bluuebunny/crossref_metadata_2025 huggingface dataset:
First overtone helium nanodroplet isolation spectroscopy of molecules bearing the acetylenic CH chromophore
Authors: Callegari C. , Conjusteau A. , Reinhard I. , Lehmann K. K. , Scoles G.
Published: July 2002

High-resolution helium nanodroplet isolation spectra of the first overtone (2ν1) of the acetylenic stretch of several substituted acetylenes (RC≡C–H) at T=0.38 K, have been observed for the first time. A tunable 1.5 μm laser is coupled, using a power buildup cavity, to a beam of He droplets seeded with the molecule to be studied. Absorption spectra are recorded by monitoring the beam depletion as a function of laser frequency with a thermal detector. The spectra of hydrogen cyanide (HCN), monodeuteroacetylene (DCCH), cyanoacetylene (NCCCH), propyne (CH3CCH), trifluoropropyne (CF3CCH), 3,3-dimethylbutyne ((CH3)3CCCH), and trimethylsilylacetylene ((CH3)3SiCCH) have been recorded. Due to the superfluid nature of the droplet, rotational resolution is achieved despite the presence of some solvent-induced broadening. The spectroscopic constants have been extracted by means of spectral simulations. The resulting rotational constants are smaller than for the bare molecule by a factor which depends on the molecule nonsphericity and its gas-phase moment of inertia. The linewidths are found to be at least twice as large as those of the corresponding fundamental (ν1) transitions observed in a helium droplet by Nauta et al. [Faraday Discuss. Chem. Soc. 113, 261 (1999) and references therein]. The helium-induced spectral shifts are found to be very small, but cannot be easily rationalized.
Variable-coefficient unstable nonlinear Schrödinger equation modeling electron beam plasma: Auto-Bäcklund transformation, soliton-typed and other analytical solutions
Authors: Gao Yi-Tian , Tian Bo
Published: July 2002

The variable-coefficient unstable nonlinear Schrödinger equation describes the nonlinear modulation of the high-frequency mode in the electron beam plasma, with a nonuniform background. In this paper, an auto-Bäcklund transformation and several families of the exact analytical solutions to the variable-coefficient unstable nonlinear Schrödinger equation are presented. One family turns out to be soliton-typed.
Photoreflectance study of the surface state density and distribution function of InAlAs
Authors: Hwang J. S. , Hwang W. C. , Chang C. C. , Chen S. C. , Lu Y. T.
Published: July 2002

Photoreflectance is used to investigate the band gap, built-in electric field, and surface Fermi level of a series of lattice-matched In0.52Al0.48As surface-intrinsic n+ structures having different undoped layer thicknesses. Experimental results indicate that, although the built-in electric field depends on the undoped layer thickness, there is a range of thickness within which the surface Fermi level is weakly pinned. From the dependence of electric field and surface Fermi level on the undoped layer thickness, we can determine that the surface states distribute over two separate regions within the energy band gap. The densities of the surface states are evaluated as well. Moreover, the dependence of the built-in electric field on undoped layer thickness is converted into the dependence of surface state density on the surface Fermi level in order to theoretically and exactly calculate the energy spectrum of the surface state density using a Guassian distribution function. The center and width of the distribution near the conduction band are obtained from the fitting parameters.
Resonant electron scattering caused by Alfvén waves
Authors: Ciurea-Borcia Rodica , Ignat Margareta , Borcia Ion-Dan
Published: July 2002

Pitch-angle scattering phenomenon caused by resonant interactions between relativistic electrons, 5 MeV<E<15 MeV, and right circularly polarized Alfvén waves, in parallel propagation, is examined for the inner Jovian magnetosphere. A steady-state situation is analyzed, assuming that hot electrons are injected by a constant source, which continuously compensates for the losses at the magnetic mirrors. A parametrical numerical study is performed for different electron energies and different values of electron source intensities. These theoretical results will be compared with experimental data.
Electrostatic potential of a particulate influenced by secondary electron emission in a streaming plasma
Authors: Nishimura Kazumi
Published: July 2002

The electrostatic potential around a particulate immersed in a streaming plasma is investigated by means of a three-dimensional particle simulation. Taking into account the attachment of plasma particles and the emission of secondary electrons in the charging process of a particulate, the particulate charge becomes not only negative but also positive at a state of equilibrium. The Mach cone excited by an ion-acoustic wave appears on the downstream side of a particulate in both cases. It is found that the scale length of the potential structure depends on both the relative velocity of the ion flow to a particulate, and the sign of the particulate charge.
The effect of load thickness on the performance of high velocity, annular Z-pinch implosions
Authors: Douglas M. R. , Deeney C. , Roderick N. F.
Published: July 2002

Numerical calculations have been performed to investigate the role that load thickness may play in the performance of fast annular Z-pinch implosions. In particular, the effects of load thickness on the mitigation of the magnetically-driven Rayleigh–Taylor (RT) instability and energy coupling between the plasma load and generator are addressed. Using parameters representative of the Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories, two-dimensional magnetohydrodynamic simulations show that increased load thickness results in lower amplitude, slightly longer wavelength RT modes. In addition, there appears to be an optimum in implosion velocity which is directly associated with the thickness of the sheath and subsequent RT growth. Thin, annular loads, which should couple efficiently to the accelerator, show a large reduction in implosion velocity due to extreme RT development and increased load inductance. As a consequence, thicker loads on the order of 5 mm, couple almost as efficiently to the generator since the RT growth is reduced. This suggests that Z-pinch loads can be tailored for different applications, depending on the need for uniformity or high powers.
Comment on “Surface diffusion potential energy surfaces from first principles: CO chemisorbed on Pt{110}” [J. Chem. Phys. 111, 9461 (1999)]
Authors: Graham A. P. , Toennies J. P.
Published: July 2002

In a recent article Ge and King [J. Chem. Phys. 111, 9461 (1999)] question, on the basis of their recent density functional calculations, the interpretation of quasielastic helium atom scattering experiments on surface diffusion. In this comment we attempt to clarify a number of misunderstandings and justify the methods used in the data analysis.
Infrared absorption study of nitrogen in N-implanted GaAs and epitaxially grown GaAs1−xNx layers
Authors: Alt H. Ch. , Egorov A. Yu. , Riechert H. , Wiedemann B. , Meyer J. D. , Michelmann R. W. , Bethge K.
Published: July 2002

Fourier-transform infrared absorption measurements have been carried out in the two-phonon region of GaAs. Implantation of the nitrogen isotopes N14 and N15, respectively, into bulk GaAs shows that a local vibrational mode at 471 cm−1 (14N) is due to isolated nitrogen. The band is also found in GaAs1−xNx(0<x<0.03) layers grown by solid-source molecular beam epitaxy. The strength of the band correlates quantitatively with the decrease of the lattice parameter determined by x-ray diffraction for x<0.01 and can be used for the assessment of the nitrogen fraction incorporated substitutionally on anion lattice sites.
Spatial, spectral, and statistical properties of the high-frequency electrostatic fluctuations in a beam-driven turbulent plasma
Authors: McFarland M. D. , Wong A. Y.
Published: July 2002

The spatial, spectral, and statistical properties of the high-frequency (ω∼ωpe) electrostatic fluctuations in an unmagnetized, statistically stable, beam-driven, strongly turbulent plasma are investigated experimentally and the results are compared to the two-component model of Robinson and Newman. The fluctuations are found to consist of low-level wave activity 〈W〉∼10−2–10−3 punctuated by semiperiodic, intense, spiky field events 〈W〉∼1, where 〈W〉 is the normalized wave intensity. The low-level wave activity has a spectral spread Δk/k∼Δω/ω∼30%, dispersion relation vbeam∼ω/k, and correlation length lc≈3λES, where λES is the electrostatic wavelength, and shows evidence of low-intensity parametric decay products. The intense field events, on the other hand, show little correlation for l>λES, have a full width at half maximum of lf<40λD, where λD is the Debye length, and are nonpropagating. The results indicate that the two-component model, and the Zakharov equations to which the model was originally verified, give an accurate description of strong Langmuir turbulence.
Ab initio study of silver bromide AgnBrp(+) clusters (n⩽6,p=n,n−1)
Authors: Rabilloud F. , Spiegelman F. , L’Hermite J. M. , Labastie P.
Published: July 2002

Ab initio calculations in the framework of the density functional theory (DFT) with 19-electron pseudopotential on silver atoms are performed to study the lowest-energy isomers of silver bromide clusters AgnBrp(+) (n⩽6, p=n, n−1). The stability, the structural and electronic properties of neutral, and positively charged systems are examined. The B3LYP functional has been used. For the smallest species, the DFT/B3LYP results are very close to those obtained with multireference configuration interaction or coupled cluster methods. In AgnBrn−1 clusters, the excess electron density shows a picture intermediate between that observed in alkali halide clusters (F-center or localization on a pending alkali atom) and that suggesting partial delocalization on a dimer or trimer silver subunit. Isomer stabilities, fragmentation energies, electric dipole moments, adiabatic and vertical ionization potentials are calculated. The use of 11-electron pseudopotential on silver is discussed.
An improved space-charge-limited emission algorithm for use in particle-in-cell codes
Authors: Watrous J. J. , Lugisland J. W. , Sasser G. E.
Published: July 2002

An improved space-charge-limited emission algorithm has been developed and tested for use in particle-in-cell codes. Comparisons to established and accepted techniques of modeling space-charge-limited emission demonstrate the new technique to be accurate and efficient. Moreover, the new technique is very insensitive to mesh resolution.
Nonvolatile two-color holography in indium-doped lithium niobate
Authors: Zhang Guoquan , Tomita Yasuo , Xu Wusheng , Yang Chunhui
Published: July 2002

We demonstrate nonvolatile two-color holography in an as-grown congruent LiNbO3:In crystal of 2.2 mm thickness. We show that the nonvolatile two-color holographic performance of the crystal is significantly improved as compared with that of other congruent LiNbO3 crystals reported so far. The figure of merit M/# and the two-color recording sensitivity S are measured to be ∼0.04 and ∼3.5×10−4 cm/J, respectively, with a writing intensity of 4 W/cm2 at 780 nm and a gating intensity of 1.15 W/cm2 at 488 nm. These values are comparable to those of unreduced near-stoichiometric LiNbO3 crystals under similar recording conditions.
Exchange of instability modes for natural convection in a narrow horizontal annulus
Authors: Mizushima Jiro , Hayashi Sachiko
Published: July 2002

Instabilities and transitions of natural convection in a narrow horizontal concentric annulus are investigated theoretically by assuming two-dimensional and incompressible flow fields. It is assumed that the inner cylinder is kept at a higher temperature than the outer cylinder. Steady solutions for the natural convection are obtained numerically by Newton–Raphson’s method for various values of Prandtl number and their linear stabilities are analyzed. It is found that there are two different instability modes for the natural convection depending on the Prandtl number, which exchange at a critical value of the Prandtl number. The origins of the two instabilities are clarified from the bifurcation and linear stability analyses of the steady-state solutions.
Observation of the limit cycle in asymmetric plasma divided by a magnetic filter
Authors: Ohi Kazuo , Naitou Hiroshi , Tauchi Yasushi , Fukumasa Osamu
Published: July 2002

An asymmetric plasma divided by a magnetic filter is numerically simulated by the one-dimensional particle-in-cell code VSIM1D [Koga et al., J. Phys. Soc. Jpn. 68, 1578 (1999)]. Depending on the asymmetry, the system behavior is static or dynamic. In the static state, the potentials of the main plasma and the subplasma are given by the sheath potentials, φM∼3TMe/e and φS∼3TSe/e, respectively, with e being an electron charge and TMe and TSe being electron temperatures (TMe>TSe). In the dynamic state, while φM∼3TMe/e, φS oscillates periodically between φS,min∼3TSe/e and φS,max∼3TMe/e. The ions accelerated by the time varying potential gap get into the subplasma and excite the laminar shock waves. The period of the limit cycle is determined by the transit time of the shock wave structure.
Role of quasineutrality in drift waves
Authors: Saleem H. , Haque Q.
Published: July 2002

The ambiguity in using the quasineutrality approximation to describe the low-frequency plasma perturbations in nonuniform magnetized plasmas is pointed out.
Path integral for relativistic Aharonov–Bohm–Coulomb system on the pseudo-sphere
Authors: Nouicer Kh. , Chetouani L.
Published: July 2002

The Green’s function for relativistic spinless Aharonov–Bohm–Coulomb (ABC) system on the pseudo-sphere Λ(2) is calculated using Kleinert’s path integral representation for relativistic spinless particles. The energy spectrum and the corresponding wave functions are extracted for bound and scattering states. The results in the nonrelativistic and flat space limits are considered.
Generator for single bubbles of controllable size
Authors: Ohl C. D.
Published: July 2002

A new type of apparatus for the production of single bubbles of adjustable size is presented. A single bubble is generated by injecting a short burst of gas into a liquid channel flow. The radius of the bubble can be varied continuously from 300 μm up to a 3 mm by adjusting the timing of a valve injecting the gas. The device works also in liquids with variable pressure and can be microcomputer controlled.
Ionization instability of acoustic modes in dusty plasma with ion drag and charging dynamics
Authors: Avinash K.
Published: July 2002

The excitation of the zero frequency mode (which may be the precursor to the void phenomenon in a dusty plasma) at high dust densities is examined taking into account the effects due to grain charging dynamics. The two acoustic modes considered are the dust acoustic mode and the dust electroacoustic mode. These effects are shown to reduce the threshold of the instability. The reduction is mainly due to the increase in the ion attachment frequency with the dust space charge. At high dust density, the destabilization caused by the reduction in the phase-velocity due to small qd and adiabatic dust charge fluctuations also become important. The relevance of these results to recently observed “voids” in dusty plasmas is briefly discussed.
Turbulence reduction and poloidal shear steepening in reversed shear plasmas investigated by light scattering on Tore Supra
Authors: Antar G. , Hoang G. T. , Devynck P. , Garbet X. , Laviron C. , Goniche M.
Published: July 2002

Infrared light scattering is used to characterize turbulence during reversed shear scenario obtained by a fast current ramp-up on Tore Supra [Hoang et al., Phys. Rev. Lett. 84, 4593 (2000)]. The preformed hollow profile is freezed using ion cyclotron resonance minority heating (ICRH). Turbulence intensity is recorded to dramatically decrease during the ramp-up and to recover smoothly as the discharge is ended. Simultaneously, a confinement improvement of about 40% is achieved for about 2 s. Although the diagnostic performs a line average and the measurement is dominated by turbulence at the edge, we show that the core behavior can be deduced. Core turbulence decrease is in agreement with the effective heat diffusivity behavior. With the same optical device, the radial electric shear is deduced from the frequency spectra Doppler shift. Turbulence reduction is thus demonstrated to be correlated with the E×B shear steepening.
Mechanisms for the anomalous dependence of carrier lifetime on injection level and photoconductance on light intensity
Authors: Karazhanov S. Zh.
Published: July 2002

Recent experiments indicated an anomalous dependence of carrier lifetime on injection level and of photoconductance on carrier photogeneration rate. In this work mechanisms for these phenomena are proposed which are based on the effects of anomalous increase of carrier lifetime and filling of the defect level by minority carriers, respectively. Distinct from previously known mechanisms, which considered two types of deep defects, traps and recombination centers, our mechanism may take place in semiconductors containing only one type of deep defect, which is a recombination center. It was shown that the anomalous injection-level dependence of lifetime occurs only when the semiconductor is exactly compensated by recombination centers. This conclusion differs from that of the previous model, which concluded that carrier trapping is responsible for the earlier phenomenon. It is shown that the injection-level dependence of carrier lifetime can be used as an experimental tool to determine the deep defect concentration.
Experimental test of the radial force balance equation in the compact helical system
Authors: Ida K. , Fujisawa A. , Iguchi H. , Yoshimura Y. , Minami T. , Okamura S. , Nishimura S. , Isobe M. , Kado S. , Liang Y. , Nomura I. , Tanaka K. , Osakabe M. , Takahashi C. , Matsuoka K.
Published: July 2002

Experimental test of the radial force balance equation was done in the Compact Helical System Heliotron/Torsatron [S. Okamura et al. Nucl. Fusion 39, 1337 (1999)]. A radial electric field is measured with a heavy ion beam probe, while plasma rotation and drift velocity of fully ionized carbon are measured with charge exchange spectroscopy. The two measurements agree with each other to within 10% of the radial electric field in a wide range of electron densities of 0.3–2.0×1019 m−3.
Many-body effects and matrix inversion in low-Reynolds-number hydrodynamics
Authors: Ichiki Kengo , Brady John F.
Published: July 2002

It is shown that the method of reflections in resistance form (with truncated multipoles) is one of many possible iterative methods to obtain the inverse of the mobility matrix (with the same truncation) in low-Reynolds-number hydrodynamics. Although the method of reflections in the mobility form is guaranteed to converge, it is found that in the resistance form the method may fail to converge. This breakdown is overcome by conjugate-gradient-type iterative methods, and the implications of the iterative method for low-Reynolds-number hydrodynamics are discussed.
Raman forward scattering in plasma channels
Authors: Shvets G. , Li X.
Published: July 2002

Raman scattering instability of an intense laser pulse in a plasma channel proceeds differently than in a homogeneous plasma: The growth rate is reduced and the scaling with the laser intensity modified. These differences, significant even for shallow plasma channels, arise because of the radial shear of the plasma frequency and the existence of the weakly damped hybrid (electrostatic/electromagnetic) modes of the radially inhomogeneous plasma. The interplay of these two effects produces double-peaked spectra for the direct forward scattering in a channel.
Onset of chaotic wave dynamics near the critical point in a glow discharge: Theory and experiment
Authors: Bruhn B. , Dinklage A. , Koch B.-P. , Wilke C.
Published: July 2002

Conditions for the onset of irregular wave dynamics are derived from a theoretical description of ionization waves in a glow discharge by means of amplitude equations. It is shown that the Benjamin–Feir condition of the cubic Ginzburg–Landau equation is a necessary but not sufficient condition for loss of stability of plane waves in a higher order amplitude equation. The onset conditions are numerically evaluated in the case of a neon discharge. Experimental findings concerning the onset of chaotic wave dynamics near the critical point agree very well with the theoretical predictions. This example reveals physical relevance of higher order nonlinearities close to the critical point.
Toroidal momentum confinement in neoclassical quasilinear theory in tokamaks
Authors: Shaing K. C.
Published: July 2002

The neoclassical quasilinear theory is employed to study the toroidal momentum confinement in tokamaks. The electrostatic fluctuation-induced toroidal stress is calculated. It is found that besides the conventional diffusion term, there is also a convective term in the toroidal momentum equation. In the case of a weak toroidal momentum source, the logarithmic gradient of the toroidal flow is proportional to the linear combination of the logarithmic gradients of the plasma pressure and the temperature. This conclusion is insensitive to both the renormalization procedure and the magnitude of the fluctuations. The theory is applied to explain the changing of the toroidal flow direction after low mode (L-mode) to high mode (H-mode) transition observed in some experiments.
Density enhancement near lower hybrid resonance layer in m=0 helicon wave plasmas
Authors: Yun Seokmin , Cho Suwon , Tynan George , Chang Hongyoung
Published: July 2002

Radial density profiles are measured with various frequencies, powers, pressures, and gas masses in m=0 helicon wave plasmas. The radial density profiles show clear mode changes for the E mode (capacitive coupling), the H mode (inductive coupling), and the W mode (wave coupling) as the rf (radio frequency) power increases. The radial density profile hardly changes as the frequency varies in the E and the H modes but changes substantially in the W mode; it changes from a central peak profile to a hollow density profile near the lower hybrid resonance frequency. The change of radial density profile near the lower hybrid resonance frequency occurs only at pressures below 20 mTorr, and the resonance effects become very weak at higher pressures. The change of the radial density profile may be explained by the density enhancement near the lower hybrid resonance layer. The experiments with various magnetic fields and gas masses show a close correlation between the lower hybrid frequencies and the density profile changes.
Ground and valence excited states of C2N and CN2 transients: Ab initio geometries, electronic structures, and molecular properties
Authors: Pd. Rajendra , Chandra P.
Published: July 2002

Geometric and vibrational characterization of CCN(X̃ 2Π,ã 4Σ−,Ã 2Δ,B̃ 2Σ−,C̃ 2Σ+), CNC(X̃ 2Πg,Ã 2Δu,B̃ 2Σu−), CNN(X̃ 3Σ−,ã 1Δ,b̃ 1Σ+,Ã 3Π,1 1Π) and NCN(X̃ 3Σg−,ã 1Δg,b̃ 1Σg+,Ã 3Πu) systems have been done using full-valence complete active space SCF (CASSCF) method. The Renner–Teller interaction parameter, ε, is calculated for Π electronic states with CASSCF potentials. Excitation energies with zero-point corrections, T0, electric field gradient (efg), and dipole moment, μ, are calculated using CASSCF, complete active space second order perturbation theory (CASPT2) and multireference singles and doubles configuration interaction (MRD-CI) levels of theory. The fact that CASSCF values of the principal components VXX, VYY, and VZZ of the efg tensor listed through two quantities eq1(=VZZ) and eq2(=VXX−VYY) are not very different from their CASPT2 counterparts, suggests that second-order perturbation involving all singles and doubles over the one-dimensional space spanned by the CASSCF wave function are not important for the efg and μ. However, the important contributions come from the higher excitations (triple, quadruples, etc.), which are included in MRD-CI wave function, by taking multireference zeroth-order wave function. The use of iterative natural orbital seems to be necessary to obtain stable values of the efg and μ in the MRD-CI method.
Generation of density irregularities and whistler waves by powerful radio waves in the polar ionosphere
Authors: Kuo S. P. , Koretzky E.
Published: July 2002

It is shown that a powerful high-frequency (HF) wave can excite a thermal instability in the E region of the polar ionosphere to introduce a significant electron temperature perturbation. This instability has a broad spatial spectrum with an upper bound. The temperature perturbation then develops nonlinearly, via the electron thermal diffusion process, into spatially periodic irregularities along the geomagnetic field, where the spatial period of the irregularities in the range between 460 m and 1.3 km is mainly governed by the minimum wavelength in the spectrum of the instability. Due to the decreasing dependence on the electron temperature of the recombination rates of electrons with the E region dominant ion species NO+ and O2+, the background plasma density as well as the electrojet current are also perturbed spatially in a similar fashion as the electron temperature irregularities. If an amplitude-modulated HF wave with a modulation frequency in the frequency range between 2 and 30 kHz is used to modulate the electrojet current in time, the density irregularities can effectively convert the current perturbation into a spatially distributed mode current of whistler waves. This direct excitation process enhances the generation efficiency of whistler waves and reduces their harmonic components.
Uncertainty for spin systems
Authors: Barros e Sá Nuno
Published: July 2002

A modified definition of quantum mechanical uncertainty Δ for spin systems, which is invariant under the action of SU(2), is suggested. Its range is shown to be ℏ2j⩽Δ⩽ℏ2j(j+1) within any irreducible representation j of SU(2) and its mean value in Hilbert space computed using the Fubini–Study metric is determined to be mean(Δ)=ℏ2j(j+1/2). The most used sets of coherent states in spin systems coincide with the set of minimum Δ uncertainty states.
Transient phenomena and secularity of linear interchange instabilities with shear flows in homogeneous magnetic field plasmas
Authors: Tatsuno T. , Volponi F. , Yoshida Z.
Published: July 2002

Transient and secular behaviors of interchange fluctuations are analyzed in an ambient shear flow by invoking Kelvin’s method of shearing modes. Because of its non-Hermitian property, complex transient phenomena can occur in a shear flow system. The combined effect of shear flow mixing and Alfvén wave propagation overcomes the instability driving force at sufficiently large time, and damps all fluctuations of the magnetic flux. On the other hand, electrostatic perturbations can be destabilized for sufficiently strong interchange drive. The time asymptotic behavior in each case is algebraic (nonexponential).
Ion beam driven ion-cyclotron waves in a plasma cylinder with negative ions
Authors: Sharma Suresh C. , Srivastava M. P.
Published: July 2002

An ion beam propagating through a magnetized plasma cylinder containing K+ positive ions, electrons and SF6− negative ions drives electrostatic ion-cyclotron (EIC) waves to instability via Cerenkov interaction. Two EIC wave modes are present, the SF6− and K+ modes. The frequencies and the growth rate of both the unstable negative ion and the positive ion modes increase, with increasing ε (=nSF6−/nK+), the relative density of negative ions. The frequencies of both the unstable modes (SF6− and K+) increase, with the magnetic field. The growth rate of both the unstable modes (SF6− and K+) scales as the one-third power of the beam density. The real frequency of both the unstable modes increases as almost the square root of the beam energy. Numerical calculations of the growth rate and modes frequencies have been carried out for the parameters of the experiment of Song et al. [Phys. Fluids B 1, 2316 (1989)] and the results are in good agreement with the experimental observations.
Photoluminescence studies of GaN layers grown by hydride vapor phase epitaxy
Authors: Yu P. W. , Park C. S. , Kim S. T.
Published: July 2002

Photoluminescence of gallium nitride (GaN) layers 7.5–870 μm thick was studied with changes in temperature and excitation intensity. These layers were grown by hydride vapor phase epitaxy on a buffer layer of aluminum nitride (AlN). The photoluminescence emission consists of the shallow-donor bound exciton at 3.471 eV and the free hole-to-electron bound to a donor (possibly a nitrogen vacancy VN or oxygen) transition at 3.40–3.433 eV. The peak position varies depending on the thickness of the GaN and AlN layers. The localized donor due to donor concentration fluctuation is attributed to the variable peak position. The observed 3.269 eV emission is attributed to a donor–acceptor pair transition. The relationship between the peak and the excitation intensity is described accurately by a theoretical description which yields Ed=32 meV and Ea=230 meV, which originate, respectively, from a silicon donor and carbon acceptor.
Ab initio studies of the reactions of M(1S,3P, and 1P) with GeH4 (M=Cd, Hg)
Authors: Luna-Garcı́a H. , Ramı́rez-Solı́s A. , Castillo S.
Published: July 2002

The interaction of Cd(1S:5s2, 3,1P:5s15p1) and Hg(1S:6s2, 1,3P:6s16p1) with GeH4 were studied by means of Hartree–Fock self-consistent field (SCF) and multiconfigurational SCF plus variational and multireference second order Möller–Plesset perturbational configuration interaction (CIPSI) calculations, using relativistic effective core potentials. It was found that both metal atoms in their 3P(ns1np1) state break spontaneously the Ge–H bond of the germane molecule, giving place to the MH+GeH3 (M=Cd, Hg) final products. For both atoms, the 1P(ns1np1) state is also inserted in the Ge–H bond and the corresponding interaction surface shows an avoided crossing with the lowest-lying X 1A′ potential surface adiabatically linked with the M(1S:ns2)+GeH4. This interaction leads eventually to the MH+GeH3 products. The HMGeH3 X 1A1 (M=Cd, Hg) intermediate molecules, diabatically correlated with the M(1P:ns1np1)+GeH4, which lie 13.6 and 21 kcal/mol, respectively, above the ground state reactants, have been carefully characterized as well as the dissociation channels leading to the MH+GeH3 and H+MGeH3 products. These products are reached from the HMGeH3 intermediates without activation barriers. Accurate energy differences for all these species are reported. This work suggests that the simultaneous photoexcitation of Cd and Hg atoms in the presence of silane and germane molecules in the gas phase could be used to produce better quality a-SiGe:H thin films.
The structure of the OCS–H2 van der Waals complex embedded inside He4/3He droplets
Authors: Grebenev Slava , Sartakov Boris G. , Toennies J. Peter , Vilesov Andrej F.
Published: July 2002

The rotational infrared spectra of complexes of OCS with single para-H2, HD, and ortho-D2 molecules inside cold (0.15 K) mixed 4He/3He droplets are used to determine the in-plane structure, in agreement with calculations for the free complex, as well as the out-of-plane amplitude imposed by the superfluid liquid 4He environment.
Ionic interactions in molten complex chlorides from vibrational dephasing
Authors: Kirillov S. A. , Voyiatzis G. A. , Musiyenko I. S. , Photiadis G. M. , Pavlatou E. A.
Published: July 2002

In this article we present the first quantitative estimates of the spectroscopically active part of the interaction potential in molten complex chlorides based on dephasing studies. We have selected the molten salt systems containing quasispherical complex MCl4−2 anions (M=Mn+2 and Zn+2) and performed the study of their ν1(A1) isotropic Raman line profiles as a function of the temperature and concentration. We have analyzed the form of the time correlation function of vibrational dephasing and determined the type of modulation events, which cause the line broadening processes in these systems; these are found to be purely discrete Markovian. Within the formalism of the purely discrete Markovian modulation, we have made a judgement about spectroscopically active interactions in these systems. Interionic potential in complex chlorides is dominated by the attraction forces, which depend on the interparticle distance r as r−4, and by repulsion of the r−7-type.
Exploring the transition state for the Li+HF→LiF+H reaction through the A←X absorption spectrum and X←A stimulated emission pumping
Authors: Aguado Alfredo , Lara Manuel , Paniagua Miguel , Roncero Octavio
Published: July 2002

The A(2 2A′) first excited electronic state of LiHF has been calculated (about 3300 points at MRDCI level) and an analytical fit of the global potential energy surface is presented. The A←X absorption spectrum is simulated at 10 K between 9000 and 13 000 cm−1, what involves the calculation of excited bound states for moderately high total angular momentum. These quasibound levels of the A state can only decay by electronic predissociation (EP) towards the X ground state, since the spontaneous radiative emission is considered to be very slow. The decay of such states has been estimated using a perturbative approach and it is found that LiF products are produced with a high efficiency (>80%). The X←A stimulated emission pumping spectra is simulated for several initial quasibound levels. These spectra allow the examination of the reaction dynamics specially near the transition state region, and the first photon excitation may act as a rotational filter, reducing the problem of the partial wave average involved in reactive collisions. The high reaction efficiency of this last process is also discussed.
Exchange energy in a double-well potential profile from fluctuation theory
Authors: Battezzati Michele , Magnasco Valerio
Published: July 2002

This article shows that the asymptotic quantum mechanical exchange energies and wave functions in a bistable potential can be obtained by computing the complex-valued first-passage time across the potential barrier from a transient state endowed with Smoluchowski boundary conditions, whose probability density is concentrated on one side of the barrier. This interpretation is validated by one of the authors’ previous work on diffusion in a random field, showing that this model yields a diffusion equation equivalent to quantum mechanical equations.
Electrostatic thermal modes in dusty plasmas
Authors: Schamel H. , Das Nilakshi , Rao N. N.
Published: July 2002

A continuum of nonlinear, undamped, ultralow-frequency acoustic waves is shown to exist, propagating below (above) the dust-thermal velocity and being characterized by deficit (surplus) of dust particles trapped in the trough of the wave potential.
Entropy driven unidirectional motion of Brownian particle inside a three-dimensional tube
Authors: Sheu Sheh-Yi , Yang Dah-Yen
Published: July 2002

We propose a three-dimensional (3-D) tube model consisting of a sequence of connected two-gate blobs for an ion channel and a microtube of a molecular motor. Due to the competition effect between two gates (or sinks) on the same blob surface, the escape pathway for the Brownian particle inside a blob is selected by these two gates owing to gate modulation. Furthermore, there exists a nonzero net flow inside the 3-D tube and this unidirectional flow is strongly dependent on the selectivity of the escape pathway for the Brownian particle inside each individual blob. In this context, the Brownian particle moves unidirectionally without any applied external fluctuation forces and the entire process is entropy driven.
On product state distributions in triatomic unimolecular reactions: Beyond phase space theory and the adiabatic assumption
Authors: Larregaray P. , Bonnet L. , Rayez J. C.
Published: July 2002

Our goal is to derive a simple dynamically corrected statistical treatment of state distributions in the products of triatomic unimolecular reactions involving efficient energy transfers between rotation and translation motions en route to products. For, phase space theory or the adiabatic channel model—the only statistical approaches of final state distributions—may not be applicable to such processes. We thus analyze and model how the departing atom perturbs the rotation of the diatomic molecule. Since the general problem is intricate, we limit our study to the basic, yet realistic and instructive case where the bending force is harmonic and the total angular momentum is zero. A remarkable fact is the one-to-one relation between the perturbation and a coefficient χ, related in a simple way to the mechanical parameters of the system. Transition state theory combined with our model—the linear transformation model—leads to the desired treatment of product state distributions. Its predictions are in good agreement with dynamical calculations. We also show that there is a close relation between the final shape of state distributions and the angular dependence of the potential energy at the transition state.
Study of the nature of glass transitions in the plastic crystalline phases of cyclo-octanol, cycloheptanol, cyanoadamantane and cis-1,2-dimethylcyclohexane
Authors: Tyagi Madhusudan , Murthy S. S. N.
Published: July 2002

Cycloheptanol, cyclo-octanol, cyanoadamantane and cis-1,2-dimethylcyclohexane are known to form plastic crystals which can be supercooled to show a glass transition at a temperature Tg. The molecular dynamics in different plastic phases is studied in their supercooled states using dielectric spectroscopy (frequency range: 106 Hz–10−3 Hz) and differential scanning calorimetry (DSC) over a wide temperature range. The kinetic freezing of the various dielectric processes have been critically examined in relation to the Tg found in the DSC experiments. The plastic phase I of cyclo-octanol shows two Tg’s: one at 148.5 K and the other at 164 K, the former of which is not found in well annealed phase I. The dielectric α-modes correspond to the latter. Cycloheptanol exhibits many Tg’s for the different plastic phases. Unlike the cyclic alcohols, the dielectric spectra of cyanoadamantane and cis-1,2-dimethylcyclohexane is clearly found to follow the Havriliak–Negami equation, both of which interestingly show very little molecular mobility, often referred to as the secondary relaxation in their glassy states. In addition, the temperature dependence of the relaxation rates and the dielectric strengths are critically examined for various supercooled phases to gain an insight into the nature of the molecular mobility in those phases.
An inhomogeneous integral equation for the triplet structure of binary liquids
Authors: Jorge S. , Lomba E. , Abascal J. L. F.
Published: July 2002

The inhomogeneous integral equation proposed by Attard for the study of triplet correlations [J. Chem. Phys. 91, 3072 (1989)] has been generalized to multicomponent systems. Defining one of the particles of a triplet as the source of an external field, the three particle distribution functions for the mixture are calculated using the inhomogeneous Ornstein–Zernike equation, an approximate closure relation and the Triezenberg–Zwanzig relation. The proposed theory performs satisfactorily for asymmetric mixtures of Lennard–Jones fluids for which other approximations at the two particle level tend to be rather inaccurate.
Comment on “Instabilities of self-gravitating dusty clouds in magnetized plasmas” [Phys. Plasmas 7, 3762 (2000)]
Authors: Ciolek Glenn E.
Published: July 2002

A study was recently presented on the linear stability of self-gravitating, magnetic interstellar dust clouds, including the effects of fluctuations of the charges of dust grains and collisions with a neutral fluid, and it was suggested that gravitational instability in such systems could lead to the formation of stars. In fact, this particular problem was solved in a series of detailed, nonlinear numerical simulations (which also included the effects of fluctuating grain charges and friction due to collisions with neutral particles) that were published years ago.
Nature of polarized excitons
Authors: Ambjörnsson T. , Apell S. P.
Published: July 2002

The electromagnetic interaction energy of a molecular aggregate consisting of pointlike molecules in the presence of an electromagnetic field is derived. The corresponding Hamiltonian consists of three parts; H0 describes the aggregate in the absence of the electromagnetic field, H1 describes the interaction of the molecules with the external field, and H2 corresponds to the induced interaction between the molecules. Based on this Hamiltonian we derive a self-consistent equation of motion for a quasiparticle, which we refer to as a polarized exciton. The equation has the same form as the one in classical dipole theory. The polarized exciton model is based on a time-dependent perturbative treatment and corresponds to the assumption H0≫H1+H2. Our model is compared to standard exciton theory, which is based on the assumption H0≫H2≫H1. In particular the differences and similarities are illustrated for a direct example, a finite linear chain. We advocate the use of polarized excitons to fully account for the physics in these systems.
Wetting transitions in symmetrical polymer blends
Authors: Engels S. M. , Leermakers F. A. M.
Published: July 2002

The characteristics of wetting in polymer blends are investigated by a self-consistent-field theory. A symmetrical system is chosen: the interface between two homopolymeric liquids A and B is wetted by a third homopolymer C which is equally insoluble in both liquids. All components have the same molecular weight (NA=NB=NC=N=10 or 100). The emphasis of this study is on the wetting transitions induced by varying the interactions between the components. Cahn’s argument, which predicts complete wetting near the critical temperature of two system components, is verified in this context. We show that it is necessary to consider the effective interaction parameters χACeff=χBCeff to verify Cahn’s argument. Since we vary the solubility of C (given by χAC=χBC) and the thickness of the AB interface (determined by χAB) independently, we have a two-dimensional parameter space. In this parameter space we can distinguish three regimes representing wetting transitions with different characteristics. One of these regimes indeed shows Cahn-type transitions. A key observation is that the wetting transitions near the simultaneous critical point of mixtures A/C and B/C are of a second-order type. A second regime in the parameter space represents wetting transitions which are understood from the high surface tension of the AB interface. In many cases these wetting transitions are also of a second-order type, but become first order when NχAB>8. In the third regime we find what might be called “pseudowetting:” from inspection of the adsorption isotherms it follows that C seems to wet the AB interface, but with increasing amount of C in the system, the wetting layer is suddenly destroyed. The reason for this is clear. Here, the apparent wetting point χACwet is close to χACeff,crit, but χAB<χAC=χBC, the wetting film is unstable. As a result, while at first Cahn’s argument seems to be fulfilled, it eventually fails in this region of the parameter space.
Beam-generated plasmas for processing applications
Authors: Meger R. A. , Blackwell D. D. , Fernsler R. F. , Lampe M. , Leonhardt D. , Manheimer W. M. , Murphy D. P. , Walton S. G.
Published: July 2002

The use of moderate energy electron beams (e-beams) to generate plasma can provide greater control and larger area than existing techniques for processing applications. Kilovolt energy electrons have the ability to efficiently ionize low pressure neutral gas nearly independent of composition. This results in a low-temperature, high-density plasma of nearly controllable composition generated in the beam channel. By confining the electron beam magnetically the plasma generation region can be designated independent of surrounding structures. Particle fluxes to surfaces can then be controlled by the beam and gas parameters, system geometry, and the externally applied rf bias. The Large Area Plasma Processing System (LAPPS) utilizes a 1–5 kV, 2–10 mA/cm2 sheet beam of electrons to generate a 1011–1012 cm−3 density, 1 eV electron temperature plasma. Plasma sheets of up to 60×60 cm2 area have been generated in a variety of molecular and atomic gases using both pulsed and cw e-beam sources. The theoretical basis for the plasma production and decay is presented along with experiments measuring the plasma density, temperature, and potential. Particle fluxes to nearby surfaces are measured along with the effects of radio frequency biasing. The LAPPS source is found to generate large-area plasmas suitable for materials processing.
Quantum transport theory of vibrons in a molecular monolayer
Authors: Pouthier V. , Light J. C. , Girardet C.
Published: July 2002

We establish a quantum kinetic equation describing the transport properties of the vibrons in a molecular monolayer adsorbed on a dielectric substrate. A renormalization procedure is applied to the Hamiltonian of the system which is then separated in a vibron Hamiltonian, a bath Hamiltonian connected the external motions and a coupling Hamiltonian between the vibrons and the external modes. A perturbative analysis based on the projector method allows us to eliminate the irrelevant information related to the bath dynamics. The use of conventional approximations (Markov limit and Wick theorem) leads us to write the kinetic equation in a form exhibiting linear and nonlinear contributions. The linear term characterizes irreversible processes connected to the bath fluctuations whereas the nonlinear term represents a self-modulation of the dynamical matrix with respect to the vibron distribution. An application of the transport of CO vibrons on NaCl(100) illustrates the method.
Numerical investigation of the scalar probability density function distribution in neutral and stably stratified mixing layers
Authors: Cortesi A. B. , Smith B. L. , Sigg B. , Banerjee S.
Published: July 2002

Direct numerical simulations of a temporally-growing mixing layer are performed to examine how the resulting concentration probability density function (PDF) of an advected, nearly nondiffusive, passive scalar (numerical dye) varies under the combined effects of stable stratification, thermal conductivity, and perturbation of the initial velocity field. In stably-stratified mixing layers convective instabilities are responsible for the development of streamwise vortices or ribs. These exhibit shorter spanwise separation than in nonstratified shear layers and lead to differences in the subsequent three-dimensionalization of the flow field. Furthermore, vortex development is very efficiently suppressed by stratification in high-thermally-conducting fluids, because only small temperature gradients arise and thus negligible baroclinic vorticity reinforcement is available to counterbalance the stabilizing effects of buoyancy. This is reflected in smaller mixed-fluid total PDF areas with decreasing Prandtl number. The effect of coherent structures on the PDF distribution is seen to be significant. The main rolls are the cause of “global-concentration” mixing, i.e., mixing of fluid lumps with vastly different species concentration, reflected in nonmarching PDF peaks, where the peak of the PDF is located at a constant concentration value. Ribs, on the other hand, having shorter spatial extent and engendering mixing on a narrower concentration range, cause “local-concentration” mixing, which translates into marching PDF peaks. The combined action of the spanwise vortices rolling up, or pairing, and the ribs, may then give rise to tilted PDF distributions, which are intermediate between nonmarching and marching. The “pairing parameter,” used to predict the transition from nonmarching to marching PDFs, was found not to be reliable, small values being sufficient to allow the PDF to be marching in stratified flow. The scalar mean, and the scalar mean-mixed-fluid, concentrations are also investigated and are seen to deviate considerably from each other, depending on the strength and coherence of the vortical structures, the imposed stratification, and the thermal conductivity.