bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibronic effects on solvent dependent linear and nonlinear optical properties of push-pull chromophores: Julolidinemalononitrile
Authors: Moran Andrew M. , Egolf Debra S. , Blanchard-Desce Mireille , Kelley Anne Myers
Published: July 2002

The linear absorption spectra and absolute resonance Raman excitation profiles of the “push-pull” chromophore julolidinemalononitrile have been measured in cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol solution at excitation wavelengths spanning the strong visible charge-transfer absorption band. Numerical simulation of the spectra using time-dependent wave-packet propagation methods yields the excited-state geometry changes along the ∼15 strongly Raman-active vibrations as well as the solvent reorganization energies. The distribution of the total vibrational reorganization energy among the various normal modes is solvent dependent, indicating solvent polarity effects on the electronic structure. These results are compared with those previously obtained for two other push-pull chromophores, p-nitroaniline and julolidinyl-n-N,N′-diethylthiobarbituric acid. The frequency dispersion of the molecular first hyperpolarizability, β, is also calculated in each solvent using a time-domain form of the standard Oudar–Chemla two-state model modified to incorporate solvent reorganization, inhomogeneous broadening, and the vibronic structure of the charge-transfer state. We show that accurate extrapolation of β measured at frequencies in the near-infrared to zero frequency requires a realistic description of the excited state as the measuring wavelength approaches a two-photon resonance. This is particularly relevant to the high chromophore concentrations needed for device applications, where intermolecular interactions can strongly perturb the electronic transitions.
Current–voltage characteristics of polar heterostructure junctions
Authors: Singh Madhusudan , Singh Jasprit , Mishra Umesh
Published: July 2002

We report calculations that show that a metal–polar semiconductor heterostructure can exhibit highly controllable nonlinear current–voltage characteristics. A change in barrier thickness can alter the characteristics from Schottky-like to ohmic in different bias regimes. The origin of these unusual effects is a large electric field (>106 V/cm) and high sheet charge(∼1013–1014cm−2) without doping, in the polar heterostructure. Theoretical calculation of the tunneling current density in these systems is done in this work. The results indicate that very interesting nonlinear behavior is shown by these systems, even in the undoped case. The choice of suitable compositions of the materials and thicknesses can be used to tailor devices with desired characteristics.
Comment on “Anti-cooperativity in hydrophobic interactions: A simulation study of spatial dependence of three-body effects and beyond” [J. Chem. Phys. 115, 1414 (2001)]
Authors: Czaplewski Cezary , Rodziewicz-Motowidło Sylwia , Liwo Adam , Ripoll Daniel R. , Wawak Ryszard J. , Scheraga Harold A.
Published: July 2002

We address the criticism of our methodology for determination of the three-body cooperative terms in the potential of mean force (PMF) of the hydrophobic interaction of methane molecules in water [Czaplewski et al., Prot. Sci. 9, 1235 (2000)] expressed in the title paper of Shimizu and Chan, as well as their conclusion that hydrophobic association is predominantly anti-cooperative. We demonstrate that their reference two-methane PMF curve is subject to a systematic error, which invalidates their conclusions about the sign of the cooperative PMF.
Reply to “Comment on ‘Anti-cooperativity in hydrophobic interactions: A simulation study of spatial dependence of three-body effects and beyond’ ” [J. Chem. Phys. 116, 2665 (2002)]
Authors: Shimizu Seishi , Chan Hue Sun
Published: July 2002

An accurate, physically justifiable procedure for determining zero-PMF baselines is essential in addressing the important issue of anti-cooperativity or cooperativity in hydrophobic interactions. For this purpose, the test-particle insertion technique is less dependent on unproven assumptions than other procedures. Its application is recommended for studies in which the technique is computationally feasible. We agree with Czaplewski et al. that much remains to be learned about hydrophobic interactions nonadditivity, and that sustained efforts are needed to overcome numerical uncertainties in the simulation results reported thus far. Nonetheless, with data available to date, there is apparently stronger support for anti-cooperativity than for cooperativity for a significant fraction of solute configurations of the three-methane system in question. This is illustrated by a reanalysis of a three-methane PMF reported previously by Czaplewski et al. [Protein Sci. 9, 1235 (2000)] in light of the new two-methane PMF presented in their Comment.
The parallel cylindrical mirror electron energy analyzer
Authors: Read Frank H.
Published: July 2002

An electrostatic energy analyzer is described that allows parallel acquisition of the energy spectrum of charged particles over a wide range of energies and over the complete range of azimuthal directions. The analyzer is similar to the cylindrical mirror analyzer except that a linear potential variation is applied in the axial direction to the outer cylinder. The analyzer can also be used in a second-order focusing mode to analyze a narrow range of energies with higher energy resolution. A position sensitive detector is required, the shape of which depends on the way in which the analyzer is used. Comparisons with existing electron energy analyzers are given.
Non-Lorentzian spectral diffusion line shapes in glasses: Analysis based on the two-level-system model
Authors: Kharlamov B. M. , Zumofen G.
Published: July 2002

The dynamics of tunneling two-level systems (TLSs) or other slowly relaxing objects in glasses cause a diffusional broadening of the line shapes of chromophore impurity molecules. The standard tunneling TLS model for glasses predicts that the diffusional broadening is Lorentzian. Recently non-Lorentzian absorption line profiles of chromophores were discovered in a series of glasses by means of optical hole burning spectroscopy. This paper attempts to analyze possible reasons for the non-Lorentzian line shapes. The standard model is modified to account for the dispersion of the chromophore-TLS coupling strengths, for TLS concentration inhomogeneities, and for chromophore-TLS interactions other than dipolar. It is found that in general the above modifications lead to line shapes, which are more peaked at the center and show more pronounced wings than a Lorentzian of the same width, in agreement with the experimental observations.
Absorption characteristics of vapor transport equilibrated Er:LiNbO3 crystals
Authors: Zhang De-Long , Pun E. Y. B. , Chen Xiao-Jun , Wang Yan , Jin Yue-Han , Zhu Deng-Song , Wu Zhong-Kang
Published: July 2002

The visible and infrared transmission spectra of vapor transport equilibration (VTE) treated Er:LiNbO3 crystals, which have different doping levels (0.2%, 0.4%, and 2.0% Er per cation site), different cut orientation (X and Z cut) and different VTE duration (80, 120, 150, and 180 h), were recorded at room temperature in the wavelength range of 250–3700 nm. All of 2.0 mol % doped VTE crystals have precipitated whether X cut or Z cut, while the others have not. Their absorption characteristics were summarized and discussed in contrast to those of corresponding as-grown crystals. The OH− absorption feature of VTE treated Er:LiNbO3 is found to be different from that of pure VTE LiNbO3 crystal. The significant reduction of OH− absorption band implies that the hydrogen content in the VTE crystals has been reduced substantially whether the crystal precipitates or not. The electron transition absorption characteristics of the lower-doped, not precipitated crystals mainly include the higher transmittance, slight shift of peak or band position, slight absorption intensity change, the appearance of some additional peaks or bands, the narrowing of the peak width (full width at half maximum), and the definite blueshift of the optical absorption edge. The spectral changes are associated with the redistribution of Er3+ spectroscopic sites induced by the VTE procedure. In comparison with those lower-doped VTE crystals, the highly doped VTE crystals display more significant absorption characteristics: the significant enhancement of 1480 nm pumping band and the obvious weakening of 1531 nm peak, the appearance of many additional peaks in the infrared region, and the interesting evolution of the transmittance with the wavelength. These substantial spectral changes are unambiguously conducted with the formation of a precipitate ErNbO4 induced by the VTE treatment in these crystals. The mechanism for the formation of the precipitate has been tentatively explained from the viewpoint of both microscopic and macroscopic domains. The higher transmittance, the narrowing of the peak width, the blueshift of the optical absorption edge, as well as the significant reduction of the OH band are the indicatives of the VTE treatment having brought these crystals closer to stoichiometric composition.
Nonlinear equation for curved stationary flames
Authors: Kazakov Kirill A. , Liberman Michael A.
Published: July 2002

A nonlinear equation describing curved stationary flames with arbitrary gas expansion, θ=ρfuel/ρburnt, subject to the Landau–Darrieus instability, is obtained in a closed form without an assumption of weak nonlinearity. It is proved that in the scope of the asymptotic expansion for θ→1, the new equation gives the true solution to the problem of stationary flame propagation with the accuracy of the sixth order in θ−1. In particular, it reproduces the stationary version of the well-known Sivashinsky equation at the second order corresponding to the approximation of zero vorticity production. At higher orders, the new equation describes influence of the vorticity drift behind the flame front on the flame velocity and the flame front structure. Its asymptotic expansion is carried out explicitly, and the resulting equation is solved analytically at the third order. For arbitrary values of θ, the highly nonlinear regime of fast flow burning is investigated, for which case a large flame velocity expansion of the nonlinear equation is proposed.
Vibrational echoes for classical and quantum solutes
Authors: Akiyama Ryo , Loring Roger F.
Published: July 2002

The infrared vibrational echo measurement has the capacity to discriminate among spectral line-broadening processes according to time scale. Modelling infrared vibrational echoes in condensed phases at the molecular level requires computing the dynamics of large, anharmonic systems, a task far more tractable in classical than in quantum mechanics. The use of classical trajectory data to compute vibrational echoes raises the issue of the quantitative relationship between classical and quantum vibrational echo calculations. This relationship is addressed here in classical and quantum calculations of the vibrational echo for an anharmonic oscillator coupled to a harmonic solvent. Our calculations demonstrate that for a high-frequency solute coupled to a low-frequency solvent, in which the echo is dominated by pure dephasing dynamics, a fully classical calculation can represent a reasonable approximation to the quantum mechanical result.
The effective group potential, a new method for the study of spectrum in large molecules: Tests and perspectives
Authors: Heully Jean-Louis , Poteau Romuald , Berasaluce Sandra , Alary Fabienne
Published: July 2002

The new method, called effective group potential, which generalizes the concept of effective core potential by including in a potential the effects of many atoms ligand, is used to calculate transition energies. Three molecules containing a metal atom (Cu, Cr, Ni) surrounded by NH3 ligands have been studied. It is shown that by replacing the NH3 group by an effective group potential, the characteristic transitions energies are very well reproduced at various ab initio levels. The transitions studied are transitions between open shells on the metallic center but also ligand to metal transitions where, though unexpected, quite reliable results were also obtained. This method is thus very well suited for the study of the spectrum of a large molecule under the condition that a separation between an active part plus some spectator groups can be made, a condition satisfied in most organometallic molecules as well as in biological molecules.
Electronic relaxation of aniline in argon matrix: A site selective laser spectroscopy
Authors: Gée C. , Cuisset A. , Divay L. , Crépin C.
Published: July 2002

Absorption (S1←S0), fluorescence (S1→S0) and phosphorescence (T1→S0) spectra of aniline isolated in argon matrices are presented and analyzed. Upon selective laser excitation, two families of sites are characterized by electronic spectroscopy and excited state dynamics. A strong electron/phonon coupling is observed in the S1↔S0 transition due to a geometry change of the molecule. Phosphorescence spectra exhibit different vibronic intensity distribution for both sites; a1 in plane and b1 out-of-plane modes are observed. A direct estimation of the inversion splitting in the ground state (46 cm−1) is obtained from the presence of an intense doublet assigned to A13(0+)→1A1(0+,0−) vibronic transitions.
Photodissociation spectroscopy of Al+-acetaldehyde
Authors: Lu W.-Y. , Acar M. , Kleiber P. D.
Published: July 2002

We have investigated the photodissociation spectroscopy of the Al+-acetaldehyde bimolecular complex over the spectral range 212–345 nm. We find evidence for three overlapping molecular absorption bands in the near UV. Two of the bands are unstructured and are assigned as 1A″ and 2A″←1A′. These continuum bands are of mixed character with contributions from Al+-centered 3pπ(A″)←3sσ(A′), acetaldehyde-centered π*(A″)←n(A′), and Al-acetaldehyde charge transfer excitation processes. The third absorption band at short wavelengths, λ>223 nm, shows a prominent vibrational progression with a mode frequency ωe=210±4 cm−1. The structured band is assigned as 2A′←1A′ and correlates to an Al+-centered 3pπ(A′)←3sσ(A′) radiative transition; the vibrational progression is assigned to the intermolecular Al+-acetaldehyde in-plane bend. Spectroscopic results are in good agreement with ab initio predictions.
Enhanced exchange coupling constant and thermal stability of antiferromagnetically coupled media with thin Co interlayers
Authors: Oh S. C. , Hong S. Y. , Lee H. J. , Lee K. J. , Lee T. D.
Published: July 2002

The magnetic properties and thermal stability of antiferromagnetically coupled (AFC) media with thin Co interlayers are investigated. Since the thermal stability is strongly dependent on the exchange coupling constant Jex in the AFC media, the thin Co interlayers were inserted on both interfaces of the Ru layer to obtain higher values of Jex. The Jex above 0.6 erg/cm2 was obtained in the AFC media with the Co interlayers above 1 nm in comparison with about 0.1 erg/cm2 in AFC media without the Co interlayers. The thermal stability of the AFC media with Co interlayers was greatly improved over those of the AFC media without Co interlayers.
Dynamics of propylene glycol and its 7-mer by neutron scattering
Authors: Swenson J. , Köper I. , Telling M. T. F.
Published: July 2002

The dynamics of propylene glycol (PG) and its 7-mer have been investigated by quasielastic neutron scattering (QENS). In the case of PG we used two techniques; ordinary QENS and neutron spin-echo (NSE). The QENS and NSE experiments were carried out at the temperature ranges 300–420 K and 260–340 K, respectively. The QENS results on both the monomer and the 7-mer showed three clear dynamical processes; a weakly temperature dependent fast local motion of hydrogens in the polymeric backbone, an almost temperature independent rotational motion of the methyl groups, and a strongly temperature dependent translational process. This latter motion exhibits a clear quasi-elastic broadening proportional to Q2 at high temperatures (T⩾380 K). At lower temperatures the diffusion becomes more difficult to characterize due to the contribution from the methyl group rotation. However, despite this difficulty it is no doubt about that the diffusion is faster for the 7-mers than for the monomers at low temperatures (T⩽340 K). The reason for this anomalous behavior may be that the OH end groups of the monomers are linked together to a network at lower temperatures, which would slow down the translational diffusion. Another possible explanation can be that the dynamics of the 7-mers is dominated by segmental motions, which might be faster than the translational diffusion of single monomers at low temperatures. The NSE data on deuterated PG were described by the Kohlrausch–Williams–Watts (KWW) stretched exponential function, with a temperature independent stetching parameter βKWW. The reason for the stretched behavior of the coherent intermediate scattering function I(Q,t) is most likely that several dynamical processes, such as the primary α-relaxation and secondary β-relaxations, are observed in the experimental time window (3–1500 ps), in accordance with the QENS results given above. An interesting observation is that the dynamics is slowest and the stetching parameter βKWW is largest for Q=1.4 Å−1, corresponding to the position of the first diffraction peak in the static structure factor S(Q), a phenomenon known as de Gennes narrowing.
Ab initio all-electron fully relativistic Dirac–Fock self-consistent field calculations for molecules of superheavy elements: Seaborgium hexabromide
Authors: Malli Gulzari L.
Published: July 2002

Gargantuan ab initio all-electron fully relativistic Dirac–Fock (DF) and nonrelativistic (NR) Hartree–Fock (HF) limit self-consistent field (SCF) molecular calculations are reported for SgBr6 at various Sg–Br bond distances assuming an octahedral geometry. Our fully relativistic Dirac–Fock and nonrelativistic HF calculations predict for SgBr6 bond distance of 2.52 and 2.59 Å, respectively. Both our DF and NR HF SCF calculations predict the ground state of SgBr6 to be bound, with the predicted atomization energy of 18.75 and 11.53 eV, respectively. A relativistic Dirac–Fock wave function predicts for SgBr6∼63% larger atomization energy than the corresponding NR HF calculation. The vertical ionization potential of SgBr6 calculated with our DF and HF wave functions is almost the same, viz., 10.60 and 10.78 eV, respectively. This is due to the fact that the HOMO consists entirely of the combination of the 4p AOs of the six Br ligands, for which relativistic effects are nominal. However, the vertical electron affinity calculated with our HF and DF wave function for SgBr6 is 5.35 and 3.80 eV, respectively. The calculated HF HOMO–LUMO gap of 7.74 eV is in fairly close agreement with that of 8.91 eV obtained from the corresponding DF relativistic MOs for SgBr6. These results can be understood in terms of the nature of the HOMOs and LUMOs calculated in our HF and DF calculations for SgBr6. Mulliken population analysis of our relativistic DF and HF wave functions yields a charge of 1.26 and 0.70, respectively on Sg in SgBr6; our DF wave function predicts SgBr6 to be more ionic (and less volatile) than that by the corresponding HF wave function. Our prediction of the bond dissociation energy of 44 and 72 kcal mol−1 with our NR HF and relativistic DF wave functions, respectively for SgBr6 is a first for a species of a superheavy transactinide element, as is our prediction of a positive electron affinity for SgBr6 with both our HF and DF wave functions.
Dalitz plot analysis of three-body fragmentation of Na3+ excited by He impact
Authors: Babikov D. , Gislason E. A. , Sizun M. , Aguillon F. , Sidis V. , Barat M. , Brenot J. C. , Fayeton J. A. , Picard Y. J.
Published: July 2002

Three-body fragmentation of Na3+ ions to Na++Na(3s)+Na(3s) following excitation by He is studied experimentally and theoretically. The three reduced kinetic energies of the products in the center-of-mass are determined for each fragmentation event, and the results are displayed in a Dalitz plot. The fragmentation involves three adiabatic A′1 electronic states of Na3+ that become degenerate at the detector. It is possible to determine the final electronic state for each event, and here we show that each of the three product states appears in a particular sector of the Dalitz plot. Theoretical and experimental Dalitz plots for the three-body fragmentation of Na3+ are presented, and the results are related to various mechanisms for three-body fragmentation of this system.
Anion photoelectron spectroscopy of I2−(CO2)n(n=1–8) clusters
Authors: Gómez Harry , Taylor Travis R. , Neumark Daniel M.
Published: July 2002

We report the anion photoelectron spectra of I2−(CO2)n clusters (n=1–8) measured at a photon energy of 4.661 eV. Assignment of the spectra is aided by electronic structure calculations on I2−(CO2). The experiment yields size-dependent vertical and adiabatic detachment energies for the formation of the ground state and low-lying valence-excited states of the neutral cluster. Vertical detachment energies are successively blueshifted with increasing cluster size, indicating a stronger stabilization of the anionic cluster relative to the neutral counterpart. In addition, a short progression in the CO2 bending mode is observed in the n=1 and 2 clusters, indicating that the CO2 solvent species are slightly bent (∼2.5°) in the anion clusters. The trends in the total and stepwise solvation energies are discussed in terms of cluster geometries solute–solvent interactions.
A self-consistent mode-coupling theory for dynamical correlations in quantum liquids: Application to liquid para-hydrogen
Authors: Reichman David R. , Rabani Eran
Published: July 2002

The self-consistent quantum mode-coupling theory developed in the preceding paper [J. Chem. Phys. 116, 6271 (2002)] is applied to the study of density fluctuations and transport in liquid para-hydrogen. The method combines exact static input generated by a path-integral Monte Carlo method, and an approximate form of the quantum memory function, for the solution of the exact quantum generalized Langevin equation under consideration. The approach provides exact results for any correlation function at short times, and is expected to provide accurate results at long times. The quantum mode-coupling theory is compared with experimental observations for a variety of dynamical correlations in liquid para-hydrogen. We find that the quantum mode-coupling theory provides quantitative agreement with experiments. Improvement of the methodology and future applications are discussed.
Statistical model of energy transfer in molecular collisions: De-energization of highly excited toluene
Authors: Nilsson Daniel , Nordholm Sture
Published: July 2002

Recent experimental data on the collisional de-energization of toluene from initial energies of between 10 000 and 49 000 cm−1 have been analyzed by the partially ergodic collision theory. The data for 11 different gases were obtained by kinetically controlled selective ionization by Luther and co-workers [J. Chem. Phys. 112, 4076 (2000)] and are of exceptional detail and reliability. The semiempirical PECT model starts from the average energy transferred per collision 〈ΔE〉 and predicts the full transition probability P(E′,E) to good accuracy. The energy transfer efficiency is about a tenth of the ergodic collision limit with only minor dependence on gas molecule or initial toluene energy.
The adsorption of acenes on rutile TiO2(110): A multi-technique investigation
Authors: Reiß S. , Krumm H. , Niklewski A. , Staemmler V. , Wöll Ch.
Published: July 2002

The adsorption of benzene, naphthalene, and anthracene on the TiO2(110) surface has been investigated using near edge x-ray absorption spectroscopy (NEXAFS), x-ray photoelectron spectroscopy, and thermal programmed desorption. For all three adsorbates a planar adsorption geometry is found. In contrast to the bonding of benzene and larger acenes to metal surfaces, we find that the interaction is dominated by electrostatic forces between the adsorbed molecules and the TiO2(110) substrate. The fact that the average tilt angle between molecular and surface plane as determined by NEXAFS is substantially different from zero indicates the presence of defect species.
Scattering from isolated molecules on metal surfaces: The relationship between elastic and inelastic intensities
Authors: Choi B. H. , Graham A. P. , Tang K. T. , Toennies J. P.
Published: July 2002

The measured angular distributions of the elastic and one-phonon T-mode vibrational inelastic scattering of helium atoms from a low coverage (ΘCO=0.03) of isolated carbon monoxide molecules on Pt(111) are analyzed with a novel scattering theory. The inelastic angular distributions of the dispersionless parallel vibrational mode (T mode) are found to have similar features as the much more intense elastic angular distribution. The new theory relates the elastic angular distribution to the inelastic T-mode angular distribution, via a form factor.
Application of the intermediate Hamiltonian valence-universal coupled-cluster method to the magnesium atom
Authors: Malinowski P. , Meissner L. , Nowaczyk A.
Published: July 2002

The atomically oriented valence-universal coupled-clusters method in the intermediate Hamiltonian formulation with one- and two-body part(s) included in the cluster operator (VU-CCSD/R) is applied to describe the ground state and some excited states of the Mg atom. Two Slater-type orbitals (STO) basis sets are employed to see the effect of the basis set on the results. In addition to that, several complete model spaces are used in the calculation starting with (3s,3p) and ending with (3s,3p,3d,4s,4p) orbitals as active. It has been found that the quality of the description of the ground- and excited states depends on the STO basis set used in the calculation. In order to obtain reliable excitation energies, special attention has been paid to the construction of well-balanced basis sets in which functions necessary for adequate description of both the ground state and excited states under consideration are included. Therefore, in spite of the moderate size of the orbital basis, our results compare quite well in terms of the total energy as well as in energy differences with results of other methods which, in some cases, use significantly larger basis sets.
Prediction of the phase behavior of acetonitrile and methanol with ab initio pair potentials. II. The mixture
Authors: Sum Amadeu K. , Sandler Stanley I. , Bukowski Robert , Szalewicz Krzysztof
Published: July 2002

The comprehensive study of the ab initio pair potentials and prediction of properties for acetonitrile and methanol [J. Chem. Phys. 116, 7627 (2002), preceding paper] is extended to examine the vapor–liquid equilibria of their mixture. An ab initio pair interaction potential is developed for the acetonitrile–methanol interaction consistent with the pure component pair potentials using symmetry-adapted perturbation theory with a double zeta quality basis set including bond functions. Interaction energies were calculated for a large number of configurations to obtain a good representation of the potential energy surface, and employed to develop a site–site pair interaction potential. The ab initio pair potentials for the like and unlike interactions were then used in Gibbs ensemble Monte Carlo simulations to predict the phase behavior of the acetonitrile–methanol mixture. Simulations were performed to determine the phase boundary of the mixture, and although the predicted equilibrium concentrations are not in perfect agreement with experimental measurements, the predicted and measured phase boundaries are similar, including the occurrence of an azeotrope in close agreement with experiment. This prediction of mixture phase behavior is the first reported using ab initio potentials for both like and unlike interactions without the use of any combining rule.
Quantitative investigation of the kinetic energy release in metastable decay reactions of (O2)n=2–10+ ions: Evidence for a change in the metastable decay mechanism as a function of cluster size
Authors: Matt S. , Parajuli R. , Stamatovic A. , Scheier P. , Märk T. D.
Published: July 2002

Using a high resolution two sector field mass spectrometer of reversed geometry we have measured metastable fractions and mass-analyzed ion kinetic energy peaks for metastable decay reactions involving monomer evaporation via (O2)n+*→(O2)n−1++O2. Both, average kinetic energy release 〈KER〉 data derived from the peak shapes and the time dependence of the metastable fractions show a characteristic dependence on cluster size indicating a change in the metastable fragmentation mechanism when going from the dimer to the dekamer ion. Moreover, the 〈KER〉 data contain information about the transition state temperature and thus one can use finite heat bath theory to calculate the binding energies of the decaying cluster ions. The results obtained are in fair agreement with (i) previous results based on gas phase ion equilibria measurements and with (ii) the corresponding bulk value.
On the hydrodynamic stability of pulsatile flow in a plane channel
Authors: Straatman A. G. , Khayat R. E. , Haj-Qasem E. , Steinman D. A.
Published: July 2002

A linear stability analysis is undertaken for plane pulsatile Poiseuille flow. The stability picture is described in terms of two distinct ranges of the Womersley number, Wo, which is the ratio of pulsatile to viscous forces. The low-frequency range, termed precritical, has dominant viscous effects, while the high-frequency range, termed postcritical, has dominant inertial effects. A critical Womersley number, Wocr=1.107, is identified, where viscous and inertial forces are in balance. In the precritical range, viscous forces become rapidly dominated by impulse forces, resulting in a sharp drop of the critical Reynolds number with increasing Wo. In the postcritical range, inertial forces become increasingly dominant over impulse forces, resulting in flow stabilization. In the limits Wo→0 and Wo→∞, the flow recovers the stability characteristics of the steady base flow, independently of the pressure amplitude. In other words, pulsating the flow is always destabilizing. The results reported in the postcritical range are in agreement with existing theoretical and experimental works.
Effect of ablation parameters on infrared pulsed laser deposition of poly(ethylene glycol) films
Authors: Bubb Daniel M. , Papantonakis M. R. , Toftmann B. , Horwitz J. S. , McGill R. A. , Chrisey D. B. , Haglund R. F.
Published: July 2002

Polymer thin films were deposited by laser ablation using infrared radiation both resonant (2.90, 3.40, 3.45, and 8.96 μm) and nonresonant (3.30, 3.92, and 4.17 μm) with vibrational modes in the starting material, polyethylene glycol. The chemical structure of the films was characterized by Fourier transform infrared spectroscopy, while the molecular weight distribution was investigated using gel permeation chromatography. The films deposited by resonant irradiation are superior to those deposited with nonresonant radiation with respect to both the chemical structure and the molecular weight distribution of the films. However, the molecular-weight distributions of films deposited at nonresonant infrared wavelengths show marked polymer fragmentation. Fluence and wavelength dependence studies show that the effects may be related to the degree of thermal confinement, and hence to the relative absorption strengths of the targeted vibrational modes.
Photonic band schemes of opals composed of periodic arrays of cored spheres depending on thickness of outer shells
Authors: Takeda Hiroyuki , Yoshino Katsumi
Published: July 2002

Photonic band schemes of opals as pseudophotonic crystals composed of three-dimensional arrays of cored spheres are calculated theoretically. Pseudophotonic band gaps are confirmed to be dependent on the thickness of outer shells and also refractive indices of the inner core and outer shells. Tunabilities of photonic band schemes of these types of opals are also discussed.
Field sums for extended dipoles in ferroelectric polymers
Authors: Al-Jishi R. , Taylor P. L.
Published: July 2002

A previously described method for calculating the Lorentz factor L for orthorhombic polar crystals is modified to allow discussion of dipoles of finite size. In this approach, the field due to a lattice of point charges ±q separated by a distance d is calculated. The Lorentz factor, which relates the field at a lattice site to the macroscopic polarization, is found as a function of the dipole length d, and is seen to vary appreciably with changes in the separation between the charges. Assumption of a value of 1/3 for L can thus lead to significant errors in determining the self-consistent polarization of crystals consisting of polarizable molecules.
Analysis of a flowing silane dc discharge in the presence of a hot surface
Authors: Longeway P. A. , Weakliem H. A. , Estes R. D.
Published: July 2002

We have measured the rates for the depletion of silane and the formation of solid material, disilane, trisilane, and hydrogen under steady-state conditions in a flowing dc silane discharge in the presence of a heated substrate. The depletion rate of silane depends linearly on discharge current whereas the film yield and hydrogen yield increase superlinearly and the disilane and trisilane yields increase sublinearly with current. The yield of disilane is reduced in the presence of a hot substrate which we interpret as evidence that a significant fraction of disilane is produced by surface combination of SiH3 radicals. We also find that the optical emission intensity of the Balmer Hα line is proportional to the partial pressure of H2, and the intensity of the SiH 414-nm band is unrelated to the film deposition rate.
Domain patterns and high-frequency magnetic properties of amorphous metal ribbons
Authors: de Wit H. J. , Brouha M.
Published: July 2002

Domain patterns and hysteresis curves were studied on field annealed amorphous ribbons of Co70.3Fe4.7Si15B10. The domain structure was observed with the longitudinal Kerr effect. Hysteresis curves and permeabilities were measured at frequencies up to 10 MHz and in fields up to 2 kA/m in the length direction of the ribbon. The frequency dependence of the permeability and the hysteresis curves in the MHz range, measured after annealing in a field perpendicular to the ribbon axis in the plane of the ribbon, can be fully explained by assuming that only rotation contributes to the magnetization and that losses are due to eddy currents. Specimens annealed in a field perpendicular to the ribbon plane behave similarly. After annealing in a field parallel to the ribbon axis, however, only wall displacements occur, leading in the MHz range to a low permeability and large additional contributions to the losses.
Finite-size rounding of first-order transitions
Authors: Privman Vladimar , Fisher Michael E.
Published: July 2002

Studies of finite-size effects at the phase boundary (H=0, T<Tc) of ferromagnetic spin systems are reported for both ‘‘block’’ and ‘‘cylinder’’ geometries in d dimensions. Explicit expressions are given for universal scaling functions describing the rounded first order transition for n-component spins: The helicity modulus and the lengthwise or longitudinal correlation length, which is directly related, enter for long cylinders when n≥2.
Optical limiters using photorefractive nonlinearities
Authors: Cronin-Golomb Mark , Yariv Amnon
Published: July 2002

We report experimental and theoretical studies of the use of the photorefractive effect in the design of optical limiters. Preliminary results indicate extinction coefficients of at least 98–99% with 1 J/cm2 transmitted before steady state is reached.
Kinetics of interstitial supersaturation and enhanced diffusion in short-time/low-temperature oxidation of silicon
Authors: Hu S. M.
Published: July 2002

The kinetics of the supersaturation of self-interstitials and the enhancement of impurity diffusivity in short-time/low-temperature oxidation of silicon is investigated analytically. It is found that, whereas in long-time/high-temperature oxidation the interstitial supersaturation and the diffusivity enhancement decrease with time as t−n (n≂0.2–0.3), in short-time low-temperature oxidation they start from zero and increase with time, until some characteristic time determined by the linear-parabolic oxide growth. This characteristic time increases rapidly with the decrease of temperature. This kinetic behavior has not been expected previously, mainly because there is no available data on stacking fault growth and on diffusion enhancement under the short-time/low-temperature condition which has become common in modern IC processings. The more general case of linear-parabolic oxidation and the effect of bulk recombination have also been analyzed.
Free electron density measurements by IR absorption in CdS
Authors: Boone Jack L. , Cantwell Gene , Shaw Michael D.
Published: July 2002

A contactless method for determining the spatial variation of the free carrier density in CdS is presented. The method involves the correlation of IR transmission with the free carrier density via the IR absorption coefficient. Preliminary measurements indicate a free electron density sensitivity of less than 5×1014 cm−3 and a spatial resolution which is limited only by the spot size of a CO2 (λ=10.6 μm) laser; this should be of the order of 100 μm. Experimental results indicate that the predominate scattering is due to impurity interaction at room temperature; this is indicative of the high degree of compensation in CdS.
Photoluminescence and Raman scattering investigations of implanted and thermally annealed InP
Authors: Olego D. J. , Serreze H. B.
Published: July 2002

Photoluminescence (PL) and Raman scattering experiments were carried out on Si-implanted and thermally annealed semi-insulating InP. The application of PL measurements to determine concentrations of free electrons at the surfaces of implanted samples is demonstrated. The carrier concentrations are estimated from the measured widths of the PL line shapes. These estimates are compared with electron concentrations obtained from the Raman scattering measurements of the coupled modes.
Raman scattering in In1−xGaxAsyP1−y quaternary alloys
Authors: Soni R. K. , Abbi S. C. , Jain K. P. , Balkanski M. , Slempkes S. , Benchimol J. L.
Published: July 2002

We report a resonant Raman scattering study of quaternary In1−xGaxAsyP1−y alloys which confirms the four-mode behavior of the alloy. The scattering amplitude shows resonance enhancement at the E1 edge of the alloy which diminishes with increasing value of x, corresponding to increasing compositional disorder. This disorder further manifests itself in broadening and line-shape asymmetry of the stronger LO-phonon lines.
Photodecomposition of Mo(CO)6 adsorbed on Si(100)
Authors: Creighton J. R.
Published: July 2002

The photochemical properties of Mo(CO)6 adsorbed on Si(100) were investigated using temperature programmed desorption (TPD) and Auger spectroscopy. TPD experiments indicate that Mo(CO)6 physisorbs on silicon and desorbs at 210–230 K. At 150 K, KrF laser radiation (248 nm) partially decomposes the adsorbed Mo(CO)6 releasing gas-phase CO in the process and TPD experiments after irradiation show that additional CO desorbs at 335 K. However, Auger analysis indicates that one CO molecule per molybdenum atom dissociates, leaving the molybdenum overlayer heavily contaminated with carbon and oxygen. The cross section for photodecomposition was measured to be 5±3×10−17 cm2. Decomposition of the excited molecule must compete strongly with energy relaxation to account for the magnitude of this cross section.
Optical characterization of pseudomorphic InxGa1−xAs–GaAs single-quantum-well heterostructures
Authors: Anderson N. G. , Laidig W. D. , Kolbas R. M. , Lo Y. C.
Published: July 2002

Strain and quantum-size effects in pseudomorphic InxGa1−xAs–GaAs single-quantum-well heterostructures (SQWHs) are examined using low-temperature photoluminescence techniques. Strain effects in InxGa1−xAs epitaxial layers are first described, then photoluminescence data for a series of MBE-grown pseudomorphic SQWHs are presented and discussed. Each SQWH consists of an unintentionally doped, highly strained (ε∼2%) In0.28Ga0.72As quantum well sandwiched between GaAs confining layers. The structures were grown consecutively under identical conditions, with quantum-well thicknesses ranging from 17 to 430 Å. The thinner quantum-well structures exhibit luminescence characteristics indicative of high-quality material (photoluminescence half width ∼6 meV for Lz ∼17 Å), whereas significant broadening and eventual quenching of the photoluminescence peak is observed as alloy layer thicknesses approach and exceed the critical value. Quantum-well luminescence from the thinner (Lz ≤38 Å) SQWHs is dominated by a single, sharp feature which we attribute to n=1 electron-to-heavy hole confined-carrier transitions. An additional shallow (∼20 meV) feature, perhaps impurity related, is present in the photoluminescence spectra of some of the thicker quantum wells, and peak emission intensities are examined as a function of excitation intensity for the various transitions. Finally, the observed dependence of the transition energies upon quantum-well thickness is compared to predictions from an effective-mass SQWH model which incorporates strain effects. Reasonable agreement is obtained for SQWHs with Lz ≤100 Å, the expected critical layer thickness for these samples. This work represents the first optical study of pseudomorphic single wells, and our results should be useful in the design of strained-layer quantum-well lasers.
Determination of electron density in discharge-pumped excimer laser using Stark broadening of Hβ line
Authors: Hiramatsu Mineo , Furuhashi Hideo , Goto Toshio
Published: July 2002

The determination of the electron density in the discharge plasma used for the compact discharge-pumped rare-gas halide excimer laser of capacitor transfer type with automatic preionization was performed first by measuring the Stark half-width of the Hβ line. For the discharge-pumped XeCl laser at a total gas pressure of 2 atm and a charging voltage of 20 kV, the measured electron densities were 2.1×1015 cm−3 for Ne diluent and 1.7×1015 cm−3 for He diluent.
Study of near-zone fields of a small aperture
Authors: Leviatan Yehuda
Published: July 2002

The structure or the near-zone field transmitted through an electrically small aperture in a perfectly conducting plane is investigated. We dwell on two issues that are of importance to engineers analyzing small aperture problems. The first question addressed is how far should one recede from the aperture in order to justifiably use the well-known aperture dipole approximations. Alternatively, how close may a source or a backing object be located without appreciably effecting the aperture quasistatic field distributions. The second consideration is to what distance the radiation emanating from the aperture remains collimated and how strongly is the near-field intensity dependent upon distance. The model used for our calculation is that of a small circular aperture for which the quasistatic aperture field distributions are analytically derivable. It is presumed that this should give an indication of the near-zone field characteristics to be expected in the general case.
I
n s
i
t
u characterization of the n-Si/acetonitrile interface by electromodulated infrared internal-reflection spectroscopy
Authors: Venkateswara Rao A. , Chazalviel J.-N. , Ozanam F.
Published: July 2002

We present a systematic experimental investigation on the infrared (IR) vibrational absorption spectroscopy of the n-Si/acetonitrile interface utilizing the attenuated total internal-reflection geometry in the near-infrared (1.1–5 μm) spectral region. The IR absorption of the interface has been isolated selectively by electromodulation, and studied as a function of modulation potential. The electrochemical behavior of the interface has been checked by current/voltage and impedance measurements. The IR spectra are composed of a broad background and various sharp vibrational peaks. The background can be analyzed as the sum of two contributions: (i) absorption by surface states at shorter wavelengths (<2 μm), (ii) free-carrier absorption at longer wavelengths. The free-carrier contribution is itself composed of a Drude-like component (proportional to λ3/2) and an interband component. The vibrational peaks can be ascribed to the C≡N, C–H, Si–H, and (Si–)O–H chemical bonds. The shapes and magnitudes of the C≡N and C–H peaks can be quantitatively understood in terms of displaced ions and acetonitrile molecules near the surface upon the electrode potential modulation. The shape of the C≡N peak also gives an indication of a weak interaction of the acetonitrile molecules with the electrode surface. The Si–H and (Si–)O–H peaks can be interpreted in terms of Stark effect modulation of the infrared absorption of these species. The shape of the O–H peak indicates the presence of nonequivalent sites at the interface. Upon the electrode aging and oxidation the magnitude of the Si–H peak decreases and the (Si–)O–H peak increases and correspondingly the surface-state density increases which provides a direct in situ physicochemical information regarding the slow oxidation of the electrode surface.
Electron swarm properties of CCl2F2/SF6 mixtures
Authors: Fréchette M. F. , Novak J. P.
Published: July 2002

The ionization and attachment coefficients for CCl2F2/SF6 mixtures were determined from measurements of prebreakdown ionization currents in a steady-state Townsend discharge at a pressure p25=4 Torr and temperature of 25 °C. The curves representing the ionization coefficient (α) for reduced fields in the range from 110 to 180 V cm−1 Torr−1 exhibit a small maximum at about 15% of CCl2F2, while those representing the attachment coefficient (η) flatten out between 0% and 50% CCl2F2 and decrease at higher partial densities. The limit field obtained using the condition α=η shows two extrema, i.e., a minimum at approximately 20% and a maximum at 75% of CCl2F2, while the curves obtained from the breakdown-potential measurements available in the literature show only a maximum, at roughly the same position. Theoretical values of the transport coefficients were obtained by numerically solving the Boltzmann equation using previously determined effective sets of electron collision cross sections [J. P. Novak and M. F. Fréchette, J. Appl. Phys. 55, 107 (1984); 57, 4368 (1985)]. The good agreement between the calculated and the experimental values provides supporting evidence for the applicability of the present calculation method and collisional cross sections to analyze electron swarms in the range of fields comparable in magnitude to the limit field.
SiO2-Si interface formation by catalytic oxidation using alkali metals and removal of the catalyst species
Authors: Soukiassian P. , Gentle T. M. , Bakshi M. H. , Hurych Z.
Published: July 2002

We studied the catalytic action of adsorbed cesium or sodium on the oxidation of Si(100)2×1 by core-level photoemission using synchrotron radiation. At room temperature, a large enhancement of the silicon oxidation rate was observed with formation of high oxidation states. By moderate thermal annealing, we were able to remove the catalyst species from the surface and to form a clean SiO2-Si interface at lower temperatures than with other processes.