bluuebunny/crossref_metadata_2025 huggingface dataset:
Translational relaxation and electronic quenching of hot O(1D) by collisions with N2
Authors: Matsumi Yutaka , Chowdhury A. M. Sarwaruddin
Published: July 2002

Translational relaxation and electronic quenching processes of translationally hot O(1D) atoms by collisions with N2 in a gas cell at room temperature are studied using a vacuum ultraviolet laser induced fluorescence technique. The initial hot O(1D) atoms which have translational energies of 18.2 and 9.8 kcal mol−1 are produced by the photodissociation of N2O at 193 nm and O2 at 157 nm, respectively. The translational relaxation processes are investigated by time resolved measurements of the Doppler profiles for the O(1D) atoms, while the quenching processes are studied by measuring both the decrease of the O(1D) concentration and the increase of the product O(3P) concentration after the photochemical formation of the hot O(1D) atoms. When the initial translational energy of O(1D) is 9.8 kcal mol−1, about 40% of the O(1D) atoms are electronically quenched before the entire thermalization of the hot O(1D) atoms takes place in a gaseous mixture with N2. This indicates that the translational relaxation rate of O(1D) by collisions with N2 is not fast enough compared with the electronic quenching by N2. It is found that the steady state distribution of the O(1D) translational energy in the upper stratosphere is superthermal and that the populations at high translational energies are higher than that estimated from an equilibrated condition with the ambient air. The cross section of the electronic quenching by N2 at the high collision energy of (8±6) kcal mol−1 is found to be (0.7±0.1) Å2, which is about 5 times smaller than that at thermal collision energy at 298 K.
Spin–orbit relaxation kinetics of Br(4 2P1/2)
Authors: Johnson Ray O. , Perram Glen P. , Roh Won B.
Published: July 2002

Pulsed and steady-state photolysis techniques have been employed to measure the rate coefficients for collisional deactivation of the spin–orbit excited state of atomic bromine, Br(4 2P1/2). Pulsed lifetime studies for quenching by Br2 and CO2 yielded absolute rate coefficients at room temperature of kBr2=1.2±0.1×10−12 and kCO2=1.5±0.3×10−11 cm3/molecule s. The rate coefficients for quenching by rare gases, N2, O2, NO, NO2, N2O, CO, CO2, COS, SO2, SF6, CF4, CH4, H2S, H2, D2, HBr, HCl, and HI, relative to that for Br2 were determined in a steady-state photolysis experiment. Correlation of the deactivation probabilities with energy defect for the case of electronic-to-vibrational energy transfer is demonstrated.
Semiclassical analysis of resonance states induced by a conical intersection
Authors: Zhu Chaoyuan , Nikitin Evgueni E. , Nakamura Hiroki
Published: July 2002

The resonance states induced by nonadiabatic coupling in the conical intersection problem are analyzed semiclassically. Not only the general framework but also the explicit analytical expressions of resonance positions and widths are presented. Interestingly, the nonadiabatic transition schemes are found to be quite different in the two representations employed, i.e., the adiabatic and generalized adiabatic (or dynamical state, or postadiabatic) representations. In the former case the transition is assigned to be of the Landau–Zener (LZ) type, and the latter case is analyzed by a mixture of LZ- and Rosen–Zener (RZ)-type in the case of m≥3/2 and by the nonadiabatic tunneling (NT) type in the case of m=1/2, where m is the angular momentum quantum number. Both of these semiclassical results agree well not only with each other in spite of the very different schemes, but also with the exact numerical results in a wide range of energy and angular momentum.
A variable charge central force model for water and its ionic dissociation products
Authors: David Carl W.
Published: July 2002

The Central Force Model for water is adjusted to contain features of the Polarization Model for water and its ionic dissociation products. An ‘‘electronegativity’’ term which permits charge transfer between dissimilar moieties (protons and oxide ions in the water system) is introduced. A parameterization of this new Variable Charge Central Force Model (VCCF) is presented, and results on water and hydroxide ion are presented which support the view that some damage has occurred, and some improvement has been accomplished.
The calculation of 77Se chemical shifts using gauge including atomic orbitals and density functional theory
Authors: Schreckenbach Georg , Ruiz-Morales Yosadara , Ziegler Tom
Published: July 2002

Calculations of 77Se nuclear magnetic resonance chemical shifts and shieldings are reported for a number of selenium containing compounds. The calculated shifts span a range of about 2800 ppm, and therefore cover almost the complete range of known 77Se chemical shifts. The calculations are based on density functional theory (DFT); gauge including atomic orbitals (GIAO’s) are used. The DFT-GIAO method is tested against other theoretical approaches. It is able to predict chemical shifts as well as individual tensor components of the shielding tensor with about the same quality as sophisticated ab initio methods. DFT is even capable of handling highly correlated systems like Se2+4. Other methods fail in this particular case.
Comparisons of results from parametrized configuration interaction (PCI-80) and from hybrid density functional theory with experiments for first row transition metal compounds
Authors: Blomberg Margareta R. A. , Siegbahn Per E. M. , Svensson Mats
Published: July 2002

Different methods and schemes have been tested for the difficult class of first row transition metal complexes. The systems investigated are the M+, MH+, MCH+3, and MCH+2 systems for the entire row and the Ni(CO)x systems with x=1–4. In general quite satisfactory results are obtained both at the PCI-80 and hybrid density functional levels. In particular, for the MH+ and MCH+3 systems the PCI-80 average deviation to experiments is of the same size as the uncertainty in the experiments. The MCH+2 systems are somewhat more difficult to describe and a rather large error is found for chromium at the PCI-80 level, due to a large coupling of exchange energy loss and change of correlation energy resulting from the formation of two covalent d-bonds. Scaling at the modified coupled pair functional (MCPF) level is also compared to scaling at the coupled cluster singles and doubles (CCSD) level. In most cases very similar results are obtained, but classes of systems can be identified where scaling works better at the MCPF level and other classes where it works better at the CCSD level.
Ar–I2 interactions: The models based on the diatomics-in-molecule approach
Authors: Buchachenko A. A. , Stepanov N. F.
Published: July 2002

Semiempirical model is developed for studying the electronic structure of the rare gas atom–halogen molecule systems. It is formulated in the frame of diatomics-in-molecule (DIM) approach and takes explicitly into account strong spin–orbit coupling pertinent to heavy halogen molecules. The consistent DIM scheme is realized for intermolecular interactions, whereas the description of valence electronic states of halogen molecule is more approximate being based on the asymptotic wave functions. The corresponding perturbation theory is also put forward. The model is applied to analysis of several features of the Ar...I2 van der Waals complex. First, the calculations on the spectroscopic constants of the B←X transition in the complex reveal the quantitative performance of the model. Second, mechanisms of nonadiabatic dynamics are examined. The results are qualitatively consistent with the current view on the Ar...I2 electronic predissociation and one-atom cage effect. Third, the prediction is made on the valence electronic spectrum of Ar...I2 complex. These examples demonstrate the reliability of the model.
Excited-state absorption spectra of V2+-doped fluoroperovskites. An ab initio model potential embedded-cluster study
Authors: López-Moraza Sara , Barandiarán Zoila
Published: July 2002

In this paper we present an ab initio model potential embedded-cluster study of the electronic structure of the local excited states of V2+-doped KMgF3, KZnF3, and CsCaF3 fluoroperovskites, which are more directly involved in their potential laser activity: the 4T2g, 4Ta1g, and 4Tb1g states of the (V F6)4− embedded-cluster. The ab initio model potential embedded-cluster model used allows for the consideration of intracluster electron correlation and Jahn–Teller coupling in the excited states, as well as of lattice effects which include quantum mechanical interactions resulting from a relaxed, dipole polarized crystal lattice. The embedding potentials enable the geometry optimization of the embedded-cluster electronic ground-state but also of the excited states; as a consequence, the ab initio calculation of the vertical ground-state absorption, excited-state absorption, and emission spectra is possible and it is done in this work, without resorting to the use of experimental structural parameters of the local defect, which, in any case, are not available for the ground state and not attainable for the excited states. The agreement of the calculated ground-state absorption, excited-state absorption, and emission transition energies with the available experimental data is satisfactory and systematic. Our results allow for the discussion of (a) the competition between the infrared 4T2g→4Ta1g excited-state absorption and the 4T2g→4A2g spontaneous emission, (b) the overlap between the excited-state absorption bands and the pumping bands, and (c) the 4A2g→4T2g reabsorption, for all three V2+-doped fluoroperovskites, as well as their host dependency. The overall conclusion is that the laser efficiency is expected to deteriorate in the investigated series, going from KMgF3:V2+ to CsCaF3:V2+.
The rovibrational spectrum of the ArCO complex calculated from a semiempirically extrapolated coupled pair functional potential energy surface
Authors: Jansen Georg
Published: July 2002

The rovibrational spectrum of the ArCO van der Waals complex has been calculated using a recently published ab initio potential energy surface determined by the coupled pair functional approach. Comparison with known experimental values for some of the transitions shows that the anisotropy of this surface comes out reasonably well, although its well depth of 72 cm−1 is too small. Based on a comparison of coupled pair functional interaction energies for Ne2, NeAr, and Ar2 with empirical potential energy curves an extrapolation scheme for the differential correlation energy is suggested. This semiempirical extrapolation scheme, with a slight modification to account for anisotropy, is also applied to the coupled pair functional interaction energies for ArCO, resulting in a surface which is characterized by a well depth of 109 cm−1 at a T-shaped geometry and a barrier of 20 cm−1 for rotation of Ar around the oxygen end of CO and of 26 cm−1 for rotation around the carbon end. The rovibrational spectrum calculated from this potential is in very good agreement with the known experimental data, so that for the first time a realistic level scheme for the ArCO complex can be presented. Couplings between rotational levels of different van der Waals modes play an important role and are analysed in some detail. The intensities of infrared transitions have also been calculated in order to help in the experimental determination of the predicted new van der Waals modes.
Fluctuations near limit cycles in chemical reaction systems
Authors: Vance William , Ross John
Published: July 2002

We investigate fluctuational properties near a limit cycle for a homogeneous chemical reaction system using a master equation approach. Our method of solution is based on the WKB expansion of the probability density in the inverse of the system size. The first two terms of this series give the leading asymptotic behavior. The eikonal equation for the leading order term has the structure of a Hamilton–Jacobi equation. Its solutions are determined by the associated characteristic equations, which also give fluctuational trajectories. In the vicinity of the limit cycle, the characteristic equations are the variational equations for the associated Hamiltonian system, and its solutions may be expressed as linear combinations of Floquet eigenfunctions. These eigenfunctions fall into three sets according to whether the real part of the characteristic exponent is less than, equal to, or greater than zero. Eigenfunctions corresponding to characteristic exponents with the real part less than zero span the stable subspace; they describe exponentially fast relaxation to the limit cycle in the deterministic system. Eigenfunctions corresponding to characteristic exponents with the real part greater than zero span the unstable subspace; they describe most probable fluctuational trajectories away from the limit cycle. The remaining two eigenfunctions are associated with a double zero characteristic exponent and span the center subspace. One eigenfunction is due to the translational invariance of the periodic orbit and the other (generalized eigenfunction) to the one-parameter family of periodic orbits in Hamiltonian systems. The generalized eigenfunction describes diffusion along the limit cycle of a probability distribution front for which the gradient is perpendicular to the isochrons of the limit cycle. We develop an explicit formula for the time evolution of an initially localized density based on all these eigenfunctions. We show that relaxation of the density is exponentially fast in directions transverse to the limit cycle and slow (linear in time) along the limit cycle. In addition, we give a simple formula for the probability diffusion coefficient that characterizes dephasing along the orbit. A formula for the stationary distribution is ob- tained from the nonstationary density by removing the center and stable subspace. For this dens- ity, we give a new derivation of an identity: The marginal probability density along the limit cycle equals a constant times the inverse of the speed on the cycle, which is the invariant density along the limit cycle of the deterministic system.
Product state distributions in the dissociation of H3 (n=2,3) Rydberg states
Authors: Müller U. , Cosby P. C.
Published: July 2002

Dissociation of the 2s 2A1′, 2p 2A2″, 3s 2A1′, and 3d 2E″ Rydberg states of the H3 molecule is investigated using a fast neutral beam photofragment spectrometer. A beam of 3–6 keV metastable H3 2p 2A2″ (N=K=0) molecules, generated by charge transfer neutralization of H+3 ions in Cs vapor, is intersected by a laser beam to selectively populate a specific rovibrational level in each of the H3 states for study. The correlated pair of fragments H+H2, created by dissociation of the H3 state, is observed by a time- and position-sensitive detector, which specifies the fragments’ center-of-mass kinetic energy release and angle of ejection. The 3s 2A1′ (N=1, K=0) and 3d 2E″ (N=1, G=0, R=1) rotational levels are prepared either in their ground vibrational states or in vibrationally excited states by pumping transitions from the 2p 2A2″ (N=K=0) level with a tunable dye laser. All of these photoexcited levels are observed to both predissociate and to radiate into the dissociative ground state. In contrast, the 2s 2A1′ (N=1, K=0) level, produced by stimulated emission pumping from the 2p 2A2″ (N=K=0) level using a CO2 laser, is observed to only predissociate. Predissociation of the H3 produces a discrete release of kinetic energy to the H+H2 fragments that uniquely identifies the production of a specific rovibrational level in the H2. Monte Carlo simulation of the detector response, combined with observation of the corresponding dissociations in D3 (where predissociation is essentially negligible), allows quantitative separation of H+H2 predissociation fragments from a background of continuous energy release produced by radiative dissociation. Branching ratios in the radiative and the predissociative decay channels are estimated and detailed distributions for the production of the H2(v,J) final state are determined. Comparison of experimental distributions with recent theoretical calculations illuminates the important role nonlinear couplings play in determining the dissociation dynamics of a particular H3 Rydberg state. Additionally, transition energies for the vibrationally excited levels of H3 are determined with improved accuracy, the relative importance of the H+H+H dissociation channel is estimated, and spontaneous dissociation of the metastable 2p 2A2″ (N=K=0) level is discussed.
Thermally activated escape processes in a double well coupled to a slow harmonic mode
Authors: Drozdov Alexander N. , Talkner Peter
Published: July 2002

We present accurate calculations of thermally activated rates for a symmetric double well system coupled to a dissipative harmonic mode. Diffusive barrier crossing is treated by solving the time-independent two-dimensional Smoluchowski equation as a function of a coupling and a diffusion anisotropy parameter. The original problem is transformed to a Schrödinger equation with a Hamiltonian describing a reactive system coupled to a one-dimensional harmonic bath. The calculations are performed using a matrix representation of the Hamiltonian operator in a set of orthonormal basis functions. An effective system-specific basis is introduced which consists of adiabatically displaced eigenfunctions of the coupled harmonic oscillator and those of the uncoupled reactive subsystem. This representation provides a very rapid convergence rate. Just a few basis functions are sufficient to obtain highly accurate eigenvalues with a small computational effort. The presented results demonstrate the applicability of the method in all regimes of interest, reaching from inter-well thermal activation (fast harmonic mode) to deep intra-well relaxation (slow harmonic mode). Our calculations reveal the inapplicability of the Kramers–Langer theory in certain regions of parameter space not only when the anisotropy parameter is exponentially small, but even in the isotropic diffusion case when the coupling is weak. The calculations show also that even for large barrier heights there is a region in the parameter space with multiexponential relaxation towards equilibrium. An asymptotic theory of barrier crossing in the strongly anisotropic case is presented, which agrees well with the numerically exact results.
Measured coexistence curves of phase-separated polymer solutions
Authors: Xia K.-Q. , An X.-Q. , Shen W.-G.
Published: July 2002

Measurements of the coexistence curves for phase-separated polymer solutions of polymethylmethacrylate in 3-octanone (PMMA/3-OCT) for six different molecular weights of PMMA have been carried out. We have tested the scaling behavior of the order parameter Δφ (the difference between polymer volume fractions in the two coexisting phases) for its dependence on the degree of polymerization N of the polymer chains and the reduced temperature ε=(Tc−T)/Tc. The experiments reveal that, in the asymptotic regime, the measured order parameter has power-law dependence on both N and ε, i.e., Δφ=3.8N−0.21ε0.34. Furthermore, when the symmerization procedure of Sanchez [J. Appl. Phys. 58, 2871 (1985)] is used to analyze the data, it suggests that εN0.46 may be used as a scaling variable for phase separated polymer solutions.
The potential energy surface of H2 16O
Authors: Polyansky Oleg L. , Jensen Per , Tennyson Jonathan
Published: July 2002

We report here a new determination of the H216O potential energy surface from experimental data. The calculations have been carried out by means of the very accurate and highly efficient method proposed and applied to H216O in a previous paper [Polyansky, Jensen, and Tennyson, J. Chem. Phys. 101, 7651 (1994)]. This previous work has been significantly improved by inclusion of additional terms in the analytical expression used to represent the potential energy surface. Previously, 1600 rotation-vibration term values for H216O were fitted with a standard deviation of 0.36 cm−1. With the extended model of the present work, this standard deviation could be improved to 0.25 cm−1. With the extended model and the new fitted potential function we have calculated a data set comprising 3200 term values, all of which can be compared with experimentally derived values. The standard deviation for this data set is 0.6 cm−1. The data set contains rotationally excited energy levels for all the 63 vibrational states which have been characterized by high resolution spectroscopy. The potential energy function obtained in the present work improves drastically the agreement with experiment for the highly excited local mode stretching states above 20 000 cm−1. For the vibrational band origins of these states, the highest of which is measured at 25 118 cm−1, our previous fitted potential produced discrepancies of more than 100 cm−1. These deviations are reduced to less than 1 cm−1 by the potential energy function of the present work. We show that no significant improvement of the fit can be obtained by extending the analytical expression for the potential energy by further high-order terms. An analysis of the residuals shows that at the level of accuracy achieved, the major contribution to the error originates in the neglect of nonadiabatic correction terms in the Born-Oppenheimer kinetic energy operator. We conclude that any further improvement of the potential energy surface requires that such correction terms be included in the Hamiltonian. With the present potential, reliable extrapolations towards higher rotational and vibrational energies can be carried out, and we expect that such calculations can be very helpful in the assignment of experimental spectra involving highly excited states.
Axis switching in the B̃ 2A′–X̃ 2A′ transition of HCO and fluorescence lifetimes of the B̃ 2A′(0,0,0) rotational states
Authors: Lee Shih-Huang , Chen I-Chia
Published: July 2002

Fluorescence spectra of the B̃ 2A′–X̃ 2A′(0,0) band of thermalized HCO at 298 K were recorded. The lifetimes of nascent HCO B̃ 2A′(0,0,0) states produced from acetaldehyde photolyzed at a wavelength of 310.9 nm in a supersonic jet were obtained for rotational states to N=26, 24, and 18 for Ka=0, 1, and 2, respectively. The lifetimes of these states decrease as the Ka and N quantum numbers increase; they vary in the ranges 24–89 ns for Ka=0 states, 12–52 ns for Ka=1 states and 14–18 ns for Ka=2 states. An a-type Coriolis interaction between the B̃ 2A′ and Ã 2A″ states is proposed to account for predissociation of the low vibrational levels of B̃ 2A′. The spectral intensity shows anomalous behavior in that the intensity ratio of transitions QR0/QP0 is nearly 2; similar phenomena are observed for other branches. The mechanism of axis switching in the B̃ 2A′–X̃ 2A′ transition is because the geometry is altered during the excitation; consequently the principal inertial axes are rotated to account for the intensity anomaly. Corrections for the lifetime variation and intensity anomaly are necessary to obtain accurate relative populations of the X̃ 2A′ state from fluorescence spectra B̃ 2A′–X̃ 2A′.
Performance of a time-independent scattering wave packet technique using real operators and wave functions
Authors: Kroes Geert-Jan , Neuhauser Daniel
Published: July 2002

We investigate the performance of a scattering algorithm which uses purely real algebra for the major part of the wave function calculation, while incorporating automatically the appropriate boundary conditions. The algorithm falls in the category of time-independent wave packet methods ([R. Kosloff, J. Phys. Chem. 92, 2087 (1988)], and, more specifically for scattering [Y. Huang, W. Zhu, D. J. Kouri, and D. K. Hoffman, Chem. Phys. Lett. 206, 96 (1993)]), and combines two previous approaches: A method [V. A. Mandelshtam and H. S. Taylor, J. Chem. Phys. 103, 2903 (1995)] in which the action of the absorbing potentials is implicitly inserted in a polynomial expansion of the Green’s function, and a real initial wave function approach, in which zero initial momenta are avoided. Compared to the conventional, multiple time-step Chebyshev method, the new algorithm required three times less Hamiltonian evaluations for a model problem involving direct scattering. The new method also showed faster convergence for a problem involving resonances. Both methods showed convergence problems in the vicinity of a very narrow resonance.
Intermolecular vibrations and relaxation dynamics in complexes of OH A 2Σ+ (v′=0,1) with N2
Authors: Schwartz Rebecca L. , Giancarlo Leanna C. , Loomis Richard A. , Bonn R. Timothy , Lester Marsha I.
Published: July 2002

The intermolecular vibrational energy levels supported by the OH A 2Σ+ (v′=0,1)+N2 potentials have been characterized spectroscopically through excitation of OH–N2 complexes in the OH A 2Σ+–X 2Π 0–0 and 1–0 spectral regions. At least 95 levels correlating with OH A 2Σ+ (v′=0)+N2 are observed in fluorescence depletion experiments. OH–N2 complexes prepared in these levels have lifetimes with lower limits ranging from 1.4 to 8 ps due to rapid electronic quenching which precludes their detection by laser-induced fluorescence. An onset of OH–N2 laser-induced fluorescence occurs at the OH A 2Σ+ (v′=0)+N2 dissociation limit, enabling determination of the ground and excited state binding energies at ∼250 and ⩾1372 cm−1, respectively. In the OH A–X 1–0 region, OH–N2 transitions originating from a common ground state level to single or groups of intermolecular vibrational levels correlating with OH A 2Σ+ (v′=1)+N2 are observed via laser-induced fluorescence and fluorescence depletion measurements. Comparison of the OH–N2 spectra obtained in the OH A–X 0–0 and 1–0 regions reveals that vibrational excitation of OH A 2Σ+ increases the OH–N2 binding energy by 139 cm−1. OH–N2 complexes excited in the OH A–X 1–0 region undergo ultrafast dynamics (<200 fs) which give rise to extensive spectral line broadening. A kinetic model indicates that vibrational predissociation is the dominant decay channel for OH–N2 prepared in the intermolecular levels derived from OH A 2Σ+ (v′=1)+N2.
Comment on ‘‘The effect of charged impurities on a glass transition in a polar medium’’ [J. Chem. Phys. 104, 664 (1996)]
Authors: Miller Danforth P. , de Pablo Juan J. , Corti Horacio R.
Published: July 2002

Dakhnovskii and Lubchenko have recently presented a theory to predict the effect of charged impurities on the glass transition temperature (Tg) in a polar medium. We present and cite more extensive sets of experimental results that show that the electrolyte concentration required to induce a 10 K increase in Tg is nearly three orders of magnitude greater than that predicted from their theory.
σ* resonances in electron impact-induced vibrational excitation of n-propane, cyclopropane, ethylene oxide, cyclopentane, and cyclohexane
Authors: Allan Michael , Andric Lidija
Published: July 2002

Electron-energy-loss spectra in the range of vibrational excitation, and excitation functions for selected vibrational peaks, were measured for the title compounds. Angular distributions of the vibrationally inelastic peaks were measured for n-propane and cyclopropane. The results in n-propane are similar to the published results in ethane, only one very broad band is observed in all channels, with gradual onset at about 3 eV and a maximum around 8 eV. In contrast, narrower resonances emerge in all cyclic compounds. The effect is most pronounced in cyclopropane, where two resonances appear, at 2.6 and 5.5 eV. The latter is exceptional in several respects. It is narrow and thus relatively long lived for a shape resonance of this energy. It causes ring stretch excitation with very high selectivity and pronounced angular distribution, which is reproduced very well by the theory of Read and Andrick, revealing dominance of a partial wave with an unusually high angular momentum, l=3, m=3, and unambiguously identifying the resonance as a2′. The resonances in ethylene oxide are similar, but somewhat broader and shifted to 3 and 4.8 eV, respectively. Resonances in cyclopentane and cyclohexane resemble loosely the cyclopropane case. It is concluded that the major cause of the dramatic differences in spectral appearance between linear and cyclic alkanes are not major changes of resonant energies, but decrease of their (lifetime-determined) width, caused by higher symmetry, rigidity, and consequently larger contribution of partial waves with high l to the scattering. This implies that the vertical electron affinity of linear alkanes is not around −8 eV, as could be assumed from the position of the peak in the attachment spectra, but higher, around −3 eV.
Density functional theory study of some structural and energetic properties of small lithium clusters
Authors: Gardet Georges , Rogemond François , Chermette Henry
Published: July 2002

Some properties of small Lin clusters (n up to 20) are theoretically investigated, within the density functional theory formalism. The structural properties are examined at the so-called local level of approximation. For very small clusters (n≤8), the Lin conformations which are well known from ab initio calculations are found at very low computational cost. For n>8, optimal starting geometries are generated from two growth patterns, based on the increase of the number of pentagonal subunits in the clusters by adsorption of one or two Li atoms. Several new stable structures are proposed, for which the corresponding vibrational analysis is performed for n up to 18. The study of energetic properties and stability requires the use of gradient-approximated functionals. Such functionals are used for the determination of the relative stability of these clusters. For example, we show that the icosahedral structure is the most favorable geometry for Li13, whereas this is not the case for Na13. Ionization potentials and binding energies are also investigated in regard to the size and the geometry of the clusters. Comparison with experimental results and other theoretical approaches (such as nonspherical jellium model) suggests that some combinations of gradient-corrected functionals are more adapted than others to describe Lin energetic and structural properties.
Kinetics of the A+B→0 reaction under steady and turbulent flows
Authors: Reigada R. , Sagués F. , Sokolov I. M. , Sancho J. M. , Blumen A.
Published: July 2002

We consider the kinetics of the two-dimensional, stoichiometric A+B→0 reaction under external flows, for which we use two models: First a steady flow, which consists of an array of eddies; second a time-dependent, stochastic flow which mimics turbulent fluid motion. We follow the reaction numerically and detect in both situations complex decay patterns, which obey different power-laws at short, intermediate and long times: While the initial stage of the reaction is dominated by mixing, c(t)∝t−1, the long-time behavior shows fluctuation-dominated kinetics, c(t)∝t−1/2. In the case of the eddy-lattice-flow a very slow intermediate regime, with c(t)∝t−1/4, emerges. This regime is explained in the framework of an analytical model for mixing along closed-flow lines.
Doubly excited valence states of neutral van der Waals molecules: Mg(3pπ,3pπ 3P
J)⋅Ar(3Σ−)
Authors: Massick Steven , Breckenridge W. H.
Published: July 2002

The first of a new class of doubly excited valence states of van der Waals molecules, Mg(3pπ,3pπ 3PJ)⋅Ar(3Σ−), has been discovered and characterized by resonance two-photon ionization spectroscopy. This state has been found to be quite strongly bound (D0=2850±100 cm−1 and to have a very short bond length (R0=2.41±0.02 Å). The analogous singly excited state, Mg(3s3pπ 3PJ)⋅Ar(3Π0−), in stark contrast, has a very much smaller bond strength (D0=160±40 cm−1) and a longer bond length (R0=3.66±0.02 Å). In fact, the new Mg(3pπ3pπ 3PJ)⋅Ar state is more than twice as bound than even the Mg(3s 2S1/2)⋅Ar+ ground-state ion (D0=1240±40 cm−1, R0=2.82 Å). The strong bonding is due both to the transverse alignment of the Mg(3pπ) electrons and particularly to the lack of a Mg(3s) electron. The resulting loss of Mg(3sσ)/Ar(3pσ)2 repulsive forces facilitates close approach of the Ar atom. This is turn allows attractive Mg(3pπ)2/Ar dispersive forces and Mg2+/Ar ion/induced-dipole type forces to continue to much smaller internuclear distances, creating a strong, short van der Waals bond. Possible mechanisms for the parity-dependent predissociation and the apparent ‘‘spin–spin’’ type splittings observed for the Mg(3pπ,3pπ 3PJ)⋅Ar(3Σ−) state are also discussed.
A density functional theory estimation of the heat of formation for FOOCl
Authors: Jursic Branko S.
Published: July 2002

The structure of FOOCl and its isodesmic heat of formation (HOF) were computed with hybrid (B3LYP), gradient-corrected (BLYP), and local (SVWN) Density Functional Theory (DFT) methods. The DFT computed HOFs were compared with the CCSD(T) ab initio HOF for FOOCl and their suitability for computing HOFs for small polar molecules was discussed.
NaK
D
1
Π
electric dipole moment measurement by Stark level crossing and
e
–
f
mixing spectroscopy
Authors: Tamanis M. , Auzinsh M. , Klincare I. , Nikolayeva O. , Stolyarov A. V. , Ferber R.
Published: July 2002

The paper presents the first permanent electric dipole moment dp measurements for NaK D1Π state rovibronic levels. Two different methods were applied to obtain dp values. Stark effect induced level crossing registered as the changes of fluorescence linear polarization P(E) with external electric field E yielded from one fit both the electric dipole moment value and the Λ-doubling splitting between e,f substates of an individual rotational state. Another method consisted of obtaining the ratio ΔefJ/dp from E-dependence of the forbidden line appearing in fluorescence as a result of e–f Stark mixing, along with direct ΔefJ measurement by RF – optical double resonance. The respective dipole moment values obtained are 5.9 – 6.4 D for the state v=7, J=23, as well as 4.5 – 4.8 D for v=12, J=7, the typical errors being ca. 12%–20%. The dp value for the latter state reflects dp diminution expected due to the admixture of the d3Π state caused by intramolecular interaction. Signal simulation and data fitting have been accomplished using direct Hamiltonian diagonalization accounting for Stark interaction within rotational states J±ΔJ,ΔJ=0,1 and 2 in the initial, excited and final state.
A helium atom scattering study of the growth and dynamics of CH4 and C2H6 on Cu(001)
Authors: Graham A. P. , Bertino M. F. , Hofmann F. , Silvestri W. , Toennies J. P.
Published: July 2002

The growth, structure and dynamics of methane and ethane monolayers on Cu(001) have been studied using high resolution helium atom scattering. Specular intensity measurements of the growth behavior reveal an island growth mode for methane and ethane, indicative of an attractive interatomic interaction. Diffraction measurements show that the methane monolayer forms a hexagonal structure, incommensurate in one direction, with a lattice constant of 4.18 Å, while ethane forms a structure with an interatomic distance of 5.10 Å. The frustrated translational vibrations perpendicular to the surface (S mode) of methane and ethane have been measured with inelastic helium scattering and have frequencies of 6.5 meV and 6.8 meV, respectively. In addition, the desorption energies were determined to be 165 meV for methane and 260 meV for ethane, in both cases independent of coverage. These results are compared with recent measurements made for C2H4 and larger alkanes adsorbed on Cu(001).
Collisions of O(1D) with HCl, Cl2, and COCl2: Total quenching, channel specific rate constants, and yields of Cl(P1/22)
Authors: Chichinin A. I.
Published: July 2002

The method of time-resolved laser magnetic resonance was used to monitor Cl atoms produced by the reactions of O(D1) with HCl, Cl2, and COCl2 at room temperature. For these reactions, the measured relative quantum yields of spin-orbit excited atoms are [Cl(P1/22)]/([Cl(P1/22)]+[Cl(P3/22)])=(0.10±0.04), <0.05, <0.10, respectively. The rate constants for total removal of O(D1) by HCl, Cl2, and COCl2 have been determined to be (1.5±0.3), (2.5±0.5) and (2.6±0.5), respectively. The reactive rate constants have been determined to be k2=(0.94±0.2), k4=(1.9±0.3), and k5a+2k5b=(2.1±0.27); here k2, k4, k5a, and k5b are rate constants of O(D1)+HCl → OH+Cl (2), O(D1)+Cl2 → ClO+Cl (4), and O(D1)+ COCl2 → CO+ClO+Cl (5a), CO2 +2Cl (5b), respectively. The rate constants are in units of 10−10 cm3/molecule⋅s, quoted errors are 2σ.
Statistical thermodynamics of end-attached chain monolayers
Authors: Li Buqiang , Ruckenstein Eli
Published: July 2002

A simple cubic lattice model and a matrix formalism are used to calculate the segment density distribution and structural properties of monolayers consisting of end-attached chains and solvent molecules. The chain stiffness, the nearest-neighbor bond correlations, and the intermolecular interactions are taken into account. The calculations are performed for chains of various lengths, from short to polymer brushes. The main difference between the present treatment and those already available consists in the more compact formulation of the basic equations. The incorporation of the nearest-neighbor bond correlations, or higher values of the interaction parameter results in a higher average segment density and a narrower distribution profile of the chain free ends. With decreasing chain length, the effect of the bond correlations becomes less important. The model allows us to calculate the density profiles of the lateral, forward, and backward bonds as a function of the layer number, and to calculate the orientational probability for a bond to be lateral, forward, or backward as a function of the bond number counted from the attached end.
Stabilization of localized states in dissipative tunneling systems interacting with monochromatic fields
Authors: Makri Nancy
Published: July 2002

We study the dynamics of an initially localized symmetric two-level system coupled to high-temperature dissipative environments and driven by a strong time-periodic force which corresponds to high-frequency monochromatic light. Qualitative arguments based on the quantized representation of the radiation field predict a wealth of intriguing behaviors which are confirmed and quantified via accurate numerical path integral calculations. With the exception of very strong friction we find that high-frequency driving always helps stabilize localized states. At intermediate friction the delocalization rate approaches a “universal’’ limiting value which is largely independent of the parameters of the environment and of the specifics of the driving force, depending only on its overall strength. This robust behavior implies that localized states can be stabilized in these systems without much finetuning of external conditions. In the weak friction regime the interplay between phase interference and dissipation results in nonmonotonic variation of the decay rate with friction and driving frequency. The path integral results are compared to those obtained earlier via analytical treatments.
Concentration and frequency-dependent trapping of reactive polymers
Authors: Fredrickson Glenn H.
Published: July 2002

The kinetics of diffusion-controlled reaction of an end-functionalized polymer chain with a fixed random array of reactive “trap’’ sites is examined as a function of trap concentration n. It is demonstrated that a frequency-dependent trapping rate coefficient is generally required to describe the trapping dynamics. Scaling methods and renormalized perturbation theory are used to examine the behavior of this rate coefficient as a function of nR3 and dimensionless frequency sτ, where R is the radius of gyration and τ is the terminal relaxation time of the polymer. The Rouse model is employed throughout, although extensions to account for hydrodynamic interactions and entanglements are straightforward. The implications of our results for reactive blending and polymer adsorption processes are discussed.
The weak interaction potential of Ar–H2S
Authors: de Oliveira Glenisson , Dykstra Clifford E.
Published: July 2002

Ab initio calculations have been carried out to generate a potential energy surface for the Ar–H2S weakly bonded cluster. The surface displays a low energy trough with small barriers for the “orbit” of Ar about H2S. Basis set and correlation effects have been analyzed through a series of calculations at different levels, and the feature of a low energy trough seems assured. The nature of this surface has only little to do with electrostatic interaction, and so, a fairly good fit of the nonelectrical part of the potential surface is obtained with atom–atom Lennard-Jones potential terms. With different surface fits and a fully anharmonic analysis of the ground vibrational state, on-average rotational constants were calculated via rigid-body diffusion quantum Monte Carlo. These calculations show that certain intriguing spectroscopic manifestations of deuterium substitution are associated primarily with the trough feature of the surface.
Surface scattering of hyperthermal (10–50 eV) neutral C60: Angular and energy distributions
Authors: Budrevich A. , Tsipinyuk B. , Bekkerman A. , Kolodney E.
Published: July 2002

The scattering dynamics of hyperthermal C60 from a carbonized nickel surface at impact energies E0=10–50 eV was studied by high resolution angular and energy distributions. The scattered energy scales linearly with E0 and kinetic energy losses vary with scattering angle from ∼85 to ∼40%. Nearly complete decoupling between normal and tangential energy losses was found. The tangential losses are described in terms of various models of rotational excitation and the involvement of translational slip is concluded.
Quantum Monte Carlo binding energies for silicon hydrides
Authors: Greeff C. W. , Lester, Jr. W. A.
Published: July 2002

Variational and diffusion Monte Carlo calculations are carried out on the molecules SiHn (n=1–4), Si2, Si2H6, and Si3H8. The anion SiH3− is also considered. Core electrons are eliminated through the use of an ab initio effective core potential. The atomization energies and various bond energies are compared with experiment and other calculations. The atomization energies of SiH–SiH4 are in good agreement with experiment and with the best other calculations. The calculated electron affinity of SiH3 is within 0.02 eV of the measured value. The calculated atomization energies of the silanes SiH4, Si2H6, and Si3H8 are more consistent with the original heats of formation of Gunn and Green [J. Phys. Chem. 65, 779 (1961); 68, 946 (1964)], without the subsequent corrections [M. W. Chase, Jr., C. A. Davies, J. R. Downey, Jr., D. J. Frurip, R. A. McDonald, and A. N. Syverud, J. Phys. Chem. Ref. Data 14, Suppl. 1 (1985)] made on the assumption that the final state of Si is amorphous.
Rheology of model confined ultrathin fluid films. I. Statistical mechanics of the surface force apparatus experiments
Authors: Bordarier Philippe , Rousseau Bernard , Fuchs Alain H.
Published: July 2002

We have studied the behavior of confined ultrathin films of a simple model system in which the fluid and wall particles were modeled as Lennard-Jones rare gas atoms by Monte Carlo simula-tions. We have used a new (Grand isoforce) statistical ensemble in these simulations, with which it is possible to simulate thin films under constraints (fixed chemical potential μ, temperature T, stress tensor components Tzz, Txz, and Tyz) that are presumably resembling the conditions under which the Surface Force Apparatus (SFA) operates. We also employ another ensemble, with fixed (μ,T,V,Txz,Tyz), which we call “Grand Isoshear.” Some important features of the SFA experiments on globular, nonpolar, molecular liquids have been reproduced in the simula-tions. Using the new statistical ensembles we have shown that a periodic solvation force can be obtained under the operating conditions of the SFA, although some additional features may exist, but remain hidden in the experiments if located in the unstable branch of the solvation force curve. We demonstrate that epitaxy between the solid walls and the fluid molecules is not required to obtain an apparently periodic solvation force curve.
Microscopic parameters influencing the phase separation in compressible binary blends of linear semiflexible polymers
Authors: Foreman K. W. , Freed Karl F.
Published: July 2002

The lattice cluster theory (LCT) is used to determine the essential microscopic parameters that influence the phase separation in binary blends of linear semiflexible lattice chains with equal polymerization indices. The LCT and the polymer reference interaction site model are shown to predict nearly identical and universal constant volume phase behaviors (after simple numerical rescaling of the polymerization indices) for “athermal” blends with vanishing van der Waals attractive energies. Phase separation in these systems is driven solely by stiffness disparities. LCT computations are extended to “thermal” systems in which the van der Waals interactions are large enough to produce liquid densities at standard temperature and pressure. Both the stiffness disparity between the blend components and the relative magnitudes of the van der Waals interaction energies influence the phase behavior of the model blends. We find a family of universal constant volume spinodals, parameterized by the exchange energy. Compressibility is shown to produce significant enthalpic contributions to phase separation, even when all van der Waals energies are identical. We also study the pressure dependence of these model blends, as well as the variety of qualitatively different phase behaviors exhibited. A future work will determine the combined influence of monomer structure, semiflexibility, van der Waals interactions, and the energetic implications of compressibility on the phase behavior of polyolefin blends.
Relative orientation of chemical shielding and dipolar coupling tensors: Mixed single- and double-quantum homonuclear rotary resonance nuclear magnetic resonance of rotating solids
Authors: Bak M. , Nielsen N. C.
Published: July 2002

A novel two-dimensional magic-angle spinning nuclear magnetic resonance (NMR) method for determination of relative orientation of dipolar and chemical shielding tensors for dipolar-coupled homonuclear spin-1/2-pairs of powder samples is described. Simultaneous recoupling of anisotropic chemical shielding and dipolar coupling interactions is accomplished using a homonuclear rotary resonance pulse sequence with the amplitude ωrf of a rf irradiation field matched to the spinning frequency ωr according to ωrf=ωr. Employing this technique in the first dimension of a two-dimensional experiment leads to powder spectra exhibiting strong dependence on the magnitudes and the relative orientation of the two shielding tensors and the dipolar coupling tensor correlated to a high-resolution spectrum in the sampling dimension. Various aspects of the recoupling experiment are described theoretically and the applicability of the method for determination of relative orientation of these three anisotropic tensors through numerical simulation is demonstrated on basis of experiments for a doubly C13-labeled powder of L-alanine. With reference to this sample (and aminoacids in general), minor effects from simultaneous recoupling of the heteronuclear dipolar coupling between Cα and the amide N14 nucleus are evaluated. In the present case, the C13-N14 dipolar interaction is used in numerical simulations to refine our structural analysis and to obtain information about the absolute orientation of the C13 chemical shielding tensors relative to the molecular coordinate system.
Neutral carbon cluster distribution upon laser vaporization
Authors: Kaizu K. , Kohno M. , Suzuki S. , Shiromaru H. , Moriwaki T. , Achiba Y.
Published: July 2002

The mass distribution of neutral carbon clusters generated by a laser vaporization of graphite in He was safely determined by a single photon (10.5 eV) ionization technique. The resulting mass distribution was surprisingly different from those reported in previous articles. It was found that the carbon clusters generated under a moderate laser fluence condition (0.1–1 J/cm2) consists of mostly even-numbered monocyclic rings with the cluster sizes of n=10–18. The formation mechanism of such neutral carbon clusters upon laser irradiation is discussed.