bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic transport in polycrystalline Pbl2 films
Authors: Street R. A. , Ready S. E. , Lemmi F. , Shah K. S. , Bennett P. , Dmitriyev Y.
Published: July 2002

Electronic transport and optical measurements in polycrystalline Pbl2 are reported as part of a study to evaluate the material for large area x-ray imaging applications. The films are deposited by vacuum evaporation with thickness 20–100 μm and have grain sizes of up to 10 μm. The room temperature hole drift mobility measured by time-of-flight is 2×10−2–1.5×10−1 cm2/V s, depending on the specific sample, with an activation energy of 0.25 eV. Hole charge collection measurement gives about 10−6 cm2/V for the mobility–lifetime product. Details of the electron transport were not determined in this study because the mobility is too small. The hole transport is discussed in terms of a trapping model with either a discrete level above the valence band or a disorder-induced band tail. Optical absorption, photoconductivity, and Hall effect measurements are also reported.
An analytical modeling of time dependent pulsed laser melting
Authors: Tokarev Vladimir N. , Kaplan Alexander F. H.
Published: July 2002

The one-dimensional time dependent heat conduction equation for surface heating and a phase boundary (the so-called classical Stefan problem) has been solved in the absence of vaporization. For a rectangular laser pulse and constant material parameters, useful solutions have been determined for melt depth as a function of time both during and following the pulse. Based on the model, the intensity dependence of the melt depth is investigated. Two melting regimes—slow and fast—have been identified by comparison with previously reported data for silicon.
Viscoelastic crack healing and adhesion
Authors: Baney J. M. , Hui C.-Y.
Published: July 2002

Closing cracks in viscoelastic materials are analyzed using a cohesive zone model. The analysis is relevant to the bonding of surfaces of viscoelastic materials, and in particular to the Johnson–Kendall–Roberts (JKR) experiment. A crack bonding theory due to Schapery is extended to include the case of the standard linear solid material, and some consequences of this material’s finite long time compliance are illustrated. The stress field behind a steadily closing crack in a standard linear solid is also investigated in detail. Finally, a method for applying Schapery’s crack bonding theory to the analysis of a JKR-type experiment involving viscoelastic cylinders is given explicitly.
Photoluminescence and Zeeman effect in chromium-doped 4H and 6H SiC
Authors: Son N. T. , Ellison A. , Magnusson B. , MacMillan M. F. , Chen W. M. , Monemar B. , Janzén E.
Published: July 2002

Photoluminescence (PL) and Zeeman effect measurements in near-infrared luminescence bands in Cr-doped 4H and 6H SiC are presented. The PL spectrum consists of two no-phonon lines (NPLs) at 1.1583 and 1.1898 eV in 4H SiC and three NPLs at 1.1556, 1.1797, and 1.1886 eV in 6H SiC. The observed Zeeman splittings and temperature dependence studies reveal the spin triplet of the ground state and the orbital doublet structure of the excited state of the Cr-related center. All the triplets have almost isotropic g values close to 2 with trigonal symmetry and small zero-field splitting values D. In contrast, the effective g values of the excited state of the center are very anisotropic with g∥ in the range of 0.22–0.64 and g⊥=0 for different NPLs in both polytypes. Based on the Zeeman results, the PL is attributed to the internal transition 1E(D)→3A2(F) within the d shell of a substitutional, neutral chromium (Cr4+) in the 3d2 electronic configuration.
Flux pinning in melt-processed ternary (Nd–Eu–Gd)Ba2Cu3Oy superconductors with Gd2BaCuO5 addition
Authors: Pradhan A. K. , Muralidhar M. , Koblischka M. R. , Murakami M. , Nakao K. , Koshizuka N.
Published: July 2002

The flux pinning behavior of ternary melt-processed (Nd–Eu–Gd)Ba2Cu3Oy superconductors is studied with varying Gd2BaCuO5 second-phase (Gd-211) defect concentrations using magnetotransport and magnetization measurements. The critical current density, Jc increases with the addition of Gd-211 particles displaying a maximum value of Jc for 30% at zero and in intermediate field range and decreases on further addition of Gd-211 particles. A pronounced field-induced bump feature in the resistivity was observed. The dynamic scaling of the resistance suggests the low temperature phase as the vortex-glass phase. The current–voltage characteristics over the whole transition temperature regime show a linear flux–flow type behavior that favors the phenomenon of vortex entanglement in the liquid phase. The Nd/Ba substitution sites along with Gd-211 second-phase particles refined by the addition of a small amount of Pt are one of the possible reasons for the vortex entanglement in the liquid phase. The in-plane and out-of-plane resistance measurements clearly show the correlation of the vortices in the liquid state favoring some influence of twin planes and occasionally occurring natural grain boundaries on the liquid state as well. These defects give rise to high critical current density at low temperature with significant enhancement in pinning favoring the glassy phase to occur.
Photocapacitive detection of hole emission from DX center in n-type Al0.3Ga0.7As doped with Te
Authors: Murai Akihiko , Oyama Yutaka , Nishizawa Jun-ichi
Published: July 2002

Photocapacitance (PHCAP) measurements are applied to liquid-phase epitaxially grown n- Al0.3Ga0.7As crystals at different temperatures. The PHCAP measurements revealed deep levels optically located at 0.5 eV below the conduction band and 1.5 eV above the valence band [Ec−0.5 eV level (Te-related DX center) and Ev+1.5 eV level]. The thermal activation energy of electron capture at the ionized Ec−0.5 eV level was found to be 31 meV. The optical hole emission process from the Ev+1.5 eV level was enhanced with increasing sample temperature. After 1.5 eV monochromatic light preirradiation, the Ec−0.5 eV level was detected in an intentionallly undoped n-Al0.3Ga0.7As sample. From these results, the structure of the Te-related DX center is discussed.
Recombination mechanisms in amorphous silicon/crystalline silicon heterojunction solar cells
Authors: Jensen N. , Rau U. , Hausner R. M. , Uppal S. , Oberbeck L. , Bergmann R. B. , Werner J. H.
Published: July 2002

This article investigates limitations to the open circuit voltage of n-type amorphous silicon/p-type crystalline silicon heterojunction solar cells. The analysis of quantum efficiency and temperature dependent current/voltage characteristics identifies the dominant recombination mechanism. Depending on the electronic quality of the crystalline silicon absorber, either recombination in the neutral bulk or recombination in the space charge region prevails; recombination at the heterointerface is not relevant. Although interface recombination does not limit the open circuit voltage, recombination of photogenerated charge carriers at the heterointerface or in the amorphous silicon emitter diminishes the short circuit current of the solar cells.
Spatial behavior of energy relaxation of electrons in capacitively coupled discharges: Comparison between Ar and SiH4
Authors: Yan M. , Bogaerts A. , Gijbels R. , Goedheer W. J.
Published: July 2002

The electron energy relaxation in space has been investigated for electropositive Ar and electronegative SiH4 discharges by a 1D Particle-in-cell/Monte Carlo code. The ionization rate has been studied since this rate is strongly influenced by the energy relaxation mechanism. The ionization rate in the two kinds of discharges at a low pressure (30 mTorr), a low power (8 W), and a low frequency (13.56 MHz) is regarded as the reference case. The effects of pressure, power, and frequency on the ionization rate have been observed and compared between the two types of discharges. With the pressure increasing from 30 to 400 mTorr, in the case of the argon discharge the ionization peak moves from the plasma bulk (nonlocal behavior of the electron energy distribution function) towards the momentary cathodic presheath (local behavior). In addition to a similar variation of the ionization rate, in the silane discharge an ionization peak occurs near the momentary anodic presheath, and at the high pressure the ionization in the plasma bulk is still considerable. The power can only influence the ionization rate quantitatively. The effect of frequency on the ionization rate is similar for both kinds of discharges. With increasing frequency the ionization rate tends to a somewhat local character, i.e., the ionization appears closer to the momentary cathode. This is attributed to the fact that at the high frequency a bulk electric field in the bulk is found clearly out of phase with the sheath fields, and the energetic electrons are pushed towards the cathode earlier in space. Meanwhile, in the silane discharge the distinctive ionization, which is strongly present at the anodic presheath and in the plasma bulk at low frequency, almost disappears.
Self-limiting and pattern dependent oxidation of silicon dots fabricated on silicon-on-insulator material
Authors: Heidemeyer H. , Single C. , Zhou F. , Prins F. E. , Kern D. P. , Plies E.
Published: July 2002

We present a systematic investigation of the oxidation properties of Si dots fabricated on a silicon-on-insulator (SOI) wafer. Dots with diameters varying from 9 to 81 nm were structured on a SOI wafer. These dots were oxidized in a dry oxygen atmosphere at 700, 850, and 1000 °C. The resulting structures were investigated using a side view transmission electron microscopy (TEM) technique in combination with energy filtered TEM. The dimensions of the residual Si and the grown SiO2 were then extracted from the micrographs and analyzed. The oxidation appears to be retarded as compared to the well-known planar oxidation. At 700 and 850 °C a self-limiting effect is observed as well as a clear pattern dependent oxidation at 850 and 1000 °C. We attribute these effects to stress buildup in the oxide. The critical stress, causing the self-limiting effect, is calculated using a model that considers the decrease of the reaction rate with increasing stress perpendicular to the Si surface.
Kinetic modeling of film growth rates of TiN films in atomic layer deposition
Authors: Lim Jung-Wook , Park Jin-Seong , Kang Sang-Won
Published: July 2002

A kinetic model has been studied for exploring the prospects of atomic layer deposition (ALD) of TiN thin films. In the present article, assuming the existence for readsorption of each reactant, we can explain and model the film growth of 0–2 at. ML in one deposition cycle in TiN-ALD. Applying the proposed model to TiN films grown by ALD using tetrakis(dimethylamido)titanium and ammonia, the parameters related to both the adsorption rate and the adsorption order of each reactant were extracted. With the extracted parameters, TiN film thickness in one deposition cycle depending on the pulse time of each reactant could be predicted in a reasonable range of accuracy.
Quantum confinement effect in heat treated silver oxide nanoparticles
Authors: Banerjee S. , Maity A. K. , Chakravorty D.
Published: July 2002

Ag 2 O particles of sizes varying from 6.0 to 16 nm have been prepared by a chemical method. These have been subjected to a heat treatment at temperatures varying from 533 to 623 K. The optical absorption spectra of the heat treated particles dispersed on a glass substrate have been delineated. The absorption peak shows a maximum in wavelength as a function of heat treatment temperature. This has been explained on the basis of formation of nanometer-sized silver layer on the Ag2O particles and the consequential electron confinement within the same.
Linear optical properties of the monoclinic bismuth borate BiB3O6
Authors: Hellwig H. , Liebertz J. , Bohatý L.
Published: July 2002

New materials for nonlinear optical (NLO) applications are still of great interest. The monoclinic BiB3O6 (BIBO) shows exceptionally large NLO coefficients. In this article we will present the linear optical properties in the wavelength range between 350 and 2400 nm, the phase matching conditions calculated for second harmonic generation, and optical parametric oscillation. Angular bandwidth data are also given. The wide tuning range of phase matched directions together with the monoclinic symmetry allow a broad variety of applications. The laser damage threshold is comparable to high quality lithium triborate.
Continuous anneal method for characterizing the thermal stability of ultraviolet Bragg gratings
Authors: Rathje Jacob , Kristensen Martin , Pedersen Jens Engholm
Published: July 2002

We present a new method for determining the long-term stability of UV-induced fiber Bragg gratings. We use a continuous temperature ramp method in which systematic variation of the ramp speed probes both the short- and long-term stability. Results are obtained both for gratings written in D2 loaded and nonloaded fibers. The results for the nonloaded fibers are in good agreement with those previously obtained. Precise predictions of the grating decay were made. We find good agreement with a broad trap energy distribution where the defects with the lowest energy decay first. For the D2 loaded fiber grating we resolve two separate energy distributions, suggesting that two different defects are involved. The experiments show that complicated decays originating from various energy distributions can be analyzed with this continuous isochronal anneal method. The results have both practical applications in determining the long-term stability of fiber gratings and fundamental importance since they can be used to determine the energy distribution when using different fiber types and writing techniques.
Molecular beam studies of the F+H2 reaction
Authors: Neumark D. M. , Wodtke A. M. , Robinson G. N. , Hayden C. C. , Lee Y. T.
Published: July 2002

The dynamics of the F+H2 reaction have been investigated in a high resolution crossed molecular beam study. Differential cross sections and kinetic energy distributions were obtained for each HF vibrational state. The v=1 and v=2 states were predominantly backward scattered, but substantial forward scattering was observed for HF (v=3) over the range of collision energies accessible in our apparatus, from 0.7 to 3.4 kcal/mol. The results strongly suggest that dynamical resonances play a significant role in the reaction dynamics of F+H2 and that resonance effects are most prominent in the v=3 product channel. Quantal reactive scattering calculations on F+H2 predict that the v=2 channel should be most strongly affected by resonances. This discrepancy is attributed to inadequacies in the potential energy surface used in the calculations, and several modifications to the surface are proposed based on the experimental results. Other features of the reaction are also discussed, including the integrated partial cross sections, the effect of H2 rotation, and the reactivity of F(2P1/2).
The psi-type circular dichroism of large molecular aggregates. III. Calculations
Authors: Kim Myeong-Hee , Ulibarri Laura , Keller David , Maestre Marcos F. , Bustamante Carlos
Published: July 2002

Computations have been carried out to determine how the magnitude and shape of the polymer and salt induced (psi)-type CD spectra depend on the structural properties of a collection of randomly oriented large chiral aggregates. Uniaxial polarizable groups located at the cubic lattice points have been used to model the aggregates. The structure of the model is similar to that of a cholesteric liquid crystal. All computations have been carried out for the case of polarizable groups possessing only one electronic transition between 200 and 320 nm. It is found that the radiation and intermediate couplings between the chromophores in the aggregate which are neglected in previous theories play an important role in determining the shape and magnitude of the psi-type CD spectrum. It is shown that when these couplings are included, only three-dimensional large chiral aggregates show huge and nonconservative psi-type CD spectra. It is shown that the magnitude of the psi-type CD spectrum is controlled by the volume, the chromophore density, and the pitch of the aggregate, while the shape of the psi-type CD spectrum is determined mostly by the pitch and the handedness of an aggregate. When the pitch is close to the center of the absorption band of the chromophore in the aggregate the most distorted (least conservative) psi-type CD spectrum is obtained. The CD spectra of aggregates with opposite handedness are mirror images of each other. It is shown that a rotationally disordered collection of chiral aggregates cannot give rise to a selective reflection of one circular polarization over the other as shown by liquid crystals. The results obtained confirm the theoretical predictions of the two previous papers in this series.
Fluids of dimerizing hard spheres, and fluid mixtures of hard spheres and dispheres
Authors: Wertheim M. S.
Published: July 2002

We continue the study of a fluid of hard spheres which contain an off-center attraction site, with the range of attraction restricted so that dimers, but no higher s-mers can be formed. Two approximations, an integral equation and a version of thermodynamic perturbation theory, were previously found to perform well when tested against Monte Carlo simulations. Here, the limit in which the attraction becomes an infinitely strong glue spot of infinitesimal extent is evaluated analytically for the two theories. The resulting equation of state may be identified with the equation of state of a mixture of hard spheres and hard dispheres. In the limit of complete dimerization the system is equivalent to a one-component fluid of hard dispheres. The equation of state of hard dispheres, known from Monte Carlo simulations, is accurately represented by the Tildesley–Streett (TS) fitting function. In the pure disphere limit, our version of thermodynamic perturbation theory predicts an equation of state which is numerically almost indistinguishable from the TS fitting function. For the integral equation, the disphere limit of the compressibility equation of state is also very good, showing a maximum deviation from the TS fitting function of 2% over the density range of the simulations. The virial equation of state is much less satisfactory, with the deviation rising to 15% at the highest density.
Reduction of 1+1 resonance enhanced MPI spectra to population distributions: Application to the NO A 2Σ+–X 2Π system
Authors: Jacobs D. C. , Madix R. J. , Zare R. N.
Published: July 2002

A two-step methodology is presented for extracting ground state population distributions and alignment factors from 1+1 resonance enhanced multiphoton ionization (REMPI) spectra. In the first step the ion signal is corrected for variation with laser intensity as it is collected, generating an isopower spectrum. In the second step populations and alignments are derived from the isopower spectrum by correcting for the interdependent effects of saturation and intermediate state alignment. This procedure is applied to a room temperature thermal distribution of nitric oxide using the 1+1 REMPI process in which lines of the NO A 2Σ+–X 2Π (0,0) band constitute the resonant transition. The present treatment is able to recover the known rovibrational population distribution, independent of branch choice, over a wide range of practical operating conditions.
Raman excitation profiles of polyatomic molecules in condensed phases. A stochastic theory
Authors: Sue James , Yan Yi Jing , Mukamel Shaul
Published: July 2002

The absorption and the Raman excitation profiles of polyatomic molecules in condensed phases are analyzed using a stochastic model of line broadening. The electronic energy gap is assumed to be stochastically modulated by the random force resulting from the interaction with the solvent. The model interpolates all the way from homogeneous broadening (fast modulation) to inhomogeneous broadening (slow modulation). We analyze the conditions whereby the Raman excitation profiles can be expressed as the square of an ensemble-averaged amplitude. An efficient procedure for modeling both absorption and excitation profiles and extracting the relevant structural and dynamical information for large polyatomic molecules is proposed. Application is made to the Raman profiles of azulene in CS2 and in methanol, and the magnitude and the inverse correlation time of the random force are extracted from the experimental data in both cases.
Vacuum ultraviolet absorption cross sections of SiH4, GeH4, Si2H6, and Si3H8
Authors: Itoh Uichi , Toyoshima Yasutake , Onuki Hideo , Washida Nobuaki , Ibuki Toshio
Published: July 2002

The vacuum ultraviolet absorption cross sections of SiH4, GeH4, Si2H6, and Si3H8 are reported for the wavelength region 107–220 nm using synchrotron radiation as a light source. Absorption maxima of these compounds were found at the exciting wavelengths of 115–119 nm. Broad peaks observed were mostly assigned as primarily Rydberg transitions of the σSiH and σSiSi bonding electrons to the 4s, 4p, and 4d orbitals. The absorption features of germane resemble those of monosilane. In the photoexcitation of monosilane, the emission of the SiH(Ã 2Δ→X̃ 2Π) transition was observed and its onset was found to be 132±2nm. The absorption spectrum of disilane showed five peaks. They were mostly assigned as 2a1g→4s, 2a1g→np(n=4−6) transitions and the strongest band was overlapped by 1eg→4d and 1eu→4p Rydberg transitions. In trisilane molecules three very weak and broad peaks were recognized and assigned as 3b2→4s, 4p and 4a1→4s, 4d Rydberg transitions. The strongest band was tentatively assigned as the superposition of 1a2→4d and 3a1→4p Rydberg excitations.
Hydrogen dynamics in hydro-sodalite Na8 [AlSiO4]6 (OH)2 by inelastic and quasielastic neutron scattering
Authors: Buehrer W. , Felsche J. , Luger S.
Published: July 2002

The dynamical properties of hydrogen in the sodalite Na8[AlSiO4]6 (OH)2 were investigated by quasielastic and inelastic neutron scattering techniques. The data were analyzed with a jump model for the hydrogen motion (OH reorientation). The characteristic time of the jump process is 2.1 ps at 295 K, the frequency of the local vibration (jump attempt frequency) is 17.4 THz. The detailed analysis shows that the hydrogen environment in the (Na4OH)3+ complex cation has lower than tetrahedral symmetry.
HTP kinetics studies of the reactions of O(2 3P
J) atoms with H2 and D2 over wide temperature ranges
Authors: Marshall Paul , Fontijn Arthur
Published: July 2002

The O+H2(1) and O+D2(2) reactions have been investigated, using the high-temperature photochemistry (HTP) technique, over the 350 to 1420 and 390 to 1420 K temperature ranges, respectively. O(2 3PJ) atoms were generated from flash photolysis of CO2 and monitored by time-resolved atomic resonance fluorescence with pulse counting. Above 430 K the rate coefficients are given by k1(T)=7.3×10−21 (T/K)2.93 exp(−2980 K/T) cm3 molecule−1 s−1 and k2(T)=3.1×10−16 (T/K)1.65 exp (−5260 K/T) cm3 molecule−1 s−1. Combination of our data with those from other experiments which isolated the reactions from secondary processes yields our recommendations k1(T)=1.5×10−12 exp (−3540 K/T)+3.7×10−10 exp (−7450 K/T) cm3 molecule−1 s−1 (300 K≤T≤2500 K) and k2(T)=1.4×10−12 exp(−4260 K/T) +2.9×10−10 exp (−7780 K/T) cm3 molecule−1 s−1 (390 K≤T≤1420 K). Accuracy assessments are discussed in the text. k1(T), k2(T), and the kinetic isotope effect compare well with calculations based on recent ICVT/LAG and CEQB ab initio methods, which suggest that the first terms of the double exponential expressions approximate the tunneling contributions.
Theory of nematic networks
Authors: Warner M. , Gelling K. P. , Vilgis T. A.
Published: July 2002

Using classical elasticity theory, the rise in free energy upon crosslinking nematogenic polymers into a network is calculated for the isotropic and nematic phases. Spontaneous strains are allowed for in the network. The consequence of network formation upon nematic–isotropic equilibria is calculated by adding these elastic contributions to a conventional Landau theory. Memory of the crosslinking conditions yields quartic and quadratic additions to the standard Landau theory. We find that crosslinking in the isotropic state lowers the nematic–isotropic phase transition temperature compared with the unlinked case and the application of suitable stress raises it again. Crosslinking in the nematic state raises the transition temperature. We recover the mechanical critical point proposed long ago by de Gennes. Our Gaussian theory encompasses both main- and side-chain polymers. The hairpin limit for main chain networks yields a modulus varying exponentially with temperature. The Landau–de Gennes free energy for comb polymers is presented for the first time.
The extended Ewald method: A general treatment of long-range electrostatic interactions in microscopic simulations
Authors: Kuwajima Satoru , Warshel Arieh
Published: July 2002

The Ewald method, which is known as a method for evaluating electrostatic interactions in periodic systems, is extended to nonperiodic systems. A general way of applying the Ewald method to nonperiodic systems is described and a detailed application is demonstrated for the treatment of ions immersed in dipolar solvent molecules. As a preliminary test, electric fields produced by point dipoles forming a simple cubic lattice are calculated, both by the new Ewald treatment and by the conventional cutoff (truncation) method. These test calculations not only demonstrate the advantage of the new Ewald method but also reveal inherent problems associated with the cutoff method.
The rate constant for the reaction O(3P)+D2 at low temperatures
Authors: Zhu Yi-Fei , Arepalli Sivaram , Gordon Robert J.
Published: July 2002

The rate constant for the reaction of O(3P)+D2 was measured over the temperature range 343 to 446 K using the laser photolysis–resonance fluorescence technique. The results can be fit well with a two-parameter Arrhenius function and are in excellent agreement with the previous measurements of Presser and Gordon and with the recent high temperature measurements of Michael. The data are also in very good agreement with the theoretical calculations of Joseph et al. and of Bowman et al. and demonstrate that tunneling makes a significant contribution to the rate coefficient over the entire range of temperatures studied in this experiment. The Arrhenius behavior of the rate constant supports the conclusions of Robie et al. in their study of the intramolecular isotope effect in O(3P)+HD, where it was found that tunneling plays a major role at low temperatures.
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o configuration interaction study of the electronic and geometric structure of small, mixed neutral and cationic MgNak and MgLik (k=2–8) clusters
Authors: Fantucci P. , Bonačić-Koutecký V. , Pewestorf W. , Koutecký J.
Published: July 2002

Neutral and cationic MgYk clusters with Y=Na, Li (k=2–8) have been investigated using all-electron ab initio configuration interaction (CI) procedures. The alkaline earth metal (or group IIa) atom takes the central position in the most stable geometries of all clusters studied except for MgNa3, MgLi3, and MgNa+3 . The importance of the chemical nature of the atoms involved in determining the stability and other properties of the clusters, is clearly demonstrated by comparing the neutral and cationic series of MgNak , MgLik , and BeLik . The properties obtained can be fully explained by considering the s–p promotion, the role of Jahn–Teller distortions, and the stereochemical aspects. The appropriate treatment of electronic correlation effects is of crucial importance for correctly predicting the stability of the clusters. The low stability of small clusters, especially MgNak (k<5), is particularly noticeable. The atomization energies per atom for MgY6, MgY7, and MgY8 with Y=Na or Li, are all found to be comparable. The general pattern for atomization energies per atom and for other stability measures as well as for fragmentation channels and ionization potentials as functions of the cluster size is explained in the paper.
Photodissociation of H2O2 and CH3OOH at 248 nm and 298 K: Quantum yields for OH, O(3P) and H(2S)
Authors: Vaghjiani Ghanshyam L. , Ravishankara A. R.
Published: July 2002

The quantum yields of the products, OH(X 2Π), O(3P) [plus O(1D)] and H(2S), in the photolysis of H2O2 and CH3OOH at 248 nm and 298 K have been measured. OH was directly observed by laser-induced fluorescence while the atomic species were detected by cw-resonance fluorescence. All quantum yield measurements were made using relative methods. The quantum yields of OH, O, and H in H2O2 photolysis were measured relative to the well known quantum yields of O(1D) and O(3P) in the photodissociation of O3, and H(2S) in CH3SH. The values we obtain are, 2.09±0.36, <0.002 and <0.0002 for OH, O, and H, respectively. For CH3OOH photolysis, the quantum yield of OH was measured relative to our value for OH quantum yield in H2O2 photolysis, and the quantum yields of O and H relative to those in O3 and CH3SH photodissociation, respectively. The values we obtain are, 1.00±0.18, <0.007 and 0.038±0.007 for OH, O, and H, respectively. In both H2O2 and CH3OOH photolysis, the observed O and H quantum yields showed an apparent dependence on the fluence of the photolysis light, the possible origin of which is discussed. The large quantum yield of OH we measure is consistent with the known continuous and unstructured absorption spectra of these molecules in this wavelength region, where the most important process is the dissociative (Ã 1A←X̃ 1A) transition to give OH(X 2Π, v″=0) fragment.
Washboard model of gas–surface scattering
Authors: Tully John C.
Published: July 2002

The conventional ‘‘hard cube’’ and ‘‘soft cube’’ models of gas–surface scattering are extended to incorporate the effects of surface corrugation. The proposed ‘‘washboard’’ model invokes the hard cube assumption of conservation of tangential momentum, but with respect to the local surface tangent at the point of impact of the molecule with the corrugated surface. Expressions are derived in the form of convenient single variable quadratures for the angular scattering distribution, the mean velocity and kinetic energy as a function of scattering angle, and the trapping probability. The model is applied to the scattering of argon atoms from smooth and corrugated faces of platinum. Results are compared to those from multidimensional stochastic trajectory simulations employing a realistic interaction potential and moving surface atoms. The washboard model is shown to have a far wider range of validity than the cube models, and to describe properly the transition from singly peaked near-specular scattering from smooth surfaces to doubly peaked ‘‘rainbow’’ scattering from highly corrugated surfaces.
Surface diffusion of n-alkanes on Ru(001)
Authors: Brand J. L. , Arena M. V. , Deckert A. A. , George S. M.
Published: July 2002

The surface diffusion of n-alkanes on Ru(001) was measured using laser-induced thermal desorption (LITD) techniques. The surface diffusion coefficients for propane, n-butane, n-pentane, and n-hexane all displayed Arrhenius behavior. The surface diffusion activation energies increased linearly with carbon chain length from Edif =3.0±0.1 kcal/mol for propane to Edif =4.8±0.2 kcal/mol for n-hexane. In contrast, the surface diffusion preexponentials remained nearly constant at D0 ≂0.15 cm2 /s. Measurements performed at different coverages also revealed that the surface diffusion coefficients were coverage-independent for all the n-alkanes on Ru(001). The surface corrugation ratio Ω was defined as the ratio of the diffusion activation energy to the desorption activation energy, Ω=Edif /Edes . The surface corrugation ratio was observed to be remarkably constant at Ω≂0.3 for all the n-alkanes. This constant corrugation ratio indicated a linear scaling between the diffusion activation energy and the desorption activation energy. This behavior also suggested that the n-alkanes move with a rigid configuration parallel to the Ru(001) surface.
Analysis of vibrational correlations and couplings in the lowest two singlet states of pentacene by high resolution, fully resonant, coherent four-wave mixing spectroscopy
Authors: Carlson Roger J. , Wright John C.
Published: July 2002

Coherent, fully resonant four-wave mixing (FRFWM) spectroscopy is used to probe the vibrational and vibronic state of the pentacene S0 and S1 electronic states. High resolution is obtained using mixed molecular crystals at cryogenic temperatures. The FRFWM spectra combined with conventional absorption and fluorescence spectra allow correlation of the S1 fundamentals to the corresponding ground state normal coordinates. Couplings between modes are indicated and may be related to Fermi resonance of the pentacene vibrations by means of host librational phonons.
Isotope effect on the glass transition and crystallization of hyperquenched glassy water
Authors: Johari G. P. , Hallbrucker Andreas , Mayer Erwin
Published: July 2002

The thermal behavior of hyperquenched glassy D2O was investigated by differential scanning calorimetry from 103 to 250 K, in order to investigate the isotope effect on the glass→liquid transition and on the liquid to cubic, and cubic to hexagonal ice. For a heating rate of 30 K min−1 the temperatures of the thermal effects are: 137 K for the onset of the reversible glass→liquid transition (Tg), 154 and 173 K for the beginning and the peak minimum of the crystallization exotherm, and ≊229 K for the peak minimum of the transformation cubic→hexagonal ice. Increase in heat capacity at Tg and the width of the glass transition are similar to those reported for glassy H2O. Tg of D2O is higher than that of H2O by an amount that is expected for isorelaxational and/or isoviscous states assuming that the functional form of the molecular reorientation rates with temperature is unaffected by the isotopic substitution. The increase in the temperature of crystallization of ‘‘fluid’’ D2O is ≊three times greater than the increase in the glass→liquid transition which causes Tg and devitrification to become more separated in D2O than in H2O. Implications of these results for both the inferred λ-type anomally at ≊228 K in H2O and at ≊233 K in D2O, and the presumed formation of a gel-like structure at these temperatures are discussed.
A comparison between the Mo/ller–Plesset and Green’s function perturbative approaches to the calculation of the correlation energy in the many-electron problem
Authors: Holleboom L. J. , Snijders J. G.
Published: July 2002

The well-known expression for the total energy in terms of the single-particle many-body Green’s function is analyzed in detail. In particular the relation between the nth order Mo/ller–Plesset energy and the energy calculated from a Green’s function generated by the nth order self-energy is investigated. It is shown how the nth order Mo/ller–Plesset energy can be expressed in terms of the Green’s function. The H2 molecule is studied in a minimal basis to serve as a model in which exact results can be easily obtained. Numerical calculations are performed for H2, He, Be, LiH, Ne, HF, H2O, NH3, and CH4 and the results are analyzed in detail.
Kinetics of low-intensity infrared laser photodissociation. The thermal model and application of the Tolman theorem
Authors: Dunbar Robert C.
Published: July 2002

Infrared multiphoton dissociation by low-power cw lasers under collision-free conditions is analyzed from the point of view of thermal kinetic theory. It is pointed out first that the laser has the same effect on the molecular population as irradiation by an equivalent black-body source, assuming only that the dominant IR emissions of the molecule at steady state are at wavelengths not far from the irradiating IR laser wavelength. Then the dissociation reaction kinetics are considered for this thermal population under the assumption that the reaction perturbs only the high-energy tail of the Boltzmann distribution. Analysis from this point of view leads to a slightly modified version of the Tolman theorem giving the activation energy as a function of temperature. A random walk simulation of the kinetics for dissociation of a model molecular system was carried out and the validity of the thermal kinetic approach was verified by the excellent agreement between the activation energies from the Tolman-theorem analysis and those from the random-walk simulations. The extent of dissociation is strongly dependent on intensity (at constant fluence) under these conditions.
Biexciton states and two-photon absorption in molecular monolayers
Authors: Spano Francis C. , Agranovich Vladimir , Mukamel Shaul
Published: July 2002

Exciton–exciton interactions in doubly electronically excited molecular monolayers, may result in the formation of bound two-exciton states (biexcitons), whose binding is induced by the change in the molecular permanent dipole moments upon electronic excitation. We solve for the biexciton energies in the dipole approximation, and explore the effects of biexcitons on the monolayer nonlinear susceptibility. Calculations are made for monolayers with molecular dipole moments oriented either parallel or perpendicular to the monolayer surface.
Reactions of boron atoms with molecular oxygen. Infrared spectra of BO, BO2, B2O2, B2O3, and BO−2 in solid argon
Authors: Burkholder Thomas R. , Andrews Lester
Published: July 2002

Boron atoms from Nd:YAG laser ablation of the solid have been codeposited with Ar/O2 samples on a 11±1 K salt window. The product infrared spectrum was dominated by three strong 11B isotopic bands at 1299.3, 1282.8, and 1274.6 cm−1 with 10B counterparts at 1347.6, 1330.7, and 1322.2 cm−1. Oxygen isotopic substitution (16O18O and 18O2 ) confirms the assignment of these strong bands to ν3 of linear BO2. Renner–Teller coupling is evident in the ν2 bending motion. A sharp medium intensity band at 1854.7 has appropriate isotopic ratios for BO, which exhibits a 1862.1 cm−1 gas phase fundamental. A sharp 1931.0 cm−1 band shows isotopic ratios appropriate for another linear BO2 species; correlation with spectra of BO−2 in alkali halide lattices confirms this assignment. A weak 1898.9 cm−1 band grows on annealing and shows isotopic ratios for a BO stretching mode and isotopic splittings for two equivalent B and O atoms, which confirms assignment to B2O2. A weak 2062 cm−1 band grows markedly on annealing and shows isotope shifts appropriate for a terminal–BO group interacting with another oxygen atom; the 2062 cm−1 band is assigned to B2O3 in agreement with earlier work. A strong 1512.3 cm−1 band appeared on annealing; its proximity to the O2 fundamental at 1552 cm−1 and pure oxygen isotopic shift suggest that this absorption is due to a B atom–O2 complex.
A simple curvilinear internal coordinate model for vibrational energy levels
Authors: Kauppi Esa , Halonen Lauri
Published: July 2002

A simple curvilinear internal coordinate Hamiltonian model for vibrational term values of well bent XY2 molecules is developed. The stretching vibrations are described in the zero-order picture by Morse oscillators and the bend by a harmonic oscillator. Coupling terms are approximated by harmonic oscillator formulas. Van Vleck perturbation theory is used to transform the Hamiltonian matrix to a block diagonal form. Analytical expressions for the matrix elements are derived. Fermi resonances between the stretches and the bend and local modes are taken into account by diagonalizing the block diagonal Hamiltonian matrix. Rotational parameters (α constants) are calculated with perturbation theory expressions. Potential energy parameters are optimized with the nonlinear least-squares method using vibrational term values and α constants as data. The model is applied to five isotopic species of hydrogen sulfide and to two isotopic species of sulfur dioxide. The potential energy parameters obtained agree well with parameters determined by other methods.
The Jahn–Teller effect in triptycene
Authors: Furlan Alan , Riley Mark J. , Leutwyler Samuel
Published: July 2002

The irregular vibronic structure resolved in the S1←S0 resonant two-photon ionization (R2PI) spectrum of supersonically cooled triptycene (9,10-dihydro-9,10[1′2′]benzenoanthracene) is assigned in terms of a single-mode E′⊗e′ Jahn–Teller vibronic Hamiltonian for the excited state, with linear and quadratic coupling terms. The Jahn–Teller active vibrational mode is a benzene wagging framework mode. To fit to the observed vibronic levels yields a very low frequency νe′ =47.83 cm−1 and linear and quadratic terms are k=1.65 and g=0.426. This fit accounts for ≊98% of the observed absorption band intensities over the observable range 0–350 cm−1. The quadratic term is unusually large, leading to localization of the lowest vibronic levels in the three symmetry-equivalent minima. Emission spectra from 13 vibronic levels in the excited E′ state show extended vibrational progressions with up to 25 members in the analogous e′ ground state vibration, which is highly harmonic in the electronic ground state. The Franck–Condon factors of the fluorescence emission spectra calculated with the E′ state Jahn–Teller parameters fitted to the absorption spectrum also yield a quantitative fit to observed emission intensities. The eigenvectors of the E′ state vibronic levels are hence determined to great precision; the lowest five can be classified as radial oscillator and/or hindered rotor states, while higher levels have mixed character. Several eigenvectors are strongly localized in the upper sheet of the adiabatic Jahn–Teller surface, corresponding to ‘‘cone’’ states.
Determination of potential energy curves for ground and metastable excited state transition metal ions interacting with helium and neon using electronic state chromatography
Authors: von Helden Gert , Kemper Paul R. , Hsu Ming-Teh , Bowers Michael T.
Published: July 2002

Reduced zero field mobilities of Cr+, Co+, and Ni+ in their ground state configuration (3dn) and their metastable excited state configuration (3dn−14s1) have been measured in He and Ne as a function of temperature. Parameters for a n−6−4 interaction potential are obtained by fitting the mobility vs temperature curve. Bond dissociation energies, vibrational frequencies, and equilibrium bond lengths are extracted from these curves and compared with data from traditional equilibrium measurements and ab initio theory. Dramatic changes in bonding are observed for the 3dn and 3dn−14s1 configurations; the 3dn configuration yielding very much stronger bonds and very much shorter equilibrium bond lengths than the 3dn−14s1 configuration. The presence of multiple potential curves in the interaction of Co+(3F,3d8) and Ni+(2D,3d9) with He/Ne make interpretation of the ground state mobility data ambiguous. The analysis of each system is discussed in some detail.
State-to-state dynamics of atom+polyatom abstraction reactions. II. The H+C2H6/C3H8→H2(v′,J′) +C2H5/C3H7 reactions
Authors: Germann Geoffrey J. , Huh Young-Duk , Valentini James J.
Published: July 2002

The rotational and vibrational quantum state distributions for the H2 products of the H+HR→H2+R reactions (HR=C2H6 and C3H8 ) at 1.6 eV collision energy have been measured using coherent anti-Stokes Raman scattering. Total reaction cross sections have also been determined. For the total cross sections we find 1.5±0.5 Å2 for the ethane reaction and 2.9±0.8 Å2 for the propane reaction. Although several vibrational states are energetically accessible, we observe H2 products only in v′=0 and v′=1, with the majority in the ground vibrational state. The H2 products are on average rotationally cold as well, and 20% or less of the total energy is partitioned to H2 internal energy. However, the quantum state distributions show a positive correlation of H2 product rotational and vibrational energy. That is, the average rotational energy of the H2 in v′=1 is substantially greater than the average rotational energy of the H2 in v′=0. Comparison with state-to-state dynamics results previously obtained for the kinematically and energetically similar H+CD4→HD+CD3 and H+HCl→H2+Cl reactions seems to indicate that this anomalous energy disposal is an intrinsic characteristic of H + alkane hydrogen atom abstraction reactions at high collision energy. We speculate that this anomalous behavior is the result of inelastic encounters between the nascent H2 and alkyl radical products in the reaction exit channel.
Perturbation-facilitated all-optical triple resonance spectroscopy of the Na2 b 3Πu state
Authors: Whang T.-J. , Stwalley W. C. , Li L. , Lyyra A. M.
Published: July 2002

The Na2 b 3Πu state has been studied by the continuous wave (cw) all-optical triple resonance (AOTR) technique. The AOTR technique used here corresponds to a perturbation-facilitated optical–optical double resonance (PFOODR) excitation through A 1Σu+∼b 3Πu mixed intermediate levels from the ground state to the 2 3Πg state and stimulated emission pumping (SEP) of the 2 3Πg→b 3Πu transition. This sub-Doppler high-resolution PFOODR-SEP technique has allowed us to reach many b 3Πu levels (Ω=0, 1, 2, perturbed and unperturbed, including low v below the A 1Σu+ potential minimum and higher vibrational levels up to v=57). Based on these new high resolution data and previous results from high resolution spectroscopy, we have determined a new set of deperturbed molecular constants from the b 3Πu state up to v=57, the corresponding RKR potential energy curve and the A 1Σu+∼b 3Πu spin–orbit interaction constants. This represents an example of a powerful and general technique for observing a ‘‘dark’’ (e.g., triplet) perturbing state when only the ‘‘bright’’ (e.g., singlet) perturbed state is well known from single photon spectroscopy.
Thermodiffusion in polymer solutions as observed by forced Rayleigh scattering
Authors: Köhler W.
Published: July 2002

Thermodiffusion, the cross coupling between temperature and concentration, is studied in dilute polymer solutions by forced Rayleigh light scattering. First a spatial temperature grating is created within the solution by absorption of an optical interference grating and subsequently a concentration grating builds up. The time-dependent diffraction efficiency of both gratings is described by a phenomenological model from which the thermal diffusivity, the ordinary diffusion coefficient, and the thermodiffusion coefficient are obtained without the need for absolute intensity measurements or calibration with a known standard. No labeling of the polymer chains with photochromic dyes is required. Experimental details and potential pitfalls are discussed. Measurements are carried out on polystyrene in ethyl acetate and the obtained values compare well with data taken from the literature.
Complete optimization of models of the Belousov–Zhabotinsky reaction at a Hopf bifurcation
Authors: Hynne F. , So/rensen P. Graae , Mo/ller T.
Published: July 2002

We analyze two reaction networks for the Belousov–Zhabotinsky reaction at a Hopf bifurcation by methods developed in a previous paper. One network is equivalent to the oregonator, the other is an extended oregonator with seven species and eleven reactions. For each model the current polytope has a simple geometry and the paper serves to illustrate the analytic methods and explain their relation to the chemistry of the reactions. All possible models based on each network are compared with experimental quenching results and other experimental data at a supercritical Hopf bifurcation. The best possible fit is obtained through a systematic search of the current cone and concentration space. For the oregonator, the optimization is essentially complete and results in a determination from scratch of all six rate constants of the model. The agreement of the optimum with the experiments is very good, and the agreement of the deduced rate constants with the most recent set determined from kinetic measurements on subsystems seems quite remarkable. The larger model is too big for a really complete search, but the best point found shows a significant improvement over our previous results based on continuation methods. Despite the good agreement with the experiments, a graphical eigenvector analysis definitively shows that perfect agreement is impossible for any of the models, no matter how the rate constants are chosen; additional reactions are needed.