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The fragmentation dynamics of two isomers of C3H6, cyclopropane and propene, induced by 4 keV/u Ar8+ are investigated employing a reaction microscope. Four two-body and two three-body dissociation channels of C 3 H 6 2 + dications are identified for each isomer, among which the channels involving CC bond breaking are found to be much more favored than H 3 + and H 2 + formation channels. The observation of the CH 3 + or H 3 + formation channels from cyclopropane are direct evidence of the proton migration within the carbon skeleton before dissociation. Obvious isomer effects are revealed by comparing the relative branching ratios of different channels of the two isomers. Moreover, it was shown that a sequential dissociation mechanism with H elimination prior to CC bond cleavage may be dominant for the two three-body dissociation channels for both isomers. [ABSTRACT FROM AUTHOR]

Sagnac interferometry is advantageous in measuring time-reversal-symmetry breaking effects in ferromagnetic and antiferromagnetic materials as it suppresses time-reversal symmetric birefringent effects that are ubiquitous and often overwhelming in optical detection systems. When its sensitivity is limited only by the amplifier noise in the photo-detector, one needs to optimize the optical power that returns to the detector. We demonstrate an experimental scheme that maximizes the returning optical power in a Sagnac interferometry when detecting the magneto-optic effect in ultrathin films. In this scheme, the optical beam bearing the Faraday effect on a thin film is reflected at a second surface coated with a highly reflective gold film. The gold film increases the returned optical power by a factor of 4–5. For a normal-incidence Sagnac interferometer, this scheme yields an increase in the signal-to-noise ratio by the same factor. For an oblique-incidence Sagnac interferometer, this scheme should yield an increase in the signal-to-noise ratio by a factor of 20–25. For illustration, this scheme is used to measure magnetization curves and Kerr rotation images of 4.5-unit-cell thick SrRuO3(001) grown on SrTiO3(001). [ABSTRACT FROM AUTHOR]

Recent experimental results showed that an electron gas in an asymmetrical double barrier heterostructure can be effectively cooled down under resonant tunneling condition, thus leading to the realization of an electronic cooler. The cooling process is a multi-parameters phenomenon and it is desirable to handle this problem through a reasonably simple approach, in order to understand the role of each parameter. To this end, we present a rate equation modeling of the electron cooling. We model the resonant tunnel injection of the electrons in the well and their thermionic emission assisted by Longitudinal Optical (LO) phonons absorption and emission. The influence of several parameters on the electronic temperature is discussed. This simple model compares rather well to the predictions of non-equilibrium Green function approach and to the experiments. [ABSTRACT FROM AUTHOR]

I present a magneto-optic effect study of magnetic domains in a ferrimagnetic yttrium iron garnet crystal at different stages of a hysteresis loop. By measuring the Faraday effect in response to the out-of-plane component and Kerr effects to the in-plane components of the sample magnetization, I examined the evolution of magnetic domains during a hysteresis loop in an in-plane external magnetic field. I found that crystalline anisotropy, magneto-static energy, and the presence of movable domain walls play indispensable roles in the domain orientation, particularly when the external field is near zero, and have led to seemingly unusual appearances of hysteresis loops. [ABSTRACT FROM AUTHOR]

In this paper, a simple model was developed to predict the dynamics of a spark-induced bubble under different ambient pressures. This work helps in developing a deep-towed plasma sparker, as the model can predict the dynamics of bubbles subjected to very high ambient pressures (about 20 MPa) which normally are difficult to obtain experimentally. Experimental results indicate that the maximum bubble radius for a fixed discharge energy decreases as a power-law function of the ambient pressure up to 1.0 MPa; the bubble period also decreases quickly with increasing ambient pressure. For a constant value of the ratio of bubble energy to discharge energy, the modeling results for both maximum radius and bubble period are in good agreement with the experimental results. Both sets of results indicate that the bubble period is proportional to the maximum radius under different ambient pressures. [ABSTRACT FROM AUTHOR]

We describe an oblique-incidence zero-area Sagnac interferometric microscope for studying spatial and temperature dependence of magneto-optic (MO) effects in samples under cryogenic conditions. The microscope is capable of independently measuring Kerr effects from three Cartesian components of a magnetization and thus can be used to map out the magnetization vector across the sample. For illustration, we present MO Kerr effect images of magnetic domains at 77 K of a LaCrGe3 crystal terminated with an a–c plane (the plane that contains the lattice a-axis and c-axis). We further present measurements of magnetization in these domains from 90 to 77 K during zero-field cooling and field cooling in an external magnetic field from 20 to 150 Oe. The inherently high sensitivity and the capability of detecting a magnetization without external modulation makes such a Sagnac interferometric microscope particularly useful for studying magnetic effects in novel materials at low temperatures. [ABSTRACT FROM AUTHOR]

I report experimental methods for measuring real and imaginary parts of the linear electric quadrupole susceptibility tensor of nonlinear optical crystals that lack inversion centers. The third-ranked tensor is related to the corresponding second-order nonlinear susceptibility tensor. For opaque materials such as GaAs, the methods involve normal-incidence reflectivity difference detection schemes. For transparent materials, quadrupole susceptibility tensor elements may be measured with similarly construed transmission difference detection schemes. [ABSTRACT FROM AUTHOR]

The results of time-of-flight diagnostics of the composition of high-intensity pulsed ion beams are presented. The experiments were performed on a diode of focusing and flat geometry, in the mode of self-magnetic insulation of electrons (accelerating voltage 250–300 kV, pulse duration 120 ns, ion current density 20–300 A/cm2), and a focusing diode in an external magnetic insulation mode (300 kV, 80 ns, 100–200 A/cm2). A delay in the registration of protons by 40–50 ns (on the drift path 14–16 cm) was found in the absence of a delay in the registration of heavy ions. It has been shown that this delay can be related to the deceleration of light ions during the transport from the diode to a collimated Faraday cup. This effect of spatial compression of the ion beam in the direction of the drift increases its pulse power but complicates the time-of-flight diagnostics of its composition. [ABSTRACT FROM AUTHOR]

PbO–MgO–Nb2O5–TiO2 (PMNT) pyrochlore thin films were prepared on Pt-coated silicon substrates by radio-frequency magnetron sputtering and postdeposition annealing method. Very interestingly, these pyrochlore-structured PMNT thin films exhibited ultralow dielectric losses, with a typical loss tangent as low as 0.001, and relatively high dielectric constants, typically [variant_greek_epsilon]r∼170. It was found that the relative permittivity slightly but continuously increased upon cooling without any signature of a structural phase transition, displaying a quantum paraelectriclike behavior; meanwhile, the PMNT pyrochlore thin films did not show any noticeable dielectric dispersion in the real part of permittivity over a wide temperature range (77–400 K). Their dielectric responses could, however, be efficiently tuned by applying a dc electric field. A maximum applied bias field of 1 MV/cm resulted in a ∼20% tunability of the dielectric permittivity, giving rise to a fairly large coefficient of the dielectric nonlinearity, ∼2.5×109 J C-4 m-5. Moreover, the PMNT pyrochlore films exhibited superior electrical insulation properties with a relatively high breakdown field (Ebreakdown∼1.5 MV/cm) and a very low leakage current density of about 8.2×10-7 A/cm2 obtained at an electric field intensity as high as 500 kV/cm. [ABSTRACT FROM AUTHOR]

The giant magnetoresistance (GMR) characteristics of hybrid perpendicular/Cu/in-plane magnetic anisotropy [Pd/Co]/Cu/Co single spin valves and [Pd/Co]/Cu/Co/Cu/[Co/Pd] dual spin valves with applied field perpendicular and parallel to the film plane are studied. Linear and nonhysteretic GMR behaviors are observed for the two spin valves in perpendicular-to-plane applied fields due to the coherent rotation of the Co layer driven by the in-plane shape anisotropy. The GMR value of the dual spin valve is 1.8 times as large as that of the single spin valve. The correlations between the magnetic configuration of the devices and the resistance evolvement are analyzed. In-plane GMR curves with maximum applied field of 30 kOe deviate from the linear behavior and can be well described by the Stoner–Wohlfarth model including high order anisotropy terms. Moreover, obvious hysteresis behavior is observed in the in-plane GMR curve of the dual spin valve. This phenomenon results from the hindered rotation of the Co/Pd moments due to the magnetostatic coupling between the top and bottom Co/Pd multilayers in the dual spin valve. [ABSTRACT FROM AUTHOR]

Dielectric characteristics of Ba6CoNb9O30 tetragonal tungsten bronze ceramics have been evaluated over a broad temperature and frequency range. A broad dielectric peak with strong frequency dispersion is observed at 140–225 K, which well follows the Vogel–Fulcher relationship and indicates the relaxor ferroelectric nature in the present ceramics. The P-E hysteresis loops varying with temperature are consistent with this dielectric relaxation process and provide the further evidence on the relaxor ferroelectric nature. The nonlinear magnetic hysteresis curve is observed in Ba6CoNb9O30 ceramics at 5 K, indicating the soft magnetic behavior which is related to the tetragonal tungsten bronze crystal structure containing magnetic ions of Co3+. [ABSTRACT FROM AUTHOR]

Dielectric properties of Ba0.7Sr0.3TiO3 (BST) thin films, which were prepared on silicon-based substrates by ion beam sputtering and postdeposition annealing method, were systematically investigated in different electrode configurations of metal-insulator-metal and coplanar interdigital capacitors. It was found that a large dielectric anisotropy exists in the films with better in-plane dielectric properties (higher dielectric permittivity and tunability) than those along the out-of-plane direction. The observed anisotropic dielectric responses are explained qualitatively in terms of a thermal strain effect that is related to dissimilar film strains along the in-plane and out-of-plane directions. Another reason for the dielectric anisotropy is due to different influences of the interfacial low-dielectric layer between the BST film and the substrate (metal electrode). [ABSTRACT FROM AUTHOR]

Dielectric characteristics of Ba4Nd2Ti4Nb6O30 tungsten bronze ceramics have been investigated over a broad range of temperature and frequency. The real part of the dielectric constant ([variant_greek_epsilon]′) shows a sharp peak at 417 K indicating the ferroelectric phase transition. Two dielectric relaxations are observed in the lower temperature range, which are very obvious in the dielectric loss (tan δ) and the imaginary part of the dielectric constant ([variant_greek_epsilon]″). The differential scanning calorimetry (DSC), Raman scattering, and ultrasonic studies of the present ceramics have also been conducted in a broad temperature range covering the ferroelectric transition and/or the dielectric relaxation temperatures. The endothermal peak in the DSC curve around the ferroelectric transition temperature confirms the first-order structural transition with the transition enthalpy of 5.72 kJ/mol. Steplike anomalies in the wave number of the Raman vibrations and abnormal drop of the intensity of the Raman spectra are observed accompanying the ferroelectric phase transition. The dielectric relaxations in the temperature range below Tc are attributed to the structure fluctuations associated with the coupling between the polarizability in the ab plane and the c axis displacement, as well as the remaining random field effects, which are suggested as the possible origin of the anomalous reduction of the Raman intensity below Tc as well as the anomalies in the ultrasonic velocity and attenuation measurements. Freezing of these structure fluctuations are detected by the ultrasonic anomaly at about 120 K and the increasing Raman intensity below 150 K. [ABSTRACT FROM AUTHOR]

Ba0.7Sr0.3TiO3 (BST) thick films with thickness up to 1 μm were deposited on Pt-coated silicon substrates by ion beam sputtering, followed by an annealing treatment. It is demonstrated that pure well-crystallized perovskite phase could be obtained in thick BST films by a low temperature process (535 °C). The BST thick films show highly tunable dielectric properties with tunability (at 800 kV/cm) up to 51.0% and 66.2%, respectively, for the 0.5 and 1 μm thick films. The relationship between strains and dielectric properties was systematically investigated in the thick films. The results suggest that a comparatively larger tensile thermal in-plane strain (0.15%) leads to the degradation in dielectric properties of the 0.5 μm thick film; besides, strong defect-related inhomogeneous strains (∼0.3%) make the dielectric peaks smearing and broadening in the thick films, which, however, preferably results in high figure-of-merit factors over a wide operating temperature range. Moreover, the leakage current behavior in the BST thick films was found to be dominated by the space-charge-limited-current mechanism, irrespective of the film thickness. [ABSTRACT FROM AUTHOR]

It was experimentally found that the evolution of dielectric loss with processing temperature displays a common trend in ferroelectric and dielectric thin oxide films; first an increase and then a decrease in dielectric loss when the processing temperature is gradually raised. Such a dielectric response of ferroelectric/dielectric thin films has been theoretically addressed in this work. We propose that at the initial stage of the crystallization process in thin films, the transformation from amorphous to crystalline phase should substantially increase the dielectric loss. Then, with further increase in the processing temperature, the coalescent growth of small crystalline grains into big ones could be helpful in reducing the dielectric loss by lowering grain boundary densities. The obtained experimental data for (Ba,Sr)TiO3 thin films 500 nm in thickness were analyzed in terms of the model developed and shown to be in reasonable agreement with the theoretical results. [ABSTRACT FROM AUTHOR]

The structural, magnetic, and transport properties of the electron-doped perovskites cobalt oxides La1-xTexCoO3 (0≤x≤0.25) have been investigated. For the structural properties, the room temperature structural transition from rhombohedra (R3C) to orthorhombic (Pbnm) symmetry is found in these samples with x>=0.10 by the Rietveld refinement of x-ray powder diffraction patterns, which is ascribed to the smaller ionic radius of Te. As for the magnetic properties, firstly the spin state transition of Co3+ ions is observed in all samples, and the transition temperature shifts up to higher temperature with Te doping, which is ascribed to the smaller ionic radius and the strong acidity of Te. Secondly, it is interesting that there exists paramagnetic to ferromagnetic phase transition in the Te-doped samples. It is suggested that such a phase transition results from the introduction of Co2+ ions and the appearance of double exchange interaction between Co2+ and Co3+ due to Te doping. Moreover, Curie temperature TC increases with the increase of the Te doping level x (for x≤0.10), and then decreases with the further increase of x. About the transport properties, the temperature dependence of the resistivity ρ shows that all samples display semiconducting behavior in the whole measured temperature region. And it can be seen that the resistivity first decreases with increasing Te doping level from x=0 to 0.10, then increases as x>=0.15. Such variations of TC and ρ with x are suggested to be related to the structure transition. [ABSTRACT FROM AUTHOR]

Large-volume atmospheric dielectric-barrier discharges (DBD) are particularly useful for processing applications when they operate in their homogeneous mode. A vast majority of their theoretical studies is currently based on the hydrodynamic treatment in which electrons are assumed to be in equilibrium with the local electric field. Recognizing that this assumption is incorrect in the sheath region, we report the development of an electron-hybrid model to treat electrons kinetically and all other particles hydrodynamically. Through numerical examples, it is shown that the mainstream hydrodynamic model underestimates gas ionization and discharge current. Using the hybrid model, it is demonstrated that variation in the amplitude of the applied voltage does not significantly alter sheath characteristics in terms of the electric field and the electron mean energy. Also gas ionization in atmospheric DBD is found to be significant only over a short timescale of 1 μs. Compared with dc atmospheric pressure glow discharges, atmospheric DBD are shown to have a smaller electron mean energy and a larger sheath thickness. [ABSTRACT FROM AUTHOR]

During the growth of indium phosphide (InP) crystals, dislocations are mostly generated in a plastically deformed crystal due to crystallographic glide caused by excessive thermal stresses. High dislocation density presented in the InP crystal can reduce the performance, lifetime, and reliability of the InP-based microelectronic and optoelectronic devices/circuits. The generation of dislocations in InP single crystals grown from the melt can be predicted by using a transient finite-element model. This model couples microscopic dislocation motion and multiplication to macroscopic plastic deformation during the crystal growth process. The temperature fields in the crystal are determined by solving the partial differential equations of heat transfer for the vertical gradient freeze (VGF) process. These temperature fields are then employed to the transient finite-element model to study the effects of doping impurities and growth parameters (i.e., imposed temperature gradient, crystal radius, and growth rate) on dislocation reduction in InP crystals grown by different VGF processes. [ABSTRACT FROM AUTHOR]

Ab initio all-electron molecular-orbital calculations are carried out to study the structures and relative stability of low-energy silicon clusters (Sin = 12-20). Selected geometric isomers include those predicted by Ho et al. [Nature (London) 392, 582 (1998)] based on an unbiased search with tight-binding/genetic algorithm, as well as those found by Rata et at [Phys. Rev. Lett. 85, 546 (2000)] based on density-functional tight-binding/single-parent evolution algorithm. These geometric isomers are optimized at the Møller-Plesset (MP2) MP2/6-31G(d) level. The single-point energy at the coupled-cluster single and double substitutions (including triple excitations) [CCSD(T)] CCSD(T)/6-31G(d) level for several low-lying isomers are further computed. Harmonic vibrational frequency analysis at the MP2/6-31G(d) level of theory is also undertaken to assure that the optimized geometries are stable. For Si12-Si17 and Si19 the isomer with the lowest-energy at the CCSD(T)/6-31G(d) level is the same as that predicted by Ho et al., whereas for Si18 and Si20, the same as predicted by Rata et at However, for Si14 and Si15, the vibrational frequency analysis indicates that the isomer with the lowest CCSD(T)/6-31G(d) single-point energy gives rise to imaginary frequencies. Small structural perturbation onto the Si14 and Si15 isomers can remove the imaginary frequencies and results in new isomers with slightly lower MP2/6-31G(d) energy; however the new isomers have a higher single-point energy at the CCSD(T)/6-31G(d) level. For most Sin, (n = 12-18,20) the low-lying isomers are prolate in shape, whereas thr Si19 a spherical-like isomer is slightly lower in energy at the CCSD(T)/6-31G(d) level than low-lying prolate isomers. [ABSTRACT FROM AUTHOR]

We report a simultaneous formation process of textured surfaces for hydrogen-free amorphous carbon (a-C) induced by an ion beam. By adjusting surface reactions via controlling temperature together with ion beams, several kinds of textured surfaces of a-C on mirror-polished Si (111) can be produced during ion-beam assisted deposition. In high-temperature regimes, submicron-sized pyramidlike a-C particles and ripples structures form. The sizes and separations of a-C particles change with substrate temperature significantly. An intrinsic model based on a natural self-organization mechanism is proposed for the production of textured surfaces, where the surface diffusion is shown to play a decisive role. In a low-temperature regime, the films commonly present mounding roughening. Likewise, self-organized nanodots appear at a very low ion flux. These nanodots are about 5±0.5 nm high with a diameter to height ratio close to 15:1 and a density of approximately 290 μm-2. The nanodot production is correlated with ion-induced nucleation. The demonstration of the textured surface formations may open a way to fabricate and tune desirable surface structures on the technically important functional materials. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Potential energy curves and spectroscopic constants for 37 low-lying electronic states of Mo[sub 2] and the ground state of Mo[sup +][sub 2] have been computed using the complete active space multiconfiguration self-consistent field followed by the multireference singles+doubles configuration interaction calculations that included up to 19 million configurations. We compute equilibrium distances (r[sub e]), vibrational frequencies (ω[sub e]), and energy separations (T[sub e]). We have suggested tentative assignments for the spectra of Mo[sub 2] including the recent work on resonance Raman and fluorescence spectra of Mo[sub 2]. Our work supports the ground state with ω[sub e] near 450-486 cm[sup -1], and we suggest that the 394.5 cm[sup -1] line in the resonance Raman spectrum of Lombardi and co-workers is for the a ³Σ[sup +][sub u] state of Mo[sub 2]. Our computations are consistent with the recently observed near-infrared spectrum of Mo[sub 2] by Bondybey and co-workers. We have computed a number of spectroscopic systems for Mo[sub 2], which are yet to be observed. Our computed adiabatic ionization energy of Mo[sub 2] is in excellent agreement with Simard and co-workers. We have computed the D[sub e]'s of Mo[sup +][sub 2] and Mo[sub 2]. [ABSTRACT FROM AUTHOR]

We report the unstable surface feature in nanometer-scale of diamondlike carbon (DLC) films deposited through C[sub 60] evaporation with simultaneous bombardment of 1.5 keV Ne[sup +] ions. The periodical ripples, commonly appearing in postprocessing of the deposited films, form directly at 550°C and 700°C, which is qualitatively consistent with the theoretical model based on sputtering yield variation with surface curvature. A dramatic transition from ripple surface to mounding roughening occurs at 400°C. The graphitization with increasing temperature induces the different dominant smoothening mechanisms, which is responsible for the morphological change observed. Further, the calculations of height-height correlation function show that the roughness exponents are around 0.8 at 200°C and 400°C, implying self-affinity of roughened surfaces. This study exhibits a potential of ion beam assisted deposition to tune DLC morphologies by controlling the deposition parameters to drive the competition between ion erosion and film deposition. [ABSTRACT FROM AUTHOR]

Studies the effects of doping impurity and growth orientation on dislocation generation in gallium arsenide crystals grown from the melt. Dislocation density generated in the crystal; Influence of the growth orientation during the crystal growth process on the level of reduction; Generation of dislocations by excessive thermal stresses during the growth process.

An exciton–polariton condensate is a hybrid light–matter state in the quantum fluid phase. The photonic component endows it with characters of spin, as represented by circular polarization. Spin-polarization can form stochastically for quasi-equilibrium exciton–polariton condensates at parallel momentum vector k|| ∼ 0 from bifurcation or deterministically for propagating condensates at k|| > 0 from the optical spin-Hall effect (OSHE). Here, we report deterministic spin-polarization in exciton–polariton condensates at k|| ∼ 0 in microcavities containing methylammonium lead bromide perovskite (CH3NH3PbBr3) single crystals under non-resonant and linearly polarized excitation. We observe two energetically split condensates with opposite circular polarizations and attribute this observation to the presence of strong birefringence, which introduces a large OSHE at k|| ∼ 0 and pins the condensates in a particular spin state. Such spin-polarized exciton–polariton condensates may serve not only as circularly polarized laser sources but also as effective alternatives to ultracold atom Bose–Einstein condensates in quantum simulators of many-body spin–orbit coupling processes. [ABSTRACT FROM AUTHOR]

Perpendicular magnetic anisotropy (PMA) in magnetic thin films has attracted much attention due to its potential applications in spintronics devices. Here, we report the continuous strain-mediated PMA in epitaxial (111) CoFe 2 O 4 (CFO) thin films grown on (0001) LiTaO 3 substrates. A large variation in lattice strain (∼ 0.9 %) in a continuous way is realized in the CFO thin films by changing substrate temperature during deposition due to the difference in the thermal expansion coefficient between CFO and LiTaO 3. As a result, the PMA of the (111) CFO thin films can be continuously mediated by the strain with uniaxial magnetic anisotropy energy in the range of 0.12 - 14.69 × 10 6 erg/cm 3. Furthermore, the strain as well as the consequent PMA in the (111) CFO thin films can be maintained within the thickness of 25–205 nm, which is consistent with the scenario of the magnetoelastic effect. Our results reveal that the CFO/LiTaO 3 system can be regarded as an ideal platform to realize robust PMA and its continuous strain tuning in the (111) CFO thin films by virtue of strain-induced magnetic anisotropy. [ABSTRACT FROM AUTHOR]

Vibrational population up to v″=3 in the umbrella mode of CD3 thermally desorbed from GaAs(100) has been determined by resonance enhanced multiphoton ionization (REMPI) spectroscopy. The population ratios are superthermal. The methyl radical must desorb promptly with a transition state geometry significantly different from the planar gas phase geometry. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Understanding photochemistry and energy transfer mechanisms in molecular solid films is of interest to many scientific issues, ranging from matrix-assisted laser desorption ionization mass spectrometry to photochemical processes on polar stratospheric cloud particles. We present a study of a model system: the photochemistry (hν=1.2–6.4 eV) of a molecular Cl2CO solid film at low laser power density, 10 μJ–1 mJ/cm2 for ∼10 ns pulses. At hν≥3.5 eV, photon absorption by Cl2CO leads to a major photodissociation channel resulting in CO (g) and Cl (g) and a minor molecular Cl2CO ejection channel. Both photodissociation and molecular ejection are observed at the lowest laser power density and their yields depend linearly on pulse energy. This result establishes a single photon photoexcitation mechanism. The electronically excited Cl2CO in the surface region of the solid film can either dissociate or convert its electronic energy to translational motion in Cl2CO. The translational energy distribution of CO (g) from the photodissociation channel is bimodal: the flux-weighted mean translational energy of the fast channel is photon energy dependent (=210, 135, and ∼90 meV at hν=6.4, 5.0, and 3.5 eV, respectively), while the slow channel is independent of photon energy and corresponds to completely thermalized CO molecules (=84±3 K). The mean translational energy of photoejected Cl2CO is =220±20 meV. In addition to photoejection, there is also a distinctively different thermal desorption channel due to transient laser heating. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The translational, rotational, and vibrational state distributions of CO (g) resulting from the single photon photodissociation of Cl2CO in the condensed phase at ∼90 K have been determined by time-of-flight (TOF) distribution measurement and resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The TOF distribution of CO (g) is bimodal. Internal state characterization of the slow channel reveals a completely thermalized origin, with a rotational temperature of Trot=88±5 K, which is equal to the translational temperature as well as the substrate temperature. We believe these slow CO molecules originate from photodissociation below the topmost surface of the molecular film and achieve thermal equilibrium with the substrate before escaping into the gas phase. Internal state characterization of the fast channel shows, on the other hand, an energetic origin: at hν=5.0 eV, the rotational distribution, with an overall flux-weighted mean rotational energy of =0.12±0.01 eV, is non-Boltzmann and can be approximated by a bimodal distribution with rotational temperatures of 210±40 K at low J″(s) and 2200±300 K at high J″(s); the relative vibrational population is Nν=1/Nν=0=0.33±0.05. Both rotational and translational distributions of fast CO show positive correlation with photon energy. These CO molecules must be promptly ejected into the gas phase, carrying nascent energetic information from the photodissociation reaction on the surface of the molecular film. For electronic excitation events that result in photodissociation, 74% of the excess excitation energy is distributed in the translational and internal motions of products (CO and Cl); only 26% of the available energy is converted to motions of surrounding molecules. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

State resolved characterization of nascent gas phase products is used as probe for the dynamics of an energetic surface reaction. This is achieved in the photodissociation of monolayer phosgene adsorbed on Ag(110). Irradiation of adsorbed Cl2CO in a broad photon energy range (hν=1.9–6.4 eV) leads to dissociation, with Cl retained on the surface and CO desorbing into the gas phase. The translational energy of product CO (g), =0.26 eV, is independent of hν, even at the threshold photon energy (1.9 eV). This result establishes a dissociative electron attachment mechanism involving a reactive intermediate, ClCO, whose prompt dissociation serves as a probe to surface dissociation dynamics. Consistent with translation, internal state distribution of product CO (g) also shows an energetic origin: The rotational distribution, with an overall flux-weighted mean rotational energy of =0.17 eV, can be approximated by a bimodal Boltzmann distribution with rotational temperatures of 700 K at low J(s) and 7000 K at high J(s); the relative vibrational population is Nν=1/Nν=0=0.30. Contrary to common expectation based on quenching rates, both translational and rotational energies of CO (g) from monolayer photodissociation are much higher than those from the direct photodissociation in multilayers. This is taken as evidence for concerted reaction dynamics on the surface: The high exothermicity in the Cl–Ag bond formation on the surface exerts part of the energy to the Cl–CO coordinate, leading to higher energies in CO (g). © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Excitation and de-excitation of the electronic states of an adsorbed molecule on a solid surface result in intramolecular vibrational excitation. The internal vibrational energy can couple to the adsorbate–substrate bond and lead to molecular desorption. In this mechanism, the isotope effect in molecular desorption is determined not by the mass ratio of the leaving particles, but by that in the internal coordinate. This process is modeled within the context of semiclassical wave packet dynamics, which, in the short-time-limit harmonic approximation, allows analytical solution. Results are obtained for the ammonia/GaAs(100) system and are compared with experimental observations. [ABSTRACT FROM AUTHOR]

UV laser irradiation of ammonia adsorbed on GaAs(100) leads to molecular desorption and dissociation. A nitride passivation layer can be formed on the GaAs surface at 100 K by simultaneous exposure to ammonia and uv photons in a UHV environment. The nitride layer consists of a mixture of Ga and As nitrides. While the dominating GaN surface species is thermally stable, AsN desorbs below 800 K. Surface NH2 is identified as an intermediate. The implication of this study for selective area passivation and GaN growth is discussed. [ABSTRACT FROM AUTHOR]

UV laser irradiation of ammonia adsorbed on GaAs(100) leads to molecular desorption, with a mean translational temperature of =300 K, independent of photon energy and isotope substitution. However, the photodesorption cross section depends strongly on isotope substitution: σNH3/σND3=4.1 at hν=6.4 eV. This isotope effect is too large to be accounted for by the mass difference in the leaving particles (NH3 vs ND3), but can be successfully explained in terms of an isotope effect in the internal N–H(D) coordinates. We take this as evidence for uv-driven photodesorption from electronically quenched, but vibrationally hot ground state ammonia. [ABSTRACT FROM AUTHOR]

Molecular AsH3, adsorbed on Ga-rich GaAs(100) at 115 K, dissociates readily upon uv irradiation with 193, 248, and 351 nm excimer laser light. In the initial photodissociation step one As–H bond cleaves, leaving all the AsH2, and a large fraction of the H, adsorbed to As. The AsH2 further photodissociates to give As–H and Ga–H. The final steps, photochemical removal of hydrogen from Ga–H and As–H, lead to As deposition. The photodissociation cross section decreases sharply with the extent of photolysis. The wavelength dependence, compared to the gas-phase absorption cross section, extends to much lower photon energies and indicates that substrate-mediated excitation dominates the observed chemistry. There are strong isotope effects in all the cross sections; these are related to mass-dependent substrate-mediated quenching of the excited states. Implications for photon-assisted organometallic chemical vapor deposition are discussed. [ABSTRACT FROM AUTHOR]

The photodissociation rate of phosgene (Cl2CO) adsorbed on Pt(111) has been measured as a function of photon energy at normal incidence and as a function of incident angle using p-polarized light. Compared to the gas phase, the wavelength dependence of the initial photolysis cross section on the surface is redshifted. The angular response to p-polarized light is wavelength dependent. Above 315 nm, the angular dependence correlates with calculated metal absorption. At 280 nm, the angular dependence is much too strong to be accounted for solely by substrate excitation. A combination of substrate and direct excitation is adequate. This is the first direct evidence, for monolayers on metals, that both direct and substrate excitation contribute to surface photochemistry but dominate at different wavelengths. [ABSTRACT FROM AUTHOR]

A Pt(111) surface saturated with molecular oxygen was irradiated by UV light from a mercury arc lamp with various cutoff filters. High-resolution electron energy-loss spectroscopy (HREELS) and temperature programmed desorption mass spectroscopy (TPD) were used to characterize the products retained on the surface. Upon UV irradiation at 95 K, chemisorbed O2 (peroxo) undergoes dissociation, desorption and rearrangement. Different wavelength dependences were observed for the three processes: dissociation was not observed at wavelengths longer than 295 nm, in agreement with gas phase photodissociation of the O–O bond in hydrogen peroxide; desorption and rearrangement became negligible at wavelengths longer than 420 nm, in agreement with the photolysis of an organometallic peroxoplatinum complex. For wavelengths between 230 and 315 nm, the average cross sections of dissociation and desorption were estimated to be 5.7×10-20 and 1.2×10-19 cm2 , respectively. The possible origins of the three processes are discussed. [ABSTRACT FROM AUTHOR]

We report evidence for the coexistence of direct and precursor dynamics in the dissociative chemisorption of H2 on Ni(l00). Hydrogen and deuterium uptakes on Ni(l00) were measured at various surface temperatures by following the secondary ion ratios, Ni2 H+/Ni+ and Ni2 D+/Ni+ , which are proportional to surface hydrogen and deuterium coverage on both clean and carbon-covered Ni(100). Between 100 and 200 K on clean Ni(l00), the initial sticking coefficient of hydrogen decreases, but only slightly, as the surface temperature increases. The decrease is more pronounced both in the presence of predosed carbon and for deuterium adsorption on Ni(100). This is interpreted as due to the involvement at low temperatures of a molecular precursor which mediates dissociative adsorption at low temperatures. The precursor probably involves surface defects and sites formed in the presence of carbon. In addition to the precursor channel, a direct dissociation channel also operates, and dominates for T≥200 K. [ABSTRACT FROM AUTHOR]

Homodyne depolarized photon-correlation functions of 1,3,5-tri-α-naphthyl benzene (TαNB) have been measured at the 90° scattering angle in the temperature range of 75–110 °C. The correlation functions are analyzed in terms of a fractional exponential function and a modified constrained regularization method originally developed by Provencher. The results from both methods of data analysis are in agreement. The temperature dependence of the mean orientational relaxation time when fitted to the Arrhenius equation, gives an apparent activation energy E=85.3 kcal/mol. The Antoine–WLF equation provides a better fit of the mean reorientational relaxation time data. It is also found that orientational time τ is linearly proportional to η/T, indicating that the Debye–Stokes–Einstein equation is applicable to describe molecular reorientation in the supercooled TαNB liquid. [ABSTRACT FROM AUTHOR]

I present a detailed account of a zero loop-area Sagnac interferometer operated at oblique incidence for detecting magneto-optic Kerr effects arising from a magnetized sample. In particular, I describe the symmetry consideration and various optical arrangements available to such an interferometer that enables measurements of magneto-optic effects due to both in-plane and out-of-plane magnetization of the sample with optimizable signal-to-noise ratios. [ABSTRACT FROM AUTHOR]

We describe a zero loop-area Sagnac interferometer at oblique incidence for detecting magneto-opticKerr effect arising from in-plane magnetization in a sample. By exploiting properties of polarization states under relevant crystal symmetry transformation, we show that contributions from longitudinal and transverse Kerr effects can be separated. In addition we can select one optical arrangement out of four that detects the longitudinal effect with the highest signal-to-noise ratio. Compared to finite loop-area Sagnac interferometers operating at oblique incidence, the zero loop-area interferometer involves significantly fewer optical elements and is thus more stable against drifts in the optical system. For demonstration, we measured the in-plane magneto-optic Kerr effect from a 42-nm Ni film. [ABSTRACT FROM AUTHOR]

The focused high-intensity pulsed ion beam (HIPIB) of 100 ns order pulse is generated with respect to its spatial stability in two types of magnetically insulated ion diodes (MIDs) with geometrical focusing configuration using the passive anode, i.e., insulation of electrons with an external magnetic-field and a self-magnetic field, respectively. Anode plasma formation for the ion beam generation is based on different processes in the two types of MIDs, as the surface breakdown on the polymer-coated anode operated in the unipolar pulse mode for the external-magnetic field MID and the explosive electron emission on the graphite anode in the bipolar-pulse mode for the selfmagnetic field MID. Typical energy density per pulse is in the range of 3-6 J/cm², at an accelerating voltage of 200-300 kV with a pulse duration of 120-150 ns. The spatial deviations of the HIPIB is evaluated by measuring the energy density distribution by using an infrared diagnostic method considering neutralizing during the ion beam propagation to the focal plane with a spatial resolution of 1 mm. The ion beam deviation is about ±1.5mm for the external-magnetic field MID and ±2.5mm for the self-magnetic field MID, leading to a fluctuation in the energy density of 1%-12%, and 9%-27% within a 10mm range at the focal point, respectively. It is revealed that the displacement of different parts of a beam spot occurs nonsynchronously, mainly attributable to the intrinsic diode processes of plasma generation and expansion, and ion beam extraction from the anode-cathode gap, while the influence of magnetic field in the transportation region is negligible. The ion beam spatial deviation has a major influence on the shot-to-shot stability of ion beam, and it is suggested that the stability can be enhanced via diode process improvement. [ABSTRACT FROM AUTHOR]

Though widely used in modelling nano- and micro- structures, Eringen's differential model shows some inconsistencies and recent study has demonstrated its differences between the integral model, which then implies the necessity of using the latter model. In this paper, an analytical study is taken to analyze static bending of nonlocal Euler-Bernoulli beams using Eringen's two-phase local/nonlocal model. Firstly, a reduction method is proved rigorously, with which the integral equation in consideration can be reduced to a differential equation with mixed boundary value conditions. Then, the static bending problem is formulated and four types of boundary conditions with various loadings are considered. By solving the corresponding differential equations, exact solutions are obtained explicitly in all of the cases, especially for the paradoxical cantilever beam problem. Finally, asymptotic analysis of the exact solutions reveals clearly that, unlike the differential model, the integral model adopted herein has a consistent softening effect. Comparisons are also made with existing analytical and numerical results, which further shows the advantages of the analytical results obtained. Additionally, it seems that the once controversial nonlocal bar problem in the literature is well resolved by the reduction method. [ABSTRACT FROM AUTHOR]

High-intensity pulsed ion beam (HIPIB) with ion current density above Child-Langmuir limit is achieved by extracting ion beam from anode plasma of ion diodes with suppressing electron flow under magnetic field insulation. It was theoretically estimated that with increasing the magnetic field, a maximal value of ion current density may reach nearly 3 times that of Child-Langmuir limit in a non-relativistic mode and close to 6 times in a highly relativistic mode. In this study, the behavior of ion beam enhancement by magnetic insulation is systematically investigated in three types of magnetically insulated ion diodes (MIDs) with passive anode, taking into account the anode plasma generation process on the anode surface. A maximal enhancement factor higher than 6 over the Child-Langmuir limit can be obtained in the non-relativistic mode with accelerating voltage of 200-300 kV. The MIDs differ in two anode plasma formation mechanisms, i.e., surface flashover of a dielectric coating on the anode and explosive emission of electrons from the anode, as well as in two insulation modes of external-magnetic field and self-magnetic field with either non-closed or closed drift of electrons in the anode-cathode (A-K) gap, respectively. Combined with ion current density measurement, energy density characterization is employed to resolve the spatial distribution of energy density before focusing for exploring the ion beam generation process. Consistent results are obtained on three types of MIDs concerning control of neutralizing electron flows for the space charge of ions where the high ion beam enhancement is determined by effective electron neutralization in the A-K gap, while the HIPIB composition of different ion species downstream from the diode may be considerably affected by the ion beam neutralization during propagation. [ABSTRACT FROM AUTHOR]

Initial anode plasma formation in a magnetically insulated ion diode (MID) of bipolar-pulse mode with self-magnetic field is studied for high-intensity pulsed ion beam generation in TEMP-6-type apparatus [X. P. Zhu, M. K. Lei, and T. C. Ma, Rev. Sci. Instrum. 73, 1728 (2002)]. The field emission characteristics on graphite anode, the self-magnetic field, the perveance, and the impedance of the MID are obtained by analysis of the time-dependent diode voltage-current characteristics during the initial negative pulse stage of 100 ns, with peak values of 250 kV and 30 kA, respectively. The anode plasma is initiated by explosive emission on the anode due to a significant field enhancement effect up to four orders of magnitude, and the subsequent merging and movement of the anode plasma are affected by the superimposing space-charge effect and magnetic insulation of the self-magnetic field. The formation process of the initial anode plasma in the MID corresponds to the diode response under the initial negative pulse of the bipolar pulse. [ABSTRACT FROM AUTHOR]

Generation of high-intensity pulsed ion beam (HIPIB) has been studied experimentally using polyethylene as the anode polymer in magnetically insulated ion diodes (MIDs) with an external magnetic field. The HIPIB is extracted from the anode plasma produced during the surface discharging process on polyethylene under the electrical and magnetic fields in MIDs, i.e., high-voltage surface breakdown (flashover) with bombardments by electrons. The surface morphology and the microstructure of the anode polymer are characterized using scanning electron microscopy and differential scanning calorimetry, respectively. The surface roughening of the anode polymer results from the explosive release of trapped gases or newly formed gases under the high-voltage discharging, leaving fractured surfaces with bubble formation. The polyethylene in the surface layer degrades into low-molecular-weight polymers such as polyethylene wax and paraffin under the discharging process. Both the surface roughness and the fraction of low molecular polymers apparently increase as the discharging times are prolonged for multipulse HIPIB generation. The changes in the surface morphology and the composition of anode polymer lead to a noticeable decrease in the output of ion beam intensity, i.e., ion current density and diode voltage, accompanied with an increase in instability of the parameters with the prolonged discharge times. The diode voltage (or surface breakdown voltage of polymer) mainly depends on the surface morphology (or roughness) of anode polymers, and the ion current density on the composition of anode polymers, which account for the two stages of anode polymer degradation observed experimentally, i.e., stage I which has a steady decrease of the two parameters and stage II which shows a slow decrease, but with an enhanced fluctuation of the two parameters with increasing pulses of HIPIB generation. [ABSTRACT FROM AUTHOR]

The optical spectra of transition metal dichalcogenide monolayers are dominated by excitons and trions. Here, we establish the dependence of these optical transitions on the disorder from hyperspectral imaging of h-BN encapsulated monolayer MoSe2. While both exciton and trion energies vary spatially, these two quantities are almost perfectly correlated, with spatial variation in the trion binding energy of only ∼0.18 meV. In contrast, variation in the energy splitting between the two lowest energy exciton states is one order of magnitude larger at ∼1.7 meV. Statistical analysis and theoretical modeling reveal that disorder results from dielectric and bandgap fluctuations, not electrostatic fluctuations. Our results shed light on disorder in high quality TMDC monolayers, its impact on optical transitions, and the many-body nature of excitons and trions. [ABSTRACT FROM AUTHOR]

A magnetically insulated ion diode (MID) with an improved external-magnetic field system has been developed and installed onto a TEMP-6-type high-intensity pulsed ion source in order to produce a high-intensity pulsed ion beam (HIPIB) for surface modification of materials. The external-magnetic field MID is operated in unipolar mode based on dielectric high-voltage flashover, and a double coaxial pulse-forming line (PFL) powered with a Marx generator is used to form the unipolar pulse of nanosecond width. A specially designed cathode has been constructed with a forked connection to two symmetrically installed transformers to improve the effect of the magnetic field and thus increase the stability of generation and propagation of the ion beam. It was found that the efficient generation of HIPIB mainly depended on the magnetic field strength, the gas pressures in reverse and output switches of PFL, and the anode-cathode (A - K) gap of the external-magnetic field MID. A proper magnetic field strength was found with magnetic field power system at dc charging voltage of 8 kV. The proper A - K gap distance is not uniform with the value varied from 6 to 8 mm. Suitable gas pressures for reverse and output switches were about 1.2 and 2.4 atm, respectively, at a charging voltage of 40 kV to the Marx generator. The most efficient plasma generation and ion extraction led to a maximum output of HIPIB with a peak ion current density of 300 A cm[SUP-2] and a beam pulse width of 80 ns (full width at half maximum), at an accelerating pulse of 300-350 kV with a pulse width of 70 ns. [ABSTRACT FROM AUTHOR]

A high-intensity pulsed ion source of TEMP-type series, operating in bipolar mode, has been developed as a unique pulsed energy source to produce a high-intensity pulsed ion beam (HIPIB) for surface modification of materials. To generate the ion beam, a specially shaped bipolar pulse, consisting of a first negative pulse and a second delayed positive pulse both of nanosecond width, is formed by a double coaxial pulse-forming line (PFL) powered with a Marx generator and supplied to a magnetically insulated ion diode (MID) by a self-magnetic field. It is found that the efficient generation of a HIPIB is mainly dependent on the delay time of the bipolar pulse, adjusted by pressure ratio in the two gas switches of a PFL, and the anode-cathode (A-K) gap distance in the self-magnetic field MID. The delay time determines the effective area on the anode surface for plasma generation and the A-K gap distance ensures the stability of the process. A proper delay time and a proper A-K gap distance are obtained by a series of experimental investigations. Under delay time from 30 to 280 ns and several different A-K gap distances, the typical wave forms of the bipolar pulses at a dc charging voltage of 45 kV to Marx generator are illustrated to clarify the effects of delay time and A-K gap distance on the ion beam generation. The proper A-K gap distance is not uniform, varied from 6 to 8 mm, and the corresponding proper delay time is 250 ns. The most efficient plasma generation leads to a maximum output of HIPIB with a peak ion current density of 350 A cm[sup -2] and a beam pulse width of 75 ns (full width at half maximum), at an accelerating pulse of 220 kV with a pulse width of 100 ns. [ABSTRACT FROM AUTHOR]

This paper presents a novel reaction microscope designed for ion–atom collision investigations, established at the Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, China. Its time-of-flight (TOF) spectrometer employs an innovative flight-time focusing method consisting of two acceleration regions, providing optimal time focusing conditions for charged fragments with diverse initial velocities. The TOF spectrometer's axis intentionally tilts by 12° relative to the ion beam direction, preventing potential obstructions from the TOF grid electrodes. The introduced focusing method allows for a flexible time-focusing TOF spectrometer design without restricting the length ratio of the two regions. In addition, this configuration in our case significantly suppresses noise on the recoil ion detector produced by residual gas in the ion beam trajectory, which is a considerable challenge in longitudinal spectrometers. In a test experiment on the single electron capture reaction involving 62.5 keV/u He2+ ions and a helium atomic beam, the recoil longitudinal momentum resolution achieved 0.068 atomic units. This novel configuration and successful test run show excellent precision for ion–atom collision studies. [ABSTRACT FROM AUTHOR]