bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic moments and anisotropies in ultrathin epitaxial Fe films on ZnSe(001)
Authors: Reiger E. , Reinwald E. , Garreau G. , Ernst M. , Zölfl M. , Bensch F. , Bauer S. , Preis H. , Bayreuther G.
Published: July 2002

The morphology, atomic magnetic moments, and in-plane magnetic anisotropies of ultrathin bcc Fe(001) films deposited by molecular beam epitaxy on ZnSe epilayers grown on GaAs(001) single crystal are reported. The growth mode and structure have been determined in situ by means of reflection high energy electron diffraction and Auger electron spectroscopy. The magnetic properties were characterized ex situ by an alternating gradient magnetometer, superconducting quantum interference device (SQUID) magnetometry, and conversion electron Mössbauer spectroscopy (CEMS). The Fe growth is epitaxial and occurs by three dimensional nucleation at the beginning. The coalescence of the islands is observed around 7 monolayers (ML). In agreement with SQUID results, CEMS measurements indicate no reduction of the Fe magnetic moment compared to the bulk value even for the first Fe monolayers. Determination of the in-plane anisotropy constants as function of the Fe thickness shows a strong interface-induced uniaxial in-plane magnetic anisotropy, which leads to a continuous evolution from a pure uniaxial anisotropy with easy axis along [110] direction for thickness below 10 ML to the pure bulk cubic Fe anisotropy above 40 ML.
Microcantilever torque magnetometry of thin magnetic films
Authors: Löhndorf Markus , Moreland John , Kabos Pavel , Rizzo Nick
Published: July 2002

We have developed a microcantilever torque magnetometer based on a torsion-mode atomic force microscope. Thin magnetic films are deposited directly onto micromachined silicon cantilevers. We have measured hysteresis loops of iron thin films with thicknesses ranging from 1 to 40 nm and total magnetic volumes ranging from 2.2×10−11 to 8.8×10−10 cm3. The magnetic moment sensitivity is estimated to be 1.3×10−12 A m2/Hz1/2 at room temperature and ambient conditions. We expect that by operating at the cantilever torsion resonance frequency and at higher torque fields sensitivity will be improved by a factor of 100–1000.
Magnetic imaging on a spin-stand
Authors: Mayergoyz I. D. , Serpico C. , Krafft C. , Tse C.
Published: July 2002

A new technique of magnetic imaging on a spin stand has been developed. In this technique, raw image acquisition is performed by scanning a target area of a disk by a conventional magneto-resistive (MR) read head in two orthogonal (along- and cross-track) directions. Due to the nonlocalized nature of the MR reading head in the cross-track direction, image reconstruction is needed in order to retrieve the actual distribution of magnetization. It is demonstrated that the image reconstruction can be performed by using the response function characterization of the MR head and the specially designed deconvolution technique, which yields the curl-free (field producing) component of magnetization. The technique developed is illustrated by the sample examples of imaging of overwritten tracks with small misregistrations.
Mössbauer spectroscopy and magnetic properties of copper-304 stainless steel multilayer films
Authors: Varasteh M. , Parvin K. , Boekema C. , Porush K.
Published: July 2002

The magnetic behavior of multilayer films of Cu-304 stainless steel (304SS) with equal layer thickness fabricated by magnetron sputtering at ambient temperature is reported. For small layer thickness (t=0.5–1.0 nm), the films show epitaxial fcc growth of 304SS on fcc copper, and multilayers are nonmagnetic due to nonmagnetic nature of fcc 304SS. For larger layer thickness (t=1.0–50.0 nm), 304SS grows epitaxially as fcc near the copper interface accompanied by a bcc 304SS growth away from the interface. As a result, two regimes are observed in 1.0–50.0 nm thickness range. Medium layer thickness (t=1.0–1.35 nm) multilayers show weak ferromagnetism characterized by small magnetization due to comparable thickness of ferromagnetic bcc 304SS and nonmagnetic fcc 304SS. A strong ferromagnetic behavior is observed for thick samples (t=1.35–50.0 nm) due to 304SS layer being primarily ferromagnetic bcc phase with very small percentage of fcc near the interface. Mössbauer effect spectroscopy and magnetometry were utilized to observe these magnetic phases. We present the dependence of magnetization on external field at room temperature for the multilayers. In Mössbauer measurements a single absorption peak for nonmagnetic films is observed. A superposition of a six-peak spectrum and a single-peak spectrum representing the existence of ferromagnetic and nonmagnetic phases in each layer were observed for all other films.
Induced easy-axis reorientation in Ni/Pd multilayers upon Ar sputtering pressure
Authors: Jeong Jong-Ryul , Shin Sung-Chul
Published: July 2002

We have studied the easy-axis reorientation of Ni/Pd multilayer with varying Ar sputtering pressure. All the Ni/Pd multilayers prepared by dc-magnetron sputtering at an Ar sputtering pressure of 2 mTorr show in-plane magnetic anisotropy. However, room-temperature perpendicular magnetic anisotropy was observed in Ni/Pd multilayers prepared at an Ar sputtering pressure of 7 mTorr. To understand the origin of the easy-axis reorientation from in-plane to out-of-plane with varying the sputtering pressure, the magnetoelastic anisotropy was quantitatively determined from delicate in situ stress and ex situ magnetostriction coefficient measurements. We have found that the observed easy-axis reorientation was ascribed to the enhancements of the surface anisotropy as well as the magnetoelastic anisotropy with increasing Ar sputtering pressure.
Effects of Pt and Ta addition on compositional microstructure of CoCr-alloy thin film media
Authors: Inaba Nobuyuki , Futamoto Masaaki
Published: July 2002

Compositional segregation structures of Co–Cr–Ta, Co–Cr–Pt, and Co–Cr–Pt–Ta thin film media with Cr concentrations of about 15 at. % were studied using an analytical transmission electron microscope. Cr atoms in the CoCrTa specimen are extremely localized around the grain boundaries with a maximum concentration of about 23 at. % with a width of 1 nm, while that inside the grain is about a half of the average composition. The Co–Cr–Pt specimen shows Cr segregation at grain boundaries with a maximum concentration by 4 at. % greater than the average composition. The Cr concentration inside the grain is kept nearly same with the average concentration. The Co–Cr–Pt–Ta specimen shows a degree of Cr segregation between the Co–Cr–Ta and the Co–Cr–Pt systems. The Cr enrichment at bicrystal grain boundaries is broader and weaker than those at the normal grain boundaries.
Dependence of Co anisotropy constants on temperature, processing, and underlayer
Authors: Yang Wei , Lambeth David N. , Laughlin David E.
Published: July 2002

The dependence of the Co anisotropy constants on the measurement temperature, deposition conditions, and underlayer materials was studied using unicrystal Co(101̄0)/Cr (or NiAl)(112)/Ag(110) films sputter deposited on hydrofluoric acid etched Si(110) substrates. The single, in-plane easy axis orientation in these films allows the direct determination of the anisotropy constants. The anisotropy constants of unicrystal Co films are smaller than those of a bulk Co single crystal, and the temperature dependence of the anisotropy constants is stronger. The K1 value drops by 50% as the temperature is increased from 25 °C to 75 °C, and then becomes negative at 135 °C. This zero-crossing temperature is considerably lower than the 250 °C at which K1 of a bulk Co single crystal decreases to zero. The anisotropy constants also vary with the film preparation substrate temperature. Applying a substrate bias during the Co deposition effectively increases K1 to near bulk material values. Unicrystal Co films grown on NiAl/Ag/HF–Si(110) show smaller K1 as compared to those on Cr underlayers. The addition of a thin Cr intermediate layer on the NiAl underlayer, however, restores K1 to the larger value obtained on Cr/Ag/HF–Si(110).
Different mechanisms of spin reorientation in exchange coupled double rare earth-transition metal layers with in-plane and perpendicular magnetic anisotropy
Authors: Stavrou Evangelos , Sbiaa Rachid , Suzuki Takao , Roell Klaus
Published: July 2002

The objective of this article is to clarify the spin reorientation effects in exchange coupled layers based on rare earth and transition metals with in-plane and perpendicular anisotropy. These systems are proposed for high density magneto-optical recording with super resolution readout. Two mechanisms are found to be responsible for this change of the magnetization direction with the temperature. The first mechanism is an intrinsic spin reorientation in the GdFeCo layer and the second is the exchange coupling force between the perpendicularly magnetized TbFeCo film and the in-plane GdFeCo film. With appropriate adjustment of the compensation temperatures and the thicknesses of the films, it is possible that the one or the other mechanism dominates. The experimental results are in agreement with a numerical calculation of the magnetization profiles in the coupled system.
Magnetic anisotropy and spin reorientation effects in Gd/Fe and Gd/(FeCo) multilayers for high density magneto-optical recording
Authors: Stavrou E. , Sbiaa R. , Suzuki T. , Knappmann S. , Röll K.
Published: July 2002

We have investigated the anisotropy behavior and temperature dependent changes of the magnetic anisotropy in Gd/Fe and Gd/(FeCo) multilayers. The spin reorientation effects are very important for the super resolution readout in new methods for high-density magneto-optical recording. Gd/(Fe, Co) multilayered films are a good alternative to the common Gd(Fe, Co) alloy films, because the magnetic anisotropy and also spin reorientation effects can be comfortably adjusted by varying the interface and volume anisotropy components and the composition using experimental parameters such as the periodicity λ=tGd+tTM [tGd,tTM: the sublayer thicknesses of the Gd and transition metal Fe, FeCo (TM)] and the ratio of the sublayer thicknesses tGd/tTM. We have found the mechanisms for spin reorientation, which are explained qualitatively with a new model.
Characterization of spin-wave dynamics near homoclinic orbits through one-dimensional mapping
Authors: Rosenblatt S. , de Aguiar F. M. , Rezende S. M. , Azevedo A.
Published: July 2002

A description is given of pulsed high-power ferromagnetic resonance experiments in a YIG sphere at room temperature. Beyond the Suhl instability threshold, evidences were found for the alternating periodic-chaotic sequence. The chaotic spin-wave self-oscillations were characterized through return maps obtained from the local maxima in the observed transient-free time series. As a result, a simple one-dimensional mapping scheme that exhibits the basic features of the observed transition is presented.
Ultrahigh coercivity of granular CoCrPt–SiO2 thin films
Authors: Xu Yingfan , Wang Jian Ping , Su Ying
Published: July 2002

Magnetic properties and microstructure of granular CoCrPt–SiO2 thin films with Cr underlayers have been investigated for potential applications in magnetic recording media. The coercivities exceeding 2500 Oe have been obtained in the as-deposited films with SiO2 varied from 8 to 16 vol %. Remarkable improvement in magnetic properties was achieved by postdeposition annealing in vacuum. The ultrahigh coercivity up to 5640 Oe was obtained in the lower SiO2 content film (CoCrPt)96(SiO2)4 which was annealed at 580 °C for 10 min. The annealing did not induce significant grain growth. The average grain size varied from 10 to 20 nm, depending on SiO2 concentration. Grain isolation in magnetic layer was achieved by the segregated SiO2 and further enhanced by annealing. The ac demagnetized magnetic clusters, observed by magnetic force microscopy, became very fine after annealing. The peak value of delta M curves changed from positive to negative value, indicating a great reduction of intergrain magnetic exchange coupling after annealing.
Hall effect due to spin injection in Co/Al planar junctions
Authors: Otani Y. , Ishiyama T. , Kim S. G. , Fukamichi K.
Published: July 2002

The Hall effect due to spin injection was investigated by using an Al Hall bar attached to a Co/Al planar wire junction. There is observed a significant change in the Hall voltage due to the spin injection when the position of the Hall bar is placed 0.5 μm away from the Co/Al interface. The obtained results were analyzed by taking into account the contributions of the enhancement of the ordinary Hall effect due to a locally increased gradient of the electrochemical potential, and of the extraordinary Hall effect due to the induced nonequilibrium magnetization. The spin diffusion length in Al in the present study is about 1 μm, and the induced difference in chemical potential between up and down spins is about 2.6×10−8 eV. The experimentally evaluated enhancement factor of the ordinary Hall effect is about 1.5, which shows a relatively small contribution to the net Hall effect compared to the extraordinary Hall effect due to the induced nonequilibrium magnetization.
The Hall effect of Fe–Al–O thin film
Authors: Xu Q. Y. , Ni G. , Sang H. , Du Y. W.
Published: July 2002

A series of Fe45.51(Al2O3)54.49 (volume fraction) nanogranular films were prepared using the ion-beam sputtering technique. A saturated Hall resistivity of about 12.5 μΩ cm was observed at room temperature. With different annealing temperature TA up to 300 °C, the saturated Hall resistivity of the sample decreases only a little, which shows good thermal stability. The transmission electron microscopy image shows that very small Fe particles (smaller than 1 nm) embedded in the Al2O3 matrix, and connected into the network, with the ρ–T curve indicates that this Hall effect may originate from the percolation phenomenon.
Magnetic interactions in nanometer-scale particle arrays grown onto permalloy films
Authors: Wirth S. , von Molnár S.
Published: July 2002

The magnetic interactions in arrays of nanometer-scale ferromagnetic iron particles enhanced by direct growth onto thin permalloy films were investigated. The magnetic measurements [Hall magnetometry up to 100 K and variable field MFM (magnetic force microscopy) at room temperature] showed that the magnetization behavior of the permalloy was strongly influenced by the presence of the small (∼13 nm in diameter) particles. The mean values of the particles’ switching fields coincided with those for noninteracting particles. The switching field distribution of the iron particles, however, was considerably broadened by their interactions. These results for strongly interacting small particles exemplify the magnetization behavior of ever smaller and more dense magnetic storage media.
The random field critical concentration in dilute antiferromagnets
Authors: Barber W. C. , Belanger D. P.
Published: July 2002

Monte Carlo techniques are used to investigate the equilibrium threshold concentration, xe, in the dilute anisotropic antiferromagnet FexZn1−xF2 in an applied magnetic field, considered to be an ideal random-field Ising model system. Above xe equilibrium behavior is observed whereas below xe metastability and domain formation dominate. Monte Carlo results agree very well with experimental data obtained using this system.
Nuclear magnetic resonance study of magnetic phase segregation in La1−xCaxMnO3
Authors: Kapusta Cz. , Riedi P. C. , Kocemba W. , Ibarra M. R. , Coey J. M. D.
Published: July 2002

A Mn55 nuclear magnetic resonance (NMR) study of the La1−xCaxMnO3 system, 0<x<1, is reported. For x<0.3 and x>0.5 the spin echo spectra contain resonance lines corresponding to both the microscopic ferromagnetic and antiferromagnetic regions. A resonance line near 380 MHz, which is similar to that of the spectrum of the ferromagnetic and metallic compounds (0.2<x<0.5) is observed for all x. This indicates the presence of clusters in which the Mn magnetic moments are double exchange coupled in the whole range of doping. Their hyperfine field exhibits a much weaker temperature dependence than that of the lines of the host. This reveals the greater strength of the magnetic coupling within the clusters than in the antiferromagnetic regions and indicates a weak magnetic coupling of the clusters to the host matrix. A comparison with the NMR results on the hole doped nonstoichiometric compounds, LaMnO3+δ is made. The temperature dependence of the line intensities corresponding to the double exchange clusters and to the host matrix is attributed to the evolution of the magnetic phases in the compounds.
Single nanoparticle measurement techniques
Authors: Wernsdorfer W. , Mailly D. , Benoit A.
Published: July 2002

Various single particle measuring techniques are briefly reviewed and the basic concepts of a new microsuperconducting quantum interference device technique are discussed. It allows measurements of the magnetization reversal of single nanometer-sized particles at low temperature. The influence of the measuring technique on the system of interest is discussed.
Magnetic properties of U2V3Ge4
Authors: Lin C. L. , Yuen Tan , Mihalisin T. , Myer G. H.
Published: July 2002

We have measured the temperature and magnetic field dependence of the magnetization for the U2V3Ge4 system. A relatively high ferromagnetic TC of 63 K is observed for this system even though the U–U distance is less than the Hill limit of 3.5 Å. The field dependence of the magnetization at low temperatures exhibits hysteresis with a large coercive field indicating a high anisotropy for this compound. Below TC both the zero field cooled (ZFC) and field cooled (FC) magnetization increase as the temperature is decreased, but the ZFC magnetization deviates from the FC data, reaches a broad maximum, and then drops significantly when the temperature is lowered further. This irreversible behavior in the ZFC and FC curves can still be observed in an applied field of H=50 kOe. The anomalous magnetization of this system can be interpreted in terms of intrinsic domain wall pinning which usually occurs in systems with high anisotropy.
Structure of plasmas generated by the interaction between metallic ions and neutral gas particles in a low-pressure arc
Authors: Kelly H. , Lepone A. , Minotti F.
Published: July 2002

A numerical solution for the metallic-plasma-neutral-gas structure generated in a low-pressure arc is presented. The equations correspond to a spherically symmetric fluid-like steady model, valid for the outer region of the arc, and describe the ion slowing down by elastic scattering with the neutral particles. Technically, the obtention of the profiles of different magnitudes is complicated due to the existence of a critical point in the steady-state system of equations. The proposed approach to overcome this difficulty is to solve instead a pseudotransient system of equations which rapidly and efficiently relax to the stationary state. By employing this numerical method of second-order accuracy in space, the plasma and neutral gas density, the electron and ion drift velocities, the electron and neutral temperatures, and the electrostatic potential profiles are obtained from the border of the arc channel up to the discharge chamber wall. It is found that the value of the neutral gas filling pressure strongly influences the plasma density and plasma potential distributions. An important result is that metallic ions emitted from the arc channel deliver their kinetic energy to the filling gas in a gradual manner, up to a pressure-dependent point beyond which they move to the walls sustained against collisions with the gas by a self-consistent electric field. Near the mentioned point, the metallic ion density presents a peculiar behavior, showing an increase that is more pronounced at high pressures; a pattern also evident in the electrostatic potential.
Direct imaging of end-of-range compaction in ion beam irradiated silica waveguides by atomic force microscopy
Authors: von Bibra M. L. , Roberts A. , Mulvaney P. , Huntington S. T.
Published: July 2002

The end-of-range compaction induced by megaelectronvolt proton irradiation of fused silica has been imaged by atomic force microscopy. A maximum surface compaction of 50 nm was measured for an ion dose of 4.0×1015ions cm−2. A linear correlation between ion dose and compaction has been observed in the range of 1.0 to 4.0×1016 ions cm−2. When the samples were left at room temperature over a period of several months, the amount of compaction appeared to decrease significantly which suggests some temporal annealing of the waveguides has occurred.
Unstable states of antiferroelectric liquid crystal devices
Authors: Judge L. A. , Beccherelli R. , Elston S. J.
Published: July 2002

The unstable states of an antiferroelectric liquid crystal cell have been investigated, mapping out the unstable branch of the hysteresis loop. It was found that the unstable branch extended over a much greater voltage range than the quasistatic hysteresis loop of the cell. The narrow hysteresis loop is attributed to the influence of defects within the cell, which can seed switching in the presence of a small energy barrier between initial and final states. The measurements indicate that the ferroelectric state may be fundamentally stable at 0 V.
A high-speed silicon-based few-electron memory with metal– oxide–semiconductor field-effect transistor gain element
Authors: Irvine Andrew C. , Durrani Zahid A. K. , Ahmed Haroon
Published: July 2002

The design and operation of a single-electron transistor-controlled memory cell with gain provided by an integrated metal–oxide–semiconductor field-effect transistor is described. The field-effect transistor has a split gate, the central section of which is addressed by the single-electron transistor. This design effectively reduces the size of the cell’s memory node such that the memory states are represented by a difference of only a few hundred electrons, while obtaining output currents in the microamp range. The hysteresis loop in the field-effect transistor current shows clear step discontinuities which arise from Coulomb oscillations in the single-electron transistor’s drain-to-source current. The cell operates with write times as short as 10 ns and voltages of less than 5 V at 4.2 K, and operation persists to 45 K. The cell design is compact and the memory is fabricated in silicon-on-insulator material by processes that are compatible with current silicon fabrication methods.
Magnetostriction and torque response of Tb0.5Dy0.5Fe2/Fe composites
Authors: Pinkerton F. E. , Herbst J. F. , Meyer M. S.
Published: July 2002

Equivolume composites of melt-spun Tb0.5Dy0.5Fe2 in an Fe matrix have been consolidated by hot pressing over a range of temperatures. We find that both the magnetic field dependence of the magnetostriction λ(H) and the torque response Rτ of these materials in a sensor configuration depend strongly on the consolidation temperature. As the hot press temperature increases, λ(H) becomes sharper, generating higher strains at lower applied fields, higher values of the maximum slope dλ/dH, and more nearly saturated λ(H) at the maximum applied field of 19 kOe. These changes in magnetostriction are closely correlated with the observed response of the composites in a torque test. For a composite hot pressed at 755 °C we observe Rτ=6.3 G/100 ppm, a value which is twice that obtained for material consolidated at 550 °C and nearly a factor of three greater than the response of maraging steel.
Magnetization reversal in irradiation-fabricated nanostructures
Authors: Devolder T. , Chappert C. , Mathet V. , Bernas H. , Chen Y. , Jamet J. P. , Ferré J.
Published: July 2002

We study the magnetization reversal mechanisms of buried submicron magnetic nanostructures fabricated by ion irradiation through a patterned mask. By means of 30 keV He+ ion irradiation of Co–Pt multilayers, we have produced a nearly optical contrast-free, planar array of magnetically hard lines embedded in a softer matrix. Magnetization reversal in these nanostructures exhibits specific features: Low field nucleation centers and preferred domain wall propagation paths are located at the borders between irradiated and nonirradiated areas. The magnetization reversal dynamics is limited everywhere by domain wall motion, ensuring a relatively weak spread of coercive forces. In contrast with usual magnetic nanostructures, the coercivity of irradiation-fabricated lines decreases when reducing their size. Modeling of the irradiation process when performed through a mask indicates that in such a geometry, the irradiation effects are not homogeneous. It arises from collateral damage around the mask. An irradiation overshoot occurs in the vicinity of the mask edges: Depending on mask width, it can extend to a few hundred nanometers and reach up to +34% of the nominal fluence. Both magnetically hard and soft area coercivities are affected by this overshoot. A semiquantitative analysis of these dependencies is presented. It allows us to understand the magnetic properties of irradiation-fabricated nanostructures in a wide size interval.
Scanning probe microscope tip-induced oxidation of GaAs using modulated tip bias
Authors: Okada Yoshitaka , Iuchi Yoshimasa , Kawabe Mitsuo
Published: July 2002

We have investigated the basic mechanisms that determine the lateral resolution limits in the tip-induced nano-oxidation process of n+-GaAs(100) substrates. For this purpose, a set of systematic experiments and measurements were conducted. First, the cross-sectional profiles of GaAs oxide nanodots were investigated upon which the relationship between the measured profiles and oxidation parameters were studied. The current that flows during oxidation was monitored and analyzed. It is shown that the fundamental mechanism that governs the lateral resolution would be a finite electrical conductance of an adsorbed thin water film existing between the substrate and the tip, which then acts to defocus the electric field required for the oxidation. Second, the use of a modulated or pulsed bias with a short voltage duration in the range of 10–100 ms upon oxidation was proposed in order to improve the lithographic performance. The lateral resolution and aspect ratio were significantly improved with the modulated/pulsed tip bias scheme.
Structural characterization of Si1−xGex alloy layers grown by molecular beam epitaxy on Si(001) substrates
Authors: Asano T. , Nakao T. , Matada H. , Tambo T. , Ueba H. , Tatsuyama C.
Published: July 2002

Residual strain, surface roughness, and dislocations of Si1−xGex alloy layers grown by molecular beam epitaxy on Si(001) substrates at 550 °C have been characterized by x-ray diffraction, atomic force microscopy, and cross-sectional transmission electron microscopy (XTEM). Two kinds of samples were grown. One is a series of Si1−xGex alloy layers with x⩽0.3 and a thickness of 5000 Å directly grown on Si(001) substrates, and the other is a series of Si0.7Ge0.3 alloy layers with a thickness of 2000 Å grown on Si(001) via compositionally graded Si1−yGey buffer layers (0⩽y⩽x). The Ge grading rate gr in the buffer layer, defined by gr% Ge/μm, ranged from 22 to 76. In the case of direct growth, the surface morphology changes from a wavy ripple pattern to a cross-hatch pattern with increase in x, and islandlike patterns appear at x=0.3. The residual strain decreases with increase in x, whereas the surface roughness increases with x. In the case of Si0.7Ge0.3 alloy layers grown with buffer layers, the surfaces of all samples display cross-hatch pattern. The surface roughness is highest for a grading rate of about 35, and it decreases for both lower and higher grading rates. The residual strain also shows a similar dependence on the grading rate. XTEM images are correlated to the residual strain and surface roughness.
Optical and structural properties of ZnO films deposited on GaAs by pulsed laser deposition
Authors: Ryu Y. R. , Zhu S. , Budai J. D. , Chandrasekhar H. R. , Miceli P. F. , White H. W.
Published: July 2002

ZnO films were synthesized on GaAs substrates at different growth conditions by pulse laser deposition. High-purity (99.999%) oxygen was used as the ambient gas. The pressure of the ambient oxygen gas for ZnO film growth was varied from 20 to 50 mTorr, and the growth temperature from 300 to 450 °C. ZnO films showed very strong bound exciton peaks located between 3.37 and 3.35 eV. The full width at half maximum of the bound exciton peak is less than 5 meV. These results indicate ZnO films on GaAs substrates can be used for optical devices such as light-emitting diodes. The other significant properties of textured ZnO films on GaAs substrates are described.
Preferential population of magnetic sublevels of photodissociated atoms. I. Predissociation of Cs2
Authors: Katô Hajime , Onomichi Kyoko
Published: July 2002

Preferential population of the magnetic sublevels of Cs(6p 2P3/2) atoms produced by predissociation of the C 1Πu state of the Cs2 molecule along the repulsive molecular state b 3Σ+u , has been found by observing the polarization of the atomic fluorescence Cs(6p 2Pm3/2→6s 2S1/2). By applying an external magnetic field, we have determined the population of the magnetic sublevels Cs(6p 2Pm3/2) directly from the intensities of the fluorescence lines Cs(6p 2Pm3/2 →6s 2Sm±11/2 ) split by the Zeeman effect. The results are analyzed theoretically by assuming the separability of electronic and nuclear motion for the dissociation along the repulsive potential curve. The effect of the external magnetic field on the photodissociation has been made clear, and the observed results could be fully explained on the basis of the adiabatic approximation.
Vibrationally resolved photoelectron spectra of jet-cooled naphthalene: Intramolecular relaxation processes in S1 and S2 states
Authors: Hiraya Atsunari , Achiba Yohji , Mikami Naohiko , Kimura Katsumi
Published: July 2002

Combining a photoelectron spectroscopic technique with a (1+1) resonant ionization method, we have investigated photoelectron spectra by ionizing naphthalene through single vibronic levels of the S1 state up to an internal energy (Evib) of about 2500 cm−1 as well as through some vibronic levels of the S2 origin region. The photoelectron spectra thus obtained have been found to show many bands which are interpreted as the vibrational structure of the naphthalene cation. The present photoelectron results also support the available spectroscopic evidence that intramolecular vibrational redistribution occurs at the energy levels higher than 2200 cm−1 above the S1 origin. Intramolecular electronic relaxation from the S2 to the S1 state has been found to be faster than ionization under the present laser irradiation conditions, suggesting that the relaxation rate is larger than an order of 1011 s−1.
Laser spectroscopy of organometallic free radicals
Authors: Brazier C. R. , Bernath P. F. , Kinsey-Nielsen S. , Ellingboe L. C.
Published: July 2002

A large number of novel organometallic radicals have been observed by laser induced fluorescence spectroscopy. These species were produced by the gas phase reaction of calcium and strontium with various aldehydes, ketones, alcohols, and carboxylic acids.
A state-to-state study of the symmetric charge transfer reaction Ar+(2P3/2,1/2)+Ar(1S)
Authors: Liao C.-L. , Liao C.-X. , Ng C. Y.
Published: July 2002

The relative state-to-state total charge transfer cross sections, σ3/2→3/2, σ3/2→1/2, σ1/2→1/2, and σ1/2→3/2, for the reactions Ar+(2P3/2)+Ar(1S0)  → Ar(1S0)+Ar+(2P3/2)  → Ar(1S0)+Ar+(2P1/2), Ar+(2P1/2)+Ar(1S0)  → Ar(1S0)+Ar+(2P1/2)  → Ar(1S0)+Ar+(2P3/2), respectively, at the laboratory collision energy range of 1–4000 eV, have been determined using the newly constructed crossed ion–neutral beam

photoionization apparatus. This apparatus is equipped with a high resolution photoionization ion source for reactant state selections and a charge transfer detector for product state identifications. The measured profile of the kinetic energy dependence for the probability for 2P3/2→2P1/2 fine-structure transitions in Ar+(2P3/2)+Ar(1S0) charge transfer collisions [σ3/2→1/2/(σ3/2→3/2+σ3/2→1/2)] is in general agreement with the theoretical prediction of Johnson. However, the theoretical probabilities are approximately 40% greater than those observed in this experiment. The total charge transfer cross section for Ar+(2P3/2)+Ar(1S0)[σ3/2→3/2+σ3/2→1/2] were found to be slightly higher than that for Ar+(2P1/2)+Ar(1S0)[σ1/2→1/2+σ1/2→3/2]. Furthermore, the experimental values for (σ1/2→1/2+σ1/2→3/2)/(σ3/2→3/2+σ3/2→1/2) indicate that the difference in the total charge transfer cross sections for Ar+(2P1/2)+Ar(1S0) and Ar+(2P3/2)+Ar(1S0) diminishes at both low and high collision energies, in accordance with the theoretical expectations. Taking into account the experimental uncertainties, the experimental results are also consistent with detailed balance which requires the value for σ1/2→3/2 to be twice that for σ3/2→1/2 at collisional energies substantially higher than the spin-orbit splitting of Ar+.
Electronic transition moments for the Lyman and Werner bands of the H2 molecule
Authors: Dressler K. , Wolniewicz L.
Published: July 2002

We report new, more accurate ab initio values for the electronic transition moments of the B–X and C–X transitions of H2 in the internuclear distance range R=[1, 12 a.u.]. Most of the values published by Wolniewicz in 1969 are confirmed within 0.3% such that the resolution of a recently published discrepancy between experimental and theoretical B and C state lifetimes should be sought in the interpretation of the experimental measurements. (AIP)
Energy partitioning in atom–radical reactions: The reaction of F atoms with NH2
Authors: Donaldson D. J. , Sloan J. J. , Goddard J. D.
Published: July 2002

An extension of the low-pressure infrared chemiluminescence technique has allowed the measurement of energy partitioning in the atom/radical reactions: F+NH2→HF+NH, F+ND2→DF+ND. A complete numerical model of the experiment is described in detail including its parametrization. This model allows the unambiguous determination of the primary energy distribution of the above reactions. These reactions give inverted product energy distributions, in contrast to the isoelectronic F+OH→HF+O reaction. The inverted primary energy distribution for F+NH2/ND2 indicates a direct abstraction mechanism. Ab initio quantum chemical computations on some features of the relevant potential energy surfaces support this direct abstraction route. An energetically accessible transition state, having approximately zero barrier, is found on the triplet surface which directly correlates reagents and products. The geometry of this triplet transition state is also suggestive of strong HF vibrational excitation. Abstraction on the triplet surface provides an alternative pathway to reaction on the lowest singlet surface, which contains a deep potential energy well corresponding to NH2F.
The photoionization yield and electron thermalization range spectra of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in tetramethylsilane
Authors: Lee Kaidee , Lipsky Sanford
Published: July 2002

The fractional change in the fluorescence yield ψF of TMPD in tetramethylsilane, due to imposition of an electric field of strength E=45.6 kV/cm, has been determined at 44 discrete excitation energies, ε, above the photoionization threshold from ε=4.77 to 7.52 eV. Two maxima are clearly resolved in the field quenching ratio, (ΔψF/ψF0)E, one at 5.71 eV and the other at 6.79 eV. Beyond 7.0 eV, (ΔψF/ψF0)E is again observed to slowly increase. For each of 12 of the previous excitation energies, (ΔψF/ψF0)E was obtained as a function of E from 18.2 to 45.6 kV/cm. Using an exponential radial distribution function of geminate pair separation distances, f(r)=β3e−βr/2, the field dependence was fitted to the Onsager theory and the average electron thermalization range, 〈r〉=3/β extracted as a function ε. With this, the yield of thermalized electrons, φ± was also obtained as a function of ε. The range spectrum is found to increase with increasing ε with an inflection at 5.9 eV, followed by a rapid rise to a maximum of 230 Å at 6.7 eV and then to decline continuously to 167 Å at 7.5 eV. The photoionization yield spectrum shows two maxima, one at 5.7 eV with φ±=0.44 and a second at 6.9 eV with φ±=0.52. Beyond 7.0 eV, φ± increases to 0.57 at ε=7.5 eV. It is concluded on the basis of an analysis of these spectra that photoionization of TMPD in tetramethylsilane from 5.9 to 7.5 eV generates in relatively high yield at least two electronically excited states of the positive ion.
Surface impact of seeded jets at relatively large background densities
Authors: de la Mora J. Fernandez
Published: July 2002

The large inertia of heavy molecules suspended in a light carrier gas is exploited in the impingement of seeded free jets against surfaces at near-continuum conditions. It emerges that the kinetic energy at impact can be close to the free stream convective kinetic energy, even at near-continuum densities, i.e., at Knudsen numbers of 10−2 or less. Thus, one can simulate collision energies previously obtained only under molecular beam conditions but with the much higher flux densities characteristic of free jets upstream of the transition to free-molecule flow. The principal design parameter in such ‘‘high density seeded free jet’’ experiments is the heavy molecule Knudsen number, analogous to the Stokes number governing the inertial impaction of aerosol particles. When the ratio of masses mp/m for the heavy and the light gases is large, the interspecies transfer of momentum and energy is rather slow, and the Stokes number is of the order of mp/m times larger than the light gas Knudsen number. Thus, there is a region in which the light gas still behaves as a continuum fluid while the heavy component penetrates through it under effectively free-molecular conditions. A hydrodynamic theory capable of predicting the distribution of impact energies for such free jets impinging on solid objects at Stokes numbers of order unity is developed.
Matrix isolation studies of the reactions of silicon atoms with molecular hydrogen. The infrared spectrum of silylene
Authors: Fredin Leif , Hauge Robert H. , Kafafi Zakya H. , Margrave John L.
Published: July 2002

Silicon spontaneously inserts into the H–H bond of molecular hydrogen to form silylene in solid argon at 10 K. Infrared frequencies for the three fundamental modes of SiH2, SiHD, and SiD2 have been measured. The band assignments in the present study are totally different from those reported from a previous vacuum-ultraviolet photolysis study of SiH4. Silylene, formed from the molecular hydrogen reaction with silicon, reacts further with a second hydrogen molecule to give SiH4. The force field of SiH2 has been determined through normal coordinate analysis. Thermodynamic functions have also been calculated for this triatomic molecule.
Autoionizing Rydberg structure observed in the vibrationally selective, two-color, threshold photoionization spectrum of jet-cooled aniline
Authors: Hager James , Smith Mark A. , Wallace Stephen C.
Published: July 2002

We report the observation of sharp autoionization structure in the two-color photoionization efficiency (PIE) spectrum of the I20 (1B2) band of jet-cooled aniline. The energy onset of this structure allows us to assign it to vibrational autoionization of high lying neutral Rydberg states with Δv=−1. Based on linewidth measurements, a lower limit of 0.5 ps can be set for the lifetime of these states.
The dynamics of carbon monoxide binding to hemoglobin and to cytochrome
Authors: Buhks Ephraim , Jortner Joshua
Published: July 2002

In this paper we apply the theory of nonadiabatic multiphonon group transfer processes to describe the low temperature recombination reaction between CO and hemoglobin (CO–Hb) and between CO and carboxylmethyl cytochrome c (CO–cmcytc). The CO–Hb recombination is accompanied by an electronic transition from an S=2 high spin ‘‘free’’ state to an S=0 low open ‘‘bound’’ state, with the iron heme moving from a domed-type structure into a planar structure, while the characteristic low frequency of relevant vibration mode(s) involving the heme iron coupled to the protein (〈ω〉∼50 cm−1). The CO-cmcytc recombination characterized by an electronic transition from an S=1 spin free state to an S=0 bound state, with the iron heme ring staying planar and configurational frequencies prevail for a high-frequency (ωc∼400 cm−1) in-plane iron-porphyrin vibrational mode. The distinct electronic and nuclear parameters for these two recombination processes account for the large difference in the onset of the temperature dependent rate for the CO–Hb (10 K) and for the CO–cmcytc (40 K) systems, as well as for the five orders of magnitude increase in the low-temperature, temperature independent tunneling rate constant for CO–cmcytc relative to the CO–Hb recombination process.
Lattice dynamics of rare gas multilayers on the Ag(111) surface: Theory and experiment
Authors: Gibson K. D. , Sibener S. J. , Hall Burl M. , Mills D. L. , Black J. E.
Published: July 2002

We discuss the lattice dynamics of argon, krypton, and xenon overlayers on the Ag(111) surface. We consider monolayer, bilayer, trilayer and 25 layer films of each of these adsorbates. Data are also presented on the dispersion relations of selected branches of the phonon spectra of these overlayers. The data have been obtained by the method of angle-resolved inelastic He scattering. Several models of the lattice dynamics are compared with the data. It is concluded that the gas phase potentials proposed by Barker and co-workers prove suitable for a description of lateral interactions between the adsorbates, within the accuracy of the available data, provided that the phonon spectra are calculated for a lattice with the experimentally determined lattice constants.
Spectroscopic study of 2-indanone: The T1 3(n,π*) and S1 1(n,π*) states
Authors: Baba Masaaki
Published: July 2002

Phosphorescence excitation and emission spectra of 2-indanone single crystal have been observed at 1.5 K. The lowest triplet state T1 is of π*←n character which is localized in the carbonyl moiety. The very weak 0–0 band is located at 28 853 cm−1. The molecule is pyramidally distorted like formaldehyde and has an active out-of-plane C=O wagging mode. The barrier height to inversion (V) and the C=O bend angle at the potential minimum (θm) have been determined to be 883 cm−1 and 37°, respectively. This distortion is consistent with the previous ODMR results. Fluorescence excitation spectrum of 2-indanone has also been observed using a pulsed supersonic nozzle beam technique. The lowest excited singlet state is also of π*←n character localized in the carbonyl moiety. The 0–0 band is relatively intense and located at 30 664 cm−1. We obtained V=1006 cm−1 and θm =37° for the S1 state. The spectroscopic results and ab initio calculation show that the S1 and T1 states of 2-indanone are very similar to those of cyclopentanone. The difference between the spectral features of the T1 and S1 transitions is explained by the symmetry lowering of the molecule and by the vibronic coupling with a higher excited state.
Simulation of an excess electron in a hard sphere fluid
Authors: Sprik Michiel , Klein Michael L. , Chandler David
Published: July 2002

A Monte Carlo simulation of an electron in a hard sphere fluid is described. Five thermodynamic states along the isotherm λ=6σ have been examined (λ is the thermal wavelength of the electron, σ is the diameter of a sphere, d=σ/2 is taken as the distance of closest approach between a sphere and the electron). At fluid densities below 0.2σ−3, the electron fluctuates in extended configurations. At higher densities, we find that the fluid of random scatterers localizes the electronic configurations into compact yet fluctuating structures occupying voids in the fluid. Between the densities 0.1 to 0.2σ−3 we observe two relatively stable yet distinct electronic structures, one compact and the other extended. This observation of apparent metastable states seems to imply nonlinearities in the fluctuations of the electron suggestive of phase transition behavior. The statistics of the fluctuations in the compact structures are in perfect agreement with earlier results obtained for an electron in rigid disordered array of scatterers. The results of the simulation for both extended and localized states are compared with those obtained from the integral equation theory of Chandler et al. The Monte Carlo calculations were made possible by the use of the staging algorithm. This renormalization procedure allows for the efficient sampling of electronic and fluid fluctuations that extend over many length scales. The competition between the variety of length scales is intrinsic to the physics of the solvated electron.
Molecular dynamics in intense fields. III. Nonadiabatic effects
Authors: Bandrauk André D. , Nguyen-Dang T. Tung
Published: July 2002

It is shown that coupled equations for multiphoton processes can be written with either momentum transition moments or dipole transition moments. In perturbation (low field intensity) theories, nonadiabatic corrections are necessary to make the adiabatic momentum electronic transition probabilities nonzero and equivalent to electronic dipole transition probabilities in infrared transitions. For strong fields, nonadiabatic interactions can become important due to multiphoton resonances, and both approaches necessitate full coupled equations. In the case of electronic excitations, adiabatic, nuclear dipole transition probabilities become nonzero and equivalent to nuclear momentum transitions probabilities only after nonadiabatic corrections are included. These corrections should become important at strong fields for electronic states coupled nonadiabatically. It is shown further using a representation which introduces field modified electronic functions and electronic potentials that unusual field effects will occur at photon frequencies in resonance with adiabatic electronic potential energy separations at pseudo (avoided) crossings. Thus, in such cases, electronic energy gaps are increased by the field and nonadiabatic couplings at these avoided crossings are reduced by the field. Strong resonant fields should therefore quench nonadiabatic couplings at avoided crossings, rendering nonadiabatic reactions more adiabatic.
Anisotropic molecular reorientation measured by NMR relaxation including cross terms between dipolar and chemical shift anisotropy interactions
Authors: Königsberger E. , Sterk H.
Published: July 2002

It will be demonstrated that the chemical shift anisotropy (CSA) relaxation mechanism for protonated aromatic 13C nuclei is by no means negligible. The cross correlation spectral density arising from dipolar and CSA interactions in the natural abundance 13C–H spin system of pentachlorobenzene is included in the complete determination of the molecular motional anisotropy. Additionally, the absolute sign of the indirect coupling constant 1JCH is shown to be positive.
Excited state dynamics and photophysics of aggregated dye chromophores in solution
Authors: Sundström Villy , Gillbro Tomas
Published: July 2002

We have studied the excited state relaxation processes of monomeric and aggregated diethylthiadicarbocyanine iodide (DTDCI) by means of picosecond absorption spectroscopy. The strongly interacting chromophores of dimers and trimers are found to have very fast and efficient radiationless processes, whose rates are independent of temperature and viscosity, but strongly dependent on solvent dielectric constant. These results and similar ones for the covalently linked bis-DTDCI dimer allow us to identify the relaxation mechanism and suggest a theoretical model which accounts for the observations. We suggest that the radiationless processes are induced by the very strong dipole–dipole interactions between the units of the aggregate; analogies to the so called medium induced radiationless processes are also made. Estimates of the relaxation rates in the aggregated dye yields values which are in order-of-magnitude agreement with the measured values.