bluuebunny/crossref_metadata_2025 huggingface dataset:
Composition of trench sidewalls and bottoms for SiO2-masked Si(100) etched in Cl2 plasmas
Authors: Bogart K. H. A. , Donnelly V. M.
Published: July 2002

We have investigated the possible role of redeposition of silicon–chloride etching products on profile evolution by studying the influence of etching product partial pressure on the surface layer formed during chlorine plasma etching of SiO2-masked p-type Si(100). Samples were etched with high and low etching product (SiCly)-to-etchant (Cl, Cl2) concentration ratios by changing the Cl2 flow rate (1.4 or 10.0 sccm, respectively) at a constant pressure of 4 mTorr. Compositional analysis was performed using angle-resolved x-ray photoelectron spectroscopy (XPS). Electron shadowing and differential charging of the insulating SiO2 regions were exploited to spatially resolve the composition of the trench sidewalls and bottoms (2.0, 1.0, 0.5, 0.3, and 0.22 μm wide). Chlorine content and stoichiometry of the etched surfaces were determined by quantifying the XPS intensities of both the Cl(2p) peak and the silicon chloride containing tail of the Si(2p) peak. Comparisons of chlorine content and stoichiometry were also made to unmasked Si areas etched on the same samples. For trenches etched with 10 sccm Cl2, the chlorine coverage (2.6×1015 Cl/cm2, equivalent to ∼3 monolayers) and the silicon chloride stoichiometry (SiCl:SiCl2:SiCl3=1:0.45:0.33) were identical for the unmasked Si areas and the bottoms of the trenches. The trench sidewalls, however, contained roughly 50% less Cl than the unmasked areas, all in the form of SiCl. Virtually identical results were obtained for trenches etched with 1.4 sccm Cl2, indicating that increased SiCly etching product concentrations do not result in the formation of a thick, passivating sidewall layer on trench sidewalls during Cl2 plasma etching of Si masked with SiO2.
Current–voltage characteristics of Schottky barrier structures on porous silicon, and effect of an organic stabilizer film
Authors: Li W. , Andrienko I. , Haneman D.
Published: July 2002

The stability and reproducibility of current–voltage curves of Schottky barrier structures on particular porous silicon surfaces used for obtaining electroluminescence is greatly improved by first coating the surface with a conducting polymer, poly-4-dicyanomethylene-4H-cyclopenta [2,1-b:3,4-b′] dithiophene. With such coated structures it is possible to fit the usual diode formula at room temperatures with a quality factor of 3.0, obviating the need for more complex theories. The stability of electroluminescence, which shows a redshift from 500 to 580 nm after coating, is also greatly improved. The coating appears to chemically react with the hydride surface and also mechanically strengthens the structure.
Ferroelectric behavior of epitaxial Bi2VO5.5 thin films on Si(100) formed by pulsed-laser deposition
Authors: Joseph M. , Lee H. Y. , Tabata H. , Kawai T.
Published: July 2002

Thin films of Bi2VO5.5 have been prepared epitaxially using a pulsed-laser deposition method on a Si(100) substrate using TiN as a buffer layer and SrTiO3 as a seed layer. The films have smooth surface morphology with atomically flat terraces and steps of 4 Å in height. The ferroelectric characterization shows a spontaneous polarization of 2.2 μC/cm2 and a coercive field (Ec) of 22 kV/cm. The leakage current obtained is about 5×10−6 A/cm2 at a drive voltage of ±2 V.
Optical nonlinearities of inorganic metal cluster μ3-MoSe4Ag3(PPh3)3Cl
Authors: Xiong Yuenan , Ji Wei , Zhang Qianfeng , Xin Xinquan
Published: July 2002

Optical nonlinearity and dynamic response of a novel inorganic metal cluster (μ3-MoSe4)Ag3(PPh3)3Cl were studied. Large optical limiting effect was observed with the inorganic metal cluster (μ3-MoSe4)Ag3(PPh3)3Cl dissolved in CH2Cl2. We also proved that nonlinear scattering contributes much to its optical limiting performance, which counts for nearly 75% of the total nonlinear energy loss used. The dynamic response of the cluster when pumped at high fluence was tested by a standard time-resolved pump-probe technique. The effective third-order susceptibility χ(3) for this sample was determined in a standard backward degenerate four wave mixing experiment with pump fluence lower than limiting threshold Fth. The conjugate signal intensity tends to saturate when the incident fluence rolls over this value (Fth) in spite of the fact that large optical limiting effect was observed in this fluence region, which implies that the nonlinearity mechanism is different for these two regions.
A study of the structure and dynamics of the hydronium ion by high-resolution infrared laser spectroscopy. I. The ν3 band of H3 16O+
Authors: Begemann Marianne H. , Saykally Richard J.
Published: July 2002

The results of an extensive study of the ν3 (asymmetric stretch) vibration of the H3O+ ion are reported. A total of 86 and 41 transitions were measured with a precision of 0.002 cm−1 for the s–s and a–a inversion subbands, respectively, and least-squares fit to an accurate Hamiltonian. Twenty-four parameters were determined in the analysis. The ν3 frequencies are 3535.974(11) and 3519.397(25) cm−1 for the s–s and a–a inversion components, respectively.
Network thermostatics
Authors: Peusner Leonardo
Published: July 2002

The fundamental topological and metric assumptions of thermodynamic theory lead to a graphical representation related to some networks previously grouped under the label of network thermodynamics. This approach contains a high level of organized information about the thermodynamic manifold, covariant and contravariant vectors, the orthogonality of intensive and extensive quantities, and the connectivity of the manifold. It is shown that (1) the model leads to all classical results regarding Maxwell’s reciprocities, the energy and entropy potentials, thermodynamic equations of state, Legendre transformations, the stability of the equilibrium state, etc.; (2) it imbeds the thermodynamic manifold of dimension n in a Euclidean space of dimension n(n+1)/2, in which all measurable quantities split in orthogonal directions—a procedure not related to the Gramm–Schmidt orthogonalization; (3) it provides a topological definition of the chemical potential which differs from the usual definition and introduces six new energy potentials which collapse into G,E,F, and H in the two-dimensional limit; (4) it introduces irreversible transformations which can transform frames of reference when no thermodynamic energy potential can be defined; (5) it summarizes all of Euclidean thermodynamics in two reciprocity relations which follow from the theorems of network theory—the general reciprocities for energy (GRE) and entropy (GRS)—and these are further unified into a single formula which relates, through a mixed bilinear expression, how interstate measurements are affected in Euclidean thermodynamics; (6) the reciprocity relation contains all variational results related to stability and is equivalent to any of the variational formulations—e.g., Euler’s. Because the approach relates directly to several topological and graphical techniques used in various fields of chemistry, it has a high potential for application, and software is readily available to treat various families of problems.
Non-Arrhenius rate constants involving diffusion and reaction
Authors: Kelly K. , Kostin M. D.
Published: July 2002

A system in which diffusion and reaction occur is investigated. Explicit results are obtained for the rate constant as a function of position and time. Under certain conditions it is found that the nonequilibrium effects produced by diffusion can lead to a rate constant that differs significantly from the Arrhenius value.
Dielectric constant of dipolar hard sphere mixtures
Authors: Cummings P. T. , Blum L.
Published: July 2002

The dielectric constant of dipolar hard sphere mixtures is calculated from the analytic solution of the mean spherical approximation (MSA) and using Monte Carlo simulation. The MSA is solved using the factorization technique pioneered by Wertheim and Baxter and extended to nonspherical interactions by Blum. The simulations are performed using the Ewald technique to take into account the long-ranged dipolar interactions. The simulation results are compared to those obtained from the MSA as well as a modified MSA and a continuum theory due to Looyenga.
The unimolecular reaction of t-BuNO on singlet and triplet surfaces: Spectroscopy, real-time rate measurements, and NO energy distributions
Authors: Noble M. , Qian C. X. W. , Reisler H. , Wittig C.
Published: July 2002

The predissociation of jet-cooled (CH3)3CNO (t-BuNO) following laser excitation in the Ã 1A″←X̃ 1A′ system has been studied in both the energy (frequency) and time domains. Unlike the smaller nitroso compounds whose predissociation has been already examined, unimolecular reaction is the rate determining step for predissociation near threshold. Consequently, it has been possible to separately measure radiationless transition rates and unimolecular reaction rates in real time. Dissociation on both the ground state (S0) and the first triplet state (T1) has been identified. At threshold, dissociation proceeds only on S0, with lifetimes >3.5 μs, but for E°≥650 cm−1, fast (<10 ns) predissociation via T1 becomes progressively the dominant dissociative route. Nascent NO photofragments have been characterized in detail using one-photon LIF. The rotational and spin-orbit distributions of NO following dissociation on S0 are statistical, depending only on E°. The NO derived from dissociation on the T1 surface is not at all statistical, consistent with a sizable (∼650 cm−1) exit channel barrier on this surface. Most notably, the [NO(2Π3/2)]/[NO(2Π1/2)] ratios are much smaller, and rotational distributions are colder than predicted by a statistical model. In order that some of the predissociating vibrational states could be assigned, the 1A″←1A′ spectrum of t-BuNO has been analyzed and the electronic origin assigned as 13 911 cm−1. The ground state dissociation energy, D0, is found to be 13 930±30 cm−1, i.e., 39.8±0.1 kcal mol−1.
Optical properties of Cr3+ in fluorite-structure hosts and in MgF*2
Authors: Payne Stephen A. , Chase L. L. , Krupke William F.
Published: July 2002

We have examined the optical properties of Cr3+ in MgF2 and in the fluorite-structure hosts: CdF2, CaF2, SrF2, and BaF2. The properties of Cr3+ in MgF2 are similar to those observed for other fluoride crystals that have octahedral substitutional metal sites. Interestingly, Cr3+ is also found to be sixfold coordinated in the fluorite hosts, despite the fact that the metal sites of these crystal lattices are eightfold coordinated. The smaller ionic radius of Cr3+ compared to, say, Ca2+, undoubtedly results in considerable relaxation at the metal site. However, the crystal field stabilization energy present in the 4A2(d3) ground state also provides for the energetic preference of sixfold vs eightfold coordination. The similarity of the observed absorption spectra of Cr3+ in MgF2 and in fluorite give evidence that the ground state is octahedrally coordinated in all of these hosts. The reduction of this electronic stabilization energy in the 4T2(d3) excited state is considered to produce a configurational shift relative to the ground state. This shift may be the reason why Cr3+ luminesces effectively in MgF2 whereas it is largely quenched in the fluorite-structure materials.
Specific rate constants k(E,J) and product state distributions in simple bond fission reactions. II. Application to HOOH→OH+OH
Authors: Brouwer L. , Cobos C. J. , Troe J. , Dübal H.-R. , Crim F. F.
Published: July 2002

Detailed and simplified statistical adiabatic channel calculations of specific rate constants k(E,J) and product quantum state distributions for the simple bond fission reaction HOOH→2 OH are compared with recent measurements of state-resolved dissociation rates, product state distributions, and thermally averaged rate coefficients. A simple modification of phase space theory based on the statistical adiabatic channel model successfully predicts product state distributions and rate constants as well. Because of the amount of experimental data and theoretical analysis available, the dissociation of hydrogen peroxide is becoming a model case for simple unimolecular bond fission processes.
Interatomic potentials for HeAr, HeKr, and HeXe from multiproperty fits
Authors: Danielson Laurie J. , Keil Mark
Published: July 2002

Crossed molecular beam measurements of differential cross sections (DCS) are reported for elastic scattering of He by Ar, Kr, and Xe at high resolution. Interatomic potentials are determined by simultaneously fitting the DCS’s, as well as mixture viscosity and interaction second virial data. Bias due to systematic and potential model errors are examined and are used to estimate the accuracy of the potential energy curves obtained. Attractive well depths are 2.59, 2.67, and 2.64 meV±3% for HeAr, HeKr, and HeXe, respectively, agreeing with the best available HeAr potential and a previously proposed HeKr potential, but significantly deeper than previously reported potentials for HeXe. The HeXe attractive well is also considerably broader than previously reported. Attractive minimum positions are 3.48, 3.70, and 4.00 Å (±0.03 Å) for HeAr, HeKr, and HeXe, respectively. Including the accurate diffusion data of Dunlop and co-workers [Physica A 95, 561 (1979)] and the absolute integral cross sections of Pirani and Vecchiocattivi [J. Chem. Phys. 66, 372 (1977) and revisions thereto] verify the error bounds for all three potentials.
Energetics and spin- and Λ-doublet selectivity in the infrared multiphoton dissociation HN3(X̃ 1A′)→N2(X 1Σ+g)+NH(X 3Σ−,a 1Δ): Theory
Authors: Alexander Millard H. , Werner Hans-Joachim , Dagdigian Paul J.
Published: July 2002

An investigation of the energetics and mechanism of the dissociation of ground state HN3(X̃ 1A′) into ground state N2(X 1∑+g)+NH(X 3∑−) products is presented. This process, which can be induced by multiphoton infrared pumping, occurs through a crossing between the lowest-energy singlet potential energy, which correlates asymptotically with electronically excited NH products (a 1Δ), and the lowest triplet surface. By means of ab initio CASSCF and MCSCF-CI calculations we have determined that the geometry at the minimum singlet–triplet crossing corresponds to an approximately linear N3 backbone with a perpendicular NH bond. The interior N–N distance is ∼3.6 bohr. This transition state lies ∼12 500 cm−1 above the energy of X̃ 1A′ state of HN3 at the experimental equilibrium geometry. Since the N–N and N–H bonds are perpendicular at this transition state, there will be no torques tending to twist the system out of a planar geometry. The crucial singlet–triplet coupling occurs because the HN3 wave function in the region of this transition state can be considered an equal admixture of N2(X)⋅NH(a 1Δ) and

N2(X)⋅NH(b 1∑+). Since the ground state HN3 wave function as well as the relevant spin–orbit Hamiltonian are symmetric with respect to reflection of the spatial and spin components of all the electrons in the plane of the molecule, and since the NH fragment must rotate in the plane of the initital HN3 molecule if the dissociation is planar, NH products can be formed only in states in which the wave function (electronic+rotational) is also symmetric with respect to this operation. For a molecule in a 3∑− electronic state the wave functions in only the F1 and F3 multiplets will be symmetric so that one would expect population in only the F1 and F3 levels. A similar symmetry argument implies that the NH products formed in the lowest spin-allowed channel [N2(X 1∑+g)+NH(a 1Δ)] will be found predominantly in the Δ(A′) Λ-doublet state, which is symmetric with respect to reflection of the spatial coordinates of the electrons in the plane of rotation. This spin- and Λ-doublet selectivity has been found experimentally by Stephenson, Casassa, and King (accompanying article). The implications of similar spin selectivity in other photodissociation processes leading to molecules in 3∑− states, e.g., SO(X 3∑−) and O2(X 3∑−g), are also considered.
Statistical thermodynamics of association colloids. III. The gel to liquid phase transition of lipid bilayer membranes
Authors: Leermakers F. A. M. , Scheutjens J. M. H. M.
Published: July 2002

A new theory is introduced to model the lipid membrane structure and stability both above and below the gel to liquid phase transition temperature. Recently, we elaborated a self-consistent-field (SCF) theory, in which the full set of conformations was generated in a rotational isomeric state scheme and Boltzmann statistics was used to determine the statistical weight per conformation. In the present paper we also take into account that the anisotropic distribution of the molecules on the lattice induce a self-consistent anisotropic molecular field. This field, which is a function of the bond orientations, leads to an extra factor in the statistical weight of each conformation and is based on a generalization of Flory’s and Di Marzio’s analysis of systems with rigid rods. This elegant refinement follows from elementary statistics, is free of new adjustable parameters, and significantly improves details of the structure of the model membranes. To examine the properties of the SCAF (self-consistent anisotropic field) theory we use a model membrane built up by lecithin-like molecules composed of apolar and polar segments. The model has three nearest-neighbor interaction parameters of the Flory–Huggins type, namely for the interaction between apolar segments and water, that between polar segments and water, and that between polar and apolar segments. A fourth parameter is the dihedral trans/gauche energy difference. The theory predicts a first order gel to liquid phase transition for the model membranes. Depending on the membrane concentration, both an intercalated (in the dilute regime) and a nonintercalated (in the concentrated regime) gel phase are observed. Detailed information on the various membrane phases is obtained. Order parameter and segment density profiles are given.
Polyparaphenylene under pressure: Optical absorption and vibrational modes
Authors: Hanfland M. , Brillante A. , Syassen K. , Stamm M. , Fink J.
Published: July 2002

Optical absorption, Raman, and infrared (IR) spectra of undoped polyparaphenylene (PPP) have been studied under pressure up to 20 GPa at 300 K. PPP shows stable behavior, in marked contrast to the reactivity observed in trans-polyacetylene under pressure. The absorption edge of PPP shifts to lower energy with a total shift of 0.55 eV at 20 GPa. Raman spectra reveal a new band at about 820 cm−1 which becomes active above ∼5 GPa. At high pressure we find a considerable broadening of the infrared-active out-of-plane mode at 805 cm−1. The results are in part interpreted in terms of the larger interchain coupling induced by pressure.
Density functional theory for the freezing of Lennard-Jones binary mixtures
Authors: Rick Steven W. , Haymet A. D. J.
Published: July 2002

The density functional theory of freezing is used to study the freezing of binary Lennard-Jones mixtures. The phase diagrams of several different Lennard-Jones systems are presented, including models of real binary mixtures of argon, krypton, and methane. The relative importance of size ratio and interaction energy are examined. Three generic types of phase diagram, found in real materials, are obtained. The phase diagram of the hard sphere binary mixture is also presented.
The primitive model of the electric double layer: Nonsymmetric electrolytes
Authors: Plischke Michael , Henderson Douglas
Published: July 2002

We solve the inhomogeneous Ornstein–Zernike equation for the pair correlation functions together with the Lovett–Mou–Buff–Wertheim equation for the density profiles for charged hard spheres in the vicinity of a charged hard wall. This constitutes the so-called primitive model of the electric double layer. In this article, we consider 2-1 electrolytes (doubly charged positive ions and singly charged negative ions) near both positively and negatively charged surfaces. We use the hypernetted chain approximation to close the Ornstein–Zernike equation. Except for very high surface charge densities and low bulk concentrations, we obtain excellent agreement with the Monte Carlo data.
Mixed state quantum mechanics in hydrodynamical form
Authors: Lill J. V. , Haftel M. I. , Herling G. H.
Published: July 2002

A newly derive set of quantum fluid dynamical equations appropriate for the description of mixed state dynamics is presented. Based essentially on moments of the Wigner function, the theory presented here uses an expansion of the pseudodistribution function in conjunction with a renormalization technique to obtain a semiclassical approximation for the dynamics of the probability, probability current, and energy densities. Particularly simple equations, ideal fluid dynamics with a nonlocal potential, result from using a local Maxwellian ansatz for the Wigner function. Further, the transformed potential may be easily computed from the fields being convected. Analogies with classical kinetic theory and fluid dynamics are exploited whenever possible.
Infrared diode-laser spectroscopy and Fourier-transform microwave spectroscopy of the (CO2,CO) dimer in a pulsed jet
Authors: Legon A. C. , Suckley A. P.
Published: July 2002

Two techniques have been used to observe the spectrum of a weakly bound dimer (CO2, CO) formed in a pulsed jet of CO2 and CO seeded in argon. The v=1←0 vibrational transition in the modified asymmetric stretching mode of the CO2 subunit was first observed at Doppler-limited resolution by investigating the pulsed jet with an infrared diode-laser spectrometer. Then the ground-state pure rotational spectrum was detected by applying Fourier-transform microwave spectroscopy to such a jet. A combined fit of the two spectra of the parent isotopomer gave the following set of ground-state (v=0) and excited-state (v=1) spectroscopic constants: A0=11 863.48(56) MHz, B0=1883.915(4) MHz, C0=1613.297(4) MHz, A1=11 772.17(37) MHz, B1=1882.76(6) MHz, C1=1611.44(5) MHz, ΔJ =8.145(2) kHz, ΔJK =339.42(1) kHz, δJ =1.26(1) kHz, and δK =309.8(4) kHz when the centrifugal distortion constants were constrained to be equal in both states. Spectroscopic constants for the isotopomers (CO2, 13CO) and (CO2, C18O) have also been determined. The nature of the observed spectra, especially the systematic absences of certain transitions, and the magnitudes of the spectroscopic constants have been interpreted in favor of a dimer with a T-shaped geometry in which the CO molecule lies along the C2 axis of CO2, with the two C atoms contiguous and r0(C⋅⋅⋅C)=3.277 Å. The intermolecular stretching force constant has the small value kσ =2.7 N m−1.
The infrared spectrum of solid CD4
Authors: Calvani P. , Lupi S. , Maselli P.
Published: July 2002

The infrared vibrational bands of solid CD4 in the ordered phases II and III have been observed, and their fine structure resolved. Evidence for the existence of free rotating molecules has been found in CD4 II. At lower temperatures, the ν4 band is well fitted by the sum of eleven lines: their number and relative intensities are consistent with a three-site model for phase III of solid methane. Overtone and combination bands up to the third order are also reported.
Proton spin–lattice relaxation rate in supercooled H2O and H2 17O under high pressure
Authors: Lang E. W. , Girlich D. , Lüdemann H.-D. , Piculell L. , Müller D.
Published: July 2002

The proton spin–lattice relaxation rate has been investigated as a function of pressure (p≤250 MPa), temperature (180 K≤T≤300 K), and oxygen-17 enrichment (25.7 atom-%, 50.7 atom-%) in metastable water. Intramolecular proton–oxygen 17 dipolar interactions in light water in relation to deuterium electric quadrupole interactions in heavy water carry similar dynamic information and allow the complicated dynamic isotope effect in liquid water to be investigated in the whole metastable range. Under hydrostatic pressure the proton relaxation rate curve R1 (T, p, ωH) has been investigated at five different Larmor frequencies (100 MHz≤ω≤500 MHz). These experiments unequivocally prove the isotropy of reorientational motions of water molecules on a nanosecond time scale and demonstrate the necessity to include fast quasilattice vibrations into a consistent interpretation of the relaxation rates in the dispersion regime.
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o study of the energetics of the spin-allowed and spin-forbidden decomposition of HN3
Authors: Alexander Millard H. , Werner Hans-Joachim , Hemmer Terrence , Knowles Peter J.
Published: July 2002

We describe an investigation of the energetics of the dissociation of ground state hydrazoic acid HN3. The study is limited to the lowest energy spin-allowed and spin-forbidden decomposition pathways, namely HN3(X̃ 1A′)→N2(X 1Σg+) +NH(a 1Δ,X 3Σ−) and HN3(X̃ 1A′) →N3(X̃ 2Πg)+H(2S). Complete active space SCF and multireference configuration interaction calculations with large basis sets are used (a) to determine the NNN–H and NN–NH bond dissociation energies of HN3, (b) to locate the geometry of the transition state for the spin-forbidden decomposition and the corresponding activation energy, and (c) to investigate the magnitude and origin of the exit channel barrier in the spin-allowed decomposition channel.
The influence of parent bending motion on branching at a conical intersection in the photodissociation of CX3I (X=H,D,F)
Authors: Person M. D. , Kash P. W. , Butler L. J.
Published: July 2002

This work measures the change in branching between the CF3+I(2P3/2) and I(2P1/2) product channels when one photodissociates vibrationally excited rather than cold CF3I at 248.5 nm. The experiment tests a model for the dependence of branching at a conical intersection on the amplitude of the dissociative wave function at bent geometries, a model which we propose here to explain previously observed differences in branching between the I(2P1/2) and I(2P3/2) channels at 248 nm for CH3I versus CD3I. In the CF3I experiment, we observe an increase in the branching from 13% to 17% I(2P3/2) products when the temperature of the CF3I parent is increased from 100 to 400 °C, in agreement with the qualitative prediction of the model. We analyze the angular distributions of the photofragments to eliminate the possibility that the change in branching is due to an increased contribution from direct absorption to the electronic state correlating with I(2P3/2) products.
Molecular theory of solvation and solvation dynamics in a binary dipolar liquid
Authors: Chandra A. , Bagchi B.
Published: July 2002

Both the equilibrium and the dynamical aspects of solvation of a classical ion in a dense binary dipolar liquid are investigated by using a molecular theory. The theory properly includes the differing inter- and intramolecular correlations that are present in a binary mixture. As a result, the theory can explain several important aspects of the nonideality of equilibrium solvation energy (broadly known as preferential solvation) observed in experiments. We find that the nonideality of solvation depends strongly on both the molecular size and the magnitude of the dipole moment of the solvent molecules. The interactions among the solvent molecules play an important role in determining the extent of this nonideality. The dynamical calculations are based on a generalized Smoluchowski equation which has been used extensively for studies in one component liquid. For binary liquid, our study reveals rich and diverse behavior such as dependencies on the sizes, the transport coefficients and the polar properties of the components. The theory offers a detailed picture of the dependence of the solvation dynamics on the composition of the mixture. It is predicted that the dynamics of solvation in a binary liquid is, in general, nonexponential and that the details of the dynamics can be quite different from those in a one component liquid. In particular, the continuum model is found to be grossly inaccurate in describing the solvation dynamics in binary mixtures and rather extreme conditions are needed to recover the predictions of the continuum model which can be attributed to the nonideality of the solvation. The predicted results are used to study the dynamic solvent effects on the rate of an adiabatic electron transfer reaction in a binary liquid. The theoretical predictions are also compared with the available experimental results.
Dissociation dynamics of I2⋅⋅⋅Nen van der Waals clusters (n=1–9): A quasiclassical approach
Authors: García-Vela A. , Villarreal P. , Delgado-Barrio G.
Published: July 2002

The dynamics of vibrational predissociation (VP) of van der Waals (vdW) I2⋅⋅⋅Nen clusters is studied by means of a quasiclassical trajectory approach. We assume a near-equilibrium geometry that accounts for the I2 stretch and the stretching and bending modes of the rare gas atoms restricted to move along a plane perpendicular to the I2 axis. A sharp increase of the halfwidth is observed for n=9 together with certain stability of the n=8 cluster size with respect to dissociation. It would indicate the existence of a first coordination shell for Ne around I2 containing eight atoms. In addition, the dissociation results less and less efficient as n increases and statistical mechanisms become important for the cluster sizes studied.
The photochemical dynamics of the Ã 2A2 state of chlorine dioxide
Authors: Richard Erik C. , Vaida Veronica
Published: July 2002

This paper presents the Ã 2A2←X̃ 2B1 direct absorption spectrum of OClO recorded at rotational resolution in a cold molecular jet. Rotational temperatures as low as 30 K are achieved and result in substantial reduction of the inhomogeneous effects that congest the room temperature spectrum. No direct spectroscopic evidence is found for the 2A1←X̃ 2B1 perpendicular transition which was previously thought to be hidden under the stronger Ã 2A2←X 2B1 parallel transition. All of the modes of the 2A2 state observed display homogeneous rotational linewidth broadening. This broadening, due to the efficient excited state chemistry of the 2A2 state, is measured and modeled for the vibrational bands involving (v1,0,0), (v1,1,0) and (v1,0,2) for v1=4–10. We confirm the lack of rotational state dependence on the predissociation within a given vibrational band by a comparison of the linewidths obtained here at low J and K with those known at high J and K. For a given symmetric stretch band, the linewidths associated with both of the corresponding combinations of ν1 with the bend and asymmetric stretch are always greater than the symmetric stretch alone. Because of this and in view of the substantial drop in frequency of both ν2 and ν3 upon the transition we discuss the possible involvement of these modes with regard to the photoreactivity of the 2A2 state. In particular, from the higher energy lifetime information obtained here and the available photoproduct information, we address the dynamics of OClO in the 2A2 state with respect to couplings to both the 2A1 and 2B2 close-lying excited states.
Structure formation in self-associating polymer and surfactant systems
Authors: Brown Gregory , Chakrabarti Amitabha
Published: July 2002

We use Monte Carlo simulation techniques to investigate the important factors controlling the aggregation of self-associating polymers and surfactants in solution. We consider both irreversible and reversible aggregation among the association sites on the polymers. For different monomer concentrations and chain lengths we compute the number of intrachain and interchain associations. It is found that the monomer concentration separating regimes of primarily intra- and interchain associations depends upon the location of association sites on the polymers. This suggests that the distribution of stickers along the chains can be an important factor in controlling macroscopic properties of these systems.
Diffusional escaping from the well. Simple model and qualitative results
Authors: Shushin A. I.
Published: July 2002

A simple two-state model of diffusional escaping from a potential well is proposed. The two states correspond to particles inside and outside the well. Time evolution of both states is described by distribution functions (DFs). In the first state (inside the well), DF is assumed quasiequilibrium (thermal) during the escaping process. In the second state (outside the well), the evolution of DF is governed by the free diffusion equation. The escaping/capture process is approximated by a simple kinetic coupling between these two states. In the limit of high escaping and capture rates, the model reproduces almost all expressions obtained earlier for escaping kinetics by rigorous solution of the Smoluchowsky equation (SE). The model is applied to 1D escaping from the well in the presence of some other wells. The effect of interaction anisotropy on escaping kinetics is also discussed. As an example, recombination of radicals with strongly anisotropic (depending on relative orientation of radicals) interaction is considered. The kinetics is shown to be very close to exponential in this process. Magnetic field effects (MFEs) in the presence of the well are discussed briefly within the two-state model. This simple model reproduces rigorous results obtained for MFEs by the SE approach and permits some generalizations of the results.
The influence of lateral interactions on the critical behavior of a dimer–monomer surface reaction model
Authors: Satulovsky J. , Albano E. V.
Published: July 2002

The ZGB model [Ziff et al., Phys. Rev. Lett. 56, 2553 (1986)] for a monomer–dimer surface reaction process of the type A+(1/2)B2→AB, exhibits two irreversible phase transitions (IPT) from a stationary regime with AB production for p1A≤pA≤p2A, to poisoned states with B (A)-species for pA≤p1A (pA≥p2A), respectively, where pA is the mole fraction of A-species in the gas phase and piA (i=1, 2) are critical points. A generalization of the ZGB model in order to account for both attractive and repulsive interactions between the reactants is presented and discussed. It is found that in most cases the first order IPT at p2A becomes of second order and the critical points are shifted. For some particular choices of the interactions energies it is found that the second order IPT at p1A becomes of first order. Also, a first order IPT from the reactive state to a effectively poisoned regime where A-species are adsorbed forming a c(2×2) metastable structure is obtained.
Real-time dynamics of clusters. II. I2Xn (n=1; X=He, Ne, and H2), picosecond fragmentation
Authors: Gutmann M. , Willberg D. M. , Zewail A. H.
Published: July 2002

In this second paper (II) of a series, we report our picosecond time-resolved studies of the state-to-state rates of vibrational predissociation in iodine–rare gas (van der Waals) clusters. Particular focus is on the simplest system, I2He, which serves as a benchmark for theoretical modeling. Comparisons with I2Ne and I2H2 are also presented. The results from measurements made in real time are compared with those deduced from linewidth measurements, representing a rare example of a system studied by both methods under identical conditions and excited to the same quantum (v′i) states. The discrepancies are discussed in relation to the origin of the broadening and preparation of the state. The rates as a function of v′i display a nonlinear behavior which is examined in relation to the energy-gap law. The measured absolute rates and their dependence on v′i are compared with numerous calculations invoking classical, quantum, and semiclassical theories. In the following paper (III in this series), the cluster size of the same system, I2Xn, is increased (n=2–4) and the dynamics are studied.
Extended transition-state theory and constant-energy chemical-reaction molecular-dynamics method for liquid-phase chemical reactions
Authors: Nagaoka Masataka , Okuno Yoshishige , Yamabe Tokio
Published: July 2002

An extension of transition-state theory for liquid-phase chemical reactions is presented. The effect of adding a second solvent water molecule on the proton-transfer reaction in a formamidine–water (FW) cluster was studied. Ab initio molecular-orbital calculations were performed for the formamidine–water–water (FWW) system to obtain the adiabatic potential-energy surface. It was expressed in two coordinate systems: (i) the total normal-coordinate system of the FWW system, and (ii) the composite normal-coordinate system consisting of two normal-coordinate systems of the isolated FW system and the isolated medium-water molecule. In either of these two systems, the solvent effect can be categorized as either (i) an equilibrium solvation effect or (ii) a frictional effect. In this article, the former effect was investigated in detail and, in the total normal-coordinate system, a frequency diagram was obtained by diagonalizing the Hessian matrix at successive geometries along intrinsic reaction coordinate and then, within the Rice–Ramsperger–Kassel–Marcus (RRkM) formalism, the rate constant was evaluated with the vibrational frequencies assigned in this manner. In the composite normal-coordinate system, the off-diagonal elements found in the Hessian matrix are due to the interaction between the FW system and the medium-water molecule at equilibrium separation. The rate constant was evaluated within the diagonal approximation. As a result, both treatments work well and yield similar conclusions about the role of the solvent to those drawn from chemical-reaction molecular-dynamics simulations. The reaction is found to be enhanced considerably by the assistance of an additional medium-water molecule. The second treatment is concluded to be reasonably applicable in the estimation of reaction rates for liquid-phase chemical reactions.
Fragment rotational state distributions from the dissociation of NeIBr: Experimental and theoretical results
Authors: Stephenson Thomas A.
Published: July 2002

The IBr fragment rotational state distributions that result when the NeIBr van der Waals molecule undergoes vibrational predissociation have been measured in a pump–probe laser-induced fluorescence experiment. Independent of initial vibrational state and the number of quanta of vibrational energy lost from the I–Br coordinate, the rotational distributions extend over the full range of energetically accessible states. From the observation of energetic constraints on the rotational distribution, the dissociation energy (D0) is calculated to be 65.5±1.2 cm−1 for the A electronic state, v=16. For the X electronic state, v=0, D0=71.8±1.2 cm−1. Quantum mechanical bound state calculations carried out on a model A electronic state potential energy surface are in quantitative agreement with this result. The rotational distributions are broader than that predicted by either a Franck–Condon or classical impulsive model for the dissociation. The distributions are qualitatively in accord with classical trajectory calculations. Analysis of specific rotational distributions in the context of the energy gap law shows poor quantitative agreement. The average fragment rotational energy relative to analogous Ne-containing complexes is, however, qualitatively predicted by the energy gap relations.
Surface scattering of x rays in thin films. Part I. Theoretical treatment
Authors: Daillant J. , Bélorgey O.
Published: July 2002

The diffuse scattering of x rays by thin films limited by rough interfaces is presented theoretically in terms of height–height correlation functions. The method employed, analogous to the use of Green functions, allows a rigourous treatment down to grazing incidences. An expression for the scattered intensity is obtained from the scattering cross section. The results are then discussed using a model correlation function, and the interpretation of x-ray reflectivity experiments is reexamined. We show that off-specular scans yield direct information about the correlations between interfaces which, in case of liquid thin films, can be used to determine elastic parameters. The generalized case of stratified media is examined in an appendix. This theory is compared to experimental results in the case of soap films in a companion article.
Convergence to the basis-set limit in ab initio calculations at the correlated level on the water dimer
Authors: van Duijneveldt-van de Rijdt J. G. C. M. , van Duijneveldt F. B.
Published: July 2002

The equilibrium structure and binding energy of the water dimer have been determined in ab initio quantum-mechanical calculations at the correlated level using second-order Mo/ller Plesset theory (MP2) and coupled-electron pair theory (CEPA-1). Basis set superposition error was avoided by applying the counterpoise procedure throughout. Basis set convergence was monitored by studying not only the total interaction energy, but also the first and higher-order Hartree–Fock interaction energies, the partitioned intra and intermolecular components of the MP2 interaction energy, and the monomer dipole moments. This was done at a near equilibrium geometry for more than 20 progressively improved basis sets. The largest set was used in MP2 and CEPA-1 geometry optimizations in Cs symmetry, keeping all intramolecular coordinates fixed, except for the donor OH length. The equilibrium geometry is found to be ROO=2.949 (6) Å, θa=55.2 (2.0)°, θd=57.6 (2.0)°. The donor OH bond is lengthened by 0.0060 (6) Å, but this has virtually no effect upon the final ROO. The equilibrium binding energy is determined as ΔE=−4.73 (10) kcal/mol. The CEPA dipole moment is 2.60 (10) D. The error bars on these results reflect the uncertainty due to the remaining incompleteness in the one-electron basis as well as in the treatment of electron correlation. Taking into account the vibrational effects present in experimental data, the calculated results lie within the error bars of the experimental data available to date. However, the present error bars are two to seven times tighter and so some of the experimental values lie outside the present ranges. The largest discrepancy is for ΔE, which is difficult to determine experimentally. This finding is of importance for the modeling of water properties where empirical potentials with ΔE ranging from −5.0 to −5.5 kcal/mol are customarily employed.
Clustered water adsorption on the NaCl(100) surface
Authors: Wassermann B. , Mirbt S. , Reif J. , Zink J. C. , Matthias E.
Published: July 2002

The adsorption dynamics of water on NaCl(100) is studied by molecular dynamics calculations as a function of coverage. We find that, starting from a critical coverage of about 1/2 monolayer, a coupling of the water dipoles sets in and the interaction between the water molecules wins over the interaction between adsorbate and substrate leading via percolation to formation of infinite cluster networks. This effect is confirmed qualitatively by surface sensitive optical second harmonic measurements with well-controlled water exposure. At a coverage of one bilayer, a two-dimensional ‘‘ice’’ structure is found to be stable. Simulated low-energy electron diffraction (LEED) patterns for this configuration are in excellent agreement with recent observation of a c(4×2) overstructure.
Visible absorption and fluorescence of AlC, B 4Σ−↔X 4Σ−, in solid argon
Authors: Thoma Anton , Caspary Nico , Wurfel Brent E. , Bondybey Vladimir E.
Published: July 2002

Spectroscopic investigations of aluminum carbide, AlC, in an argon matrix have been performed. The visible B 4Σ−↔X 4Σ− transition has been observed in absorption and laser induced fluorescence with a Fourier transform spectrometer. Carbon-13 isotopic substitution has confirmed the carrier to be AlC. The spectroscopic constants in solid argon are found to be X 4Σ− (ωe=639.3 cm−1, ωexe=4.5 cm−1) and B 4Σ− (Te=22 610.5 cm−1, ωe=746.2 cm−1, ωexe=10.3 cm−1).
Centroid-density quantum rate theory: Variational optimization of the dividing surface
Authors: Messina Michael , Schenter Gregory K. , Garrett Bruce C.
Published: July 2002

A generalization of Feynman path integral quantum activated rate theory is presented that has classical variational transition state theory as its foundation. This approach is achieved by recasting the expression for the rate constant in a form that mimics the phase-space integration over a dividing surface that is found in the classical theory. Centroid constrained partition functions are evaluated in terms of phase-space imaginary time path integrals that have the coordinate and momenta centroids tied to the dividing surface. The present treatment extends the formalism developed by Voth, Chandler, and Miller [J. Chem. Phys. 91, 7749 (1989)] to arbitrary nonplanar and/or momentum dependent dividing surfaces. The resulting expression for the rate constant reduces to a strict variational upper bound to the rate constant in both the harmonic and classical limits. In the case of an activated system linearly coupled to a harmonic bath, the dividing surface may contain explicit solvent coordinate dependence so that one can take advantage of previously developed influence functionals associated with the harmonic bath even with nonplanar or momentum dependent dividing surfaces. The theory is tested on the model two-dimensional system consisting of an Eckart barrier linearly coupled to a single harmonic oscillator bath. The resulting rate constants calculated from our approximate theory are in excellent agreement with previous accurate results obtained from accurate quantum mechanical calculations [McRae et al., J. Chem. Phys. 97, 7392 (1992)].
Multichannel quantum interference in the predissociation of Cs2: Observation of q-reversal in a complex resonance
Authors: Kim Bongsoo , Yoshihara Keitaro
Published: July 2002

State-specific photofragment yield spectrum is obtained in the 580 nm system of Cs2 by monitoring Cs (6 2P3/2). Strongly asymmetric line shapes are observed and interpreted as being due to resonances. We observed a gradual sign change of the Fano line shape parameter, which is called ‘‘q-reversal’’, for the first time in predissociation. Some of other lines show complex line shapes that cannot be fitted by a usual Beutler–Fano profile. These complex line shapes may be attributed to the interfering resonances.
Multiple quantum nuclear magnetic resonance of solids: A cautionary note for data analysis and interpretation
Authors: Lacelle Serge , Hwang Son-Jong , Gerstein Bernard C.
Published: July 2002

The conventional method of data analysis and interpretation of time-resolved multiple quantum (MQ) nuclear magnetic resonance (NMR) spectra of solids is closely examined. Intensity profiles of experimental 1H MQ NMR spectra of polycrystalline adamantane and hexamethylbenzene serve to test the Gaussian statistical model approach. Consequences of this model are explored with a least-squares fitting procedure, transformation of data to yield linear plots, and a scaling analysis. Non-Gaussian behavior of the MQ NMR spectral intensity profiles, as a function of order of coherences, is demonstrated with all these methods of analysis. A heuristic argument, based on the multiplicative properties of dipolar coupling constants in the equation of motion of the density operator, leads to the prediction of exponentially decaying MQ NMR spectral intensity profiles. Scaling analysis and semilog plots of experimental time-resolved MQ NMR spectra of adamantane and hexamethylbenzene support this deduction. Dynamical scale invariance in the growth process of multiple spin coherences is revealed with this new approach. The validity of spin counting in solids with MQ NMR is discussed in light of the present results.