. These high Bs values are almost the same as that of a Fe-6.5 wt % Si alloy. The alloys also exhibit low core loss. Therefore, nanocrystalline Fe–V–(Mn)–Zr–B alloys are expected to be applied to power electronic devices such as power transformers.
Optical properties of CdTe1−xSx (0⩽x⩽1): Experiment and modeling
Authors: Wei K. , Pollak Fred H. , Freeouf J. L. , Shvydka Diana , Compaan A. D.
Published: July 2002

Spectral ellipsometry at 300 K, in the range 0.75–5.4 eV, has been used to determine the optical constants ε(E)[=ε1(E)+iε2(E)] of a series of CdTe1−xSx (0⩽x⩽1) films fabricated by a laser-deposition process. The measured ε(E) data reveal distinct structures associated with critical points (CPs) at E0 (direct gap), spin-orbit split E1, E1+Δ1 doublet and E2. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Phys. Rev. B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs (also E0+Δ0 CP) plus excitonic and band-to-band Coulomb enhancement (BBCE) effects. In addition to evaluating the energies of these various band-to-band CPs, our analysis also makes it possible to obtain information about the binding energies of not only the three-dimensional exciton associated with E0 but also the two-dimensional exciton related to the E1, E1+Δ1 CPs. Our results will be compared to previous experiments and modeling (which neglect the BBCE terms) of ε(E) of CdTe and CdS as well as optical absorption measurements of E0 of CdTe1−xSx (0⩽x⩽1). The results of this experiment demonstrate conclusively that the band-to-band line shape at E0 is BBCE even if the exciton is not resolved.
Field-induced structural anisotropy in magnetic fluids
Authors: Hong Chin-Yih
Published: July 2002

Under parallel magnetic fields, a periodic long-chain structure pattern will form in the magnetic fluid film when the field strength exceeds some critical value Hc. The distance d between the chains and the chain width a are inversely proportional to the field strength and the sweep rate. Due to the structural formation inside the film, the degree of anisotropy of the magnetic fluid film and the intensity of the transmitted polarized light passing through the film will increase as the distance between chains and chain width decrease. The curves of the normalized light intensity and the ratio a/(d−a) exhibit a similar behavior. Moreover, the normalized transmitted intensity is a function of the angle between the direction of H and of the polarization of the incident light and is observed as a universal law sin2 2θ. Thus the field-induced structural anisotropy in the magnetic fluid film under parallel magnetic fields was established.
Magnetic properties of ultrathin Co films on Si(111) and CoSi2 surfaces
Authors: Tsay J. S. , Yang C. S. , Liou Y. , Yao Y. D.
Published: July 2002

The orientation of the magnetization and the occurrence of interfacial ferromagnetic dead layers for ultrathin Co films on Si(111) and CoSi2 surfaces have been systematically studied using in situ surface magnetic-optic Kerr effect. We have experimentally demonstrated that an in-plane magnetization can be obtained by using CoSi2 as a buffer layer for ultrathin Co films between 2.8 and 10.5 monolayers (MLs) deposited on Si(111) at 300 K. The ferromagnetic dead layers at the interface are most likely due to the formation of a Co–Si alloy. This region can be reduced from 2.1 to 1.4 ML by lowering the substrate temperature from 300 to 120 K. From a dynamic study of the silicide formation in Co/Si(111), a two-step diffusion mechanism is suggested with two different diffusion activation energies of the Co atoms resulting from different chemical environments.
Microstructural and nonlinear optical properties of silica–titania sol-gel film doped with PbS quantum dots
Authors: Martucci A. , Fick J. , Schell J. , Battaglin G. , Guglielmi M.
Published: July 2002

Thin silica–titania films doped with different concentration of PbS quantum dots (PbS/oxide molar ratios ranging from 5% to 25%) were fabricated via a sol-gel route. The structural properties were studied by x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering spectrometry. The PbS crystals were found to have a mean diameter in the range 2.3–3.5 nm with narrow size distribution. The resonant nonlinear optical properties were studied by the nonlinear m-line technique and degenerate four-wave mixing. High negative nonlinear refractive indices (n2) were measured at 1.064 μm. Different n2 values were obtained for nanosecond excitation (n2=10−7–10−8 cm2/kW) and for picosecond excitation (n2=10−9–10−10 cm2/kW). The differences can be explained by saturation effects. Measurements at 532 nm showed n2 values ten times higher than at 1.064 μm. The response time of the nonlinearity for both wavelengths was below 35 ps.
Correlation between tip-apex shape and surface modification by scanning tunneling microscopy
Authors: Heike Seiji , Wada Yasuo , Hashizume Tomihiro
Published: July 2002

The correlation between the shape of a probe tip apex for scanning tunneling microscopy and the surface modification patterns on a Si(111)-7×7 surface is demonstrated by using an in situ tip evaluation technique. The tip apex is inversely imaged by using a very sharp nanoneedle fabricated on the sample surface by applying a high voltage between the tip and the surface. When a large current of 100–300 nA flows between the tip and the surface, silicon atoms on the surface are extracted and a trench pattern is formed. The trench pattern agrees well with the geometry of the tip apex. The correlation between the tip shape and the fabrication pattern is discussed in terms of the electric field under the tip.
Diffusion and relaxation of excess carriers in InGaN quantum wells in localized versus extended states
Authors: Vertikov A. , Ozden I. , Nurmikko A. V.
Published: July 2002

We investigate diffusivity of excess carriers in InGaN multiple quantum wells by near-field optical imaging of photoluminescence profiles created with spatially inhomogeneous photoexcitation, complemented by spatially integrated time-resolved measurements. Nominally similar samples display a wide range of behavior in terms of the impact of localized states under moderate (n∼5×1017 cm−3) electron-hole (e-h) injection. By contrast, in the high density regime where present laser diodes operate (n∼1019 cm−3), radiative recombination is dominated by electronic action within the extended states.
Activator ionization and deep level ionization in SrS:Ce ac thin-film electroluminescent devices
Authors: Peter Manuela , Nishimura Satoru , Murayama Masatoshi , Ohmi Koutoku , Tanaka Shosaku , Kobayashi Hiroshi
Published: July 2002

This report focuses on the ionization mechanism of dynamic space charge formation during ac thin-film electroluminescent device operation. Laser excitation under operating condition is used to identify the ionization mechanism. It is concluded, that Ce3+ ionization and ionization of other deep levels are forming the dynamic space charge. The charge emission process is impact excitation in combination with tunnel emission. The Ce3+ excited state is more than 0.23 eV below the conduction band and the ground state of other deep levels is more than 3.6 eV below the conduction band.
Interdiffusion up to the eutectic composition and vitrification in Gd/Co multilayers
Authors: Andrés J. P. , Sacedón J. L. , Colino J. , Riveiro J. M.
Published: July 2002

We have performed composition depth profiles and other structural experiments on Gd/Co multilayers and bilayers. Analysis of Auger depth profiles indicates a strongly asymmetric diffusion, especially of Co into the Gd layers. In multilayers, it is found that Co diffuses throughout the Gd layers and forms a Gd1−xCox amorphous alloy (x=0.29–0.35) that is limited at nearly the eutectic composition (x=0.37). Therefore, if the Co layers are nominally not too thin (≳3 nm), the multilayer mainly comprises an amorphous layers of composition around Gd0.7Co0.3 and layers of pure Co. Thin Co layers (3–7 nm) present broad halos in x-ray diffraction scans at wave vectors characteristics of pure cobalt. Thick layers (≳10 nm) show crystalline peaks with a coherence size (ξ⊥) shorter than its own layer thickness, suggesting the existence of amorphous phases next to the interface. This can be explained by the growth of pure and amorphous Co up to a thickness of 2–4 nm on the amorphous Gd alloy seed, but further Co deposition consists of hcp crystalline grains.
Impact of magnetic fields on the spatial relaxation of electrons
Authors: Winkler R. , Maiorov V. A. , Sigeneger F.
Published: July 2002

The spatial relaxation of the electrons in weakly ionized plasmas of atomic and molecular gases acted upon by spatially homogeneous electric fields has been the subject of detailed kinetic studies in recent years. In particular, the relaxation behavior of the electron velocity distribution and of relevant macroscopic transport and dissipation properties of the electrons, initiated by different disturbances, have been systematically investigated and its basic mechanisms analyzed. These studies have revealed that, particularly in atomic plasmas, the electrons generally respond to disturbances with weakly damped, spatially periodic relaxation processes that are characterized by large relaxation lengths and pronouncedly nonlocal properties. Currently, the superposition of magnetic fields gets increasing interest [S. Nakamura, P. L. G. Ventzek, and K. Kitamori, J. Appl. Phys. 85, 2534 (1999)] to additionally control or tune those plasmas that are especially used in plasma processing and laser applications by the imposed magnetic force. This evolution has motivated to extend the spatial relaxation studies to plasmas acted upon by crossed electric and magnetic fields. Thus, based on a consequent kinetic treatment, the present article reports on the considerable modifications of the relaxation behavior of the electrons enforced by the additional electron cyclotron motion in the magnetic field.
Nordheim dependence in the surface resistivity of disordered overlayers
Authors: Rous P. J.
Published: July 2002

A multiple scattering calculation is used to compute the surface resistivity of Cu(100), Al(100), and Al(111) surfaces as a function of coverage by a disordered overlayer of Cu or Al adatoms. The Bloch states of the semi-infinite bulk are described by a layer-Korringa–Kohn–Rostoker calculation combined with the coherent potential approximation to represent the carrier scattering by the disordered overlayer. The diffuse scattering of carriers by the surface disorder results in a Nordheim, or quasiparabolic, dependence of the induced surface resistivity upon the coverage. This result confirms the qualitative behavior observed in a prior calculation using the average t-matrix approximation where the surface was modeled as a random distribution of s-wave scatterers in front of a hard-wall potential representing the surface barrier [D. L. Lessie and E. R. Crosson, J. Appl. Phys. 59, 504 (1986)].
Microstructure of columnar crystallites in Ni80Fe20/Cu magnetic multilayers
Authors: Sun H. P. , Zhang Z. , Wang W. D. , Jiang H. W. , Lai W. Y.
Published: July 2002

We have used electron microscopy to investigate the microstructure of Ni80Fe20/Cu magnetic multilayers which were synthesized by dc magnetron sputtering. Columnar structure was found in the specimen with and without giant magnetoresistance (GMR). All the columnar crystallites (CCs) originate from the Fe buffer layer on silicon wafer or glass substrate and penetrate though all the multilayers up to the surface of the film. The lateral size of the CCs ranges from 10 to 30 nm. Cross-sectional high-resolution electron microscopy study shows that the CCs are single-crystal-like with fcc structure resulting from the epitaxial growth of NiFe and Cu sublayers. Electron diffraction contrast imaging and electron energy filtered elemental mapping confirmed that multilayer nature is maintained throughout the entire NiFe/Cu film. Grain boundaries between CCs can be the most likely place where NiFe or Cu bridging will occur. Columnar structure was also found in a Ta/NiFe/Cu/NiFe/FeMn/Ta spin valve film. The possible influence of the columnar crystalline structure on the GMR related problems is discussed. The microstructure results revealed in this article provide useful information for the GMR property investigation of NiFe/Cu based metallic multilayers.
Domain nucleation processes in mesoscopic Ni80Fe20 wire junctions
Authors: Lee W. Y. , Yao C. C. , Hirohata A. , Xu Y. B. , Leung H. T. , Gardiner S. M. , McPhail S. , Choi B. C. , Hasko D. G. , Bland J. A. C.
Published: July 2002

The magnetization reversal process in permalloy (Ni80Fe20) wire junction structures has been investigated using magnetoresistance (MR) measurements and scanning Kerr microscopy. A combination of electron beam lithography and a lift-off process has been utilized to fabricate wires consisting of two 200 μm length regions with distinct widths w1 and w2 in the range 1–5 μm. Longitudinal MR measurements and magneto-optic Kerr effect hysteresis loops demonstrate that the magnetization reversal of the complete structure is predominantly determined by the wider region for fields applied parallel to the wire axis. Magnetic force microscopy and micromagnetic calculations show that several domain walls nucleate in the wider part and are trapped in the junction area. This implies that domain nucleation at the junction of the wire initiates magnetization reversal in the narrow half. As a consequence, the switching fields are found to be identical in both halves in this case. These results suggest the possibility of designing structures which can be used to “launch” reverse domains in narrow wires within a controlled field range.
Enlargement of nontransmission frequency range in photonic crystals by using multiple heterostructures
Authors: Zhang Chun , Qiao Feng , Wan Jun , Zi Jian
Published: July 2002

We show theoretically that the nontransmission frequency range of photonic crystals can be substantially enlarged as desired by using multiple heterostructures. This photonic multiple heterostructure consists of different photonic crystals. The constituent photonic crystals are chosen such that the nontransmission frequency ranges of the adjacent constituents are arranged in a way similar to the type-II lineup of the band-gaps in semiconductor heterostructures. Examples in the two-dimensional systems are demonstrated theoretically by using transfer matrix methods.
X-ray scattering study of interfacial roughness correlation in Mo/Si multilayers fabricated by ion beam sputtering
Authors: Ulyanenkov A. , Matsuo R. , Omote K. , Inaba K. , Harada J. , Ishino M. , Nishii M. , Yoda O.
Published: July 2002

Specular and diffuse x-ray scattering are used to probe the mesoscopic structure of interfaces within two 30-period Mo/Si superlattices, grown on silicon and glass substrates by ion beam sputtering. The data are evaluated qualitatively and quantitatively on the basis of a distorted-wave Born approximation, which includes a correlating behavior of interface roughness in both the lateral and vertical directions. Different initial conditions of the substrate’s surface result in distinguishable characters of roughness replications in the direction of growth. The average value, lateral correlation and fractal dimension of roughness are found to be different in the two samples, which leads to differences in the reflective properties of multilayer mirrors.
Landau–Zener method to study quantum phase interference of Fe8 molecular nanomagnets (invited)
Authors: Wernsdorfer W. , Sessoli R. , Caneschi A. , Gatteschi D. , Cornia A. , Mailly D.
Published: July 2002

We present details about an experimental method based on the Landau–Zener model which allows one to measure very small tunnel splittings Δ in molecular clusters Fe8. The measurements are performed with an array of microsuperconducting quantum interference devices. The observed oscillations of Δ as a function of the magnetic field applied along the hard anisotropy axis are explained in terms of topological quantum interference of two tunnel paths of opposite windings. Transitions between M=−S and (S−n), with n even or odd, revealed a parity (symmetry) effect which is analogous to the suppression of tunneling predicted for half integer spins. This observation is the first direct evidence of the topological part of the quantum spin phase (Berry phase) in a magnetic system. The influence of intermolecular dipole interactions on the measured tunnel splittings Δ is shown.
Electrical characterization of Pb centers in (100)Si–SiO2 structures: The influence of surface potential on passivation during post metallization anneal
Authors: Ragnarsson Lars-Åke , Lundgren Per
Published: July 2002

Capacitance–voltage measurements were made on Cr-gated metal–oxide–silicon structures with ultrathin (∼30 Å) thermal oxides. Using an empirical model, activation energies for the passivation of the Pb center were determined and found to be dependent on the charge state of the defect. Depassivation was found to occur at positive gate biases.
Higher excited states of benzene: Polarized ultraviolet two-photon absorption spectroscopy
Authors: Whetten R. L. , Grubb S. G. , Otis C. E. , Albrecht A. C. , Grant E. R.
Published: July 2002

The Rydberg spectrum of benzene is explored by means of linear and circular polarized ultraviolet two-photon absorption spectroscopy of the jet-cooled molecule. A strong gerade Rydberg series previously designated nRg and all its associated vibronic structure seen in linear polarization is shown to have symmetry A1g, but analysis of spectra obtained under circular polarization demonstrates that E2g origins lie just to lower energies. These combined facts give an unambiguous assignment of this series as nd1 (E2g, A1g, A2g). Four new Rydberg series, each having vibrational structure nearly identical to each other and to the ground state ion, are also identified and have E1g or E2g symmetry. The five Rydberg series converging to the first ionization potential have quantum defects 0.77, 0.06, 0.05, 0.02, and −0.11. Bandwidths indicative of ultrafast radiationless decay processes are observed for low-n vibronic states, but higher states are sharp. In addition, an Eg band system is observed at 68 980 cm−1 (69 330 for C6D6) whose term value, vibrational structure, bandwidth, and isotope shift on deuteration are all consistent with its assignment as the lowest Rydberg state converging to the first excited ionic state (second ionization potential).
Variational transition state theory calculations of the reaction rates of F with H2, D2, and HD and the intermolecular and intramolecular kinetic isotope effects
Authors: Steckler Rozeanne , Truhlar Donald G. , Garrett Bruce C.
Published: July 2002

We use variational transition state theory to calculate rate constants and kinetic isotope effects for the reactions F+H2→HF+H (with rate constant k1), F+D2→DF+D(k2), and two other isotopic analogs as functions of temperature. The calculations are performed using a recently proposed partly empirical, partly ab initio potential energy surface, called surface No. 5, and also using a new surface, called surface No. 5A, introduced here to test the effect of a higher classical saddle point on the reaction rates, kinetic isotope effects, and reaction thresholds. The various theoretical results are compared to the available experiments to test the validity of these potential energy surfaces. For those rate constants and kinetic isotope effects for which there is more than one experimental value at a given temperature, the theoretical results for reactions on surface No. 5 agree with experiment about as well as the individual experiments agree with each other. At T>373 K where there is only one experimental measurement for k1 and k2, the theoretical rate constants for surface No. 5 are up to 44% lower than experiment, and at T<190 K where there is only one experimental measurement of k1/k2, the theoretical rate constant ratio is as much as 43% low. These discrepancies could be due to experimental error or they could indicate that the theoretical activation energy is low, but by no more than 0.5 kcal/mol, which is considerably less than the 2.5 kcal/mol discrepancy from the most recently published large-scale calculation. If, however, the potential energy surface were adjusted to raise the activation energy by even 0.5 kcal/mol above the value for surface No. 5, the threshold energies would no longer be in such good agreement with experiment, and several other aspects of the results presented here would be in much worse agreement with experiment. Taken as a whole then, we interpret the present calculations as providing further confirmation of the methods used to obtain surface No. 5. With this interpretation, the most important implication about the experiments is that Persky’s low-temperature measurements of k1/k2, especially those for T≤200 K, are systematically high.
Molecular distribution among regions of different curvature in aggregates of a lyotropic ternary system: A NMR measurement
Authors: Chidichimo G. , Golemme A. , Doane J. W. , Westerman P. W.
Published: July 2002

A lyotropic phase consisting of elongated ribbon aggregates on a rectangular lattice is induced when a small amount (∼1 mol %) of potassium laurate (KL) is added to a potassium palmitate (KP)/water mixture. By selective deuteration it is shown that the KL and KP molecules are not uniformly distributed throughout the ribbon aggregate. The less abundant KL molecules with the shorter hydrocarbon chain are more concentrated at the edges of the ribbons where there is high curvature, whereas the more abundant KP molecules are more concentrated in the central lamellar region of the aggregates. A quantitative measurement is described. The deuterium spectra also show how the KP and KL lipids are distributed among coexisting phases when an uneven distribution also occurs. The KP molecules prefer the lamellar structures, whereas the KL molecules prefer the aggregate structures.
Molecular dynamics study of the liquid and plastic phases of neopentane
Authors: Mountain Raymond D. , Brown Alan C.
Published: July 2002

Molecular dynamics has been used to investigate a model for neopentane. The velocity–velocity and angular momentum–angular momentum time correlation functions were constructed for the liquid and the single particle time correlation functions for the orientation of twofold and threefold axes of symmetry of the molecules were constructed for both the liquid and plastic phases. The model produces liquid properties that are in agreement with those of liquid neopentane. The dynamics of the molecular reorientations in the plastic phase has been examined. These reorientations are found to consist of jumps between equivalent orientations by a rotation of 120° around a threefold molecular symmetry axis.
Multiphoton ionization photoelectron spectroscopy of phenol: Vibrational frequencies and harmonic force field for the 2B1 cation
Authors: Anderson Scott L. , Goodman Lionel , Krogh-Jespersen Karsten , Ozkabak Ali G. , Zare Richard N. , Zheng Cheng-fa
Published: July 2002

A molecular beam of phenol, cooled by a supersonic expansion, is crossed at right angles by the output of a pulsed frequency-doubled dye laser, causing 1+1 resonance enhanced multiphoton ionization. The kinetic energy of the resulting photoelectrons is determined as a function of laser wavelength with time-of-flight analysis, permitting the assignment of 11 vibrational frequencies for the 2B1 phenol-h6 cation and ten vibrational frequencies for phenol-d5. Of these, all but the lowest frequency one in each case are in-plane vibrations of which phenol has a total of 19. An approximate harmonic force field for the in-plane modes of the phenol cation is derived along with its associated frequencies and mode forms. This in turn facilitates the vibrational analysis. Analogous force field calculations have been carried out on the ground (1A1) and first excited (1B2) states of the neutral parent, permitting conclusions to be reached concerning bonding changes upon removal of an electron from the phenol electron system.
Electron transfer between molecules randomly distributed in a glass
Authors: Domingue R. P. , Fayer M. D.
Published: July 2002

A theoretical and experimental study of electron transfer from an optically excited donor to randomly distributed acceptors in a glassy medium is presented. The influence on time dependent observables of an electron transfer rate which is dependent on the orientation of donor and acceptors in a system with random distance and angular distributions is examined. It is formally proven that an angular dependent electron transfer rate will affect the ensemble averaged functional form of the time dependence of donor emission. Calculations based on models of P orbital–P orbital and P orbital–S orbital angular overlap factors demonstrate that only at short time (less than a few hundred picoseconds) and high concentrations of acceptors does the time dependence deviate from that predicted by the angle independent Inokuti–Hirayama theory. At longer times (>1 ns) angle averaged electron transfer parameters can be extracted from time dependent data. Experimentally, the system pentacene (donor) and duroquinone (acceptor) in sucrose octa-acetate glass is examined. Excitation of pentacene to its first singlet state initiates the electron transfer which is monitored by observing the time dependence of fluorescence emission. The transfer parameters, R0 and a, are extracted from the time dependent curves and are used to reproduce the results of fluorescence quenching measurements on the same system. Values of R0=14.3 Å and a=0.35 Å are obtained.
Nonperturbative theory of temperature-dependent optical dephasing in crystals. II. Pseudolocal phonons
Authors: Hsu D. , Skinner J. L.
Published: July 2002

We continue our discussion of the nonperturbative theory of zero-phonon impurity lines in crystals formulated by Osad’ko and more recently by us. In Paper I of this series we evaluated our general results for the linewidth and shift for models of electronic coupling to both acoustic and optical phonons. Here we evaluate our general results for a simple model of electronic coupling to a pseudolocal phonon. The pseudolocal phonon is characterized by its frequency and lifetime in both the ground and excited electronic states. In addition to providing exact analytic expressions (reduced to quadrature) for the width and shift, we derive an approximate analytic expression for the linewidth that is bi-Arrhenius, with activation energies corresponding to the ground and excited state pseudolocal phonon frequencies. We discuss how our theoretical approach and results are related to the exchange theory of Harris, the optical Redfield theory of deBree and Wiersma, and the perturbation theory of Small and Jones and Zewail. In Paper III of this series, we compare the results of this paper and of Paper I to experiment.
Orientational disordering of an octupolar solid: NMR study of (CH4)xKr1−x alloys
Authors: Calvani P. , Glättli H.
Published: July 2002

The proton NMR susceptibility, linewidth, T1, and spin conversion rates down to 0.4 K have been measured in (CH4)xKr1−x mixtures for 0.6<x<0.89. The temperature dependence of the proton susceptibility 〈I(I+1)〉 could be fitted with a mean tunneling splitting EAF=5.8 K, independent of x. The linewidth has been found proportional to (x)1/2 as expected for random dilution. At low T, a sharp increase in T1 around x=0.8 could be due to transition to an octupolar glass. An attempt has been made to explain in terms of existing theories the spectacular increase in conversion rate upon dilution.
A rapid method for determining rate constants by molecular dynamics
Authors: Straub John E. , Berne Bruce J.
Published: July 2002

A simple method is presented for determining rate constants in activated barrier crossing in a computer simulation. Instead of calculating the reactive flux, this method focuses on dynamics in the system with an absorbing barrier placed at the transition state. This method leads to a large reduction in CPU time for the low and high friction regimes.
Vibrationally excited populations from IR-multiphoton absorption. II. Infrared fluorescence measurements
Authors: Zellweger Jean-Michel , Brown Trevor C. , Barker John R.
Published: July 2002

Infrared emission spectra were obtained for 1, 1, 2-trifluorethane (TFE) excited by infrared multiphoton absorption (1079.85 cm−1). The emission features show that the HF reaction product is formed in vibrational states up to about v=3. Furthermore, emission attributed to F–C≡C–H was observed near 3320 cm−1, indicating that the difluoroethylene primary products of TFE decomposition undergo secondary photolysis; since the difluoroethylene products at room temperature do not absorb laser light, they must be formed vibrationally excited. The emission from the C–H stretch modes of TFE was readily identified near 2980 cm−1 and the emission intensity was obtained as a function of laser fluence. These data are in excellent agreement with predictions based on the theoretical expression for fluorescence intensity and the reconstructed populations determined by the Master Equation calculations described in the preceding paper. These results provide additional support for the accuracy of the reconstructed population distributions and for the theory relating infrared fluorescence intensity to total vibrational energy in polyatomic molecules.
Variational calculation of dynamic second-order susceptibilities
Authors: Svendsen E. No/rby , Willand C. S. , Albrecht A. C.
Published: July 2002

A theoretical method for the calculation of the dynamic second-order susceptibility tensor using a variational approach is presented. In the actual calculations, the CNDO approximation including an extended basis set is used. A calculation on the water molecule compares well with experimental and ab initio values. The dispersion of the tensor components is calculated for the urea molecule.
Photofragmentation dynamics of ketene at 308 nm: Initial vibrational and rotational state distributions of CO product by vacuum UV laser-induced fluorescence
Authors: Nesbitt David J. , Petek Hrvoje , Foltz M. Frances , Filseth Stephen V. , Bamford Douglas J. , Moore C. Bradley
Published: July 2002

The vibration-rotation state distribution of CO(v,J) from CH2CO flash photolysis at 308 nm has been determined by pulsed VUV laser-induced fluorescence. The excited ketene molecule appears to internally convert to vibrationally excited ground singlet state and to dissociate on that surface, CH2CO+308 nm→CH2(1A1)+CO+6.7 kcal/mol. There are no energetic CO molecules which would indicate formation of the lower energy CH2 triplet state. The observed rotational distributions of CO(v=0) and CO(v=1) are roughly matched by rotational temperatures of 1300 and 550 K, respectively; the data are accurately matched by phase space theory (PST). PST assumes that all energetically accessible product states have equal probability; it contains no adjustable parameters. PST predicts the ratio of v=1 to v=0 populations to be 0.013 compared to an observed value of 0.09±0.05. A Franck–Condon calculation gives too much vibrational excitation, 0.22±0.07. The separate statistical ensemble (SSE) model of Wittig which decouples overall ketene rotation from the other internal degrees of freedom gives 0.082 in agreement with experiment. This type of statistical distribution is likely to characterize bond fissions for which the barrier is just the dissociation limit itself.
Rydberg states of the ArCO2 and KrCO2 van der Waals molecules
Authors: Dehmer P. M.
Published: July 2002

Relative photoionization cross sections were determined for ArCO2 at a wavelength resolution of 0.24 Å in the regions of both the Ar*+CO2 and the Ar+CO*2 dissociation limits (690–920 Å) and for KrCO2 at a wavelength resolution of 0.4 Å in the region of the Kr+CO*2 dissociation limits (690–780 Å). The dimer ionization potentials are 13.59±0.04 eV (912.5±2.5 Å) for ArCO2 and 13.33±0.04 eV (930.0±3.0 Å) for KrCO2. Combining these values with the known value of the CO2 ionization potential and the known (or estimated) values of the ArCO2 and KrCO2 dissociation energies yields values for the dimer ion ground state dissociation energies of 0.20±0.04 eV for ArCO+2 and 0.47±0.05 eV for KrCO+2. Rydberg structure is observed in the photoionization cross section of ArCO2 corresponding to dissociation limits of the type Ar*(nd,ns)+CO2; however, discrete structure is observed only near the Ar*(3d,5s)+CO2 dissociation limits. van der Waals structure corresponding to the higher Ar*+CO2 limits is diffuse and red degraded. Molecular Rydberg structure also is observed in the photoionization cross sections of both ArCO2 and KrCO2 corresponding to dissociation limits of the type Ar+CO*2. Prominent van der Waals structure is observed corresponding to dissociation limits based on members of the CO2 Henning sharp and diffuse Rydberg series, which converge to CO+2B 2Σ+u; however, no van der Waals structure is observed corresponding to dissociation limits based on members of the CO2 Tanaka–Ogawa Rydberg series, which converge to CO+2A 2Πu. Various experimental evidence indicates that neither vibrational predissociation of the excited van der Waals dimer nor vibrational predissociation of the dimer ion can account completely for the absence of van der Waals structure corresponding to the Tanaka–Ogawa series in CO2.
The neon–argon interatomic potential
Authors: Candori Roberto , Pirani Fernando , Vecchiocattivi Franco
Published: July 2002

Due to some new recent and accurate experimental results the Ne–Ar interatomic potential has been reexamined. The two multiproperty potentials available in the literature have been tested in their ability to reproduce all the experimental data available today . A new potential has been determined which allows a very good description of all the available experimental results, i.e., transport properties, second virial coefficients, and differential and integral cross sections in the thermal energy range; the new potential does not exhibit the inadequacies of the earlier multiproperty potentials.
Resonance state energies and lifetimes via analytic continuation of stabilization graphs
Authors: Frey Regina F. , Simons Jack
Published: July 2002

In this paper, we determine the lifetimes of three chemical reaction resonance states by analytically continuing real-valued stabilization graphs into the complex-energy plane. Extending the method developed by Simons in 1981, we estimate the widths of the resonances by fitting the stabilization graphs to a two-branched quadratic equation whose coefficients are polynomials in the real-scaling parameter α. We also study what criteria affect the quality of the resonance-state lifetime and develop a systematic procedure that could be followed to determine such lifetimes.