bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of highly nonlinear surface-acoustic waves on single crystal lithium–niobate plates by means of an optical sampling probe
Authors: Kavalerov Vadim , Fujii Toshitaka , Inoue Mitsuteru
Published: July 2002

A new type of laser probe for observation of surface-acoustic waves (SAWs) is presented. The probe operation is based on the knife-edge technique combined with an optical sampling method that employs a pulsed laser beam. The spatial resolution of the probe is 1.3 μm and its bandwidth is greater than 1 GHz. A highly nonlinear SAW involving 10 harmonics on single-crystal lithium-niobate (LiNbO3) plates has been studied at the input frequency 200 MHz. It is shown that the initially sinusoidal wave breaks into a train of pulses up to four per period with the evolution of time. The spectrum data were related to SAW harmonic generation studied previously under similar conditions. Head-to-head collision of two SAW wave packets of 500 ns duration has been studied. It was confirmed that the wave forms are preserved in the collision. In addition, the effect of nonlinear diffraction of a SAW driven by high input power levels was observed. The present probe is found to be a powerful tool for elucidation of SAW solitons.
Correlation between optical and compositional properties of Ti:LiNbO3 channel optical waveguides
Authors: Caccavale F. , Morbiato A. , Natali M. , Sada C. , Segato F.
Published: July 2002

Optical, compositional, and structural characterization of titanium-diffused LiNbO3 channel waveguides has been performed. The refractive index profiles have been reconstructed from near field measurements using a well-adapted mathematical approximation. Crystal lattice deformations have been measured by x-ray diffraction. Titanium concentration profiles measured by secondary ion mass spectrometry have been correlated to the refractive index profiles. The correlation between compositional, structural, and optical properties is discussed.
Comparison of optical beam smoothing techniques for inertial confinement fusion and improvement of smoothing by the use of zero-correlation masks
Authors: Lehmberg R. H. , Rothenberg J. E.
Published: July 2002

We present analytic theory and numerical simulations comparing the optical beam smoothing capabilities of the smoothing by spectral dispersion (SSD) technique using random temporal phase modulation, with that of the induced spatial incoherence technique. The analytic theory provides a simple formula for the SSD mode spectrum in the usual case where the phase mask at the focusing lens is random, and its asymptotic limit quantitatively relates the long wavelength mode smoothing to the width of the angular dispersion. With parameters and phase aberration relevant to the National Ignition Facility beams, the SSD simulations show that the large long wavelength components, which are also found in earlier simulations, can be significantly reduced by replacing the independent random phase masks in each pair of adjacent beams by a conjugate pair of zero-correlation masks. These simulations suggest that one can combine zero-correlation masks with random temporal phase modulation and multiple color cycles to achieve SSD smoothing approaching the optical bandwidth limit at all spatial frequencies, without using large angular dispersions.
Nanometer-scale surface element analysis in polymers using laser ablation atomic fluorescence spectroscopy
Authors: Kim Min Kyu , Ishii Hiroyuki , Taoka Kazuyoshi , Oki Yuji , Maeda Mitsuo
Published: July 2002

In this paper, laser ablation atomic fluorescence (LAAF) spectroscopy has been applied for a nanometer-scale solid surface analysis of Na-doped polymethyl methacrylate (PMMA). LAAF spectroscopy is a new sensitive element detection technique which involves atomizing of a sample by the laser ablation and detection of the ablated plume by laser-induced fluorescence (LIF) spectroscopy. Using this technique in the detection of Na atoms with a sample of Na-doped PMMA, an ablation rate of 4.4 nm/shot is attained and a detection limit is estimated to be 36 fg for a single laser shot. Further, a two-layer PMMA sample is ablated and by observing the shot-by-shot LIF intensity from Na atoms, the depth distribution in the sample is measured with a very high spatial resolution.
Mobility of sodium in SrS based thin film electroluminescent structures grown by atomic layer epitaxy
Authors: Lappalainen R. , Karjalainen M. , Serimaa R. , Sevanto S. , Leskelä M.
Published: July 2002

Mobility of sodium in SrS-based electroluminescent thin film stacks grown by atomic layer epitaxy was studied in the temperature range 600–850 °C using ion implantation for doping and nuclear resonance broadening for profiling of sodium. The crystallinity and orientation of SrS and SrS:Ce films were studied using wide angle x-ray scattering (WAXS). It was found that sodium diffuses rapidly to the interfaces between SrS and upper and lower Al2O3 insulators. These insulators act effectively as diffusion barriers for sodium up to a temperature of about 700 °C. In the case of SrS:Ce films, even after annealing at high temperatures for long times, the Na concentration in the bulk of the film remained in the range 0.4–0.6 at. % which is about two to three times the concentration of Ce. WAXS measurements revealed the preferred [111] orientation of SrS and SrS:Ce films with an average size of crystallites ⩾120 nm. It turned out that annealing for a few hours at about 700–750 °C was needed to recover the implantation damage and restore the crystallinity of SrS to its original level.
Nonlinear characteristics of magnetooptic Bragg diffraction in bismuth substituted yttrium iron garnet films
Authors: Su Jun , Tsai Chen S.
Published: July 2002

In this article, the nonlinear characteristics of magnetostatic forward volume wave (MSFVW)-based guided-wave magnetooptic Bragg cell modulators in bismuth-substituted yttrium iron garnet-gadolinium gallium garnet waveguides using nonuniform bias magnetic field are reported. First, the dispersion characteristics of the MSFVW under nonuniform bias magnetic field are analyzed, and the explicit expression for its bandwidth is determined. The transmission measurements of the MSFVW show that owing to the nonuniform magnetic field, the bandwidth is significantly increased. Next, the results of noncollinear magnetooptic (MO) Bragg diffraction experiments using the MSFVW in the frequency range from 2.0 to 4.0 GHz are presented. Two types of nonlinear process, namely, the four-magnon decay and modulation instabilities, are observed. However, the MO experiments at the carrier frequency of 2.85, 3.10, and 3.25 GHz suggest that the decay instabilities did not play a significant role in the MO interaction because of the larger degree of phase mismatch induced by the satellite waves generated during the nonlinear processes. We find that despite the presence of the decay instabilities, the MO Bragg diffraction characteristics still comply with that predicted by the coupled-mode theory before the nonlinear processes evolve into the modulation instabilities. Once the four-magnon modulation instabilities set in at the threshold powers, the MO Bragg diffraction will incur a drop in diffraction efficiency by as much as 9%. This feature results from perturbation of the satellite waves of smaller wave numbers induced in the modulation instabilities that lead to the MO phase mismatch. A model is established to describe the combined contributions of the initial MSFVW and the excited satellite waves associated with the modulation instabilities to the MO Bragg diffraction characteristics.
Ultraviolet electroluminescence in GaN/AlGaN single-heterojunction light-emitting diodes grown on Si(111)
Authors: Sánchez-Garcı́a M. A. , Naranjo F. B. , Pau J. L. , Jiménez A. , Calleja E. , Muñoz E.
Published: July 2002

GaN:Mg/AlGaN single-heterojunction light-emitting diodes were grown on Si(111) substrates by plasma-assisted molecular-beam epitaxy. High-quality Mg-doped GaN layers with hole concentrations up to 1.2×1017 holes/cm3 and intense low-temperature photoluminescence, which increases in annealed samples, were obtained. Smooth AlGaN layers, with surface roughness below 5 nm, were used as buffer layers. Continuous-wave room-temperature ultraviolet electroluminescence was observed at 365 nm with a full width at half maximum of 8 nm. An estimated optical power output of 1.5 μW was achieved under 15 V/35 mA operation.
Observation of the resonant optical interactions in active dielectric cylinder
Authors: Yi Soung-soo , Stafsudd Oscar M.
Published: July 2002

A theoretical investigation of plane-wave electromagnetic scattering in active dielectric cylinder predicted the existence of anomalous resonances that occur at discrete plane-wave angles of incidence. These resonances may be understood from a “leaky wave” phase matching boundary condition. Experiments using finite-diameter pump and probe laser beams verified the existence of these resonances with enhancements of the scattered field. Optical pumping of dye solution was used to provide the gain. The threshold of each resonant mode was found to be a function of the radius of cylinders used.
Mechanism of forward development of a plasma produced by an excimer laser in high-pressure argon gases
Authors: Tsuda Norio , Yamada Jun
Published: July 2002

In high-pressure argon gases up to 150 atm, the plasma produced by a XeCl excimer laser developed not only backward but also forward. The model for the forward development, called a forward breakdown wave, could not predict the developmental behavior in the latter stage. The development model was modified, newly taking into account the laser absorption by the plasma when the laser light was transmitted through the backward plasma. The time variation of the absorption coefficient at each distance was calculated using the inverse bremsstrahlung by the electron–ion impact. This modified model could better predict the forward development behavior than the previous model.
Electrorefraction in strained InGaAs/InP chopped quantum wells: Significance of the interface layers
Authors: Dorren B. H. P. , Silov A. Yu. , Leys M. R. , Haverkort J. E. M. , Wolter J. H.
Published: July 2002

We present a model for electrorefraction based on the quantum confined Stark effect (QCSE) in strained InGaAs/InP chopped quantum wells (CQWs) consisting of three 27 Å InGaAs wells separated by 15 Å InP barriers. The model fully takes into account the influence of the thin interface layers around each well. We experimentally verify the model on a InGaAs/InP CQW which combines a large 60 meV QCSE redshift at 11.7 V bias with waveguide transparency at 1.55 μm, which is two times larger than in a InGaAsP quaternary well. The calculated electroabsorption spectra of the CQWs are in good agreement with experiment. We finally applied the Kramers–Kronig transformations for calculating the switching voltage in a Mach–Zehnder switch employing CQWs in the phase shifting section. The model was found to be in good agreement with experiment for both polarizations.
Photoreflectance studies of Te-doped GaSb at the E+Δ transition
Authors: Iyer S. , Mulugeta S. , Collis W. , Venkatraman S. , Bajaj K. K. , Coli G.
Published: July 2002

Photoreflectance (PR) response of bulk and epitaxially grown Te-doped GaSb samples at the higher energy E0+Δ0 transition has been investigated from 4 K to room temperature. The PR spectra did not exhibit any Franz–Keldysh oscillations and are described using the third derivative of the Lorentzian functional form of the dielectric function. Using line shape analysis of the PR spectra, the temperature dependence of the E0+Δ0 transition energy has been obtained. The value of the E0+Δ0 transition energy in a bulk grown sample, with an electron concentration of about 1.6×1017/cm3, is found to be 1.583 eV at 4 K. For more heavily doped epitaxially grown samples with an electron concentration of 7.4×1017/cm3, the value of E0+Δ0 is determined to be 1.613 eV. The contributions of the many-body effects such as band filling and band gap renormalization on the E0+Δ0 transition are calculated and are used to explain the measured values.
Mode gaps in the refractive index properties of low-dose ion-implanted LiNbO3 waveguides
Authors: Rams J. , Olivares J. , Chandler P. J. , Townsend P. D.
Published: July 2002

Data are presented for the refractive index profiles for low-dose He+ ion-implanted LiNbO3 waveguides. In the nuclear stopping region, the extraordinary index is increased for low ion doses, by contrast with index reduction normally associated with ion-implanted waveguiding structures. The index increase was confirmed by fabricating a buried waveguide for the extraordinary index by use of multi-energy implants. For single-energy implants, data are shown which map the extraordinary index at the surface together with that in the nuclear collision zone, as a function of angle relative to the z axis of light propagation in surface waveguides for X and Y cut LiNbO3. These indices cross over near 45°, which results in a mode gap for which waveguide modes are not supported. A mechanism for this behavior is discussed based on defect-induced lattice relaxation. The phenomenon of a controlled mode gap may have applicability for optoelectronic and nonlinear materials and devices.
Optical quenching of the photoconductivity in n-type GaN
Authors: Lin T. Y. , Yang H. C. , Chen Y. F.
Published: July 2002

Results of optical quenching of photoconductivity measurements in undoped n-type and Se-doped GaN epitaxial thin films are presented. The spectral distribution of quenching phenomena shows a broadband centered around 1.26 eV. Transient changes in photoconductivity on application or removal of the quenching radiation are shown to exhibit a metastable behavior. The results reveal that the origin of the optical quenching phenomena is closely related to the defects corresponding to the persistent photoconductivity effects and the yellow luminescence band observed in most n-type GaN. In addition, this result indicates that these defects can have multiple charge states. It is found that the quenching ratio increases with increasing Se-doping concentration. We point out that the origin of the defects responsible for the optical quenching can be attributed to nitrogen antisite and/or Ga vacancy.
A neutron study of magnetic domain correlations in antiferromagnetically coupled multilayers
Authors: Langridge S. , Schmalian J. , Marrows C. H. , Dekadjevi D. T. , Hickey B. J.
Published: July 2002

The observed magnetotransport properties of magnetically coupled multilayers depends not only on the nature of the magnetic coupling but also the magnetic domain correlations and disorder. Neutron scattering gives access to the magnetic coupling through the specular reflectivity and the domain correlations through the diffuse scattering. Sputtered multilayers of Co/Cu and Co/Ru have been investigated as a function of the applied magnetic field. A simple domain model relates the observed scattering to the domain correlation length and the magnetic disorder. In both systems highly vertically correlated magnetic domains are observed with in-plane correlation lengths, at remanence, of 1.5 and 7 μm for the Cu and Ru systems, respectively. In both systems the Co domains order antiferromagnetically across the nonmagnetic spacer. The remanent vertically correlated state is recovered after saturating the sample.
Magnetotransport properties of MnAs/GaAs/MnAs ferromagnet/semiconductor trilayer heterostructures
Authors: Takahashi K. , Tanaka M.
Published: July 2002

We have succeeded in growing MnAs/GaAs/MnAs ferromagnet/semiconductor trilayer heterostructures on GaAs(111)B substrates by molecular-beam epitaxy. Double-step features were observed in magnetization characteristics due to the difference in coercive force between the top and bottom MnAs layers. Magnetoresistance (MR) curves in current-in-plane geometry showed the spin-valve effect, which was caused by the change of the magnetic alignment of the two ferromagnetic MnAs layers from parallel to antiparallel orientation. Temperature dependence of the MR was also investigated. We infer that the positive temperature coefficient of the MR by the spin-valve effect suggests the heat (carrier) induced interlayer exchange coupling.
Large low-field magnetoresistance in strained ultrathin Pr0.67Sr0.33MnO3 films
Authors: Wang H. S. , Wertz E. , Hu Y. F. , Li Qi
Published: July 2002

Strain effect on the low-field magnetoresistance (LFMR) in epitaxially grown Pr0.67Sr0.33MnO3 thin films has been studied. Very large LFMR and MR hysteresis have been found in compressive-strain ultrathin films grown on LaAlO3 (001) substrates when a magnetic field is applied perpendicular to the film plane. The LFMR ratio as high as 360% at H=1600 Oe and T=30 K was obtained from the MR hysteresis curve. The large LFMR depends strongly on the applied magnetic field direction as well as the film thickness. It is reduced to less than 10% when the film thickness is about 20 nm. In comparison, tensile-strain films on SrTiO3(001) show positive LFMR, and almost strain free films on NdGaO3 (110) show very small LFMR (<2%), at comparable magnetic fields and temperatures. These effects were found to be closely related to the strain-induced magnetic anisotropy.
Easy axis distribution in interacting fine-particle systems: Two and three dimensions
Authors: Rubio H. , Suárez S.
Published: July 2002

We describe a two- and three-dimensional theory applicable to fine-particle systems, which directly includes interactions between particles through a mean field model. Expressions that allow for the experimental determination of the easy-axis distribution are provided. When the driving magnetic field is decreased from a saturation value, interaction and disorder give rise to a rotation of the integration region that must be considered for the evaluation of the average values. Differences between parallel and transverse remanences are predicted, resulting in good agreement with experiments found in the literature. Finally, this model reproduces the results of the preceding noninteracting theories when interaction between particles is not taken into account.
Composition of a solid phase growing under nonequilibrium conditions
Authors: Lábár J. L. , Radi Zs. , Barna P. B.
Published: July 2002

The composition of a binary solid phase is predicted in a multiphase environment during phase formation by a steady-state process in linear growth geometry characterized by planar interfaces. In this situation, Gibbs free energy is also dissipated at the opposite interfaces of the forming phase which is not in thermodynamic equilibrium with its surroundings. Although the common tangent rule of an equilibrium situation cannot be applied to the steady-state process, the composition is derived here from the free energy curves of the equilibrium phase by a self-consistent, iterative solution of a new model that combines thermodynamic and diffusional considerations. Accurate composition—as proven experimentally—is predicted for the amorphous Al–Pt phase using a reasonable assumption for the ratio of chemical potential drops at the opposite interfaces of the forming phase. Although our numerical example is the amorphous Al–Pt system, the model can also be applied to other amorphous or solid solution phases which exist in a wide compositional range.
Gyroresonance in unsaturated composite bodies: Experiments and theory
Authors: Mattei Jean-Luc , Bariou David , Chevalier Alexis , Le Floc’h Marcel
Published: July 2002

The permeability spectra of composite magnetic materials (NiZn ferrite composites and YIG composites) has been experimentally studied on a wide range in magnetic load C. The resonance frequency decreases with the increasing magnetic load of the mixture. This is attributed to the fading of the magnetic pole on the surface of the magnetic inclusions. A theory is developed which is intended to give an expression for the gyroresonance frequency of magnetic composites in their unsaturated magnetic state. It fits the experimental data over its full range of volume fraction C without any corrective factors. The proposed relation, which is an extension of the Kittel’s expression for the resonance frequency of a saturated body, points out a noteworthy close link between the concept of reciprocity and the one of demagnetization.
Magnetization and dissipation measurements in the quantum Hall regime using an integrated micromechanical magnetometer
Authors: Harris J. G. E. , Awschalom D. D. , Maranowski K. D. , Gossard A. C.
Published: July 2002

We present low-temperature (365 mK) magnetization measurements of 40×100 μm2 mesas of two dimensional electron gases (2DEGs) integrated into micromechanical cantilever magnetometers. Over a wide range of applied magnetic field, the cantilever resonance frequency reveals the thermodynamic magnetization of the 2DEG. Upon illumination of the sample, we observe the appearance of both cyclotron and Zeeman gaps in the density of states. We attribute this to the narrowing of the disorder-broadened Landau levels as the carrier concentration is increased. Additionally, we observe strong peaks in the dissipation of the system at small integer filling factors which we associate with eddy currents excited by the cantilever motion.
Vacuum ultraviolet to visible emission from hydrogen plasma: Effect of excitation frequency
Authors: Fozza A. C. , Moisan M. , Wertheimer M. R.
Published: July 2002

The expanding use of low pressure (p⩽10 Torr), high frequency plasmas in various applications has stimulated research toward increased operating efficiency. In order to optimize a particular plasma process, the operator can vary several “external” (operator-set) parameters, among which the excitation frequency f has received relatively little attention in the literature over the years, probably due to the difficulties encountered in designing meaningful frequency-dependent experiments. These difficulties can be avoided by the use of surface-wave discharges (SWDs), which possess great flexibility: a very broad (continuous) range of excitation frequencies, and wide ranges of operating pressures and plasma densities, under noncritical, almost perfect impedance matching with the power source. In earlier work in these laboratories, we have examined the f dependence of plasma deposition and etching experiments; the present experiments have been designed to investigate the f dependence more “directly” by turning to the plasma through its optical emission. The vacuum ultraviolet to visible emission from SWD plasmas in pure hydrogen or 7%H2 in Ar mixture has been investigated over a broad range of excitation frequency (50⩽f⩽200 MHz) using a spectrophotometer with a known transfer function. The observed f dependence of emission intensity (atomic lines and molecular bands) as f is increased is interpreted in the case of the pure H2 discharge in terms of changes from a nonstationary to a stationary electron energy distribution function (EEDF) while, in the 7%H2/Ar mixture, it is related to changes in the form of the stationary EEDF.
Role of the buffer layer in the active junction in amorphous-crystalline silicon heterojunction solar cells
Authors: Pallarès J. , Schropp R. E. I.
Published: July 2002

We fabricated pn and pin a-SiC:H/c-Si heterojunction solar cells following two different processes. In the first approach, wafers were subjected to an extra atomic hydrogen (produced by hot wire chemical vapor deposition) prior to the deposition of the amorphous layer. A reduction in the open-circuit voltage was observed for the passivated cells due to their higher leakage current. In the second process, pin solar cells with two different quality intrinsic a-Si:H buffer layers were fabricated using plasma enhanced chemical vapor deposition. The cells with a device quality buffer layer (deposited at higher temperature) showed better performance than those with a buffer layer with high hydrogen content and higher defect density (deposited at lower temperatures).
Increased mobility anisotropy in selectively doped AlxGa1−xAs/GaAs heterostructures with high electron densities
Authors: Reuter D. , Versen M. , Schneider M. D. , Wieck A. D.
Published: July 2002

We have investigated the electron transport properties of a two-dimensional electron gas in a selectively doped AlxGa1−xAs/GaAs heterostructure with reduced Al content in the doped layer by recording the longitudinal resistance as a function of magnetic field and performing Hall measurements at low magnetic fields on the same sample. The electron density was varied by stepwise illumination and we observe a dip in the mobility versus density dependence around a density of 5.0×1011 cm−2. As a reason for this mobility decrease we identify by magnetotransport measurements the onset of parallel conductance in the Si-doped AlxGa1−xAl layer. With the onset of parallel conductance, the anisotropy in the electron mobility increases significantly. We attribute this to an increase of interface roughness scattering induced by the parallel conducting channel.
Shape anisotropy induced modification of the magnetization reversal processes in epitaxial microstripes
Authors: Ahmad E. , Lopez-Diaz L. , Gu E. , Bland J. A. C.
Published: July 2002

The magnetization reversal processes of a 150 Å thick epitaxial Fe/GaAs(001) film are modified by controlling the lateral dimensions of the continuous film to create rectangular microstripes of constant length and varying width. Size dependent transitions from (a) two-jump to single-jump, and (b) single-jump to two-jump reversal processes are observed for magnetization reversal along the two mutually perpendicular cubic easy axes (parallel and perpendicular to the easy axis of the shape anisotropy, respectively). This behavior can be explained by a simple model of the effective anisotropy energy contributions including the shape anisotropy energy.
Fabrication and characterization of three-dimensional periodic ferroelectric polymer-silica opal composites and inverse opals
Authors: Xu T.-B. , Cheng Z.-Y. , Zhang Q. M. , Baughman R. H. , Cui C. , Zakhidov A. A. , Su J.
Published: July 2002

Using a high temperature solution infiltration process, ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer is infiltrated into three-dimensional (3D) periodic opal lattices with the silica opal diameters of 180, 225, and 300 nm to form periodic composite structures. By etching out the silica opal, inverse copolymer opals can be fabricated, which retains the 3D periodic structure of the original silica opal lattice. In addition to the optical observation, x-ray diffraction and dielectric study were carried out to characterize the change in the ferroelectric behavior of the composites and inverse opals. Although the copolymer in the composites and inverse opals remains ferroelectric, the ferroelectric transition in the composites and inverse opal becomes diffused and moves to a lower temperature, which is due to the random stress introduced by the irregular voids and interfaces and may be made use of to facilitate the transformation of the copolymer into a relaxor. These results suggest the feasibility of using ferroelectric copolymer to form 3D photonic crystals.
Modeling of the formation of cationic silicon clusters in a remote Ar/H2/SiH4 plasma
Authors: Leroux A. , Kessels W. M. M. , Schram D. C. , van de Sanden M. C. M.
Published: July 2002

Cationic silicon clusters, containing up to ten silicon atoms, have been measured by mass spectrometry in an argon/hydrogen/silane expanding thermal plasma. A quasi-one-dimensional model, based on the idea that the clustering process initiated by argon or hydrogen ions depends on the path length of the plasma in the deposition chamber and on silane density, is presented. The chemistry is described by ion–molecule reactions between the formed clusters and silane and by dissociative recombination. The model is able to reproduce fairly well the experimental data for various plasma conditions. It is shown that reaction rates for the clustering process do not strongly depend on the number of silicon atoms in the cluster. This result is in contrast with rates published in the previous literature. For the conditions investigated, the consumption of silane by cationic cluster formation is not significant. The contribution of neutral clusters is investigated and recombination proves to be an important process.
Nanocrystalline Si thin films with arrayed void-column network deposited by high density plasma
Authors: Kaan Kalkan A. , Bae Sanghoon , Li Handong , Hayes Daniel J. , Fonash Stephen J.
Published: July 2002

High porosity nanocrystalline Si thin films have been deposited using a high density plasma approach at temperatures as low as 100 °C. These films exhibit the same unique properties, such as visible luminescence and gas sensitivity, that are seen in electrochemically etched Si (i.e., porous Si). The nanostructure consists of an array of rodlike columns normal to the substrate surface situated in a void matrix. We have demonstrated that this structure is fully controllable and have varied the porosity up to ∼90% (as derived from optical reflectance) by varying the deposition conditions. In particular, the impact of plasma power has been found to reduce porosity by increasing the nuclei density and therefore the areal density of columns. Humidity sensors have been demonstrated based on the enhanced conductivity of our films (up to 6 orders of magnitude) in response to increase in relative humidity. Depending on the porosity, the conductivity-relative humidity behavior of our films shows variations which can be correlated with the nanostructure. Also, these variations indicate that the dominant charge transport is limited by the dissociation of water into its ions at the column surfaces.
Magnetic field modulated infrared laser spectroscopy of molecular ions: The ν2 band of HCO+
Authors: Kawaguchi Kentarou , Yamada Chikashi , Saito Shuji , Hirota Eizi
Published: July 2002

The magnetic field effect on dc discharge production of molecular positive ions was investigated using infrared diode laser spectroscopy. When a magnetic field of about 100 G was applied, the ion signal strength in a system where the ions were generated by a hollow cathode discharge was found to decrease by an order of magnitude. This effect was used to discriminate absorption lines of ions from those of neutral species; an ac magnetic field was superimposed on the dc field and the signal was detected with a lock-in amplifier. Using this magnetic field modulation technique, the ν2 band of HCO+ was observed for the first time and the following molecular parameters were determined: ν0=828.2305(9), B′=1.490 274(14), q=0.007 051 0(44), and D′=2.822(45)×10−6 in cm−1 units with three standard errors in parentheses.
Structure of bimetallic clusters. Extended x-ray absorption fine structure (EXAFS) studies of Ag–Cu and Au–Cu clusters
Authors: Meitzner G. , Via G. H. , Lytle F. W. , Sinfelt J. H.
Published: July 2002

Extended x-ray absorption fine structure (EXAFS) studies were conducted on silica supported silver–copper and gold–copper bimetallic clusters to obtain information on their structures. The atomic ratio of copper to either silver or gold was close to one in the materials investigated. The EXAFS results, which were obtained in the presence of hydrogen, indicate extensive segregation of the components in both the silver–copper and gold–copper clusters, although it is much more pronounced in the former. The greater segregation in the silver–copper clusters is readily understandable, since silver and copper are only slightly miscible in the bulk, whereas gold and copper are completely miscible. The EXAFS results on the silver–copper clusters suggest that the copper-rich region is in the interior of the clusters, with the silver concentrating at the surface. The location of the gold-rich region in the gold–copper clusters is less clear from the EXAFS data, but there is a slight indication that it is present at the surface. However, for air exposed gold–copper clusters, x-ray diffraction data suggest that the gold-rich region is in the interior.
Determination of the Born–Oppenheimer potential function of CCl+ by velocity modulation diode laser spectroscopy
Authors: Gruebele Martin , Polak Mark , Blake Geoffrey A. , Saykally Richard J.
Published: July 2002

Over 70 transitions among the lowest six vibrational states of C35Cl+ and C37Cl+ have been measured between 1070–1210 cm−1. The spectrum has been fitted to a sixth order Dunham expansion to yield an accurate mapping of the Born–Oppenheimer potential function of CCl+. The spectroscopic constants obtained are ωe =1177.7196(8) cm−1, ωexe =6.6475(3) cm−1, and Be =0.797 940(3) cm−1. The rotational constants for both CCl+ isotopes reported here show the results of the previous electronic emission studies to be incorrect. A fit of the data to a Morse function yields a dissociation energy D of 52 828(50) cm−1. The rotational temperature has been determined as 540 K±30%. The increase in the effective vibrational temperature with vibrational excitation indicates that CCl+ is formed with high internal energy.
Monte Carlo simulation of adhesive spheres
Authors: Seaton N. A. , Glandt E. D.
Published: July 2002

A recently reported method for the Monte Carlo simulation of adhesive particles is applied to a system of adhesive spheres. Results for the pair correlation function and the equation of state are reported. The Percus–Yevick solution is closest to the simulation results at higher temperatures. The critical point determined by the simulation is close to the Percus–Yevick compressibility equation result and is quite different from the Percus–Yevick energy equation result.
Infrared diode laser spectroscopy of SO in the a 1Δ state generated by the photolysis of Cl2SO at 193 nm
Authors: Kanamori Hideto , Tiemann Eberhard , Hirota Eizi
Published: July 2002

The SO radical in the a 1 Δ state was produced by photolyzing Cl2 SO at 193 nm, and its vibration–rotation spectrum was observed for the bands up to v=6–5 by infrared diode laser kinetic spectroscopy. Analysis of the observed spectrum yielded precise Dunham coefficients, which were used to calculate an RKR potential energy curve for the a 1 Δ state. The branching ratio of the two Cl2 SO photolysis processes yielding SO in a 1 Δ and X 3 Σ−, respectively, was estimated to be 1:4, in sharp contrast with the value of smaller than 0.01 for the photolysis of SO2 at 193 nm.
The comet-tail (A 2Πi–X 2Σ+) system of 13C18O+
Authors: Prasad C. V. V. , Reddy S. Paddi
Published: July 2002

The comet-tail (A2Πi–X2Σ+) system of the molecular ion 13C18O+, excited in the cathode glow of a hollow-cathode discharge tube, was recorded in the spectral region 3620–6165 Å. Of the nine bands observed with v′=0 to 5 and v″=0 to 2, the rotational structure of 8 bands was analyzed. Brown’s effective Hamiltonian [J. M. Brown, E. A. Colbourn, J. K. G. Watson, and F. D. Wayne, J. Mol. Spectrosc. 74, 294 (1979)] was used to obtain the molecular constants of the individual bands from their wave number data. By ‘‘merging’’ the molecular constants thus obtained, a unique set of constants for the X2Σ+ and A2Πi states was obtained. Some of the derived molecular constants of 13C18O+ in units of cm−1 (except re in Å) are: X2Σ+: Be=1.794 28(5), re=1.1156, αe=0.016 66(3), ΔG(1/2)=2082.69; A2Πi: Be=1.441 73(5), re=1.2445, αe=0.016 71(1), ωe=1488.07, ωexe=12.16.
A time resolved electron energy loss spectroscopy study of CO on Pt(111): Adsorption site occupations versus coverage and temperature
Authors: Mieher W. D. , Whitman L. J. , Ho W.
Published: July 2002

The intensities of the intramolecular vibrational modes obtained by electron energy loss spectroscopy (EELS) for CO adsorbed in top and bridge sites on Pt(111) are presented as a function of coverage and temperature. The top site is predominantly occupied at low coverages. A well ordered half top–half bridge structure is formed with a c(4×2) low-energy electron diffraction (LEED) pattern following adsorption of 0.5 monolayers (ML) at 95 K. The ratio of the EELS cross section of the C–O stretch in the top site to that in the bridge site is found to be σt/ σb =1.8±0.2. The occupations of bridge and top sites are observed to vary reversibly during temperature programmed EELS (TP-EELS) when the temperature is linearly ramped up from 95 K to just below the onset of desorption and back down for coverages from 0.09 to 0.44 ML; the difference in the adsorption enthalpy between top and bridge sites is found to vary with coverage from ΔH=0.62±0.05 kcal mol−1 at 0.09 ML to −0.14±0.05 kcal mol−1 at 0.44 ML, and returns to near zero (−0.03±0.05 kcal mol−1) at 0.50 ML. The ratio of the preexponential factors for site change is νb→t/ νt→b =2.0±0.3. The utility of modulating the EEL spectrometer lens potentials to minimize the effects of changes in surface order and work function changes on the observed normalized loss intensities is investigated and discussed. The possible effects of the low frequency frustrated translational and rotational vibrations on the measurement of the enthalpy difference derived from the EELS intensities are examined.
Excited state proton transfer in the S1 state of 2-allylphenol, 2-propenylphenol, and 2-propylphenol and their van der Waals clusters with water and ammonia
Authors: Kim S. K. , Hsu S. C. , Li S. , Bernstein E. R.
Published: July 2002

Excited state intramolecular and intermolecular proton transfer reactions in cold, isolated 2-allylphenol, 2-propenylphenol and 2-propylphenol, and their clusters with water and ammonia are investigated employing a combination of spectroscopic techniques (mass resolved excitation, threshold photoionization, dispersed emission), a semiempirical calculation (MOPAC 5) and a potential energy calculation of cluster structure. Threshold photoionization spectroscopy proves to be useful for the identification of molecular conformers in these systems but has mixed results for the identification of proton transfer in their clusters. The total collection of generated data suggests the following conclusions: (1) isolated, cold 2-allylphenol displays only one conformation which appears to have a significant stabilizing intramolecular interaction between the allyl group double bond and the hydroxyl group hydrogen atom; (2) 2-propenylphenol displays only one conformer; (3) 2-propylphenol has many conformations—probably more than five under the experimental conditions; (4) no evidence of intramolecular proton transfer can be found for these three isolated cold molecules; (5) no evidence for intermolecular proton transfer in water clusters has been found by any of the above techniques; and (6) evidence is found for intermolecular proton transfer in 2-allyl- and 2-propenylphenol(NH3)n, n≥3, in dispersed emission spectra. Dispersed emission spectra of 2-propylphenol(NH3)n n≥3 are too weak to yield conclusive evidence for intermolecular excited state proton transfer. Potential energy minimization calculations of cluster geometry suggest that the difference between water and ammonia cluster behavior with regard to proton transfer arises because water molecules hydrogen bond with the hydroxyl group (both OH⋅⋅⋅OH2 and HO⋅⋅⋅HOH) and each other while ammonia molecules are more evenly distributed over the entire molecular structure of the phenol moiety. Apparently, for efficient proton transfer to occur in clusters, the proton affinity of the solvent must be large and both the anion and the proton must be well solvated (stabilized) by the solvent.
Effect of temperature and small-scale defects on the strength of solids
Authors: Blumberg Selinger Robin L. , Wang Zhen-Gang , Gelbart William M.
Published: July 2002

Using a statistical-thermodynamic formulation, we investigate the failure of ideal and almost-ideal solids at finite temperature. We propose that the onset of failure in a defect-free crystal is associated with the loss of a metastable minimum in the free energy at a critical value of the applied tensile force. Using a mean-field approximation, we estimate the free energy of the two-dimensional Lennard-Jones crystal under stress and derive the temperature dependence of its ideal strength and other properties. These results are compared to Monte Carlo simulations of this system, and the mean-field estimate of the ideal strength is shown to be an upper bound to the values observed via simulation. We also show that atomic-scale defects such as vacancies and substitutional impurities significantly reduce the crystal’s strength as a result of stress enhancement effects. While the overall strength of a defective crystal depends strongly on both temperature and the nature of the defects, the maximum local stress that the crystal can sustain without failure is essentially independent of these factors.
Multiphoton ionization studies of C6H6–(CH3OH)n clusters. I. Comparisons with C6H6–(H2O)n clusters
Authors: Garrett Aaron W. , Severance Daniel L. , Zwier Timothy S.
Published: July 2002

Resonant two-photon ionization (R2PI) scans of the S0–S1 spectra of C6H6–(CH3OH)n clusters with n=1–5 have been recorded. These scans provide an interesting comparison with earlier spectra from our laboratory on C6H6–(H2O)n clusters. A variety of vibronic level arguments are used to constrain the geometries of the C6H6–(CH3OH)n clusters. The 1:1 and 1:2 clusters possess vibronic level features which are very similar to their aqueous counterparts. The 1:1 cluster places the methanol molecule in a π hydrogen-bonded configuration on or near the sixfold axis of benzene. The spectral characteristics of the 1:2 cluster are consistent with both methanol molecules residing on the same side of the benzene ring as a methanol dimer. Higher C6H6–(CH3OH)n clusters show distinct differences from the corresponding C6H6–(H2O)n clusters. Vibronic level arguments lead to the following conclusions: the methanol molecules in the 1:3 cluster show the strongest hydrogen bonding to the π cloud of any of the clusters and attach to benzene in such a way as to strongly break the sixfold symmetry of its π cloud. The 1:4 clusters are at most only very weakly hydrogen bonded to the π cloud, break benzene’s sixfold symmetry moderately well, and possess strong activity in a very low frequency intermolecular mode. The methanol molecules in the 1:5 cluster show no hydrogen-bonding interaction with benzene’s π cloud, induce remarkably little asymmetry in the π electron density, and produce very little van der Waals’ activity. Monte Carlo simulations using intermolecular potentials developed for liquid simulations serve as a guide to the possible minimum-energy structures for the clusters. The experimental results are used to distinguish between the possible structures. In all cases, the lowest energy structures produced by the calculations satisfactorily fit the vibronic level constraints placed on the structures by our data.
Product distributions in the 157 nm photodissociation of CO2
Authors: Miller R. L. , Kable S. H. , Houston P. L. , Burak I.
Published: July 2002

The vibrational and rotational distributions of CO(1Σ+g) produced in the 157 nm photodissociation of CO2 have been determined by measuring vacuum-ultraviolet laser-induced fluorescence spectra of the CO photoproduct. The photodissociation of CO2 is known to occur via two pathways; one yielding O(1D) and the other yielding O(3P). Spin conservation and previous experimental studies confirm that dissociation via the O(1D) channel is the dominant process. The available energy for this channel is sufficient to populate only the ground and first excited vibrational levels of CO. We measured the rotational distributions for CO in v=0 and v=1 and found them to be nonBoltzmann. In fact, a highly structured distribution with distinct peaks at J=10, 24, 32, and 39 is observed for CO in v=0. A less structured population is displayed by molecules in v=1. The relative vibrational population (v=0/v=1) was determined to be 3.7±1.2. Doppler spectra of individual rovibronic transitions were also recorded. The profiles have widths in accord with the available translational energy, display the expected v⊥J correlation, and are best described by an isotropic distribution of the velocity vectors with respect to the polarization direction of the dissociation light.
Triplet excited states of the NH(ND) radical revealed via two photon resonant multiphoton ionization spectroscopy
Authors: Clement Simon G. , Ashfold Michael N. R. , Western Colin M. , Johnson Russell D. , Hudgens Jeffrey W.
Published: July 2002

We report the first spectroscopic characterization of any triplet Rydberg state of the imidogen radical. The B 3Π state of NH and ND is identified via the two photon resonance enhancement it provides in the MPI spectrum of ground (X 3Σ−) state NH(ND) radicals, and shown to be well described (at least near its equilibrium geometry) as a 3p Rydberg state built on the ground (X 2Π) state ion core. We consider also the origin of an overlapping 3Σ−X 3Σ− two photon resonance apparent only in the spectrum of the ND radical.
Measurement of the Kerr constant for H2 and D2 gases
Authors: Carusotto S. , Perrone F. , Polacco E.
Published: July 2002

The Kerr electro-optic constant of deuterium and hydrogen gases have been measured at room temperature and atmospheric pressure for λ=632.8 nm, by using a very sensitive ellipsometrycal technique. A significant difference between the Kerr constants of the two gases has been found.
Optical–optical double resonance spectroscopy of the Cl2 1g(1D) ion-pair state
Authors: Ishiwata Takashi , Kasai Yasuko , Obi Kinichi
Published: July 2002

The 1g(1D) ion-pair state of Cl2 has been analyzed by optical–optical double resonance using the 1g(1D)–A 3Π(1u)–X 1Σg+ photoexcitation sequence. We recorded the spectra for both the 35–35Cl2 and 37–35Cl2 isotope species and established the absolute vibrational numbering of the 1g(1D) state from the isotope difference. A set of molecular parameters was derived from the data set in the 0≤v≤6 range, and used to construct the potential function of the 1g(1D) state. We interpreted the Ω-doubling of the 1g(1D) state in terms of the heterogeneous coupling with an unknown 0g+(1D) state correlating to the same ionic asymptote. The potential function of the 0g+(1D) state is predicted in a pure precession analysis.
Theoretical characterization of the potential energy surface for NH+NO
Authors: Walch Stephen P.
Published: July 2002

The potential energy surface (PES) for NH+NO has been characterized using complete active space self-consistent field (CASSCF) gradient calculations to determine the stationary point geometries and frequencies followed by CASSCF/internally contracted configuration interaction (CCI) calculations to refine the energetics. The present results are in qualitative accord with the BAC-MP4 calculations of Melius and Binkley, but there are differences as large as 8 kcal/mol in the detailed energetics. Addition of NH to NO on a 2A′ surface, which correlates with N2+OH or H+N2O products, involves barriers of 3.2 kcal/mol (trans) and 6.3 kcal/mol (cis). Experimental evidence for these barriers is found in the work of Böhmer et al. The 2A″ surface has no barrier to addition, but does not correlate with ground state products. Surface crossings between the barrierless 2A″ surface and the 2A′ surface may be important. Production of N2+OH products is predicted to occur via a planar saddle point of 2A′ symmetry. This is in accord with the preferential formation of Π(A′) Λ doublet levels of OH in the experiments of Patel-Misra and Dagdigian. Addition of NH 1Δ to NO is found to occur on an excited state surface and is predicted to lead to N2O product as observed by Yamasaki et al.