bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial and temperature dependence of carrier recombination in an InGaAs/InP heterostructure
Authors: Monte A. F. G. , da Silva S. W. , Cruz J. M. R. , Morais P. C. , Cox H. M.
Published: July 2002

The microluminescence surface scan technique has been used to investigate ambipolar carrier diffusion, photon diffusion, and photocarrier recombination in a nominally undoped InGaAs bulk layer lattice matched to InP grown by vapor levitation epitaxy. Measurements taken at different temperatures between 75 and 300 K are discussed in terms of the relative contribution of the two distinct mechanisms to the spectrally integrated luminescence intensity, namely, photon diffusion and photocarrier diffusion.
Properties of pulsed laser deposited scandium-doped barium hexaferrite films
Authors: Oliver S. A. , Chen M. L. , Vittoria C. , Lubitz P.
Published: July 2002

The properties of thin films of BaScxFe12−xO19 (x=0.4) were determined by structural and magnetic measurements. Films were deposited by pulsed laser ablation deposition onto c-plane sapphire at oxygen pressures between 10 and 100 mTorr. X-ray diffraction measurements showed all films to be single-phase c-axis oriented hexaferrites with expanded c-axis lattice constants compared to x=0 films. Magnetometry measurements showed that all films had the easy axis (c-axis) normal to the film plane, with a mean saturation magnetization value of 3.8 kG. The mean uniaxial anisotropy field value was 10.6 kOe. This ability to adjust the uniaxial anisotropy field in hexaferrite films through selective substitution will be important for future planar microwave devices.
Addressing and sustaining in alternating current coplanar plasma display panels
Authors: Punset C. , Cany S. , Boeuf J.-P.
Published: July 2002

In a coplanar plasma display panel the discharges in each pixel are sustained between two parallel electrodes on the same substrate. A third electrode perpendicular to the sustaining electrodes and placed on a facing substrate is used to address the pixel. A self-consistent two-dimensional model of the microdischarge has been used to simulate and study the addressing and sustaining phases in an alternating current coplanar cell. The formation and decay of the transient plasma during the address and sustain discharge pulses are described. The time evolution of the charges on the dielectric surfaces above each electrode is also discussed and the model is used to derive the voltage margins of the address and sustain regimes.
Temperature and field dependence of microwave losses in manganite powders
Authors: Srinivasu V. V. , Lofland S. E. , Bhagat S. M. , Ghosh K. , Tyagi S.
Published: July 2002

We report systematic measurements of the temperature and field dependence of microwave losses in micron-size powders (∼3 μm) of the manganites Nd0.7Sr0.3MnO3, La0.7Ca0.3MnO3, La0.8Ba0.2MnO3, and La1.4Ca1.6Mn2O7. We fully confirm previous findings of a large increase in zero-field absorption, in La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3, accompanying the onset of ferromagnetism and a concomitant colossal low-field magnetoimpedance (up to 80% at 600 Oe at room temperature). An oversimplified set of computations renders plausible support for interpreting these observations as arising from absorption in the spin system. Possible applications of these novel phenomena are also described.
Analysis of the Stokes shift in InAsP/InP and InGaP/InP multiple quantum wells
Authors: Aït-Ouali A. , Brebner J. L. , Yip R. Y.-F. , Masut R. A.
Published: July 2002

Photoluminescence (PL) and optical absorption spectra from samples containing InAsP/InP and InP/InGaP quantum well stacks reveal the presence of band tails in the excitonic density of states. At low temperature, radiative recombinations from these band-tail states contribute to the energy separation (redshift) between PL and optical absorption peaks for the fundamental transitions. This band-tail localization contribution is evaluated quantitatively by using a potential fluctuations model [developed by D. Ouadjaout and Y. Marfaing, Phys. Rev. B. 46, 7908 (1992)] to analyze the line shape of the low temperature PL peaks. The residual energy separation, after removing the band-tail localization component, arises from thermalization processes alone, demonstrating the validity of the model used to evaluate the band-tail induced redshift. We also found that the free excitons’ effective temperature varies linearly with the optical absorption peak width, dominated by inhomogeneous broadening at low temperatures. This empirical relation suggests an explanation for the difference on the origin of the Stokes shift reported in the literature.
Rapid thermal annealing effects on the structural properties and density of defects in SiO2 and SiNx:H films deposited by electron cyclotron resonance
Authors: San Andrés E. , del Prado A. , Martı́nez F. L. , Mártil I. , Bravo D. , López F. J.
Published: July 2002

The effect of rapid thermal annealing processes on the properties of SiO2.0 and SiN1.55 films was studied. The films were deposited at room temperature from N2 and SiH4 gas mixtures, and N2, O2, and SiH4 gas mixtures, respectively, using the electron cyclotron resonance technique. The films were characterized by Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance spectroscopy. According to the FTIR characterization, the SiO2.0 films show continuous stress relaxation for annealing temperatures between 600 and 1000 °C. The properties of the films annealed at 900–1000 °C are comparable to those of thermally grown ones. The density of defects shows a minimum value for annealing temperatures around 300–400 °C, which is tentatively attributed to the passivation of the well-known E′ center Si dangling bonds due to the formation of Si–H bonds. A very low density of defects (5×1016 cm−3) is observed over the whole annealing temperature range. For the SiN1.55 films, the highest structural order is achieved for annealing temperatures of 900 °C. For higher temperatures, there is a significant release of H from N–H bonds without any subsequent Si–N bond healing, which results in degradation of the structural properties of the film. A minimum in the density of defects is observed for annealing temperatures of 600 °C. The behavior of the density of defects is governed by the presence of non-bonded H and Si–H bonds below the IR detection limit.
Experimental studies of the multimode spectral emission in quantum dot lasers
Authors: Patanè A. , Polimeni A. , Eaves L. , Henini M. , Main P. C. , Smowton P. M. , Johnston E. J. , Hulyer P. J. , Herrmann E. , Lewis G. M. , Hill G.
Published: July 2002

We investigate the electroluminescence spectra of edge-emitting lasers having self-assembled quantum dots as the active medium. A broad laser emission is observed with a modulation of intensity corresponding to single or bunches (supermodes) of Fabry–Pérot modes. The variation of the laser spectra with magnetic field shows that the supermodes originate from laser cavity effects and are not related directly to the electronic properties of the quantum dots. Measurements taken on devices of different cavity height, length, and lateral width indicate that the important parameter controlling the laser multimode emission is the cavity height, effectively the substrate thickness. In particular, the period of the supermodes is inversely proportional to this thickness, indicating that the modulation of the laser emission intensity is due to the leakage of modes into the transparent substrate.
Magnetic properties of c-axis oriented SmFe12/α-Fe nanocomposite films
Authors: Kato Hiroaki , Nomura Takuya , Ishizone Masahiko , Kubota Hitoshi , Miyazaki Terunobu , Motokawa Mitsuhiro
Published: July 2002

Thin-film nanocomposite magnets, composed of α-Fe and c-axis oriented SmFe12 phases, have been synthesized by rf magnetron sputtering onto heated glass substrates. For samples with the α-Fe volume fraction VFe<30%, the demagnetization curves with fields applied normal to the film plane exhibited good squareness, while the hard-axis curves did not saturate even in a field of 140 kOe at room temperature. Both the coercivity and maximum energy product showed a maximum for VFe≈25%.
Magnetic ordering in GaAlAs:Mn double well structure
Authors: Boselli M. A. , da Cunha Lima I. C. , Ghazali A.
Published: July 2002

The magnetic order in the diluted magnetic semiconductor barrier of double AlAs/GaAs:Mn quantum well structures is investigated by Monte Carlo simulations. A confinement adapted RKKY mechanism is implemented for indirect exchange between Mn ions mediated by holes. It is shown that, depending on the barrier width and the hole concentration, a ferromagnetic or a spin-glass order can be established.
Permanent magnet spheres: Design, construction, and application (invited)
Authors: Leupold H. A. , Tilak A. , Potenziani E.
Published: July 2002

A summary of the salutary properties of the strong permanent magnet field sources known as magic spheres is given. Topics of discussion include: obtainable field strengths, field uniformity, field insensitivity to structural modifications, methods of avoidance of field distortions due to access holes, tapering of field strength with progression along the field axis, field augmentation in different working space geometries, simplified manufacture of the magic spheres, and potential employment for various purposes. Field measurements made on a recently completed prototypical augmented magic sphere for a microwave radiation source are reported and compared with theory.
Electroluminescence at Si band gap energy based on metal–oxide–silicon structures
Authors: Lin Ching-Fuh , Liu C. W. , Chen Miin-Jang , Lee M. H. , Lin I. C.
Published: July 2002

Room-temperature electroluminescence corresponding to Si band gap energy from metal–oxide–semiconductor structures on both p-type and n-type Si is observed. With very thin oxide grown by rapid thermal oxidation, the metal–oxide–semiconductor structures behave like light emitting diodes. Luminescence is observed under forward bias even with a current density as low as 0.67 A/cm2. The physical reason for the electroluminescence is discussed and attributed to the localized wave function that leads to the spread of momentum. As a result, the spread momentum causes the electron–hole radiative recombination to occur relatively easily.
Hard magnetic properties of melt-spun R(Fe,M)12 nitrides (R=Nd or Pr; M=Mo or V)
Authors: Yang Jinbo , Oleinek Ph. , Müller K.-H.
Published: July 2002

The hard magnetic properties of nitrided melt-spun R(Fe,M)12 (R=Pr and Nd, M=V and Mo) ribbons have been investigated. Hard magnetic powders were prepared by vacuum annealing of ground ribbons in the temperature range from 700 to 950 °C and subsequent nitrogenation. For NdFe10.5Mo1.5Nx and PrFe10.5Mo1.5Nx powders, coercivities as high as 10.4 and 4.5 kOe at room temperature, respectively, were achieved by carefully controlling the nitrogenation process. The coercivity strongly depends on the preparation route, i.e., annealing temperature as well as nitrogenation temperature and time. By the detailed investigation of the influence of these conditions, the optimum process parameters were determined. The coercivity mechanism of melt-spun NdFe10.5Mo1.5Nx powders shows characteristics of a randomly oriented single domain particle system. Poor coercivities have been obtained for Nd(Fe,V)12 and Pr(Fe,V)12. In these cases as well as for not optimally nitrogenated Nd(Fe,Mo)12, the low coercivity of the nitrided powders is related to (i) too large grain sizes, (ii) the presence of α-Fe additional to the ThMn12-type phase, or (iii) a nonhomogeneous nitrogenation of the powder.
The monoenergetic vibrational predissociation of expansion cooled NCNO: Nascent CN(V,R) distributions at excess energies 0–5000 cm−1
Authors: Nadler I. , Noble M. , Reisler H. , Wittig C.
Published: July 2002

We report detailed vibration, rotation distributions for nascent CN(X 2∑+), following the one-photon photodissociation of expansion cooled NCNO via π*←n excitation throughout the region 450–585 nm. At the observed threshold for dissociation (585.3 nm), >90% of the CN product is in v″=0, N″=0, with the remainder in N″=1, corresponding to 〈Erot〉 <0.4 cm−1. CN(X 2∑+, v″=0) rotational distributions are obtained at many photolysis wavelengths and rotational levels are observed up to, but never above, the limit imposed by energy conservation: [B″vN″(N″+1)]<E p−D0(v″), where D0(v″) is the dissociation energy to produce CN(X 2∑+,v″) and Ep is the photon energy. CN(X 2∑+,v″=1) and CN(X 2∑+,v″=2) thresholds are observed at photolysis wavelengths which correspond exactly to Ep−D0(v″=1) and Ep−D0(v″=2). These observations can only be reconciled with a vibrational predissociation mechanism and spectroscopic observations suggest that this occurs following internal conversion to the ground state surface. With Ep−D0(v″) less than ∼2000 cm−1, the phase space theory of unimolecular reactions (PST) predicts the CN rotational distributions with high accuracy. However, when product vibrations are accessible, PST cannot be used, since it does not take proper account of the parent being vibrationally excited but rotationally cold. When explicitly taking this into account, we are able to reconcile the present experimental findings with a statistical model and we believe that the behavior observed for NCNO has a sound physical basis and is quite general.
Effect of temperature on nondissociative electron attachment to perfluorobenzene
Authors: Spyrou S. M. , Christophorou L. G.
Published: July 2002

Experiments are reported for the rate constants for electron attachment to perfluorobenzene in the temperature range 400 to 650 K. It is found that the rate constant actually decreases by more than two orders of magnitude when T is increased from 400 to 650 K. (AIP)
The solution of the hypernetted-chain approximation for fluids of nonspherical particles. A general method with application to dipolar hard spheres
Authors: Fries P. H. , Patey G. N.
Published: July 2002

In this paper we describe a general method for the numerical solution of the full hypernetted-chain (HNC) theory for fluids characterized by angle-dependent pair potentials. This method is also applicable to the closely related reference hypernetted-chain (RHNC) approximation. The only formal restriction is that the pair potential and correlation functions must be expandable in a basis set of rotational invariants. We present explicit numerical solutions of the RHNC theory for dense dipolar hard sphere fluids and detailed comparisons are made with previous theories and computer simulation results. It is found that the full RHNC theory generally improves upon the previous reference linearized and quadratic HNC approximations. The values given by the RHNC theory for the static dielectric constants are smaller than those given by these earlier approximations and are in much better agreement with computer simulations.
Effect of an electric field on a methane molecule. II. Calculation of the degeneracy splitting of the ν3 band. Expression of the second derivatives of the CH4 dipole moment and evaluation of the second derivative of the C–H bond polarizability
Authors: Kahn R. , De Lara E. Cohen , Möller K. D.
Published: July 2002

In order to interpret infrared spectra of methane adsorbed in NaA at low temperature, we have calculated the frequencies of CH4 and CD4 perturbed by an electric field. This calculation needs the expressions of the second derivative of the dipole moment and the polarizability. These later were given by Montero and we have expressed the terms ∂2m/∂Si∂Sj using the same approximations: additivity of the CH bond dipole derivatives. We find for the CH bond derivatives: μ′=−0.7×10−10 esu, μ″=+1.5×10−2 esu. The magnitude of the field (3×105 esu) has been determined by the intensity of the induced ν1 band. The orientation of the field deduced from the experiments, is along a C3 axis of the molecule. Adjusting theoretical and experimental results, we have calculated the parallel and perpendicular second derivatives of the CH bond polarizability 2β″+δ″=−6.4×10−8 cm, δ″− β″=+5.3×10−8 cm.
Mechanisms of nonadiabatic transitions in photoisomerization processes of conjugated molecules: Role of hydrogen migrations
Authors: Ohmine Iwao
Published: July 2002

Mechanism of nonadiabatic transition in the C=C torsional photoisomerization process of ethylene and polyenes is investigated by using the ab initio configuration interaction calculation method. We have calculated the low-lying singlet state potential energy surfaces and their nonadiabatic couplings. A multidimensional search for the molecular configurations yielding strong nonadiabatic couplings is performed to find the origin of very fast photoisomerization kinetics, which are experimentally observed to be typically in the order of a few or a few tens of picoseconds. It is found that the ‘‘pseudo’’ migration motion of a hydrogen adjacent to the twisted C=C bond causes a potential surface crossing of the low-lying excited and ground states and thus induces a sufficiently large nonadiabatic coupling to explain this experimental evidence. The hydrogen migration motion is facilitated by the so-called zwitterionic character of the low-lying excited states near the 90° C=C twisted conformation, proceeds almost without an energy barrier and involves large molecular conformational changes. The shape of potential surface crossing is found to be multidimensional in nature and so that, once a molecule reaches this crossing region by the hydrogen migration, almost all the internal molecular motions cause the strong nonadiabatic couplings between the excited and ground states. The role of the ‘‘2 1A−g’’ state in the photoisomerization is also discussed.
Mössbauer spectroscopy studies of selected conducting polypyrroles
Authors: Proń A. , Kucharski Z. , Budrowski C. , Zagórska M. , Krichene S. , Suwalski J. , Dehe G. , Lefrant S.
Published: July 2002

Polypyrrole obtained by chemical oxidation of pyrrole in ethanol–water solution containing FeCl3 can be reduced chemically in Na+Naph−/THF solution and then reoxidized with FeCl3, SnCl4, and SbCl5. 57Fe and 119Sn Mössbauer spectra of the reaction products show only one type of iron and tin with Mössbauer parameters characteristic of FeCl−4 and SnCl−5. In the case of the oxidation with SbCl5, two nonequivalent antimony nuclei have been detected by 121Sb. The calculated isomer shift and quadrupole splitting values are consistent with SbCl−6 and SbCl3. The resonant absorption in all three systems studied is strongly temperature dependent and can be observed only at temperatures significantly lower than RT. This behavior indicates a low Mössbauer lattice temperature and an extremely loose bonding between the inserted anions and polymer chains.
NCNO → CN+NO: Complete NO(E, V, R) and CN(V, R) nascent population distributions from well-characterized monoenergetic unimolecular reactions
Authors: Qian C. X. W. , Noble M. , Nadler I. , Reisler H. , Wittig C.
Published: July 2002

We report detailed vibrational, rotational, and electronic (V, R, E) distributions of nascent NO(X 2Π) deriving from monoenergetic unimolecular reactions of jet-cooled NCNO. Excitation is via the Ã 1A″ ← X̃ 1A′ system above dissociation threshold (17 085±5 cm−1), and vibrational predissociation occurs following radiationless decay of the initially excited Ã 1A″ state. These results are combined with data on the corresponding CN(X 2Σ+) nascent V, R distributions, thereby providing a complete description of the energy partitioning into the various degrees of freedom of both products. The data presented here support our previous conclusion that dissociation is ‘‘statistical.’’ All the V, R distributions of both products can be predicted accurately using a modification of the phase space theory of unimolecular reactions (PST), which we call the separate statistical ensembles (SSE) method; it is expected that this method will have quite general applicability. NO spin-orbit excitation is ‘‘cold’’ relative to the V, R degrees of freedom, and although no detailed explanation is offered, the origin of this observation is discussed.
Zero field NMR and NQR
Authors: Zax D. B. , Bielecki A. , Zilm K. W. , Pines A. , Weitekamp D. P.
Published: July 2002

Methods are described and demonstrated for detecting the coherent evolution of nuclear spin observables in zero magnetic field with the full sensitivity of high field NMR. The principle motivation is to provide a means of obtaining solid state spectra of the magnetic dipole and electric quadrupole interactions of disordered systems without the line broadening associated with random orientation with respect to the applied magnetic field. Comparison is made to previous frequency domain and high field methods. A general density operator formalism is given for the experiments where the evolution period is initiated by a sudden switching to zero field and is terminated by a sudden restoration of the field. Analytical expressions for the signals are given for a variety of simple dipolar and quadrupolar systems and numerical simulations are reported for up to six coupled spin-1/2 nuclei. Experimental results are reported or reviewed for 1H, 2D, 7Li, 13C, and 27Al nuclei in a variety of polycrystalline materials. The effects of molecular motion and bodily sample rotation are described. Various extensions of the method are discussed, including demagnetized initial conditions and correlation by two-dimensional Fourier transformation of zero field spectra with themselves or with high field spectra.
Molecular dynamics investigation of the crystal–fluid interface. IV. Free energies of crystal–vapor systems
Authors: Broughton J. Q. , Gilmer G. H.
Published: July 2002

The excess free energy, entropy, and energy have been obtained along the coexistence curve for crystal–vapor interfaces by molecular dynamics. The face centered cubic (111), (100), and (110) faces of a crystal composed of Lennard-Jones particles were investigated. At low temperatures the free energies of the three faces are ordered as expected, in accord with the density of atoms in the layers. But within 20% of the triple point the free energy is nearly isotropic. The surface stress remains highly anisotropic at all temperatures. Comparisons are made with analogous quantities obtained for the liquid–vapor system near the triple point.
High-density amorphous ice. III. Thermal properties
Authors: Handa Y. Paul , Mishima Osamu , Whalley Edward
Published: July 2002

The thermal properties of high-density amorphous ice, which is made by ‘‘melting’’ ice I at 77 K by applying a pressure of ∼10 kbar, have been measured in an automated Tian–Calvet calorimeter. The samples were heated from 78 to 270 K at a rate of 10 K h−1 and heat amounting to 544, 1425, and 35.5 J mol−1 was evolved in one run at the sharp transitions from high-density to low-density amorphous ice, from the low-density amorph to ice Ic, and from ice Ic to Ih, respectively. In addition, heat was spontaneously evolved by the high-density amorph before the transition, and by the sample over a range of ∼50 K around the temperature of the transition from ice Ic to Ih.
A state-to-state study of the electron transfer reactions Ar+(2P3/2,1/2)+N2(X̃,v=0)→Ar(1S) +N+2(X̃,v′)
Authors: Liao C.-L. , Shao J.-D. , Xu R. , Flesch G. D. , Li Y.-G. , Ng C. Y.
Published: July 2002

The vibrational state distributions of N+2(X̃,v′) ions resulting from the reactions, Ar+(2P3/2)+N2(X̃,v=0)→Ar(1S0) +N+2(X̃,v′) [reaction (1)] and Ar+(2P1/2)+N2(X̃,v=0)→Ar(1S0) +N+2(X̃,v′) [reaction (2)], over the center-of-mass collisional energy (Ec.m.) range of 0.25–41.2 eV in a crossed ion–neutral beam experiment have been probed by the charge exchange method. The experimental results obtained for reaction (1) are in accord with the predictions of the semiclassical multistate calculation of Spalburg and Gislason that N+2 ions are formed predominantly (≳85%) in the v′=1 state and that the production of N+2(X̃,v′=0) becomes more important as Ec.m. is increased. The experiment also supports the theoretical results for reaction (2) at Ec.m.=1.2 and 4.1 eV showing that ≳80% of N+2 product ions are in the v′=2 state. However, the calculation is found to either over-estimate the populations for N+2(v′<2) or underestimate the populations for N+2(v′>2) resulting from reaction (2) at Ec.m.=10.3

and 41.2 eV. Absolute spin-orbit-state-selected total cross sections for reactions (1) and (2), σ3/2 and σ1/2, respectively, at the Ec.m. range of 0.25–115.3 eV have also been measured using a tandem photoionization mass spectrometer which is equipped with a radio frequency (RF) octopole ion guide reaction gas cell. The measured values for σ3/2 at Ec.m.=4.1, 10.3, and 41.2 eV and σ1/2 at 41.2 eV are in reasonable agreement with the theoretical cross sections. However, the experimental values for σ3/2 at 1.2 eV and σ1/2 at 1.2, 4.1, and 10.3 eV are approximately a factor of 2 higher than the theoretical predictions. A model analysis, which takes into account possible collision-induced spin-orbit mixings of the reactant Ar+ states in the RF octopole gas cell, shows that the values for σ1/2/σ3/2 and σ1/2 determined using the ion beam–RF octopole gas cell arrangement can be strongly susceptible to gas cell pressure effects whereas the experimental values for σ3/2 are reliable. The values for σ1/2 deduced by multiplying the values for σ3/2 and the ratios σ1/2/σ3/2 determined in the crossed ion–neutral beam experiment are in agreement with the theoretical cross sections. Both σ3/2 and σ1/2 are found to increase as Ec.m. is increased from 41.2 eV. This observation is interpreted as due to the formation of N+2 in the Ã 2Πu state at high Ec.m. . Combining the measured vibrational state distributions of product N+2(X̃,v′) ions and the absolute state-selected total cross sections, absolute state-to-state total cross sections for reactions (1) and (2) at selected Ec.m. are determined.
Interdimensional degeneracies, near degeneracies, and their applications
Authors: Doren D. J. , Herschbach D. R.
Published: July 2002

Recently developed approximation methods for quantum mechanical problems which treat the spatial dimension D as an expansion parameter offer approximations to energy levels at arbitrary D. Rather than simply being a detour to the D=3 case, there is physical interest in nonphysical values of D due to degeneracies between states in different dimensions. For example, such degeneracies make it possible to calculate some excited states of two-electron atoms in three dimensions from the ground state energy at nonphysical values of D. Such relationships can be exploited in a simple derivation of the hydrogen atom spectrum in arbitrary D, using only the solution at D=1 and a combination of inter- and intradimensional symmetry arguments. Applications to the Yukawa potential and an anharmonic oscillator are also presented. A large class of interdimensional degeneracies is found for two-electron atoms. Approximate degeneracies are also identified for these atoms which allow highly excited D=3 states to be treated as perturbed low-lying states in another dimension. The approximate degeneracies also serve to generalize the treatment of the hydrogen atom spectrum in a way appropriate to two-electron atoms.
The vibration–rotation emission spectrum of CH(X 2Π)
Authors: Bernath P. F.
Published: July 2002

The vibration–rotation emission spectrum of the X 2Π state of CH was observed with the McMath Fourier transform spectrometer at Kitt Peak. The 1–0, 2–1, and 3–2 bands were detected in a microwave discharge of allene in Ar and methane in He. A simultaneous fit of all of the line positions provided spectroscopic constants for v=0, 1, 2, and 3. Equilibrium molecular constants (in cm−1) include ωe=2860.4118(98), ωexe=64.1082(46), ωeye=0.2406(10), Be=14.458 62(48), αe=0.534 16(58), γe=0.001 98(15), re=1.119 83(2) Å. An RKR potential curve was calculated from the equilibrium constants.
Impulsive stimulated light scattering. II. Comparison to frequency-domain light-scattering spectroscopy
Authors: Yan Yong-Xin , Nelson Keith A.
Published: July 2002

A comparison between time-domain impulsive stimulated light-scattering (ISS) and frequency-domain, spontaneous light-scattering (LS) spectroscopies is carried out in theoretical and practical terms. In some cases, the two experiments probe different material responses. In many cases the information content of ISS and LS data is identical in principle. The results can be related to each other through the time- and frequency-dependent response functions Gεε(q,t) and Gεε(q,ω), or through the time-correlation function Cεε(q,t). Simulated ISS and LS data from vibrational and Debye relaxational modes are compared in view of experimental considerations, including wave vector and time or frequency resolution and range, and sources of ‘‘noise.’’ In many cases, one or the other experimental approache offers significant advantages in practice. The complementary nature of the techniques is illustrated.
Infrared Stark spectroscopy of the hydrogen–HF binary complex
Authors: Jucks K. W. , Miller R. E.
Published: July 2002

A sub-Doppler resolution infrared spectrum has been measured for the HF stretch of the H2–HF complex using the optothermal detection method. Infrared Stark spectra have also been obtained from which both the ground and first vibrationally excited state dipole moments have been obtained. It is clear from these measurements that the HF subunit undergoes wide amplitude bending motion even in the zero-point bending state. Broadening of the transitions due to the finite lifetime (27 ns) of the excited vibrational state is just discernible under the highest resolution conditions.
Fluctuation effects in the theory of microphase separation in block copolymers
Authors: Fredrickson Glenn H. , Helfand Eugene
Published: July 2002

The effect of composition fluctuations on the microphase separation transition in diblock copolymers is investigated. Such fluctuation corrections, which were neglected in the mean field treatment of Leibler, are found to be significant for the molecular weights usually encountered. The analysis is facilitated by reducing the block copolymer Hamiltonian to a form previously studied by Brazovskii. Our principal results are the following: (i) A symmetric diblock copolymer is predicted to undergo a first order phase transition at a larger value of χN than the second order transition found by Leibler. The Flory interaction parameter is denoted χ and N is the number of statistical segments per chain. The location of the transition is predicted to be at (χN)t=10.495+41.022 N−1/3, where the peak in the scattering function attains its maximum value of 0.12328 N1/3. (ii) We find windows in composition, with finite width, through which it is possible to pass from the disordered phase to each of the ordered microphases (lamellar, hexagonal, and body-centered-cubic) by changing temperature. (iii) All the results of Leibler are recovered in the limit of block copolymers with infinite molecular weight.
The microwave spectrum of formamide–water and formamide–methanol complexes
Authors: Lovas F. J. , Suenram R. D. , Fraser G. T. , Gillies C. W. , Zozom J.
Published: July 2002

The microwave spectra of the formamide–water and formamide–methanol complexes have been investigated with a pulsed beam Fabry–Perot cavity Fourier transform microwave spectrometer. The observed hyperfine structure due to the 14N nuclear quadrupole interaction was used to assign the rotational transitions for both species. For formamide–water the rotational analysis of ten transitions provides the constants: A=11 227.931(1) MHz, B=4586.9628(10) MHz, C=3258.8278(7) MHz, eQqaa =1.332(3) MHz, and eQqbb =2.037(3) MHz. The formamide–methanol spectrum exhibits an additional splitting from internal rotation of the methyl group. Eighteen observed transitions from the A and E symmetry states have been assigned and fitted with the rotational constants: A=10 186.594(6) MHz, B=2090.36(59) MHz, and C=1762.80(56) MHz with hyperfine constants close to those of formamide–water. By assuming a methyl top moment of inertia Iα =3.206 uÅ2, the barrier to internal rotation V3=231.01(17) cm−1 is obtained. This barrier height is about 36% smaller than that of methanol. The structures determined for these complexes agree well with prior ab initio calculations which indicate essentially planar, double hydrogen bonded structures for both species.
Time-dependent coupled cluster method: A new approach to the calculation of molecular absorption spectra
Authors: Prasad M. Durga
Published: July 2002

The time-dependent coupled cluster method is developed to provide a formally exact theory of quantum-mechanical motion of a vacuum state on multidimensional anharmonic surfaces with a view to calculate the molecular absorption spectra. The exact time evolution operator is represented as an exponential of creation operators. Since the separability requirments of many-particle systems are built into such an ansatz, it is possible to develop approximations which can take into account any number of modes with much less computational effort than in a calculation by basis-set expansion. The method gives a closed set of equations for harmonic surfaces and finite ordered equations for anharmonic surfaces when truncated at some finite rank.
Quantum scattering via the S-matrix version of the Kohn variational principle
Authors: Zhang John Z. H. , Chu Shih-I. , Miller William H.
Published: July 2002

The S-matrix version of the Kohn variational principle is used to obtain a very effective method for quantum scattering calculations. The approach is especially useful for the nonlocal (i.e., exchange) interactions that arise in chemically reactive scattering (and also in electron–atom/molecule scattering). The particular version developed in this paper has a more general structure than an earlier one by Miller and Jansen op de Haar [J. Chem. Phys. 86, 6213 (1987)], and applications to an elastic scattering problem, and also to three-dimensional H+H2 reactive scattering, show that it is also more useful in practice.
Sound propagation in water–ethanol mixtures at low temperatures. I. Ultrasonic velocity
Authors: D’Arrigo G. , Paparelli A.
Published: July 2002

Ultrasonic velocity measurements in ethanol aqueous solutions from +30 to −40 °C over the entire composition range and in the frequency range 10–70 MHz are presented. The results, in combination with previous determinations up to +80 °C, allow us to follow the peculiar behavior of the adiabatic compressibility of this system in a very large temperature range extending from a region where the anomalies of pure water tend to disappear, down to the supercooled region where these anomalies are noticeably enhanced. A ‘‘normal’’ and an ‘‘anomalous’’ contribution to the isothermal compressibility of water, as derived from these data, is compared with previous evaluations and discussed in the framework of the existing theoretical models. The compressibility of the diluted solutions is interpreted in terms of stabilization of water into ordered, less dense, and more rigid structures around the solute. We show that a simple two-state model based on this concept and on the hypothesis of a structural breakdown at the concentration of maximum sound absorption can explain the main experimental features of the volumetric properties of these solutions.
A computational study of the TiO2 molecule
Authors: Ramana M. V. , Phillips D. H.
Published: July 2002

We have carried out a computational investigation of the ground 1A1 and lowest energy B2 states of the titanium dioxide molecule. The treatment utilized SCF calculations in an extended basis followed by a CI treatment for each geometry. Our ground state geometry agrees well with experiment, while the agreement between the computed vibrational frequency ν1 and the experimental value for a matrix isolated TiO2 is less satisfactory. Population analysis for the ground state indicates less than one excess electron on each oxygen atom. The first excited state has a linear geometry and the singlet and triplet are essentially degenerate.
Ultraviolet resonance Raman study of the pyrene S4, S3, and S2 excited electronic states
Authors: Jones Colleen M. , Asher Sanford A.
Published: July 2002

UV resonance Raman excitation within the S2, S3, and S4 π→π* electronic transitions of pyrene results in strong enhancement of totally symmetric ring vibrations. The Raman modes most strongly enhanced within these electronic transitions derive from vibrations exhibiting Franck–Condon vibronic structure in the absorption spectrum. The 1597 cm−1 (b3g) mode shows selective enhancement between the S3 and S4 transitions, and between the S2 and S3 transitions due to Herzberg–Teller coupling between these symmetry-allowed states. The experimentally observed S4 resonance Raman excitation profiles of the totally symmetric pyrene fundamentals agree closely with those calculated using resonance Raman transform theory if non-Condon contributions are included. We see an increased non-Condon contribution with an increasing vibrational frequency, indicating nonadiabatic vibronic interactions. The high incident energy fluxes of the Nd-YAG laser-based excitation source cause saturation of the pyrene Raman intensities since molecular absorption depletes the ground state population. The long-lived S1 singlet excited state bottlenecks relaxation back to the ground electronic state. Formation of pyrene phototransients is also observed with high incident energy fluxes at particular excitation wavelengths.
Quantum mechanical calculations on the Ar++N2 charge transfer reaction
Authors: Clary David C. , Sonnenfroh David M.
Published: July 2002

Calculations of cross sections for the charge transfer reaction Ar+(2P3/2)+N2(v=0, j) →Ar+N+2(v′=1, j′) are reported for thermal collision energies. A three-dimensional quantum-mechanical method is used in which separate rotational close-coupling calculations are performed for the Ar++N2(v=0)→Ar+N+2(v′=0) and Ar++N2(v=1) →Ar+N+2 (v′=1) channels, and the cross sections for the v=0→v′=1 channel are computed using a coupled channel-distorted wave Born approximation. Potential energy surfaces and couplings are taken from ab initio data. The predicted rotational product distributions for N+2 (v′=1, j′) agree fairly well with those measured in a molecular beam laser-induced fluorescence experiment.
Dynamics of unimolecular dissociation of sodium cluster ions
Authors: Bréchignac C. , Cahuzac Ph. , Leygnier J. , Weiner J.
Published: July 2002

We investigate the unimolecular dissociation dynamics of energy-rich sodium cluster ions, Na+n (5≤n≤40) by measuring the time evolution of their sequential monomer or dimer evaporative cooling. The experimental technique, tandem time-of-flight mass spectroscopy, measures the relative rate of competing dissociation channels from metastable ion clusters selected during an initial sampling time interval immediately following the creation of the ion cluster ensemble. Pulsed laser UV photoionization converts the distribution of neutral clusters emerging from a free-jet expansion to the distribution of ion clusters from which the initial selection takes place. For the smaller clusters, 3≤n≤14, we compare the measured dissociation rates with those calculated from a modified version of the RRK theory of unimolecular dissociation. In applying the theory we use monomer and dimer binding energies determined from theoretical calculation. For larger clusters, 15≤n≤40, the binding energies are not known, and we invert the calculation, using measured dissociation fractions, to determine the binding energies of the cluster ions.
Spin depolarization by random walks in lattice gases
Authors: Czech R.
Published: July 2002

The transverse spin depolarization of spin particles that perform tracer diffusion in lattice gases is studied. The lattice gas particles are noninteracting except that double occupancy of lattice sites is forbidden. The depolarization function P(t) is discussed for a model in which random local spin–rotation frequencies taken from a Gaussian probability distribution of width σ are associated with the lattice sites. The results of simulations of P(t) in dimensions d=1, 2, and 3 are compared with recent results for simple, uncorrelated random walks and analyzed by scaling expressions. In d=2 and 3, the dependence of the decay constant of the observed exponential polarization decay on σ and c, the concentration of the lattice gas particles, can be described by a simple scaling law involving the tracer-diffusion correlation factor. In d=1, where the tracer diffusion is anomalously slow, a faster than simple-exponential decay is found at large times t. A theory based on the scaling behavior of the distribution of sites visited in d=1 allows for a qualitative understanding of the observed polarization decay.
Crossed molecular beam study of the reaction O(3P)+C2H2
Authors: Schmoltner A. M. , Chu P. M. , Lee Y. T.
Published: July 2002

The reaction between ground state atomic oxygen and acetylene was studied using the crossed molecular beam method with an average collision energy of 6 kcal/mol. The two major primary reaction channels are (a) formation of CH2 and CO and (b) formation of HCCO and H. Product angular distributions and time-of-flight spectra were measured and the translational energy release was determined for each channel. The reaction proceeds primarily on the triplet surface through a long-lived intermediate. For both channels the translational energy distributions were found to peak at about 30% of the total available energy, indicating the existence of an exit channel barrier in each case. The branching ratio between channel (a) and (b) was found to be 1.4±0.5.
Theoretical investigation of model polymers for eumelanins. I. Finite and infinite polymers
Authors: Galvão D. S. , Caldas M. J.
Published: July 2002

We investigate the electronic structure of ideal ordered polymers of 5,6-indolequinone, in one or other redox form. These molecules are the most abundant constituents of eumelanin, the pigment appearing in human skin. Our study is carried out through Hückel π-electron theory, which allows us to follow the trends in electronic structure from a single monomer—the isolated molecules—to finite polymers of up to 10 units, and to infinite polymers. We have chosen different polymerization directions which produce semiconducting chains. The comparison between finite and infinite polymers is very useful and leads us to propose a model that accounts for some of the known properties of eumelanins.
Hypernetted-chain closure with bridge diagrams. Asymmetric hard sphere mixtures
Authors: Attard Phil , Patey G. N.
Published: July 2002

Methods of calculating the first two terms in the density expansion of the bridge function are given. The closure to the Ornstein–Zernike equation is now exact to two orders in density beyond the hypernetted-chain or Percus–Yevick approximations. The bridge function is resummed as a Padé approximant, and the results for hard spheres are relatively accurate over the whole density regime. The closure is shown to yield physically reasonable results for highly asymmetric mixtures. Infinitely dilute hard sphere solutes with diameters up to 30 times that of the hard-sphere solvent are also considered. The Derjaguin approximation for rescaling the force between spheres to the interaction free energy between planes is examined and found to give the dominant curvature correction.
Use of Rydberg atoms to probe negative ion lifetimes
Authors: Kalamarides A. , Marawar R. W. , Durham M. A. , Lindsay B. G. , Smith K. A. , Dunning F. B.
Published: July 2002

The lifetimes of the transient negative ions formed during dissociative electron capture by CHCl3 and CCl4 have been investigated through studies of electron transfer in collisions with K(nd) Rydberg atoms. Measurements of the velocity and angular distributions of the product positive and negative ions reveal that transient CHCl−*3 ions formed by Rydberg electron attachment to CHCl3 undergo very rapid dissociation. Similar data indicate that CCl−*4 ions formed through Rydberg electron capture by CCl4 have much longer lifetimes, on the order of 10 to 30 psec, consistent with the predictions of RRKM theory.
Electron transfer to oriented molecules: K+CF3I and K+CH3I
Authors: Harland Peter W. , Carman Howard S. , Phillips Leon F. , Brooks Philip R.
Published: July 2002

K+ ions have been detected from the intersection of a beam of K atoms (5–30 eV) with beams of CH3I and CF3I molecules which had been oriented prior to the collision. Collisional ionization is found to be favored for both molecules when the fast K is incident at the I end of the molecule, even though the electrical polarity of the I end is different for the two molecules. For both molecules, the effect of molecular orientation is most pronounced near threshold (≊5 eV) and almost disappears at higher (30 eV) energies. For CF3I, the threshold for impact at the I end is ≊0.7 eV less than the threshold for impact at the CF3 end. We interpret these results using a ‘‘harpoon’’ mechanism in which the electron jump during the initial approach is probably independent of orientation, but as the charged particles separate, the electron may jump back to the K+. For impact at the I or ‘‘head’’ end, the I− is ejected backwards towards the incoming K+. This increases the final relative velocity of the ions and lowers the probability of neutralization. The ion signal is greater and the electron thus appears to favor the I end of the molecule.
Linear and nonlinear optical properties of small silicon clusters
Authors: Rantala Tapio T. , Stockman Mark I. , Jelski Daniel A. , George Thomas F.
Published: July 2002

Electronic contributions to the optical properties of small silicon clusters are examined. Geometries and the electronic structures of the clusters are established using the tight-binding model, and linear as well as nonlinear polarizabilities of the clusters are evaluated using one-electron density matrix techniques. Kleinman’s conjecture for hyperpolarizabilities is shown to be violated in the frequency-degenerate case, which is of practical importance. The nonlinear polarizabilities are found to depend primarily on the symmetry of the cluster and prove to be high for the low-symmetry clusters. Possible experiments and applications are discussed.
On the derivation of the Debye theory of dielectric relaxation from the Langevin equation in the presence of the driving field
Authors: Coffey W. T.
Published: July 2002

It is shown how the Debye results for the behavior of an assembly of dipoles subjected to step-on, step-off, and ac fields which are usually obtained by calculating the linear response from the Smoluchowski equation may also be obtained from the Langevin equation by using a transformation of that equation suggested by Frood and Lal. This transformation allows one to calculate the linear response from the Langevin equation even in the presence of a driving field.
Resonance-enhanced multiphoton ionization-photoelectron spectra of CO2. I. Photoabsorption above the ionization potential
Authors: Wu Ming , Taylor David P. , Johnson Philip M.
Published: July 2002

Photoabsorption above the first ionization potential of CO2 was observed at relatively low laser intensity, detected via resonant-enhanced multiphoton ionization-photoelectron spectra through several Rydberg states. This phenomenon can be explained by the presence of accidental resonances with long-lived autoionizing states which make photon absorption within the ionization continuum possible. Laser powers are too low for this to be explained in terms of a ponderomotive potential and conventional above-threshold ionization. This resonance-enhanced above-threshold absorption phenomenon is potentially useful in the study of excited and superexcited states. Photoelectron energies can be assigned to terminations on CO+2 ionic states at both the four- and five-photon levels, allowing measurement of states up to 22 eV. Two unassigned bands may represent terminations on a new state of CO+2, with an ionization potential of 21.4 eV.