bluuebunny/crossref_metadata_2025 huggingface dataset:
Production, thermalization, and transport of sputtered lithium atoms across a glow discharge plasma, studied by concentration modulated absorption spectroscopy
Authors: Mason Rod S. , Naylor John C. , Hatcher Michael G. , Jones W. Jeremy
Published: July 2002

Sputtered Li atom density profiles have been measured, absolutely, using concentration modulated absorption spectroscopy (COMAS), across a planar Ar glow discharge. They are found to fit very well to a simple analytical solution to the sputtering model, previously proposed, of atom recoil from the surface and thermalization as the kinetically energetic atoms are stopped by collision and subsequent diffusive transport. “Stopping” or thermalization cross sections have thus been experimentally measured and are found to be a factor of between four and eight times less than the hard sphere collision cross section, depending on the recoil energy. The actual particle flux bombarding the cathode and causing sputtering is shown to be greater than the ion flux, depending on conditions.
Optical and structural characterization of ZnSe films grown by molecular beam epitaxy on GaAs substrates with and without GaAs buffer layers
Authors: Luyo-Alvarado J. , Meléndez-Lira M. , López-López M. , Hernández-Calderón I. , Constantino M. E. , Navarro-Contreras H. , Vidal M. A. , Takagi Y. , Samonji K. , Yonezu H.
Published: July 2002

ZnSe films with thickness between 800 and 7500 Å were grown on GaAs(100) by molecular beam epitaxy (MBE), and characterized by photoluminescence (PL), photoreflectance (PR), transmission electron microscopy (TEM), and high resolution x-ray diffraction. A first set of films was prepared with ZnSe directly grown on the GaAs substrate. Another set was prepared using an arsenic capped GaAs buffer layer grown on the GaAs substrate in a separated MBE system. PL studies at 18 K showed that the ZnSe films have more defects for samples grown directly on the GaAs substrate. The behavior of stacking faults and dislocations as a function of film thickness were investigated by TEM, and by the variation of the intensity of PL signals related to these defects. For both sets of samples the intensity of these signals decreased with increasing film thickness, but the decrease is steeper for films grown on GaAs buffer layers. A signal in PL spectra at ∼2.7 eV was observed only for the samples grown directly on GaAs substrates, it was associated with donor–acceptor transitions involving GaZn and VZn. The room temperature PR spectra showed, besides the GaAs and ZnSe band-gap signals, oscillations associated with the Franz–Keldysh effect due to internal electric fields. The strength of these fields was obtained by employing the asymptotic Franz–Keldysh model. A signal 22 meV below the GaAs band-gap energy was observed only in the PR spectra of the samples grown directly on GaAs substrates. This signal was associated with Zn interdiffused into the GaAs, and is correlated to the PL signal observed at 2.7 eV for the same set of samples.
Thermal stability of Pd/Ge-based ohmic contacts to n-type GaAs
Authors: Wang Y. G. , Wang D. , Ivey D. G.
Published: July 2002

The microstructural changes that occur during high temperature annealing (450–600 °C) of Pt/Ti/Ge/Pd ohmic contacts to n-type GaAs have been studied using transmission electron microscopy (TEM). The metal layers were deposited sequentially by electron beam evaporation onto GaAs doped with Si to a level of ≈5×1018 cm−3. The deposition sequence and metal layer thicknesses were: Pt (50 nm), Ti (30 nm), Ge (90 nm) and Pd (45 nm). The contact microstructure remained uniform up to 550 °C and consisted of a continuous polycrystalline layer of GePd at the semiconductor surface, along with pockets of epitaxially grown Ge. Annealing at 550 °C resulted in the nucleation of a Ge-deficient phase, Ge8Pd21, at the GePd/GaAs interface, which protruded into the GaAs and deteriorated the uniformity of the contact. At higher annealing temperatures, the contact degraded rapidly due to inward diffusion of Ti and Pt and outward diffusion of As, leading to the formation of several binary and ternary phases.
Plasma-sheath resonances and energy absorption phenomena in capacitively coupled radio frequency plasmas. Part I
Authors: Ku Victor P. T. , Annaratone Beatrice M. , Allen John E.
Published: July 2002

A plasma-sheath resonance has been observed at low pressures in argon plasmas. This phenomenon was known in a different context, that of the plasma resonance probe, but was hitherto unknown in parallel-plate plasma reactors. Two different models have been used to resolve the plasma-sheath resonances. A simple linear model is used to represent the plasma system as a series circuit of two capacitors (sheaths) and an inductor (plasma region). The plasma-sheath resonance occurs at a frequency where the cold plasma behaves inductively and resonates together with the two sheaths. The second model treats the plasma bulk as a series combination of an infinite number of elementary cells each one constituted by an inductor and a capacitor in parallel. By assuming an electron density profile the total impedance of the plasma can be obtained. Furthermore, the calculations show that the resonant energy is absorbed at an infinitely thin resonance plane. This causes the impedance of the system, filled with a lossless inhomogeneous plasma, to have a real part. This resonance can be used as a diagnostic, to measure the electron density. It could perhaps be exploited as the basis of a new plasma reactor.
Thermophysical characterization of artificially aged papers by means of the photothermal deflection technique
Authors: Bertolotti M. , Ligia S. , Liakhou G. , Li Voti R. , Paoloni S. , Sibilia C. , Ricciardiello G. , Alessi P.
Published: July 2002

The photothermal deflection technique has been used to determine the thermophysical properties of artificially aged paper samples. The deflection amplitude ratio and the phase difference of the deflection angle between front and rear surface illumination have been measured as a function of the pump beam modulation frequency, in order to avoid the dispersion effects mainly introduced by the probe beam vertical offset with respect to the sample surface. A theoretical model for the photothermal signal generation, taking into account the unavoidable light scattering occurring at the paper fibers–air interfaces, has been developed. A photothermal deflection method has been also proposed to measure the transmittance and the reflectance values of a given paper sheet, thus offering a proper constraint to the whole fitting procedure. In this way it has been possible to estimate the thermal diffusivity, the optical absorption and the scattering coefficient values of all the samples examined. A correlation between these values and the aging state of the paper has been found.
Photon energy dependence of vacuum ultraviolet light-induced stress in silica glass
Authors: Morimoto Yukihiro , Yasuda Yukio , Nasu Shoichi
Published: July 2002

The photon energy dependence of vacuum ultraviolet light-induced stress was investigated for two types of silica glass, type I (GE214) and type III (Suprasil P20). Samples of both glass types were irradiated under three conditions: (1) with xenon (Xe) arc light directly, (2) through three kinds of filtered Xe arc light, and (3) with a high-pressure mercury (Hg) lamp. To evaluate the samples, the absorption spectra were measured in vacuum ultraviolet (VUV), UV, and infrared regions. The irradiation-induced stress was also measured. The relative shape of the Xe arc spectrum was derived, and the absorption spectra of GE214 and Suprasil P20 were determined in the VUV region at the Xe lamp operation temperature. The measured stress was converted to strain energy, which was then corrected with respect to the hydroxyl group (≡ SiOH) content and the irradiation time. The results revealed that the corrected strain energy was linearly proportional to the amount of absorbed light within the measurement error. The photon energy threshold for inducing the strain was evaluated to be 5.6 eV for GE214, at which the absorption coefficient was ∼10 cm−1. In contrast, light absorption occurred at photon energy not less than 6.5 eV for Suprasil P20. Therefore, there is a photon energy dependence on the VUV induced stress. This dependence varies not only with the type of silica glass but also its ambient temperature which, in turn, controls the absorption coefficient. In fact, the estimated absorption coefficient at 7.5 eV and at 743 K for GE214 was about two orders of magnitude higher than that measured at room temperature. The bond cleavage and rearrangement of the silica glass network should be enhanced by heat. The analysis of these photon energy dependences also suggested that Suprasil P20 was strained more effectively than GE214, probably due to absorption of VUV light by ≡ SiOH s contained in silica glass.
Structural, optical, and electrical properties of laser deposited FeTiO3 films on C- and A-cut sapphire substrates
Authors: Dai Z. , Naramoto H. , Narumi K. , Yamamoto S. , Miyashita A.
Published: July 2002

FeTiO 3 (FTO) thin films were epitaxially grown on two kinds of α-Al2O3 substrates [(0001) and (112̄0)] through pulsed laser deposition under a mixtured gas ambient of 5% oxygen with argon used as the balance. The x-ray diffraction (XRD) and XRD pole figure measurements show parallel growth relationships of the films on these two kinds of substrates, (0006)∥(0006), (112̄0)∥(112̄0), respectively. The interplane XRD pole texture measurement also shows that a parallel relationship exists. Rutherford backscattering spectrometry and Auger electron spectrometry analyses show that the films are stoichiometric and without distinguishable impurities. The optical properties were characterized in the ultraviolet-visible region, and 3.55 eV was obtained for the first time as the band gap of these epitaxial and stoichiometric FTO films. This value in the present high quality films is higher than the band gap reported for bulk material, 2.58 eV. The measurement of the electrical resistance in a quite wide temperature range shows typical semiconductor properties.
Simplified scattering rate calculation in nonparabolic hydrodynamic equations
Authors: Weng X. M. , Quan H. J. , Cui H. L.
Published: July 2002

A method is presented to simplify the calculation of the scattering rates in a hydrodynamic model based on the nonparabolic balance equations. A four-dimensional integral to calculate the scattering rates is reduced to a two-dimensional integral. This method has been applied to calculate the linear and nonlinear electron transport in a SiC system. The results obtained using the simplified method and the regular method differ only slightly up to an electric field strength of 200 kV/cm, both being in good agreement with Monte Carlo simulation, but the computation time is reduced by two orders of magnitude. Since the momentum and energy relaxation times are functions of these scattering rates, this method will be very useful in device simulations for nonparabolic semiconductors.
Spectroscopic study of self-organized quantum dot like structures in Ga–In–P superlattices on (311) GaAs
Authors: Ghosh Sandip , Arora B. M. , Kim Seong-Jin , Noh Joo-Hyong , Asahi Hajime
Published: July 2002

We report temperature dependent photoluminescence, contactless electroreflectance and photoluminescence excitation study of (GaP)2(InP)2.5 strained short period superlattices sandwiched between GaxIn1−xP alloy layers grown on GaAs (311)A substrates. Transmission electron microscope pictures of these samples reveal the presence of self-organized In rich globular structures with Ga rich surroundings in the superlattice planes. The variation of the peak position of the photoluminescence band with decreasing temperature has an anomalous dip. We show that this is not due to an anomalous change in the band gap with temperature but is due to the interplay between two luminescence pathways associated with two phases, one which has the original (GaP)2(InP)2.5 superlattice and the other being the self-organized composition modulated In rich regions within the superlattice layers. We also present spectroscopic results which indicate quantum dot like nature of the self-organized In rich structures in these samples.
Characterization of transport and magnetic properties in thin film La0.67(CaxSr1−x)0.33MnO3 mixtures
Authors: Broussard P. R. , Qadri S. B. , Browning V. M. , Cestone V. C.
Published: July 2002

We have grown thin films of (100)-oriented La0.67(CaxSr1−x)0.33MnO3 on (100) NdGaO3 substrates by off-axis sputtering. We have looked at the changes in the resistivity and magnetoresistance of the samples as the Ca/Sr ratio was varied. We find that as the calcium fraction is decreased, the lattice match to the substrate decreases, and the films become more disordered, as observed in transport measurements and the variation in Curie and peak resistance temperatures. We find a correlation between the temperature independent and T2 terms to the low temperature resistivity. The room temperature magnetoresistance exhibits a maximum as the peak temperature is increased by the substitution of Sr for Ca, and a change in the field dependence to the resistivity at room temperature is observed.
Strain-induced surface morphology of slightly mismatched InxGa1−xAs films grown on vicinal (100) InP substrates
Authors: Dumont H. , Auvray L. , Dazord J. , Souliere V. , Monteil Y. , Bouix J.
Published: July 2002

A surface investigation by atomic force microscope of InP and slightly lattice-mismatched InGaAs epilayers grown by metalorganic vapor phase epitaxy shows the effect of strain on InGaAs/InP surface morphology. Epilayers were grown at 600 °C with arsine at 760 Torr. We could clearly observe the regular step/terrace-like feature of the vicinal surface morphology for InP and InGaAs lattice-matched epilayers. It is shown that the step flow mode appears with a step edge with regular spacing for lattice-matched epilayers. A crosshatch pattern, i.e., an array of perpendicular lines visible at the surface, were obtained for strain-relaxed InGaAs epilayers (|(Δa/a)⊥|>10−3) with an average separation of 1000–1500 nm. The detailed surface structure of the step edge presents a specific bending of terraces. The influence of step kinetics on the morphological stability of the surface will be illustrated. The relationship between the misfit dislocation network and anisotropic growth rate of the front step will be discussed.
Electroabsorption in extremely shallow quantum wells: Comparison between theory and experiment
Authors: Chen X. , Earnshaw M. P. , Goossen K. W. , Batty W. , Allsopp D. W. E. , Grey R.
Published: July 2002

It is shown, via detailed comparison between measured electroabsorption spectra and electroabsorption spectra calculated by a full excitonic Green’s function method, a decoupled excitonic Green’s function method and a variational method, that accurate description of the Coulombic coupling between different subband pairs, including unbound quantum well states above the quantum well edge, is crucial for a correct prediction of electroabsorption in extremely shallow quantum well structures.
Raman studies on ferroelectric phase (phase III) of KNO3
Authors: Murugan Ramaswamy , Ghule Anil , Chang Hua
Published: July 2002

In order to understand the nature, stability, and existence temperature width of phase I, phase II, and phase III (ferroelectric phase) of KNO3, a detailed Raman investigation was carried out with various heating/cooling rates, repeated thermal cycles, and different preheating temperatures. The increase in the existence temperature width of the phase III of KNO3 was observed with the increase of heating/cooling rate but the repeated thermal cycling reduced it. The phase III to phase II transformation extended to a lower temperature in the cooling process when the sample was subjected to a higher preheating temperature. The phase III could be retained down to room temperature with higher cooling rate and higher preheating temperature. This could be important in its applications. The coexistence of phase III with phase II was observed with all the heating/cooling rates. In this work, special attention was also paid to study the behavior of the lattice modes of KNO3 during the successive phase transformations.
Finite-cell-gap corrections to the capacitance analysis of liquid crystal displays with periodic interdigital electrodes
Authors: Chen C.-J.
Published: July 2002

Corrections to the capacitance analysis of liquid crystal displays with periodic interdigital electrodes are presented, accounting for the finite thickness of the liquid crystal (LC) cell gap. Conformal mapping calculations of the corrected equivalent dielectric constant for the LC material and glass substrate are carried out in terms of the aperture ratio, cell gap and LC effective dielectric constant. Departure from the infinite-cell-gap approximation occurs as the gap thickness approaches the electrode width, or the aperture ratio approaches unity. The capacitance–voltage characteristic curve obtained from the conformal mapping calculations is very close to that obtained from the two-dimensional simulations with less than 5% deviation.
Electric resistance of magnetic domain wall in NiFe wires with CoSm pinning pads
Authors: Nagahama T. , Mibu K. , Shinjo T.
Published: July 2002

The contribution of a magnetic domain wall to electric resistivity was measured using NiFe wires (width: 1 μm) partially covered with hard magnetic pads (CoSm). When the wire is covered with N pinning pads, 2N domain walls can be produced in the wire by reversing the magnetization only at the uncovered parts. The resistance for the magnetically saturated state (no domain wall structure) and that for the magnetic structure with 2N domain walls were compared at zero applied field. It was found that the resistance is smaller when magnetic domain walls exist, and that the domain wall resistance is almost temperature independent.
Probing microstructure of magnetic dispersions via transverse susceptibility: Experiment versus simulation
Authors: Benakli M. , Visscher P. B. , Chae B. S. , Lane A. M.
Published: July 2002

Recently transverse susceptibility has been measured to test the quality of dispersions of magnetic particles. In this measurement, a dc magnetic field is applied to orient the particles. To monitor their microstructure, we record the transverse susceptibility (the response to a small ac magnetic field transverse to the dc field). To help interpret the susceptibility measurements, we have done extensive simulations with a realistic computer model that includes steric, magnetostatic (via a fast multipole method), hydrodynamic, and Brownian forces and torques on a system of 100 particles. We find that although a conventional noninteracting particle picture is quite useful for describing the high-field behavior of the system, at low magnetic field the structure and dynamics are complex and require a full-scale simulation in order to understand the experimental results.
Correlation of surface roughness with the recording characteristics of thin metal-particle tape
Authors: Roesler Alex , Zhu Jian-Gang
Published: July 2002

The presence of surface roughness in tape media results in a head-to-medium spacing that fluctuates throughout the recording process, which can have a large impact on the recording performance of the tape. To provide an understanding of the fluctuations introduced by the roughness, a self-consistent recording model combined with a micromagnetic MR readback module was used to study the recording (write) process on thin-layer metal-particle media with spatially correlated surface roughness. Correlation was made between the surface roughness and the recording characteristics through a spatial analysis of the recordings. Media with both long and short roughness correlation were investigated. Discussions on recording schemes that offer better immunity to the fluctuations introduced by the surface roughness are provided, and in particular the impact on write equalization is evaluated.
Dependence of switching coefficient on damping constant of thin-film magnetic recording media
Authors: Igarashi Masukazu , Yoshida Kazuetsu , Sugita Yutaka
Published: July 2002

We have investigated the relationship between the switching time of thin film media and the Gilbert damping constant α through numerical calculations. The switching coefficient, Sw (the required energy for switching) of thin-film media was estimated from the time integration of the external field over switching field, Hcro(∫[H(t)−Hcro]dt). With decreasing α, Sw of a two-dimensional random thin-film medium with an uniaxial anisotropy field Hk of 6.5 kOe, reached a minimum (fastest) value of 50 ns Oe at α of about 0.1 and then increases slightly to 88 ns Oe at α=0.03. This value was nearly equal to the Sw value of 101 ns Oe at α=1.0. The value of α (about 0.1) that gave a minimum Sw did not change significantly with Hk or saturation flux density Bs. We showed analytically that the value of α for the fastest switching was less than 1.0, when an effective field was applied in a direction different from the easy axis of a magnetic grain. The switching time was in proportion to the rate of Sw/Hk of a thin-film media. These results show that high-speed recording at frequencies of up to 1 GHz can be achieved at α=0.03 in terms of the switching time of longitudinal thin-film media.
Collimated deposition of hard bias magnetic films via long-throw techniques
Authors: Rook K. , Gupta S. , Feng Y. , Hieronymi R. , Wagner I. , Sanders S. C. , Watson M. L.
Published: July 2002

We present an optimization of long-throw sputter-deposited Cr/CoCrPt films, specifically for (lifted-off) hard bias applications in an AMR/GMR head. Deposition was carried out in the target-to-substrate (T/S) range of 7–9 in., with pressure down to 0.25 mTorr. On increasing the T/S by 1 in., a 5% reduction in coercivity was observed, and shown to be only partially explained by the deposition rate reduction. On the other hand, lower gas pressure during the CoCrPt deposition significantly increased the coercivity. Higher target power and higher substrate bias also increased the coercivity. All the observed T/S, pressure, target power, and bias dependencies suggest enhanced coercivity is associated with higher CoCrPt adatom mobility. X-ray diffraction data showed the relative intensity of the (0002) to the (1010) peak to be decreased both with higher CoCrPt deposition power or substrate bias; more in-plane c-axis texture could explain the increased coercivity. A sputter etch of the wafer before Cr deposition increased the coercivity by about 300 Oe. A combination of these techniques may be used to compensate for the deterioration in magnetic properties due to the long-throw. Optimal properties of 1790 Oe coercivity and 3.5 memu/cm2 Mrt were achieved.
Studying the reversal mode of the magnetization vector versus applied field angle using generalized magneto-optical ellipsometry
Authors: Pufall M. R. , Berger A.
Published: July 2002

We used the technique of vector generalized magneto-optical ellipsometry to study the behavior of the magnetization vector of a 50 nm Co thin film as a function of external field magnitude and direction. We determined the relative contributions of magnetization rotation and domain formation to the reversal of M. The Co sample had a uniaxial in-plane anisotropy. When the angle between the applied field and the easy axis was greater than ∼40°, the reversal occurred primarily by rotation of the magnetization, accompanied by a small reduction of the magnitude of M. In this angular region, the field at which there is a large jump in the angle of M as a function of applied field angle followed a single domain coherent rotation model. However, at applied field angles less than 40° to the easy axis, a larger reduction in |M| occurred during the jump in the magnetization angle. The jump also occurred at fields much lower than those predicted by the coherent rotation model, indicating a reversal mode initiated by domain formation.
Dynamic measurement of the thermal stability of the magnetic anisotropy in FeTaN films
Authors: Byeon S. C. , Rantschler J. , Alexander C. , Doyle W. D. , Barnard J. A. , Minor K.
Published: July 2002

In earlier publications it has been shown that the value and direction of the magnetic anisotropy in FeTaN films can be changed by thermal annealing in a dc magnetic field. These films have potential as future write head materials, and this type of instability could pose a problem. In the current work dc and ac annealing experiments have been performed to test the stability of the anisotropy. Our results confirm the model of interstitial-nitrogen-induced anisotropy in the FeTaN films by illustrating that the magnitude of the anisotropy is reduced at higher temperatures, and that the direction of the anisotropy is stable if the time in the switched direction is less than the time required for diffusion of the nitrogen to a different interstitial position. The direction is stable at 100 °C, for square wave magnetic field switching between the easy and hard axes, at frequencies above 500 Hz. The direction is unstable at 100 °C for switching between the hard axes at frequencies less than 5 kHz.
Synthesis and reactivity of Fe nanoparticles embedded in a semi-insulating matrix
Authors: McIlroy D. N. , Zhang D. , Norton M. Grant , O’Brien W. L. , Schwickert M. M. , Harp G. R.
Published: July 2002

The technique of plasma enhanced chemical vapor deposition has been used to synthesize Fe nanoparticles in an amorphous boron carbide matrix. The nanoparticles range in size from approximately 0.7 to 4.5 nm. It has been demonstrated that the size of the nanoparticles is proportional to the density of the Fe precursor (ferrocene) in the vapor. The matrix films have been found to be extremely hygroscopic. Upon absorption of moisture the films buckle and delaminate from the Si substrate. The buckling is a consequence of induced strain arising from the transformation of the Fe nanoparticles into α-Fe2O3. The formation of α-Fe2O3 has been verified by electron diffraction and near edge x-ray absorption fine structure measurements. Examination of the magnetic properties of the α-Fe2O3 using the technique of magnetic circular dichroism indicate that the nanoparticles are not ferromagnetic.
Electrical and optical properties of Si-doped InP grown by solid source molecular beam epitaxy using a valved phosphorus cracker cell
Authors: Zheng H. Q. , Radahakrishnan K. , Yoon S. F. , Ng G. I.
Published: July 2002

We report on the electrical and optical properties of silicon (Si)-doped InP layers grown by solid-source molecular beam epitaxy using a valved phosphorus cracker cell. Within the range of Si effusion cell temperatures investigated (900–1200 °C), the highest electron concentration obtained was 1.1×1020 cm−3. A saturation phenomenon was observed for the electron concentration at higher Si cell temperatures. 300 and 77 K Hall mobility data were used to determine the compensation ratios by comparing them with the theoretical data. Although the Hall data show that the compensation ratio increases with the increase in carrier concentration, the exact values are not certain because the theoretical calculation overestimates the mobility values at higher carrier concentrations. The saturation phenomenon of electron concentration in InP may be considered due to the Si atoms occupying both the In and P lattice sites, or Si donors located at the interstitial sites. The 300 K Hall mobility and the concentration data measured were found to fit the Hilsum expression well. The mobility values obtained in this study are better than or comparable to reported data in the past, indicating good material quality. 5 K photoluminescence (PL) measurements showed two peaks for the undoped and low doped InP layers corresponding to the neutral donor-bound exciton transitions (D0–X) and the acceptor-related transitions (D–A), respectively. When the doping level was increased, the near-band edge (D0–X) recombination peak becomes broadened and asymmetric due to changes in the donor level density of states and relaxation of the wave vector conservation rule. The full-width at half-maximum (FWHM) value of the PL peak position increased when the doping concentration was increased. An empirical equation was developed to fit this variation, which provides a convenient way of determining the dopant concentration from the experimental FWHM value. The near-band edge peak positions shifted to higher energy with the increase of doping level due to the band filling effect. This shift agreed well with the calculations based on the Burstein–Moss shift and the band gap narrowing effect considering a nonparabolic conduction band.
Basis set and electron correlation effects on the electron affinities of first row atoms
Authors: Raghavachari Krishnan
Published: July 2002

The electron affinities of boron, carbon, oxygen, and fluorine atoms have been evaluated by Mo/ller–Plesset perturbation calculations through complete fourth order using several large basis sets. The convergence of the perturbation series has been evaluated carefully by means of modified coupled cluster methods. Large basis sets and electron correlation effects involving higher substitutions are found to be important in the accurate calculation of electron affinities. Triple substitutions are particularly important in the correlation treatment, contributing more than 0.2 eV to the electron affinities of O and F. The electron affinities of all the systems are calculated to be within 0.1 eV of the corresponding experimental values.
Nonperturbative theory of temperature-dependent optical dephasing in crystals. III. Comparison with experiment
Authors: Hsu D. , Skinner J. L.
Published: July 2002

We have applied the results of the nonperturbative theory of zero-phonon linewidths of impurities in crystals discussed by Osad’ko and us, and further developed by us in papers I and II of this series, to the analysis of several absorption, photon echo, and hole burning experiments. Two (relatively) high temperature absorption experiments on 1,3-diazaazulene in naphthalene and dilute ruby were analyzed with a model of Debye acoustic phonons. In both cases the Debye temperature was obtained from independent experiment or theory, and a one-parameter fit was performed on the temperature-dependent linewidth. It was found that (especially for diazaazulene) the systems are not in the weak coupling limit. For several low temperature experiments, where the dephasing is presumably due to pseudolocal phonons, the nonperturbative theory, coupled with the results of deBree and Wiersma, provides a reasonably complete understanding of the observed dephasing rates.
Pure dephasing and nonradiative decay processes in the excited electronic states of NaF:Cu+
Authors: Pack Dee William , McClure Donald S.
Published: July 2002

We report the first optical dephasing study of an inorganic impurity system possessing sharp, low frequency mode structure in its ground and excited state spectra. The total dephasing times T2 and population relaxation times T1 for the 1T2g and 1Eg d9s excited states of NaF:Cu+ are determined at a series of temperatures between 1.8 and 296 K. The T2 values are determined by extracting the Lorentzian components from one- and two-photon excitation line shapes. The T1 values caused by nonradiative decay rates are obtained by detecting very low quantum yield emission from the fast-relaxing excited states and applying the formula tnr=Qtr, where t’s are radiative and nonradiative lifetimes and Q is the quantum yield. T1(1T2g)=4.6 ps and T1(1Eg )=2.0 ps at 8 K. Significantly, these are very similar to the T1 values calculated from lowest temperature Lorentzian linewidths by the relationship 1/T2=1/T′2 +1/(2T1). The T1 values stay approximately constant over the temperature range 1.8–45 K, while the linewidths rapidly increase indicating that pure dephasing dominates. Using ground and excited state information on the low frequency modes, we test optical Redfield theory and the nonperturbative harmonic theory for pure dephasing by pseudolocal phonons against data for this system which displays strongly anhamonic progressions. The nonperturbative theory fits the line broadening data to higher temperatures than optical Redfield theory for the least anharmonic excited state potential, 1T2g. Both theories underpredict the broadening with temperature of the extremely anharmonic 1Eg state. A simple anharmonic theory including scattering to overtone levels also fails to predict the observed linewidth temperature behavior, although it is demonstrated these processes should be occurring.
A three-dimensional L2 simulation of the photodetachment spectra of CIHCI− and IHI−
Authors: Gazdy Bela , Bowman Joel M.
Published: July 2002

We report three-dimensional L2 basis-set calculations of eigenvalues and eigenfunctions of CIHCI and IHI for zero total angular momentum. Comparisons are made to previous calculations of resonance energies and the bound state in IHI. These eigenfunctions are used in simulations of the photodetachment spectra of ClHCl−→CIHCI+e− and IHI−→IHI+e−. The spectra are convoluted with Gaussian weight functions as was done in very recent simulations of Schatz, based on coupled-channel scattering calculations, and in the experiments of Neumark and co-workers.
Solution of reference interaction site model for mixtures of short-chain polyatomic molecules
Authors: Wu Rong-Song , Lee Lloyd L. , Harwell Jeffrey H.
Published: July 2002

Mixtures of chain molecules—monomers, dimers, trimers, and tetramers—are studied using the soft interaction site model. The site–site Ornstein–Zernike equations are solved using the Percus–Yevick closure. The site–site potential is of the Lennard-Jones 12-6 type. The method of solution,

based on the efficient algorithm of Labik and employing Newton–Raphson accelerations, is shown to be fast, accurate and stable; it also shows good convergence behavior even with inaccurate initial estimates. New symmetrical properties among the atom–atom pairs are used to simplify the Jacobian matrix of solution. Pure as well as mixture systems are investigated. Comparison with

simulation data of Bañon et al. and Massobrio et al. is made. The structure is qualitatively described by the integral equations. The internal energy is well predicted by the reference interaction site model calculations.
Polarized and depolarized Raman scattering of 35Cl2 with excitation from the visible to the ultraviolet region
Authors: Strempel J. , Kiefer W.
Published: July 2002

We have recorded high-resolution polarized and depolarized Raman spectra of isotopically pure 35Cl2 using the argon ion laser lines 501.7, 496.5, 488.0, 476.5, 457.9, 363.8, 351.1, and 333.6 nm and the krypton ion laser line 413.1 nm. Overtone progressions can be observed for 413.1 nm and UV excitation. The spectral band shapes of the overtones show a systematic variation with excitation frequency. The relative intensities of the overtone bands have been calculated with second order perturbation theory. The spectral feature of the overtones can be explained by resonance Raman scattering via the repulsive 1Π1u state. The 1Π1u potential was determined with a high accuracy. This is possible because the Raman band intensities are very sensitive to the location of the potential curves of the involved excited scattering states. The spectral feature of the fundamental band excited in the UV region can be explained assuming a destructive interference effect between resonant and nonresonant Raman scattering resulting from adjacent highly lying states. This effect can be monitored via the depolarization ratio of the fundamental transitions, the change in relative intensities between the different Q branches of the fundamental bands and comparison of the observed and calculated overall band intensities of the fundamental and overtone bands excited with different laser lines.
Half-collision studies: Action spectroscopy of electronic energy transfer within the Cd⋅CH4 van der Waals complex
Authors: Wallace Ingvar , Breckenridge W. H.
Published: July 2002

The technique of action spectroscopy has been used to characterize the ground state and excited states of the Cd–CH4 van der Waals complex and to study ‘‘half-collision’’ electronic energy transfer processes within the complex. By tuning one laser pulse in the spectral region near an atomic transition of the free Cd atom and monitoring a predissociation product with a second, delayed laser pulse, it has been possible to obtain ‘‘action’’ spectra which provide information about the excited Cd–CH4 potential surfaces from which electronic energy transfer processes occur. Excitation of the Cd⋅CH4 complex to the red of the Cd(5s5p 3P1 ← 5s5s 1S0) transition while monitoring Cd(5s5p 3P0) produces an anharmonic series of blue-shaded bands which have been assigned to the Cd⋅CH4(A 30+ ← X 10+) transition.

The spectra are similar to the A 30+ ← X 10+ laser-induced fluorescence (LIF) spectra of the Cd⋅Ar and Cd⋅Kr complexes, except that there are weaker subbands ∼9 cm−1 to the blue of each main band. Computer simulations of the rotational structure of several of the bands were successful, and spectroscopic parameters for both the X 10+ ground state and the A 30+ upper state are reported. It is postulated that Cd⋅CH4 is a hindered rotor with C3v (facial) geometry in both ground and upper states. Detailed considerations of the possible nuclear-spin isomers of symmetries A, F, and E, using the CH4 hindered-rotor calculations of Endo and Ohshima, lead to an assignment of the main bands as A 30+(E,K′ = 1) ← X 10+(E,K″ = 1) transitions and the subbands as A 30+(2F,K′ = 1) ← X 10+(2F,K″ = 1) transitions. It is suggested that the expected A 30+(F,K′ = 0) ← X 10+(F,K″ = 0) and A 30+(A,K′ = 0) ← X 10+(A,K″ = 0) transitions were not observed because the K′=0 states lack the rotational motion about the C3v axis which can couple the A 30+ (A1) state with the a 30− (A2) state [which correlates with Cd(5s5p 3P0)]. Action spectra of Cd⋅CH4 were also obtained to the red of the Cd(5s5p 1P1 ← 5s5s 1S0) transition when the probe laser was monitoring Cd(5s5p 3P2) products. The severely broadened spectra are consistent with extremely rapid predissociation of the C 1Π1 (C 11) van der Waals state by the repulsive c 3Σ1 (c 31) state, which correlates with Cd(5s5p 3P2) + CH4.
High-resolution isotope selective laser spectroscopy of Ag2 molecules
Authors: Beutel V. , Krämer H.-G. , Bhale G. L. , Kuhn M. , Weyers K. , Demtröder W.
Published: July 2002

Silver dimers formed in a seeded supersonic argon beam are examined with two laser spectroscopic methods. Excitation fluorescence spectra of the A-X system excited with a narrow band cw dye laser yield accurate constants of the X 1Σg+ and the A 1Σu+ state. From isotope selective resonant two-photon ionization spectra of the B-X and the E-X system excited by a pulsed dye laser and monitored with a time-of-flight mass spectrometer, improved vibrational constants and rotational constants of the B 1Πu and the E 1Πu state are derived. Rotational constants of the C and the D state could be determined from the spectral separations between bandhead and band origin in partly rotationally resolved bands. Autoionizing Rydberg states of Ag2 are stepwise excited with two pulsed dye lasers in two resonant steps. From the convergence limits of different Rydberg series converging towards different vibrational levels v+ in the X 2Σg+ ground state of Ag2+ the rotational constants of the ion ground state and the adiabatic ionization potential IP(Ag2+)=61 747±4 cm−1 could be accurately determined.
Rotational Zeeman effect in carbon monoxide as determined by magnetic vibrational circular dichroism
Authors: Wang Baoliang , Keiderling Timothy A.
Published: July 2002

The magnetic vibrational circular dichroism (MVCD) of carbon monoxide has been measured by use of a Fourier transform infrared vibrational circular dichroism spectrometer with an 8 T magnet. At 0.5 cm−1 resolution, the spectra are rotationally resolved, and both the sign and the magnitude of the g-factor for the rotational Zeeman effect in C≡O can be determined using moment analysis to deconvolve the MVCD band shape. A rotational g value of −0.262 was obtained with an estimated error in accuracy of ±0.026 and a precision of ±0.011. Averaging individual gJ determinations for more than 30 transitions in the R- and P-branches showed a much higher precision in the measurement. That this value agrees with the results of earlier, more precise, microwave-based measurements as well as theoretical determinations of gJ confirms that MVCD can be used as an alternate method to determine molecular Zeeman g factors.
Isomerization reaction dynamics and equilibrium at the liquid–vapor interface of water. A molecular-dynamics study
Authors: Benjamin Ilan , Pohorille Andrew
Published: July 2002

The gauche–trans isomerization reaction of 1,2-dichloroethane at the liquid–vapor interface of water is studied using molecular-dynamics computer simulations. The solvent bulk and surface effects on the torsional potential of mean force and on barrier recrossing dynamics are computed. The isomerization reaction involves a large change in the electric dipole moment, and as a result the trans/gauche ratio is considerably affected by the transition from the bulk solvent to the surface. Reactive flux correlation function calculations of the reaction rate reveal that deviation from the transition-state theory due to barrier recrossing is greater at the surface than in the bulk water. This suggests that the system exhibits non–Rice–Ramsperger–Kassel–Marcus behavior due to the weak solvent–solute coupling at the water liquid–vapor interface.
Wavelength dependence of external-field effects on persistent hole-burning spectra: Analytical approximation
Authors: Kador L.
Published: July 2002

External perturbations such as, for instance, hydrostatic pressure in general give rise to a shift and a concomitant broadening of persistent hole-burning spectra. The magnitude of both effects usually varies with the optical frequency in the inhomogeneous absorption band. Within the framework of a stochastic theory of dye-matrix interaction, an extended Gaussian approximation is used to derive general analytical expressions for the variation of both the shift and the broadening parameter. The results are compared with numerical calculations. Furthermore, the obtained expressions are generalized to take correlations between the matrix molecules into account.
Potential energy surfaces for Ar–OH (X 2Π) obtained by fitting to high-resolution spectroscopy
Authors: Dubernet Marie-Lise , Hutson Jeremy M.
Published: July 2002

Empirical potential energy surfaces for Ar interacting with OH (X 2Π) are obtained by fitting to experimental results from microwave and stimulated-emission pumping (SEP) spectra of the Ar–OH complex. The sum VΠ and difference V2 of the potentials for A′ and A″ symmetry are determined. The sum potential is 126 cm−1 deep, which is about 24 cm−1 deeper than the ab initio CEPA potential of Degli Esposti and Werner [J. Chem. Phys. 93, 3351 (1990)]. The equilibrium geometry is near-linear, Ar—H–O, with a barrier to internal rotation of 44 cm−1. The potential satisfactorily reproduces all the parameters obtained from the SEP experiments, but not the parity doubling obtained from the microwave spectrum.
Stress-promoted interface diffusion as a precursor of fracture
Authors: Yakobson B. I.
Published: July 2002

Stress-modulated surface mass transport offers a possible path to the ultimate fracture of brittle solids. A positive feedback between the local geometry, stress concentration, and chemical potential on a rough surface results in the formation of fracture precursor. Starting from general kinetic equations, we propose a reduced two-parameter description of a growing concavity. A suggested interpolation enables us to obtain an ordinary differential equation describing the precursor evolution; it starts from a smooth boundary and under certain conditions generates a cusp singularity. This sharp notch becomes a stress concentrator and facilitates direct brittle fracture. The diffusion-limited prefracture mechanism, though slow, circumvents a crack nucleation barrier and can be a determining factor for the durability of materials.
Control of molecular vibrational excitation and dissociation by chirped intense infrared laser pulses. Rotational effects
Authors: Chelkowski Szczepan , Bandrauk André D.
Published: July 2002

We extend our previous studies on control of dissociation and vibrational excitation of a diatomic molecule using chirped, intense, infrared laser pulses [Phys. Rev. Lett. 65, 2355 (1990)]. The present model includes molecular rotations and a realistic molecular dipole function. The results obtained from numerical integration of the time-dependent Schrödinger equation show a considerable sensitivity of dissociation probabilities to the initial rotational quantum number. Although rotational effects generally decrease the excitation efficiency compared to previous nonrotating molecule results, the dissociation probability induced by chirped pulses is still four to eight orders of magnitudes greater than that for monochromatic pulse dissociation.
Ne++C60 collisions: The dynamics of charge and energy transfer, fragmentation, and endohedral complex formation
Authors: Christian James F. , Wan Zhimin , Anderson Scott L.
Published: July 2002

Interaction of Ne+ with C60 has been studied for collision energies ranging up to 100 eV. The dominant process is charge transfer, with C60+ accounting for 85% to 100% of the total product signal in this energy range. At 25 eV collision energy, C60-2n+ dissociative charge transfer products appear, accounting for ∼13% of the total product signal at high energies. At ∼25 eV, NeC60+, NeC58+, and NeC56+ all appear together, followed by a series of NeC60-2n+ products where n increases with increasing collision energy. These products are believed to be endohedral complexes, Ne@C60-2n+, where neon is physically trapped inside the fullerene cage. The production efficiency of the endocomplexes reaches a maximum of ∼1.2% at 60 eV, and decreases to 0.5% at 100 eV. Modeling the results leads to the following picture of Ne++C60 collision dynamics: Charge transfer occurs in nearly all collisions, and is the only significant process for high impact parameters. At collision energies below ∼25 eV, translational-to-internal energy transfer is inefficient, even for low impact parameter collisions. At higher energies, the inelasticity jumps to near 100%, leading to fragmentation of the nascent products. Both endohedral penetration and efficient energy transfer appear to require bond rupture in the C60.
X-ray scattering in liquid water at pressures of up to 7.7 kbar: Test of a fluctuation model
Authors: Okhulkov A. V. , Demianets Yu. N. , Gorbaty Yu. E.
Published: July 2002

The pair correlation functions g(r) of liquid water obtained at pressures of up to 7.7 kbar at a constant temperature of 20 °C are discussed. A variety of evidence of the very strong effect of pressure on the structure of liquid water has been found. In particular, an unusual pressure dependence of the shortest intermolecular separation was observed. The results are interpreted in terms of fluctuations of the nearest environment, assuming that some of the structural configurations arising may be distinguished as preferred ones. The SIMPLEX procedure has been used to fit g(r) to the sum of correlation functions corresponding to the heavily destroyed structures of the solid phases of water. The results correlate with the phase diagram for water fairly well. This gives rise to an idea that water may be a suitable object for understanding the very nature of the liquid state.