bluuebunny/crossref_metadata_2025 huggingface dataset:
Thin-film wafer fusion for buried-heterostructure InP-based lasers fabricated on a GaAs substrate
Authors: Ohiso Yoshitaka , Amano Chikara
Published: July 2002

We present experimental results on thin-film wafer fusion of InP/GaAs to fabricate InP-based lasers on a GaAs substrate. We have studied the load pressure dependence of the photoluminescence intensity (PL) of the InP-based layers and electrical properties at the fused interface. Although a higher load pressure results in better electrical contact, it can degrade the PL intensity of InP-based quantum-wells structure fused to a GaAs substrate due to the generation of recombination centers. Buried-heterostructure InP-based lasers are consequently fabricated on a GaAs substrate by thin film wafer fusion, and these lasers are demonstrated to have good performance under continuous-wave operation.
Modulated photoconductivity study of electron drift mobility in amorphous silicon
Authors: Hattori K. , Iida M. , Hirao T. , Okamoto H.
Published: July 2002

The modulated photoconductivity technique, a convenient means of measuring the drift mobility of photocarriers, has been applied to investigate carrier transport in hydrogenated amorphous silicon. The frequency resolved spectra of drift mobility that can be obtained from the measurements were analyzed in accordance with a generalized transport model that included possible carrier interactions between localized states through tunneling transitions. Theory suggests that a tunneling-assisted thermalization of nonequilibrium carriers appreciably affects the transport process. The experimental results are reasonably accounted for by the introduced model, leading to quantitative assessments for transport mechanisms.
Pressure sensors based on silicon doped GaAs–AlAs superlattices
Authors: Robert J. L. , Bosc F. , Sicart J. , Mosser V. , Lasseur J.
Published: July 2002

We show how GaAs–AlAs short period superlattices, in place of AlGaAs thin layers, improve the performances of n-type III–V semiconductors as pressure sensing material. Pressure induced electron capture on relaxed silicon donor sites (so called DX center) is responsible for the high pressure coefficient of resistance (30% per kbar). In comparison to AlGaAs, band gap engineering is employed to optimize both pressure and temperature sensitivities of GaAs–AlAs pseudoalloys between 0 and 200 °C under pressures up to 2000 bars. An electrical characterization is made by performing resistance and Hall effect measurements as functions of hydrostatic pressure and temperature on two microstructures forming the monolithic transducer. The heterostructures consist of (GaAs)9–(AlAs)4 superlattices doped with silicon at concentrations of 1.4×1017 and 2×1018 cm−3, respectively. Accurate pressure measurements (resolution less than 0.2 bar) are performed on two resistors patterned on these microstructures. Monolithic microsensors can be designed on such a stacked GaAs–AlAs two-resistor microstructure.
Multiquantum well gain modeling using a Green’s function-based fractional dimensional approach
Authors: Vallone M. , Campi D. , Cacciatore C.
Published: July 2002

In this article we investigate semiconductor optical amplifiers (SOAs), based on strained InGaAs quantum wells. Their optical properties are computed and discussed using a new, analytical model of the gain, based on a Green’s function formalism developed in a space with a noninteger effective dimensionality, comprised between 2 and 3. Starting from the Bethe Salpeter Equation, a closed-form expression for the interband density of states in presence of carrier plasma of arbitrary density is obtained. The influence of the nonperfect two-dimensionality of quantum wells is shown to have a great influence on gain and ASE spectral shapes, making it necessary to account for it in most cases of practical interest. Experimental optical properties of SOAs are reported and modeled in the described formalism.
Optical properties of InGaAs/InAIAs diffused double quantum wells
Authors: Choy Wallace C. H.
Published: July 2002

The effects of interdiffusion on the subbands and optical properties of InGaAs/InAIAs double quantum well (QW) have been theoretically studied. The results show that since a double QW (DQW) can be diffused to become an effectively single QW structure, the characteristic features in the subbands structure and optical properties of a strongly coupled DQW structure and that of a single QW structure can be obtained by a suitable annealing time. Moreover, the increase in separation between the first symmetric and antisymmetric heavy hole subbands and the increase in the spin splitting of the valence subbands of the diffused DQW, due to an applied electric field, diminish when annealing time increases. In optimizing the In0.53(AlaGa1−a)0.47As/In0.52Al0.48As DQW structure, the results show that symmetric DQW with no Al content in wells can provide large material gain and radiative spontaneous recombination rate. With interdiffusion, the material gain and recombination rate reduce but the reduction saturates when the DQW structure is diffused to effectively become a single graded QW. By subjecting the DQW and an as-grown single QW to the same annealing conditions (where the summation of the width of the two wells and the separation barrier of the DQW equals the well width of the single QW), the diffused DQW can provide a larger material gain and radiative recombination rate than the diffused single QW when the annealing time is short. Therefore, the short-time diffused DQW is more useful for laser applications. Besides, since Al diffuses into the wells, the transition energy of the QW structure increases so that the operating wavelength of the optical devices can be adjusted.
Anomalous internal friction in lead metaniobate ceramics
Authors: He Lian-Xing , Li Cheng-En , Yan Hai-Xue , Chen Ting-Guo
Published: July 2002

Anelastic behavior of lead metaniobate ceramics has been studied by measurements of internal friction Q−1 and oscillation frequency f as a function of temperature. Two kinds of internal friction peaks are observed. One kind of peak, designated as P1, P2, and P4, is clearly related to the working process. These peaks are considered to be caused by the interaction between dislocations and point defects. The other kind of peak, designated as P3, is a broadened Debye relaxation peak in nature. The corresponding activation energy and pre-exponential factor are 1.01 eV and 2.5×10−12 s, respectively. This peak is attributed to the interaction between domain walls and oxygen vacancies.
The effect of Ta on the magnetic thickness of permalloy (Ni81Fe19) films
Authors: Kowalewski M. , Butler W. H. , Moghadam N. , Stocks G. M. , Schulthess T. C. , Song K. J. , Thompson J. R. , Arrott A. S. , Zhu T. , Drewes J. , Katti R. R. , McClure M. T. , Escorcia O.
Published: July 2002

The effect of Ta and Ta/Cu seed layers, and Ta and Cu cap layers on the effective magnetic thickness of ultrathin permalloy (Ni81Fe19) was investigated for MRAM applications. The films were deposited by Ion Beam Deposition. The magnetic moment of each as-deposited permalloy film was measured using a B-H looper and a SQUID magnetometer. The films were further annealed at either 525 K for 1/2 h or 600 K for 1 h to study the effect of thermally driven interdiffusion on the magnetic moment of the permalloy film. Our theoretical calculations showed that the presence of 12% intermixing at the interface reduces the Ni moments to zero. Experimentally, it was shown that the tantalum rather than the copper interfaces are primarily responsible for the magnetically dead layers. The Ta seed layer interface produces a loss of moment equivalent to a magnetically dead layer of thickness 0.6±0.2 nm. The Ta metal in the cap layer results in a loss of moment equivalent to a dead layer of thickness 1.0±0.2 nm. Upon annealing, thermally driven interdiffusion is concluded to have a strong effect on the Ta(seed)/ Ni81Fe19 as-deposited interface, based on the doubling of the magnetically dead layer to 1.2±0.2 nm. The Ni81Fe19/Ta(cap) as-deposited interface slightly increases its equivalent magnetically dead layer upon annealing to 1.2±0.2 nm. As-deposited interfaces of Ta(seed)/permalloy and permalloy/Ta(cap) are not chemically equivalent and result in different magnetically dead layers, whereas after annealing to 600 K both interfaces attain comparable intermixing and magnetically dead layers. It was also shown that a half-hour anneal at the lower 525 K annealing temperature, which is closer to the actual processing temperature, results in only slight increase of the magnetically dead layer at both interfaces.
Effects of misfit strain due to epitaxial growth on interface perpendicular magnetic anisotropy in ultrathin Co/Au/Cu(111) films
Authors: Murayama Akihiro , Hyomi Kyoko , Eickmann James , Falco Charles M.
Published: July 2002

We observe a systematic increase in interface perpendicular magnetic anisotropy (PMA) with increasing Au-interlayer thickness tAu ranging from 1 to 5 monolayers (MLs) in Cu/Co/Au/Cu(111), where misfit strain in Co due to the epitaxial growth increases with increasing tAu. This result can be understood within the framework of the magneto-elastic contribution to the interface PMA. With a Cu overlayer, a constant contribution of interface PMA as well as of volume anisotropy is observed when the Co thicknesses are greater than 1.3 ML. However, we have found an unexpected suppression of this variation of interface PMA with the use of Au overlayers instead of Cu ones. With Au or Pd overlayers, the PMA starts to degrade as the Co thickness is reduced below 2.5 ML.
High coercivity CoCrPt films achieved by post-deposition rapid thermal annealing
Authors: Zou Jie , Lu Bin , Leonhardt Todd , Laughlin David E. , Lambeth David N.
Published: July 2002

Post-deposition rapid thermal annealing (RTA) was performed on multilayered films consisting of a Mn capping layer on top of CoCrPt/CrTi/NiAl. High coercivities were achieved even at low Mrt values and coercivity values increased more than that would have been predicted by the decrease in magnetization values. This is most likely due to the significant decrease in intergranular exchange coupling, as shown by the δM measurement. Transmission electron microscopy characterization indicates no apparent grain growth and no visible improvement in CoCrPt crystallographic texture after RTA. It is believed that the preferential diffusion of Mn into CoCrPt grain boundaries promotes magnetic grain isolation.
Effect of chromium addition on the properties of sputtered strontium ferrite films
Authors: Morisako Akimitsu , Saitoh Makoto , Matsumoto Mitsunori
Published: July 2002

Hexagonal strontium ferrite (SrM) thin films with various Cr composition in the range of 0–36 at. % were sputtered on the thermally oxidized silicon wafers at the substrate temperature of 550 °C using targets of composition of SrFe8Ox. It was found that the SrM grows with its c axis oriented perpendicularly to the film plane and its c-axis orientation was improved by Cr addition in the range of 9–36 at. %. The saturation magnetization and coercivity in the perpendicular direction were about 150 emu/cm3 and 2.8 kOe, respectively for 9 at. % of Cr addition. The grain size observed at the surface was 80 nm and the surface roughness was 11 nm.
A generalized lumped element modeling of magnetostatic wave resonators
Authors: Bartolucci Giancarlo , Marcelli Romolo
Published: July 2002

A lumped element modeling of a typical distributed electromagnetic structure, namely a band-stop magnetostatic wave resonator, is presented here and validated by experimental results. The definition of a turns ratio and data on the quality factor of the resonator versus frequency allow a wideband approach to the prediction of its performances between 1 and 12 GHz.
Micromagnetic study of CoPt–SiO2 granular films by spin-wave Brillouin scattering
Authors: Kikitsu A. , Murayama A. , Hyomi K. , Falco C. M.
Published: July 2002

A spin-wave Brillouin scattering study of a CoPt–SiO2 granular magnetic recording medium was made. This film contains ferromagnetic CoPt particles in a SiO2 matrix, and has an extremely low medium noise property due to little exchange coupling between magnetic grains. Spin waves of both the propagating surface mode and standing wave mode were found to be excited in granular magnetic films with various microstructures. A possible origin of the spin wave is a magnetostatic coupling between regularly ordered CoPt grains, as reported for artificially patterned magnetic thin films. This result shows two promising features of the CoPt–SiO2 granular film for high density recording medium: It is an ordered media obtained in a self-organizing manner, and it is less influenced by the thermal fluctuation effect, which is a serious problem for current high density magnetic recording.
Magnetic properties of cluster-beam-synthesized cobalt: Noble-metal films
Authors: Meldrim J. M. , Qiang Y. , Liu Y. , Haberland H. , Sellmyer D. J.
Published: July 2002

A cluster-beam deposition technique has been used to produce magnetic clusters embedded in a nonmagnetic matrix. We report here on films with cobalt clusters of average diameter of 5.5 nm embedded both in Cu and Ag. Volumetric concentrations of Co ranged from 10% to 50%. Magnetization and low temperature hysteresis loops, both field cooled (FC) and zero-field cooled (ZFC), have been measured between 4.2 and 300 K. The FC and ZFC magnetization bifurcate at or above room temperature with the clusters having a nonzero remanence at room temperature. Low temperature hysteresis loops exhibit a two-phase nature with one phase displaying exchange bias upon field cooling, suggesting the presence of an oxide phase. Conditions under which the oxide is present have been studied.
Noise and linearity of a fluxgate magnetometer in racetrack geometry
Authors: Hinnrichs C. , Pels C. , Schilling M.
Published: July 2002

Magnetic fluxgates provide very good noise properties together with a large dynamic range in small, solid-state sensors used in many applications. We investigate racetrack shaped fluxgate sensors with a core of (CoFe)70(MoSiB)30 amorphous metal. We investigate the influence of the excitation current in the drive coil of our fluxgates on the sensor noise properties. The output signal is obtained by a modulation technique with demodulation from the second harmonics of the excitation. To achieve a large dynamic range with good linearity the fluxgates are operated in a feedback loop. The linearity is characterized by measurements of the total harmonic distortion in dependence on the signal amplitude. The noise properties are investigated in shielded and unshielded environment. In both cases the obtained noise level is SB(2 kHz)=960 fT/Hz. We compare our noise properties to recent data on superconducting quantum interference devices from high temperature superconductors operated in unshielded environment.
Characterization of amorphous FeZrB(Cu) alloys by the inductance spectroscopy method
Authors: Garcia Tello P. , Gonzalez J. , Blanco J. M. , Valenzuela R.
Published: July 2002

Impedance spectroscopy measurements were carried out on amorphous ribbons of nominal compositions Fe93−x−yZr7BxCuy (x=6,8 and y=0,1,2), in the frequency range of 5 Hz–13 MHz. Two different geometries were used for measurements: a longitudinal one by using an 80-turn solenoid and the magnetoimpedance geometry, where the ac current flows through the ribbon itself. The obtained results showed a larger initial permeability value for longitudinal domain walls, as compared with transverse ones. Results concerning the propagation field showed the opposite behavior. These results are interpreted in terms of the short-range order arrangements of Fe atoms in these alloys, also known as “nanoamorphous demixing structures.”
Striations in a coplanar ac-plasma display panel
Authors: Cho Guangsup , Choi Eun-Ha , Kim Young-Guon , Kim Dae-Il , Uhm Han S. , Joo Young-Dae , Han Jung-Gwan , Kim Min-Chul , Kim June-Dong
Published: July 2002

Striations observed in a coplanar or a wedge-shaped planar ac-type discharges are reported in this article. The observed data indicate that the striations are related to ion charge waves of the wall generated by the self-sustained perturbations that originate from the force balance between the ion and the electron wall-charge clouds accumulated on the dielectric layers over the metal electrodes.
Surface anisotropy in ferromagnetic prolate spheroids
Authors: Aharoni Amikam
Published: July 2002

The eigenvalue for nucleation by the curling mode, in the presence of surface anisotropy, is approximated by a rational function of both the radius and the aspect ratio of a general prolate spheroid. This approximation makes it easy to take the surface anisotropy into account without getting into cumbersome and complicated computations, and should facilitate the check of computational micromagnetics results against the analytic constraints. The approximation is rather crude, but it should be useful for cases in which the computations need to be repeated many time, as is the case of finding the best fit of the anisotropy constant to a set of experimental data.
Exchange bias relaxation in CoO-biased films
Authors: McMichael R. D. , Lee C. G. , Stiles M. D. , Serpa F. G. , Chen P. J. , Egelhoff W. F.
Published: July 2002

Because the memory of the bias direction is carried by the antiferromagnetic order in exchange biased films, the stability of the antiferromagnetic order is critical to the existence of the exchange bias field. Ferromagnetic resonance was used to measure the relaxation behavior of polycrystalline CoO films coupled to films of Ni80Fe20, probing the system on the time scale of the experiment, ≈103 s, and the time scale of the magnetic precession, ≈10−10 s. Unidirectional anisotropy (exchange biasing) and isotropic resonance field shifts are observed at the lowest temperatures. Above the apparent exchange bias blocking temperature, isotropic resonance field shifts persist. At still higher temperatures, diminishing resonance field shifts are accompanied by peaks in the FMR linewidth. The results highlight the effects of varying relaxation rates in the CoO relative to the two experimental time scales.
The construction of orientation-dependent crystal growth and etch rate functions II: Application to wet chemical etching of silicon in potassium hydroxide
Authors: van Veenendaal E. , van Suchtelen J. , van Enckevort W. J. P. , Sato K. , Nijdam A. J. , Gardeniers J. G. E. , Elwenspoek M.
Published: July 2002

In Part I we introduced a construction method for analytical orientation dependent growth and etch rate functions. In this article, this network construction principle is applied to wet chemical etching of silicon in concentrated aqueous potassium hydroxide. Detailed measurements of the etch rate as a function of crystal surface orientation are used to fit the phenomenological parameters in the network etch rate function. In this function, for each crystal facet, two surface processes are accounted for, etching through misorientation step flow and etching through nucleation of pits. The fitting procedure identifies additional mesoscopic, surface processes which influence the orientation dependence of the etch rate. These processes correspond to instabilities of the surface. In the {111} region step bunching occurs which evolves into microfaceting for larger inclination angles. Moreover, for certain experimental circumstances, the fast etching {110} region breaks up into a staircase structure of terraces. Additional network elements are defined to account for these instabilities. The step-bunching instability is treated using an ad hoc approach. With these amendments the experimental etch rate functions can be fitted to an accuracy of about 5% by a network function with nine parameters. This shows that it is possible to reproduce the essential features of an experimental growth or etch rate function using an analytical function with a limited number of physically meaningful parameters.
Dynamics of concentration fluctuations for butylcellosolve in water
Authors: Hamano K. , Kawazura T. , Koyama T. , Kuwahara N.
Published: July 2002

The coexistence curve, the shear viscosity, and the angular and the spectral distribution of scattered light have been measured for butylcellosolve in water near its lower critical mixing point. We have examined the validity of the pseudospinodal concept for the interpretation of the diffusion coefficient near the critical point. The critical exponents obtained with the assumption of the pseudospinodal agree with the recent experimental and the theoretical values. The universal dynamic amplitude ratio R=1.03±0.06 obtained in the present work is in disagreement with the value R=1.2 calculated from the renormalization-group theory but in good agreement with R=1.03 of the prediction by Burstyn et al. The diffusion coefficient as a function of temperature and concentration is well represented in terms of a scaled function.
Homogeneous nucleation rate measurements. I. Ethanol, n-propanol, and i-propanol
Authors: Kacker Atul , Heist Richard H.
Published: July 2002

Homogeneous nucleation rate measurements have been made on ethanol, n-propanol, and i-propanol vapors over a range of temperatures and supersaturations. The Becker–Doring–Zeldovitch nucleation rate expression has been scaled so as to give good agreement with the measured nucleation rates. The same scaling factors are also used to compare calculated critical supersaturations with experimental data obtained from this investigation and a variety of other sources. The agreement is good in all cases. The nucleation rate measurements and the critical supersaturation data from this investigation reveal anomalous behavior for ethanol near 260 K and for n-propanol near 275 K.
The kinetic isotope effect in the reaction of O(3P) with H2, D2, and HD
Authors: Presser Nathan , Gordon Robert J.
Published: July 2002

The reaction of O(3P)+H2, D2, and HD was studied by the flash photolysis-resonance fluorescence method. The rate constants kXY(T) were measured over the temperature ranges 297–471 K for XY=H2, 422–472 K for D2, and 422–473 K for HD. Above 400 K kH2 are in excellent agreement with earlier flow measurements, but below this temperature most of the flow data are systematically lower. A small deviation from the Arrhenius function was observed for kH2 at 297 K. Our measurements of kD2 are in excellent agreement with the results of Westenberg and deHaas and differ from those Clyne and Thrush. The measured values of kHD equal the arithmetic mean of kH2 and kD2 within experimental error. A detailed comparison is made between the present results and the CEQB calculations of Bowman, Wagner, Walch, and Dunning and the ICVT/LAG calculations of Garrett and Truhlar. Good overall agreement is obtained with both theories.
Rotational tunneling linewidth studies by NMR polarization transfer experiment: Estimates of the interaction among tunneling CH3 groups in Ge(CH3)4
Authors: Sridharan K. R. , Sobol W. T. , Pintar M. M.
Published: July 2002

The results of the proton relaxation time dispersion measurements on Ge(CH3)4 at temperatures below 26 K show that the maximum of the T−1ZT dispersion curve decreased and the linewidth increases as the temperature is decreased. This cannot be accounted for by any version of the correlation function perturbation approach which at low temperatures, on tunneling resonance nω0=ωt gives a relaxation rate proportional to the correlation time. However if the tunneling linewidth is due to the torsion–torsion interaction—which is averaged out at higher temperatures—the observed behavior is explained. From the observed low temperature tunneling linewidth the torsion–torsion interaction in this material is estimated to be 30 MHz. The results of multipulse saturation sequence measurments at the maximum of the T−1ZT dispersion curve are presented and discussed in terms of a population-transfer model.
Resonance Raman scattering study of azulene. II. Nonzero temperature multimode model calculations
Authors: Chan C. K. , Page J. B. , Tonks D. L. , Brafman O. , Khodadoost B. , Walker C. T.
Published: July 2002

In a recent paper (Az I), well-structured T=300 K resonance Raman (RR) profiles for the 1400, 1260, 900, and 2×825 cm−1 lines of azulene in CS2 and for the 825 cm−1 line of azulene in methanol were reported. Previously developed transform techniques were used to (1) compute RR profile line shapes directly from measured optical absorption spectra, and (2) extract ratios of Stokes loss parameters from the line shape scale factors. The transform analysis indicated that (1) our model assumptions (adiabatic and Condon approximations, harmonic phonons, atomic equilibrium position shifts, and small vibrational frequency shifts upon excitation to a single electronic state) are basically correct allowing for

minor modifications, and (2) any deviations from these assumptions are likely to be larger for the 900 cm−1 mode and smaller for the 1400 and 1260 cm−1 modes. In this paper (Az II), we report model calculations of the optical absorption spectra, RR profile line shapes, and relative RR intensities. In these calculations, we use a recently proposed nonzero temperature multimode time-correlator modeling procedure. Compared with the conventional sum-over-states method, our time-correlator modeling procedure is superior in that (1) our optical absorption spectra and RR profiles computed via fast Fourier transform techniques have a practically unlimited spectral range and (2) the computing times are short for nonzero temperature multimode calculations. In our basic model, we adopt the assumptions of Az I and use seven azulene modes to obtain simultaneous good fits of the well-structured RR profile line shapes and optical absorption spectra. However, we find that the basic model does not account for the intensity of the 900 cm−1 Raman line relative to that of the 1400 cm−1 line, even though the individual profile line shape fits for these modes are very good. The basic model is therefore modified to allow mixing of the normal coordinates of these two modes. By introducing a single, relatively small mode-mixing parameter, we obtain a good fit of the relative RR intensities in addition to simultaneous detailed fits of the optical absorption spectra and RR profile line shapes. In an alternate approach, we modify our basic model and find that the inclusion of two relatively small non-Condon parameters, instead of one mode mixing parameter, can also produce simultaneous detailed fits of all of our optical absorption and RR data.

A comparison of the two modified models solely on the basis of simplicity favors the mode-mixing model, since only one extra parameter is required to modify our basic model.
Measurements of second- and third-order nonlinear polarizabilities for HF and HCl
Authors: Dudley J. W. , Ward J. F.
Published: July 2002

Measurements of second- and third-order nonlinear polarizabilities (hyperpolarizabilities) for HF and HCl using dc electric-field-induced second-harmonic generation are presented: χ(3)∥(HF)=70(10)×10−39 esu/mol, χ(2)∥ (HF)=−4.70(41)×10−32 esu/mol, χ(3)∥(HCl)= 347(15)×10−39 esu/mol, χ(2)∥(HCl)= −4.22(50)×10−32 esu/mol. In the case of HF this allows a critical comparison with theory. HF has fewer electrons than any polar molecule previously studied experimentally and the small size of HF has made it an attractive candidate for theoretical investigation. Christiansen and McCullough have used numerical Hartree–Fock techniques to establish generally accepted criteria for basis set selection; and Bartlett and Purvis have applied to HF the most elaborate technique applied so far to the calculation of any molecular hyperpolarizability (CHF SDQ-MBPT[4]). Experimental corrections and uncertainties are carefully considered as are several other factors relevant to a comparison of these experimental and theoretical data. The theoretical results are about a factor of 2 smaller than the experimental data and none of the factors considered seems to offer a resolution of this discrepancy.
Solution of the Ornstein–Zernike equation in the vicinity of the critical point of a simple fluid
Authors: Cummings P. T. , Monson P. A.
Published: July 2002

We present a comparison of the results obtained from numerical and analytical solutions of the mean spherical approximation for hard spheres with a Yukawa tail. The emphasis is on the location of the liquid–vapor critical point and the calculation of the associated critical exponents. This comparison allows us to draw some general conclusions concerning the accuracy of numerical solutions of the Ornstein–Zernike equation in the critical region. One important conclusion is that for any closure correctly describing the divergence of the isothermal compressibility along the critical isotherm, extremely accurate numerical work will be required to calculate the value of the exponent delta. We have also compared numerical solutions obtained inside the two phase region with those from the analytical continuation of the analytical solutions into this regime.
On the critical behavior of the Percus–Yevick equation for nontruncated potentials
Authors: Brey J. J. , Santos A.
Published: July 2002

We present a qualitative analysis on the influence of truncating a long-ranged potential on the critical behavior of a fluid described by the Percus–Yevick equation. It is shown that a nonclassical equation of state for truncated potentials can be compatible with a classical one in the long-range limit. Our main assumption is that the dominant part of the difference between both equations of state is a regular function driven by the asymptotic behavior of the direct correlation function. The results are applied to the case of a Lennard-Jones potential. Comparison with available numerical results is quite satisfactory.
The potential of DABCO for two-photon amplification
Authors: Glownia J. H. , Arjavalingam G. , Sorokin P. P.
Published: July 2002

A study is made of the potential of triethylenediamine (DABCO) vapor for amplification of resonantly tuned picosecond pulses by two-photon stimulated emission (TPSE). Essential parameters needed to assess the feasibility of this experiment were determined. These include the absorption spectrum and population dynamics of the excited  Ã state, and the (transient regime) Ã→X̃ two-photon cross section B. It is concluded that B is ∼5× too small to allow a demonstration of TPSE in a multipass ∼300 K DABCO cell, pressurized to ∼2 atm of H2, and end pumped with a ∼1 J KrF laser beam. Much higher gain exists for stimulated anti-Stokes Raman emission, which is predicted to be a practicable generating mechanism of picosecond UV (λ∼2600 Å) pulses, if a picosecond pump source near ∼2.5  μm can be found.
An experimental and theoretical study of the deuterium quadrupole coupling constants of glycine
Authors: Weeding Tina , Kwiram Alvin L. , Rawlings Diane C. , Davidson Ernest R.
Published: July 2002

Single crystals of α-glycine with two, three, and five of the protons replaced by deuterium are studied at 4.2 K using electron–nuclear double resonance (ENDOR) spectroscopy to detect the NMR transitions of the distant deuterons on diamagnetic molecules. Complete deuteron quadrupole interaction tensors, and indirectly the electric field gradient tensors, are determined for all the labeled positions. Proton dipolar coupling tensors are also obtained for the methylene hydrogens in the case of glycine-d3. The relationship between the deuterium quadrupole coupling constants and the inverse cube of the bond length is refined and discussed. The resulting structural information including X–D bond lengths and bond angles is compared with data obtained by neutron diffraction. In addition, extensive calculations at both the semiempirical and ab initio levels are carried out to determine the electric field gradient at the hydrogen positions. Both intramolecular and lattice contributions are considered.
Thermal rate constant for H+CH3 → CH4 recombination. Comparison of quasiclassical trajectory and variational transition state theory
Authors: Hase William L. , Duchovic Ronald J.
Published: July 2002

The variational transition state theory rate constant is calculated for the H+CH3 → CH4 bimolecular reaction and compared with the quasiclassical trajectory value reported previously. The calculations are performed on two different potential energy surfaces, MAPS/CH4-I and -II. To calculate the variational transition state theory rate constant, the reaction path and frequencies orthogonal to the reaction path are required. The general classical trajectory computer program mercury was modified to perform these calculations. Good agreement is found between the variational transition state theory and quasiclassical trajectory rate constants for both the MAPS/CH4-I and -II surfaces. The rate constants for the two surfaces differ by an order of magnitude. These calculated values are compared with a recent experimental measurement.
Electronic properties of polyacetylene doped with FeCl3
Authors: Jones T. E. , Ogden T. R. , McGinnis W. C. , Butler W. F. , Gottfredson D. M.
Published: July 2002

T r a n s and cis (CH)x were chemically doped with FeCl3 using nitromethane solutions as well as by using the vapor pressure of solid FeCl3. Electrical conductivities were measured vs temperature for all samples. Room temperature conductivities of approximately 100 mho/cm were achieved. Thermoelectric power was also measured as a function of temperature. The room temperature thermoelectric power starts at +850 μV/K for the dilutely doped samples and decreases to +12 μV/K for the more heavily doped samples which exhibit metallic thermoelectric power, proportional to the absolute temperature.
Products and yields from O3 photodissociation at 1576 Å
Authors: Taherian M. R. , Slanger T. G.
Published: July 2002

An analysis has been made of the primary atomic and molecular products arising from O3 photodissociation at 1576 Å. The yield of oxygen atoms is 1.90±0.30, of which 71% are O(3P) and 29% are O(1D). Since a primary yield greater than unity can only be a consequence of three-fragment dissociation, these results suggest that fragmentation into three O(3P) atoms, and production of O(1D) plus a singlet oxygen molecule, have comparable yields. Observation of prompt emission in the 7300–8100 Å spectral region indicates that the singlet O2 is O2(b 1∑+g). Vibrational levels in the range v=0–6 have been detected, the distribution corresponding to a vibrational temperature of 104 K. This study is the first to show photodissociative production of the O2(b 1∑+g ) state in any system.
Vibrationally excited populations from IR-multiphoton absorption. I. Absorbed energy and reaction yield measurements
Authors: Zellweger Jean-Michel , Brown Trevor C. , Barker John R.
Published: July 2002

The molecule 1,1,2-trifluroethane (TFE) was used in experiments to determine the population distribution of excited molecules produced by infrared multiphoton absorption induced by high power TEA CO2 lasers operating at 1079.85 cm−1 [9.6 μm R(22) line]. Optoacoustic measurements of absorbed laser power provided a measure of the mean energy of the population distribution, while very low pressure photolysis measurements of the collision-free decomposition yield gave information about the high-energy tail of the distribution. The experimental results were accurately simulated using a Master Equation model that incorporated Quack’s statistical–dynamical theory of infrared multiphoton absorption (cases B and C), RRKM unimolecular reactions (three channels), and collisional energy transfer. The computer simulations included known TFE molecular properties and only four adjustable parameters, which were very highly constrained in order to fit the experimental data. From the simulations, we conclude that the optical coupling matrix elements are dramatically reduced in magnitude for energies above the reaction thresholds. This effect is symptomatic of the vibrational anharmonicity due to the presence of the reaction channels, even in molecules that have not yet reacted, resulting in vibrational frequency shifts of the absorption lines out of resonance with the laser line. This effect is expected to be present and observable in other highly vibrationally excited molecules.
Lattice vibrations of a monolayer of n-hexane adsorbed on graphite
Authors: Danner Horace R. , Kim Hyunyong
Published: July 2002

The incoherent inelastic neutron spectrum of a monolayer of n-hexane adsorbed on graphite has been analyzed. The two-dimensional lattice structure that was used in the analysis of a monolayer of n-butane on graphite is adopted. The intramolecular valence force fields of the n-butane system require only a minor modification of the CCC/CCC bending interaction and HCCH torsion force constants, while the intermolecular pairwise potentials transfer with no modification. The frequency dispersions and hydrogen amplitudes calculated from normal mode analysis are used to construct the neutron scattering cross sections. These are then summed for all normal modes and folded with both the instrumental resolution function and the incident beam profile to form a composite time-of-flight spectrum. The agreement between the calculated and observed spectra is even better than that obtained in our earlier analysis of n-butane. The intermolecular potential constants for the hydrogen–substrate and hydrogen–hydrogen pairs are 0.04 and 0.003 mdyn/A, respectively.
Structure and motion of hydrogen in molybdenum bronzes HxMoO3 as studied by nuclear magnetic resonance
Authors: Ritter Cl. , Müller-Warmuth W. , Schöllhorn R.
Published: July 2002

1H NMR solid state techniques have been used to study bonding properties, location, and mobility of hydrogen in various phases of the hydrogen bronze HxMoO3. Temperature-dependent spectra characteristic of different degrees of intercalation have been observed, and furthermore, from measurements of the relaxation rate, dynamic properties have been derived. There is strong evidence of intralayer hydrogen positions on a quasi-one-dimensional zig–zag line connecting the vertex-sharing oxygen atoms of the MoO6 octahedra and these are first occupied if the degree of intercalation x is low. For x>0.85 the hydrogen in excess starts to fill up interlayer positions coordinated with terminal oxygen atoms at the van der Waals gap. Both isolated and paired protons have been detected in the interlayers, whereas clusters or pairs appear along the zig–zag lines. Hydrogen separations within the clusters, bonding with oxygen and charge transfer to the conduction band of the host lattice, are discussed. Hydrogen diffusion changes from being predominantly one-dimensional to three-dimensional in character as x increases. The activation energies of the motion are of the order of magnitude of 15 to 30 kJ/mol.
Perturbational approach to the interaction between a polymer and a small molecule
Authors: Imamura Akira , Aoki Yuriko , Suhai Sándor , Ladik János
Published: July 2002

The perturbational method developed in the present paper is applied to the interaction between a polymer and a small molecule using a few simple model systems. The validity of our method is studied by comparing the results obtained with the perturbation method and the tight binding SCF crystal orbital method. The total electronic energies and charge distributions obtained are in good agreement with each other for the two methods. This result leads to the conclusion that the present perturbational approach is promising for application to interactions between real polymers and impurities.
Characterization of the lowest-lying Π bending state of Ar–HCl by far infrared laser–Stark spectroscopy and molecular beam electric resonance
Authors: Marshall Mark D. , Charo Arthur , Leung Helen O. , Klemperer William
Published: July 2002

Far infrared laser–Stark spectroscopy is used to populate an excited vibrational state in the van der Waals molecule, ArHCl, in a molecular beam. Microwave–far infrared double resonance allows the identification of this state as the first excited bending state of Π symmetry. Subsequent radio frequency electric resonance of the vibrationally excited molecule combined with far infrared–radio frequency two photon experiments gives the following spectroscopic constants: ν0−B′=33.9248(7) (cm−1), B′=1695.(20) (MHz), μ=0.265(3) (D), eqQaa=12.(7) (MHz), eqQbb−eqQcc=−73.927(23) (MHz), q1=−49.583(2) (MHz). These constants are compared with theoretical predictions obtained using previously suggested potential energy surfaces and are related to the presence of other nearby vibrational states.
‘‘Ordered’’ structure in dilute solutions of biopolymers as studied by small-angle x-ray scattering
Authors: Matsuoka Hideki , Ise Norio , Okubo Tsuneo , Kunugi Shigeru , Tomiyama Hiroshi , Yoshikawa Yukihiro
Published: July 2002

Dilute aqueous solutions of bovine serum albumin, lysozyme, chondroitin sulfate, and tRNA were measured by small-angle x-ray scattering. The scattering curves showed a single, broad peak as was observed for synthetic polyelectrolytes, indicating the presence of an ordered distribution of charged solutes. The intermolecular distance evaluated from the peak position (2Dexpt) increased with decreasing polymer concentration and with increasing salt concentration. Except for chondroitin sulfate, 2Dexpt values were nearly equal to the interparticle distance (2D0) calculated based on the assumption of a uniform distribution. The observed relationship between 2Dexpt and 2D0 was in agreement with the proposal that intermacroion attraction is weak for low-charge density particles under discussion. This attraction and repulsive interparticle interaction create a ‘‘secondary’’ minimum in the potential curve enabling ordering to take place. For tRNA, the scattering peak became lower with rising temperature. The fact that only a single, broad peak could be observed was rationalized by invoking the concept of distortions of ordering particles such as the paracrystallinity, and the thermal motion and the crystalline size effect. The correlation hole theory based on repulsive interaction is critically discussed, particularly in light of the experimental fact that the peak position of albumin shifts toward wider angles with increasing number of charges.
Vibrational state distributions following the photodissociation of (collinear) triatomic molecules: The vibrational reflection principle in model calculations for CF3I
Authors: Hennig Steffen , Engel Volker , Schinke Reinhard
Published: July 2002

Vibrational state distributions following the direct photodissociation of a collinear, triatomic molecule is investigated with particular emphasis on the so-called final state interaction, i.e., the translational–vibrational coupling due to the excited state interaction potential. In order to separate the various effects which determine the state distribution we performed calculations on three levels of accuracy: The energy sudden (ES) approximation, the modified sudden (MS) approximation, and the exact close-coupling (CC) formulation. The pure ES distributions peak at high states and are very broad. They are explained within the semiclassical limit as a mapping of an amplitude onto the quantum number axis. We call this effect vibrational reflection principle in analogy to the equivalent effect in rotational excitation processes. It is a direct and sensitive probe of the parameters of the system, most importantly the potential energy surface. Energy conservation strongly modifies the ES distributions. The MS and CC distributions are much narrower and peak at considerably lower states. A detailed analysis is given within the MS approximation. Based on these general conclusions we suggest a particular excited state potential for the dissociation of CF3I which qualitatively reproduces the recently reported experimental CF3 distribution. The essential feature of this potential is a distance dependent local frequency which we find necessary to obtain distributions as broad as in the experiment. Because of the inherent difficulties with the time-of-flight technique if several energy transfer channels are involved we certainly do not know how realistic our final potential energy surface is. However, the general trends found in this study should be valid for a larger class of systems.
Overtone band shapes and IVR: C–H stretch overtones in CHCl3
Authors: Baggott J. E. , Clase H. J. , Mills I. M.
Published: July 2002

The intracavity photoacoustic dye laser spectrum of CHCl3 in the gas phase at 16 350 cm−1 is reported. The v=6–0 overtone of the CH stretch is observed, and found to exhibit a rotational band contour closely analogous to the v=1–0 fundamental. The implication of this result for intramolecular vibrational energy redistribution is discussed.
Excess electrons in simple fluids. III. Role of solvent polarization
Authors: Nichols Albert L. , Chandler David
Published: July 2002

We have extended the RISM-polaron theory of excess solvated electrons to treat the effects of quantum mechanical charge density fluctuations in the solvent particles. Our numerical results examine a model in which the solvent is composed of hard spheres with internal Drude oscillators. By varying the fundamental Drude oscillator frequency, one may explore the role of solvent polarization as it depends upon the time scale of the dipolar motions. We describe general trends predicted by the theory for this model, and we discuss the sensitivity of our results to changes in the model characterizing the interactions between the electrons and the solvent molecules.
IR double resonance study of rotational energy transfer in pure HCl
Authors: Menard-Bourcin F. , Delaporte T. , Menard J.
Published: July 2002

The rotational relaxation of HCl (v=1) by HCl is studied by infrared double resonance using two chemical lasers. State-to-state rate constants are derived from the measurements by comparison with a computer simulation based on two different scaling laws. The ECS theory based scaling law seems to provide a better fit to the experimental data than does the exponential gap law. With both scaling laws a correct description of the double resonance is obtained by including multiple rotational quantum transitions.
Monte Carlo investigation of a kinetic Ising model of the glass transition
Authors: Fredrickson Glenn H. , Brawer Steven A.
Published: July 2002

A detailed Monte Carlo study of the equilibrium and dynamical properties of the two-spin facilitated kinetic Ising model proposed by Fredrickson and Andersen (FA) is presented. The model Hamiltonian is that of a spin-1/2 Ising model in an external magnetic field and, in the present study, contains no interactions between spins. The kinetic properties of the model are described by a master equation with single-spin-flip dynamics and highly cooperative flip rates. In particular, the rate at which a spin flips is chosen to be zero unless at least two of its neighbors are spin up. Monte Carlo simulations of the model on a square lattice demonstrate that although there is no evidence for a kinetic singularity (as predicted by FA), the model has dynamical properties much like those of viscous liquids. We find non-Arrhenius temperature dependence of the average relaxation time and highly nonexponential decay of various relaxation functions. The expression derived by Adam and Gibbs is found to describe the relationship between the average relaxation time and the entropy of the spin model. It is shown that the equilibrium time correlation functions can be accurately fit to the Kohlrausch–Williams–Watts (KWW) expression. The exponent in the KWW function is found to decrease as the temperature is lowered. Studies of multiple-spin correlation functions demonstrate how spatial correlations develop in the model as it is perturbed from equilibrium and how these relax as the system equilibrates.
The far infrared spectrum of magnesium hydride
Authors: Leopold K. R. , Zink L. R. , Evenson K. M. , Jennings D. A. , Mizushima M.
Published: July 2002

The rotational spectrum of MgH in its ground 2∑ state has been observed for the first time using a new tunable far infrared spectrometer. The molecular constants derived are of sufficient accuracy to permit astrophysical identification of this species.