bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies of spin relaxation and molecular dynamics in liquid crystals by two-dimensional Fourier transform electron spin resonance. I. Cholestane in butoxy benzylidene-octylaniline and dynamic cage effects
Authors: Sastry V. S. S. , Polimeno Antonino , Crepeau Richard H. , Freed Jack H.
Published: July 2002

Two-dimensional Fourier transform (2D-FT) electron spin resonance (ESR) studies on the rigid rodlike cholestane (CSL) spin-label in the liquid crystal solvent 4O,8 (butoxy benzylidene octylaniline) are reported. These experiments were performed over a wide temperature range: 96 °C to 25 °C covering the isotropic (I), nematic (N), smectic A (SA), smectic B (SB), and crystal (C) phases. It is shown that 2D-FT-ESR, especially in the form of 2D-ELDOR (two-dimensional electron–electron double resonance) provides greatly enhanced sensitivity to rotational dynamics than previous cw-ESR studies on this and related systems. This sensitivity is enhanced by obtaining a series of 2D-ELDOR spectra as a function of mixing time, Tm, yielding essentially a three-dimensional experiment. Advantage is taken of this sensitivity to study the applicability of the model of a slowly relaxing local structure (SRLS). In this model, a dynamic cage of solvent molecules, which relaxes on a slower time scale than the CSL solute, provides a local orienting potential in addition to that of the macroscopic aligning potential in the liquid crystalline phase. The theory of Polimeno and Freed for SRLS in the ESR slow motional regime is extended by utilizing the theory of Lee et al. to include 2D-FT-ESR experiments, and it serves as the basis for the analysis of the 2D-ELDOR experiments. It is shown that the SRLS model leads to significantly improved non-linear least squares fits to experiment over those obtained with the standard model of Brownian reorientation in a macroscopic aligning potential. This is most evident for the SA phase, and the use of the SRLS model also removes the necessity of fitting with the unreasonably large CSL rotational asymmetries in the smectic phases that are required in both the cw-ESR and 2D-ELDOR fits with the standard model. The cage potential is found to vary from about kBT in the isotropic phase to greater than 2kBT in the N and SA phases, with an abrupt drop to about 0.2kBT in the SB and C phases. Concomitant with this drop at the SA–SB transition is an almost comparable increase in the orienting potential associated with the macroscopic alignment. This is consistent with a freezing in of the smectic structure at this transition. The cage relaxation rate given by Rc, its ‘‘rotational diffusion coefficient,’’ is of order of 107 s−1 in the I and N phases. It drops somewhat in the SA phase, but there is a greater than order of magnitude drop in Rc for the SB and C phases to about 105 s−1. This drop is also consistent with the freezing in of the smectic structure. The rotational diffusion tensor of the CSL probe is significantly larger than Rc which is consistent with the basic physical premise of the SRLS model. In particular, R⊥o and R∥o are of order 108 s−1 and 109 s−1 respectively.
Application of integral equation theories to the nitrogen molecule
Authors: Pádua A. A. H. , Trusler J. P. M.
Published: July 2002

The Percus–Yevick (PY) and the hypernetted chain (HNC) integral equations have been applied to realistic, non-spherical, intermolecular potentials for nitrogen. Non-additive contributions, in the form of the Axilrod–Teller three-body interaction, have been considered. The integral equations were solved after expansion in spherical harmonics and the solution procedure is described in detail. Thermodynamic properties of the gas calculated through the compressibility and the virial routes are in surprisingly good agreement with experimental data. Typical deviations are within ±0.2 per cent for both pρT and sound-speed data at sub-critical densities. Some improvement may be achieved by fitting the three-body dispersion coefficient. We conclude that a simple integral-equation theory and a reasonably sophisticated anisotropic pair potential, coupled with the Axilrod–Teller three-body term, can offer a good description of the thermodynamic surface at sub-critical densities. A closure more accurate than either PY or HNC will be required if good results are to be obtained at densities above the critical.
Three-spin interaction in liquid state nuclear magnetic resonance
Authors: Brüschweiler R.
Published: July 2002

The imaginary part of the magnetic dipole–dipole cross correlation spectral density in spin 1/2 systems generates a novel type of three-spin Hamiltonian with terms IizIjzIkz, IizIjxIkx, and IizIjyIky and permutations in the spin labels i, j, k. These terms cause asymmetric positions of the spin multiplet components, which could not be explained by scalar J-coupling and chemical shift terms.
Energy redistribution in cluster–surface collision: I2− (CO2)n onto silicon surface
Authors: Yasumatsu Hisato , Koizumi Shin’ichi , Terasaki Akira , Kondow Tamotsu
Published: July 2002

Fragmentation of I2−(CO2)n (n=1−30) by its collision on a silicon surface was investigated by measuring the fragment anions and their translational energy parallel to the surface (surface–parallel translational energy) in a tandem time-of-flight mass spectrometer equipped with a collision chamber evacuated down to ∼10−8 Pa. At the collision energy (per I2−) of 50 eV and the incident angle of 26° with respect to the surface normal, the distributions of the surface–parallel translational energies of the fragment anions from a given parent cluster anion were found to obey the one-dimensional Maxwell–Boltzmann distribution with the same translational temperature, Ts∥ The results show that the cluster anion and its neighboring surface atoms reach quasiequilibrium before the fragment anions leave the surface. A general increasing trend of Ts∥ (6000–12 000 K) with n is interpreted as an increasing extent of cluster–impact heating with n, while the reduction of Ts∥ in the 13≤n≤∼19 range is attributable to efficient transmission of the I− and I2− translational energies to the CO2 solvent cage. The effective volume and pressure of I2−(CO2)n colliding on the surface were estimated; at n=10, the volume and the pressure were 100 nm3 and 10 MPa, respectively.
Analytic theory of surface segregation in compressible polymer blends
Authors: Freed Karl F.
Published: July 2002

We present an analytical theory for the competing influences of polymer–surface and polymer–polymer interactions, density and composition variations, and blend asymmetries on the surface profiles of a multicomponent polymer blend near an interacting, impenetrable interface. The theory is explicitly applied in the limit of small continuum model polymer–surface interaction parameters, a limit which still enables treating all qualitative behaviors of polymers that individually tend either to aggregate toward or to segregate from the surface. The formulation is based on an analytic combined self-consistent field-density functional theory for inhomogeneous polymer systems. The theory describes the compressible polymer system with a generic Gaussian chain–random mixing type model, which in the bulk phase reproduces a Sanchez–Lacomb-type description of the bulk thermodynamics. The analytic expressions for the density profiles, surface excesses, surface densities, and surface correlation lengths are all presented explicitly for binary compressible systems, but we note how to make the trivial extension to more components. The surface excess and surface correlation length are shown to diverge along the bulk phase spinodal, in rough accord with the increases found in recent experiments by Genzer and Composto. The relation between the continuum model interaction parameters and microscopic (e.g., lattice-model-type) interaction parameters is used to understand recent observations of a surface free energy that displays a higher than quadratic dependence on the surface composition and of the strong surface segregation exhibited by isotopic polymer blends.
Collisional removal of O2 (c 1Σ−u, ν=9) by O2, N2, and He
Authors: Copeland Richard A. , Knutsen Karen , Onishi Marc E. , Yalçin Talat
Published: July 2002

The collisional removal of O2 molecules in selected vibrational levels of the c 1Σ−u state is studied using a two-laser double-resonance technique. The output of the first laser excites the O2 to ν=9 or 10 of the c 1Σ−u state, and the ultraviolet output of the second laser monitors specific rovibrational levels via resonance-enhanced ionization. The temporal evolution of the c 1Σ−u state vibrational level is observed by scanning the time delay between the two pulsed lasers. Collisional removal rate constants for c 1Σ−u, ν=9 colliding with O2, N2, and He are (5.2±0.6)×10−12, (3.2±0.4)×10−12, and (7.5±0.9)×10−12 cm3 s−1, respectively. As the rate constants for O2 and N2 are similar in magnitude, N2 collisions dominate the removal rate in the earth’s atmosphere. For ν=10 colliding with O2, we find a removal rate constant that is 2–5 times that for ν=9 and that single quantum collision cascade is an important pathway for removal.
Photochemistry of adsorbed molecules. XVI. Photolysis of HX (X=Cl, Br, I) adsorbed on LiF(001), by Rydberg-atom time-of-flight spectroscopy
Authors: Giorgi Javier B. , Kühnemuth Ralf , Polanyi John C. , Wang Ji-Xing
Published: July 2002

Hydrogen Rydberg-atom time-of-flight spectroscopy was used to study the photolysis of HX (X=Cl, Br, I) adsorbed on LiF(001). Adsorption characteristics were investigated by temperature programmed desorption. While HCl and HBr appeared to form stable but perforated monolayers on LiF(001), HI did not wet the surface under equilibrium conditions. This behavior of HI was ascribed to the mismatch of its van der Waals diameter and the spacing of the adsorption sites on the substrate lattice. The photolysis experiments were performed with 193 nm excimer laser radiation incident on submonolayers at low temperatures (30–60 K). The H-atom translational energy distributions, P(ET′), showed a marked decrease in the fraction of H-atoms scattered inelastically in going from HCl(ad) to HBr(ad), followed by a similar increase in going from HBr(ad) to HI(ad). These nonmonotonic changes in PINEL down the series HX(ad) (X=Cl, Br, I) were thought to be due to the opposed effects of decreasing adsorbate density (which decreases PINEL by reducing the probability that recoiling H⃑ encounters a neighboring HX(ad)), and increasing collision energy (which increases PINEL). At H-atom energies of ET≈3.3 eV vibrational structure in the H-atom translational energy distribution, P(ET′), following photolysis of HI(ad) gave evidence of efficient T→V energy transfer to neighboring adsorbate molecules.
Microphase separation of diblock copolymer induced by directional quenching
Authors: Zhang Hongdong , Zhang Jianwen , Yang Yuliang , Zhou Xuedong
Published: July 2002

Computer simulation is carried out for studying the microphase separation of a two-dimensional diblock copolymer (DBCP) system under directional quenching. By setting the quenching boundary between the stable and the unstable phase, and shifting the boundary with a constant velocity, the time evolution of the domain morphologies is examined numerically on the basis of the time-dependent Ginzburg–Landau type equation with the free-energy functional for the DBCP. Three different types of morphologies are found for the symmetric (i.e., f=0.5) DBCP system. One is the irregular lamellar morphology and is essentially equivalent to that produced by homogeneous quenching. The other two are regular and are characteristics of directional quenching process. One of the regular lamellar morphologies is perpendicular to the quench boundary on the average, whereas the other one is parallel to the quench boundary. For the asymmetric DBCP system with f=0.4, which forms the equilibrium morphology of triangular phase, the mode of regular lamellar morphology with the normal of lamellae perpendicular to the quench boundary appears first under the condition of directional quenching when the initial thermal fluctuation is very small. The growth rate of the triangular phase gets faster when the initial thermal fluctuation increases. Therefore, our results reveal that the thermal fluctuation promotes the appearance of the mode of triangular morphology for the case of f=0.4.
The transition from single phonon to multiphonon energy transfer in atom–surface collisions
Authors: Hofmann F. , Toennies J. P. , Manson J. R.
Published: July 2002

The angular distributions and time-of-flight spectra of nearly monoenergetic He atoms with incident energies of 82 meV and 112 meV have been measured after scattering from a clean Cu(001) surface over a large range of crystal temperatures from 100 K to 1000 K. With increasing temperatures the sharp diffraction and phonon peaks of the low temperature quantum regime become broad and featureless as expected for the multiphonon classical regime. The results are compared with a quantum mechanical theory which is able to explain the height, position, width, and area under the multiphonon maximum. In the classical regime, the temperature dependence of the inelastic intensity indicates that the He atoms are reflected by a smooth vibrating barrier presumably due to the surface electron density, and not by a lattice of discrete repulsive surface atomic cores.
On cluster embedding schemes based on orbital space partitioning
Authors: Gutdeutsch Ulrich , Birkenheuer Uwe , Krüger Sven , Rösch Notker
Published: July 2002

The embedding approach to the electronic structure of local perturbations in extended systems is based on the fundamental assumption that beyond a certain region around the defect, the properties of the environment are not altered by the presence of the defect. In many computational schemes the resulting subdivision of the defect system into a central and an external region is defined in terms of orbital basis functions. The fundamental embedding assumption then translates into a partitioning of matrix representations, accompanied by fixing the external region contributions to their values in the unperturbed reference system. With the help of density functional cluster-in-cluster embedding calculations we have investigated the quality of this assumption without introducing any additional approximation as usually done to arrive at a computationally feasible embedding scheme. The fundamental embedding assumption is found to cause spurious virtual orbital admixtures to the density matrix which lead to artifacts in the results of embedding calculations. To minimize these undesirable effects, a special “class orthogonalization” scheme has been employed. It allows a perfect reproduction of the defect induced charge density changes as judged by cluster-in-cluster model calculations for a hydrogen substitutional defect in large Lin clusters (with n up to 309). However, equilibrium geometries, total energies, and vibrational frequencies calculated with this embedding scheme do not exhibit any improvement over results from calculations employing the corresponding nonembedded model clusters. The reason for this failure which prevents the expected convergence of the calculated results with increasing cluster size is analyzed. Thus, from a pragmatic point of view, “naked” cluster models are preferable, at least for metal substrates, due to their relative computational simplicity. Possible techniques to either avoid the virtual orbital admixtures or to improve the quality of the total energies obtained from the embedding calculations are discussed together with the drawbacks of these schemes.
All-electron Dirac–Fock–Roothaan calculations on the electronic structure of the GdF molecule
Authors: Tatewaki Hiroshi , Matsuoka Osamu
Published: July 2002

The electronic structure of the GdF molecule is investigated using all-electron Dirac–Fock–Roothaan calculations. It is found that, in the ground state, the Gd atom transfers a 5d electron to the 2p spinors of the F atom, so that the molecule is ionic, having the configuration of Gd+F−. However, the molecule is not purely ionic, since the electrostatic field produced by Gd+ and F− causes the spinor energies of F 2s and one of the Gd 5p to be almost energetically degenerate so that these spinors strongly mix with each other and form covalent bonds. The electrostatic field also causes a large energy lowering for one of the 4f spinors, giving further stability to GdF. The 4f electrons of Gd should be regarded as valence electrons. The lower excited states and positively and negatively ionized states are found to be roughly described by Gd atomlike excitations, ionization, and electron attachments.
Electron energy loss rates in gaseous argon determined from transient microwave conductivity
Authors: Shimamori Hiroshi , Sunagawa Takeyoshi
Published: July 2002

Thermalization of high-energy electrons in gaseous Ar at room temperature has been investigated by analyzing the imaginary component of the transient microwave conductivity produced by pulse radiolysis. The conductivity signal amplitude showing a peak due to the Ramsauer minimum has been correlated with the amplitude derived from calculations of the effective collision frequency using Margenau’s formula assuming Maxwellian velocity distribution of electrons. Two approaches, using the peak and the plateau of the signal, for absolute normalization of the conductivity amplitude give results consistent with each other. It has been found that the excess mean electron energy drops very rapidly to about 0.2 eV and then decreases exponentially with a constant relaxation time. The thermalization time for 1 eV electrons to relax to 10% thermal energy has been determined to be 5.8 ms at 1 Torr Ar. Electron energy loss rate coefficients have been derived as a function of the mean electron energy.
A diffusion process-controlled Monte Carlo method for finding the global energy minimum of a polypeptide chain. I. Formulation and test on a hexadecapeptide
Authors: Derreumaux Philippe
Published: July 2002

Finding the global energy minimum region of a polypeptide chain, independently of the starting conformation and in a reasonable computational time, is of fundamental interest. To approach this problem, a new Monte Carlo method is proposed and applied to the hexadecapeptide model Ac-(AAQAA)3Y(NH2), in which the global energy minimum conformation, an α helix, is known. In order to reduce the available conformational space, the backbone dihedral angles φ and ψ are restricted to a discrete set of ten regions and the side chains are modeled by a two-point representation. The energy used in these off-lattice simulations is of Amber type with a simplified hydrophobic potential. The novelty of the method is that, prior to the minimization of the energy, the move from the current conformation to the next must satisfy a kinetic requirement. The kinetic requirement is that there exists an upper bound on the escape time from the current conformation. From diffusion consideration it is shown that the escape time correlates with the angular deviations of the residues. The effectiveness of the approach is illustrated by a total of 25 biased simulations (i.e., using specific probabilities for the ten φ–ψ regions) and five unbiased simulations (i.e., the 10 regions are equiprobable before application of the kinetic requirement), starting from various conformations. It is found that all biased and unbiased simulations find the global minimum energy structure in ∼102–103 Monte Carlo steps, although the estimated probability of getting the full α helix is ∼10−11–10−16.
A Monte Carlo (N,V,T) study of the stability of charged interfaces: A simulation on a hypersphere
Authors: Delville A. , Pellenq R. J.-M. , Caillol J. M.
Published: July 2002

We have used an exact expression of the Coulombic interactions derived on a hypersphere of an Euclidian space of dimension four to determine the swelling behavior of two infinite charged plates neutralized by exchangeable counterions. Monte Carlo simulations in the (N,V,T) ensemble allows for a derivation of short-ranged hard core repulsions and long-ranged electrostatic forces, which are the two components of the interionic forces in the context of the primitive model. Comparison with numerical results obtained by a classical Euclidian method illustrates the efficiency of the hyperspherical approach, especially at strong coupling between the charged particles, i.e., for divalent counterions and small plate separation.
Rovibronic analysis of the laser induced fluorescence excitation spectrum of the jet-cooled methoxy radical
Authors: Powers David E. , Pushkarsky Michael B. , Miller Terry A.
Published: July 2002

The laser induced fluoresence excitation spectrum for the Ã 2A1↔X̃ 2E transition of the methoxy radical has been reinvestigated. An extensive set of vibrational levels has been assigned with the aid of increased vibrational and rotational cooling. Many of these vibrational assignments are confirmed by rotational analysis of bands involving both the symmetric and asymmetric fundamentals of the Ã state as well as vibrations containing two quanta of the e modes. Although parts of the vibrational structure have been assigned previously, several discrepancies are identified and corrected. Vibrational frequencies have been obtained for all the modes in the Ã 2A1 state of the molecule. The Fermi resonance that exists between ν3 and ν2 has been investigated and interaction constants describing it have been obtained.
A microcanonical Monte Carlo method for simulating vibrationally excited molecules embedded in clusters
Authors: Marks Alison J.
Published: July 2002

A microcanonical (fixed energy) Monte Carlo method is described for the study of weakly bound heteroclusters. A model for calculating classical vibrational frequency shifts is also presented. Preliminary results are reported for He8I2 and Ar8I2 clusters. In the argon clusters, the I2 frequency is red shifted but for He8I2 a blue shift is found at higher solvent energies.
Measurement of absolute differential cross sections for the excitation of the π,π* triplet state of ethene by electron impact at 0° and 180°
Authors: Asmis Knut R. , Allan Michael
Published: July 2002

A recently developed magnetically collimated electron impact spectrometer with pulsed incident electron beam and time-of-flight selection of electrons scattered in the forward and backward directions was used to measure differential cross sections (DCSs) for the excitation of the valence triplet state of ethene at scattering angles of 0° and 180°. The measurements are complemented by data at intermediate angles measured with a conventional spectrometer using hemispherical electron-energy analyzers. A pronounced trend is found in the DCS measured as a function of incident electron energy at 0°, 90°, and 180°: a band assigned as the (π,π*2)2 core excited shape resonance becomes substantially wider and moves to higher energies with increasing angle of observation. The DCS have also been measured as a function of scattering angle, in the range 0°–135° and at 180°, at 1.5, 3, and 10 eV above threshold. The DCS was found to be strongly backward peaked at energies higher than 5 eV above threshold. The results at 0° and 180° are found to be a sensitive test of the theory. All qualitative trends are reproduced well by recent published Schwinger multichannel and complex Kohn theories, but quantitative differences are found in the absolute magnitude of the DCSs and the position of the resonances.
New statistical mechanical treatment of systems near surfaces. II. Polydisperse linear and branched polymers in an athermal solution
Authors: Chhajer Mukesh , Gujrati P. D.
Published: July 2002

We apply a recently developed analytic but approximate method to study the behavior of polydisperse linear and branched polymers in an athermal solution and near various kinds of surfaces. We consider equilibrium polydispersity controlled by a set of activities. The method allows us to account for polymer connectivity and excluded-volume effects and goes beyond the random mixing approximation. The density profiles of various kinds exhibit oscillations for bulk densities φmb larger than some threshold bulk density φmT. The origin of these oscillations is related to the decreasing branch of the recursion function, as explained in the text. The correlation length ξ related to these oscillations increases as φmb increases. On the other hand, the correlation length ξ controlling the approach of various density profiles to their respective bulk values in the range φmb<φmT increases as φmb decreases. The free energy and the entropy are uniquely determined. Various surface properties are also easily determined. We demonstrate the existence of surface polymerization transition in the case of linear chains as surface interactions are varied, even though there is no bulk transition. The method allows us to probe the entire density range which is not possible in a Monte Carlo simulation. Our computations are ultrafast by at least three orders of magnitude compared to rival methods and easy to implement. Our results agree with Monte Carlo simulations but disagree with the mean-field predictions.
Response to “Comment on ’On the relation between unimolecular reaction rates and overlapping resonances’ ” [J. Chem. Phys. 106, 4810 (1997)]
Authors: Peskin Uri , Reisler Hanna , Miller William H.
Published: July 2002

The unimolecular decay rate k(E) saturates with increasing bound-continuum coupling. This saturation was observed in various models including a random matrix version of the optical model. The phenomena was previously interpreted in terms the transition state theory for unimolecular decay. The mechanism for the saturation is associated with a broadening of the resonance widths distribution, but not with the “bifurcation” of the decay time scales into two groups as suggested by the “trapping effect.” The onset of the trapping effect within random matrix simulations leads to a nonphysical decrease of the effective decay rate with increasing bound continuum coupling. This nonphysical tendency is explained in terms of a physical inconsistent matrix approximation of the optical model for too large bound-continuum coupling.
The calculation of O17 chemical shielding in transition metal oxo complexes. I. Comparison of DFT and ab initio approaches, and mechanisms of relativity-induced shielding
Authors: Kaupp Martin , Malkina Olga L. , Malkin Vladimir G.
Published: July 2002

The performance of different DFT (UDFT-IGLO, UDFT-GIAO, SOS-DFPT-IGLO) and hybrid-DFT approaches, as well as of HF-GIAO and MP2-GIAO methods has been compared for the calculation of O17 chemical shielding in the series of tetrahedral d0 oxo complexes MO4(M=Fe,Ru,Os), MO4−(M=Mn,Tc,Re), and MO42−(M=Cr,Mo,W). While HF-GIAO and MP2-GIAO fail for systems with low-lying excited states (e.g., MnO4−, CrO42−, or MO4), the DFT methods consistently remain remarkably stable. Larger basis sets change the results little, SOS-DFPT correction terms are small, and differences between different local and gradient-corrected exchange-correlation functionals are also minor. The inclusion of CHF-type coupling terms for DFT-HF hybrid functionals leads to a significant overestimate of the paramagnetic contributions, the neglect of these terms to an equally large underestimate. DFT-IGLO results for the 3d complexes show an unexpectedly large dependence on whether the metal semicore shells are localized separately or together with the valence orbitals. In this context, the Pipek–Mezey localization scheme appears to be more stable than the more common Boys localization. The origin of the large scalar relativistic effects found for the 5d complexes is discussed by comparing results for the shielding tensors obtained with quasirelativistic and with nonrelativistic metal ECPs. Periodic trends are analyzed in terms of MO contributions to the shielding tensors.
Relation between fluxes for membrane reaction-diffusion transport studied by the matrix method
Authors: Chen Jenn-Shing , Chang Wen-Yih
Published: July 2002

The previous matrix-theoretical analysis of the time lags and mean first passage times for reaction-diffusion transport in the Laplace domain [J. Chem. Phys. 106, 8022 (1997)] has been extended to the problems of membrane permeation, absorption and desorption. A matrix transport equation has been derived to calculate the flux (in the Laplace domain) for specified initial and boundary conditions corresponding to membrane permeation, absorption or desorption. Seven equality relations between fluxes for reaction-free diffusion and one for diffusion accompanying reaction have been found. Among them are two new equality relations which have been discovered by this matrix analysis.
Frequency dependent equation-of-motion coupled cluster hyperpolarizabilities: Resolution of the discrepancy between theory and experiment for HF?
Authors: Rozyczko Piotr , Bartlett Rodney J.
Published: July 2002

We formulate and apply the EOM-CC method to determine, recursively, any order and any process frequency dependent hyperpolarizability. Combining the theory with systematically chosen n-aug-cc-pVXZ bases, we attempt to converge to the basis set limit. Our best “relaxed” result is −9.8±0.5 a.u. for βSHG compared to an experimental value of −11.0±1 a.u. and 720±40 a.u. for γdc-SHG compared to 840±120 a.u. If orbital relaxation could be neglected, the results are −10.2±0.5 a.u. and 730±40 a.u., falling within the error bars.
Luminescence spectroscopy of atomic zinc in rare-gas solids. I
Authors: Bracken Veronica A. , Gürtler Peter , McCaffrey John G.
Published: July 2002

Steady-state and time-resolved luminescence spectroscopy of atomic zinc isolated in thin film samples of the solid rare gases, prepared by the cocondensation of zinc vapor with argon, krypton, and xenon has been recorded at 6.3 K using synchrotron radiation. Pairs of emission bands result from photoexcitation of the singlet 4p 1P1←4s 1S0 resonance transition of atomic zinc, even in annealed samples. In Zn/Ar the pair of emission bands were observed in the uv at 218.9 and 238 nm and for Zn/Xe in the near-uv at 356 and 399 nm. For the Zn/Kr system two emission bands were observed in the uv region at 239.5 and 259 nm but in addition, a weaker band was present in the near-uv at 315.6 nm. In a given annealed rare-gas host, the excitation profiles recorded for all the emission bands are identical, exhibiting the threefold splitting characteristic of Jahn-Teller coupling in the triply degenerate excited P11 state. These excitation profiles are identified as the solid phase equivalent of the 4p 1P1←4s 1S0 resonance transition of atomic zinc occurring at 213.9 nm in the gas phase. Based on their spectral positions and temporal decay characteristics, the emission bands observed in the uv and near-uv spectral regions have been assigned as the singlet and triplet transitions, respectively, of atomic zinc. The origin of the pairs of emission bands is ascribed to the Jahn-Teller coupling between noncubic vibronic modes of the lattice and the excited 4p orbital of the P11 state of atomic zinc, resulting in the coexistence of two energy minima. In Zn/Ar, the effects of slow vibrational relaxation in the excited singlet state were evident in the relative intensities and temporal decay profiles of the pair of emission bands. Specifically, the lower energy emission band was favored with excitation of the highest energy component of the threefold split Jahn-Teller absorption band, while the higher-energy emission was favored with excitation of the lowest-energy component. The intensity of the triplet state emission was observed to be enhanced in the heavier rare gases, being completely absent in Ar, weak in Kr, and the only emission observed in Xe.
Luminescence spectroscopy of atomic zinc in solid rare gases. II. Temperature dependence
Authors: Bracken Veronica A. , Kerins Paul N. , Gürtler Peter , McCaffrey John G.
Published: July 2002

The temperature dependence of the pairs of emission bands present for atomic zinc isolated in annealed solid argon, krypton, and xenon samples is examined in steady-state and time-resolved luminescence spectroscopy. The pairs of emission bands in all the Zn/RG systems exhibited a reversible temperature dependence whereby the intensity of the high-energy band decreased, while the low-energy band gained in intensity with increasing temperature. In the Zn/Ar system, the decrease in the intensity of the 218.9 nm emission band observed between 9 and 28 K was coupled with a concomitant increase in the intensity of the band at 238 nm. In this temperature range the decay times of the 218.9 nm band decreased while the 238 nm band exhibited a constant decay time of 1.41 ns and a rise time correlated with the decay of the 218.9 nm band. The interdependence exhibited by the intensities and decay times of the two emission bands is modeled by an activated nonradiative process with a barrier height of 130.6 cm−1 for population interconversion between the pairs of emitting levels on of the spin singlet adiabatic potential energy surface. Similar behavior was observed in Zn/Kr between 6.3 to 20 K, but at higher temperatures this system also exhibited enhanced intersystem crossing. Likewise, for Zn/Xe, the low-energy 399 nm emission increased in intensity at the expense of the high-energy 356 nm emission up to a temperature of 40 K. For the Zn/Kr pair of singlet emissions and the Zn/Xe pair of triplet emissions, barrier heights of 78.1 and 42.6 cm−1 were evaluated, respectively.
The influence of local electronic character and nonadiabaticity in the photodissociation of nitric acid at 193 nm
Authors: Myers T. L. , Forde N. R. , Hu B. , Kitchen D. C. , Butler L. J.
Published: July 2002

The dissociation of nitric acid upon πnb,O→πNO2* excitation at 193 nm has been studied in a crossed laser-molecular beam apparatus. The primary reaction channels are OH+NO2 and O+HONO. We measure the branching ratio between these two competing processes and determine (OH+NO2)/(O+HONO)=0.50±0.05. Our experiments provide evidence of a minor O+HONO pathway, which we assign to O(3P) and HONO in its lowest triplet state. The dominant pathway correlates to O(1D)+HONO(X 1A′). The translational energy distributions reveal two distinct pathways for the OH+NO2 channel. One pathway produces stable NO2 fragments in the 1 2B2 electronic state. The second pathway produces unstable NO2 fragments which undergo secondary dissociation to NO+O. We examine the influence of nonadiabaticity along the OH+NO2 reaction coordinate in order to explain the significant branching to this other channel. Finally, we introduce a new method for generating correlation diagrams for systems with electronic transitions localized on one moiety, in which we restrict the changes allowed in remote molecular orbitals along the reaction coordinate. Analysis of previously measured X+NO2 photofragment pathways in nitromethane and methyl nitrate provides further support for using a restricted correlation diagram to predict the adiabatic and nonadiabatic product channels.
Efficient calculation of free-energy barriers in quantum activated processes. A path-integral centroid approach
Authors: Ramı́rez Rafael
Published: July 2002

The key quantity in the study of rates of activated processes by the quantum transition-state theory based on the Feynman path-integral formulation is a free-energy barrier associated to a reaction coordinate. The free-energy barrier represents the reversible work done against the quantum potential of mean force acting on thermal paths whose centroid (center of mass) is held fixed, along a reaction coordinate defined by the centroid. A reversible thermodynamic cycle leads to a simple method to calculate this barrier by thermodynamic integration. The capability of the method is demonstrated in three models: a flux of protons impinging on a symmetric Eckart barrier; a particle in a double-well potential; and a point defect in a silicon lattice. Analysis of the temperature dependence of the free-energy barrier shows a crossover from a high-temperature regime, where the potential energy increment gives a good approximation to the barrier, to a low-temperature one, where the barrier is close to the difference between potential and kinetic energy increments. Each regime displays a characteristic ratio between the kinetic energy and the heat exchanged as the system moves reversibly along the reaction coordinate.
Solvation and the excited states of formamide
Authors: Krauss M. , Webb S. P.
Published: July 2002

Excited state geometries of formamide have been explored using the multiconfiguration self-consistent-field method. Optimized equilibrium geometries for the S1 and T1 states are nonplanar with the C–O and C–N bond distances substantially increased from the ground state values. The excitation energies at the ground and excited state geometries are calculated to vary dramatically with nonplanar rotation. Raman scattering from the S2 state depends on the transition moment which is shown to vary strongly with geometry. Experimental analyses that project out restricted planar conformations can fit the Raman vibrational pattern but do not inform us about the complicated energy surface for the S2 state which is a resonance embedded in a Rydberg series. Constrained optimizations are used to explore this surface and the variation in the oscillator strength with geometry. Effective fragment potentials (EFP) model the waters in the solvation models. Comparison of the EFP and all-electron structures and energy of binding shows that the EFP adequately replace the all-electron waters. The use of constrained C2v geometries for the EFP water does not significantly affect either the optimized structure or the energetics of the complex.
Orientational correlations and hydrogen bonding in liquid hydrogen chloride
Authors: Andreani C. , Ricci M. A. , Nardone M. , Ricci F. P. , Soper A. K.
Published: July 2002

Neutron diffraction data, obtained by employing the hydrogen isotope substitution technique, on orthobaric hydrogen chloride at two state points are presented. The site-site radial distribution functions are derived from these data and are compared with similar results obtained previously for hydrogen iodide and hydrogen bromide. Comparison of the measured hydrogen-hydrogen and chlorine-hydrogen radial distribution functions with those derived assuming uncorrelated orientations demonstrates the presence of strong orientational correlations between first neighboring HCl molecules at both states. An analysis of molecular configurations compatible with the experimental data as generated by a Monte Carlo simulation routine is used to discuss the extent of hydrogen bonding in liquid HCl.
Electronic absorption spectra of C4− and C6− chains in neon matrices
Authors: Freivogel Patrick , Grutter Michel , Forney Daniel , Maier John P.
Published: July 2002

The absorption spectra of the electronic transitions A 2Σg+←X 2Πg, B 2Σu+←X 2Πg, (2) 2Πu←X 2Πg, and (3) 2Πu←X 2Πg of C4−, as well as A 2Σg+←X 2Πu, (2) 2Πg←X 2Πu, and (3) 2Πg←X 2Πu of C6− have been obtained in neon matrices. The spectra were measured after mass-selected deposition of the anions with excess of neon at 5 K. The assignments are based on mass-selection, spectroscopic evidence, photobleaching behavior, and ab initio calculations. Vibrational frequencies in the electronically excited states have been inferred.
H atom formation dynamics in the dissociation of CH3–CF2Cl (HCFC-142b) after UV and VUV laser photoexcitation
Authors: Brownsword R. A. , Hillenkamp M. , Laurent T. , Volpp H.-R. , Wolfrum J. , Vatsa R. K. , Yoo H.-S.
Published: July 2002

Using the laser photolysis/laser-induced fluorescence (LIF) “pump-and-probe” technique, the dynamics of H atom formation in the photodissociation of CH3–CF2Cl (HCFC-142b) after excitation at 193 nm and the Lyman-α wavelength were studied under collision-free conditions in the gas-phase at room temperature. The H atoms produced were detected by (2p2P←1s2S)-LIF using tunable narrow-band Lyman-α laser radiation (λLα≈121.6 nm) generated by resonant third-order sum-difference frequency conversion of pulsed dye laser radiation. In the VUV photodissociation experiments the Lyman-α laser radiation was used both to photodissociate the parent molecules and to detect the produced nascent H atoms via laser induced fluorescence. The following quantum yields ΦH for H atom formation were determined by a photolytic calibration method: ΦH(193 nm)=(0.06±0.02) and ΦH(Lα)=(0.53±0.12). From the measured H atom Doppler profiles the average H atom kinetic energy was determined to be ET(193 nm)=(51±10) kJ/mol and ET(Lα)=(72±4) kJ/mol, respectively.
Infrared laser absorption spectroscopy of rotational and vibration rotational transitions of HeH+ up to the dissociation threshold
Authors: Liu Zhuan , Davies Paul B.
Published: July 2002

Many high J pure rotational transitions of HeH+4 up to the dissociation threshold have been measured by infrared diode laser absorption spectroscopy between 590 and 920 cm−1 in an air cooled H2/He discharge. Additional centrifugal distortion terms up to J12 have been included in the rotational Hamiltonian to fit these transitions. Ten new vibration rotational transitions have also been detected in the same spectral region, arising from both low (v=1,2) and high (v=4–7) vibrational states. Some of the transitions involving high v (v=6,7) are observed as stimulated emission signals. The measured rotational transitions are in excellent agreement with ab initio calculations based on the form of the adiabatic potential model developed by Fournier and Richard.
Mesoscopic interpretation of Fokker-Planck equation describing time behavior of liquid crystal orientation
Authors: Muschik W. , Su B.
Published: July 2002

Starting out with the mesoscopic mass balance for liquid crystals we derive the balance equation for the orientation distribution function (ODF) of the molecules which is of Fokker-Planck type. Comparison of this mesoscopic generated Fokker-Planck equation to well-known statistically induced balance equations of the ODF results in a slip relation between the rotation of the liquid crystal and the angular velocity of molecules of different orientations. The difference between these two angular velocities is caused by the torque of the meanfield, by the velocity gradient, by the alignment, and by the non-isotropy of the ODF.
Specular x-ray reflectivity studies of microstructure and ordering in self-assembled multilayers
Authors: Malik A. , Lin W. , Durbin M. K. , Marks T. J. , Dutta P.
Published: July 2002

Specular x-ray reflectivity has been used to probe the structures of self-assembled monolayers and multilayers deposited using a three-step siloxane-based self-assembly technique that is repeated to form periodic multilayers. In films containing up to ten trilayers, it is found that the film thickness increases linearly as a function of the number of trilayers with no observable change in the surface roughness. Bragg peaks corresponding to the inter-trilayer spacing are observed. Both of these results indicate high structural regularity in these self-assembled multilayers. In self-assembled films with different constituent molecular building blocks, substantial and unexpected changes in the film structure occur as a result of subtle changes in the layers.
Vacuum-UV fluorescence spectroscopy of SiF4 in the range 10–30 eV
Authors: Biehl H. , Boyle K. J. , Seccombe D. P. , Smith D. M. , Tuckett R. P. , Yoxall K. R. , Baumgärtel H. , Jochims H. W.
Published: July 2002

The vacuum-UV and visible spectroscopy of SiF4 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following excitation with synchrotron radiation from the BESSY 1, Berlin source in the energy range 10–30 eV with an average resolution of ∼0.05 eV. By comparison with vacuum-UV absorption and electron energy loss spectra, all the peaks in the Rydberg spectra that photodissociate to a fluorescing state of a fragment have been assigned. Dispersed emission spectra have been recorded at the energies of all the peaks in the excitation spectra. Four different decay channels are observed: (a) SiF3 fluorescence in the range 380–650 nm for photon energies around 13.0 eV, (b) SiF2 ã 3B1–X̃ 1A1 phosphorescence in the range 360–440 nm for photon energies in the range 15.2–18.0 eV, (c) SiF2 Ã 1B1–X̃ 1A1 fluorescence in the range 210–270 nm for photon energies in the range 17.0–20.0 eV, and (d) emission from the D̃ 2A1 state of SiF4+ predominantly in the range 280–350 nm for photon energies greater than 21.5 eV. These assignments are confirmed by action spectra in which the excitation energy of the vacuum-UV radiation is scanned with detection at a specific (dispersed) wavelength. Using the single-bunch mode of the synchrotron, lifetimes of all the emitting states have been measured. The lifetimes of the unassigned emitting state in SiF3, the Ã 1B1 state of SiF2, and the D̃ 2A1 state of SiF4+ are 3.9±0.7, 11.2±1.5, and 9.16±0.02 ns, respectively. This is the first measurement of the lifetimes of these excited states in SiF3 and SiF2. The decay from the ã 3B1 state of SiF2 has a fast component of 2.6±0.4 ns. We conclude that the lifetime of the ã 3B1 state of SiF2 is either as low as 2.6 ns or too high (τ>∼200 ns) to measure with the timing profile of the single-bunch mode of BESSY 1. If the latter interpretation is correct, as seems likely for a spin-forbidden phosphorescence to the A11 ground state, the 2.6 ns component could be the lifetime of intersystem crossing from higher vibrational levels of the ã 3B1 state of SiF2 into its ground state.
Gas phase ion chemistry and ab initio theoretical study of phosphine. I
Authors: Antoniotti Paola , Operti Lorenza , Rabezzana Roberto , Splendore Maurizio , Tonachini Glauco , Vaglio Gian Angelo
Published: July 2002

Gas phase ion processes of phosphine have been studied by theoretical calculations and experimental techniques. Ab initio quantum chemical calculations have been performed on the ion/molecule reactions starting from P+ in PH3, as they have been observed by ion trapping. P+ gives P2Hn+ (n=1,2) product ions with loss of H2 or H in different pathways and also reacts in charge-exchange processes to form PH3+. The energies of transition structures, reaction intermediates, and final products, as well as their geometrical structures have been determined by theoretical methods. The initial step is formation of a triplet P2H3+ adduct of C3v symmetry (P–PH3+). A hydrogen atom can either be directly lost from the tetracoordinated phosphorus, or first undergo a shift to the other P atom (HP+–PH2), followed by P–H bond dissociation. Dissociation of H2 from P2H3+ can also occur from both the initial P–PH3+ and HP+–PH2 species yielding PPH+. The heats of formation of the P2Hn+ ionic species have also been computed and compared with experimental data reported in the literature.
A Monte Carlo finite size scaling study of charged hard-sphere criticality
Authors: Caillol J. M. , Levesque D. , Weis J. J.
Published: July 2002

Monte Carlo simulations of the critical region of the restricted primitive model are reported. Using mixed-field finite size scaling analysis we show that the critical behavior is compatible with Ising like behavior although due to statistical error on the simulation data and large correction-to-scaling contributions mean-field behavior cannot be totally excluded. With the assumption of Ising criticality the critical temperature is estimated to be 0.0488±0.0002 and the critical density 0.080±0.005.