bluuebunny/crossref_metadata_2025 huggingface dataset:
Nascent vibrational and rotational distributions of NH(A 3Π,c 1Π) in the dissociative excitation of NH3 by Ar(3P2,0) at thermal energy
Authors: Sekiya Hiroshi , Nishiyama Nobuaki , Tsuji Masaharu , Nishimura Yukio
Published: July 2002

The Ar(3P2,0)+NH3 reaction has been studied by optical emission spectroscopy. Experiments have been performed by using a flowing afterglow apparatus and a new low-pressure apparatus. In the latter experiment the Ar(3P2,0) atoms generated by a microwave discharge were expanded into a high vacuum chamber through an orifice. Essentially the same results have been obtained from the two experimental systems. The NH(c 1Π–a 1Δ) emission as well as the NH(A 3Π–X 3∑−) emission was observed. The emission intensity ratio INH(c)/INH(A) was measured to be 0.31±0.03. The vibrational population ratio P(v′=1)/P(v′=0) for the NH(A 3Π) state was determined to be 0.35±0.05, while the effective rotational temperatures for the NH(A 3Π,v′=0) and NH(c 1Π,v′=0) states were estimated to be 2000±300 K and 1050±70 K, respectively. The observed vibrational and rotational distributions of the NH(A 3Π) state and the rotational distribution of the NH(c 1Π) state were much colder than those predicted from a simple statistical theory. The fraction of the available energy converted into vibration 〈fv〉 and rotation 〈fr〉 of NH(A 3Π) were estimated to be 3% and 4%, respectively, suggesting that a large amount of the available energy was transferred into relative translation between NH(A 3Π) and H2.
Density expansion in a disordered system and its application to nonlinear spectroscopy
Authors: Deng Z.
Published: July 2002

We study nonlinear spectroscopy processes in a disordered system. A density expansion with respect to the equations of motion is developed. In low density limit we derive explicitly the reduced equations for zero-, one-, and two-body variables. Our reduced equations contain the information of the transport property of the disordered system. By solving these equations we obtain the steady state four-wave mixing spectrum of the disordered system and study its aspect.
A CI study of the CuCO and CuCO+ complexes
Authors: Merchán Manuela , Nebot-Gil Ignacio , González-Luque Remedios , Ortí Enrique
Published: July 2002

MO CI calculations are carried out using an optimal space of valence virtual MOs obtained by means of a projection technique, as a linear combination of the AOs which are more occupied in the molecular Fock space. Localization of the occupied MOs and nonvalence virtual MOs is also achieved. The overall procedure is proven to be quite advantageous and well suited to obtain potential energy curves which keep the same physical meaning along the range of distances studied. Using a slightly better than double-zeta quality basis set, a valence CAS-CI, and selected CI wave function by the CIPSI algorithm have revealed a possible weak van der Waals interaction for the 2Σ+ state of CuCO, which remains when polarization functions are added to the basis set for the carbon and oxygen atoms. Even though the CuCO 2Π and CuCO+ 1Σ+ states are energetically close, the nature of the interactions is quite different, π bonding and mainly electrostatic, respectively. The results give further support to the view of the neutral metal–CO interaction as a balance of σ repulsion and π backbonding. However, it is proposed that the driving force for the positive ion metal–CO interaction becomes essentially electrostatic.
Critical lines for a generalized three state binary gas–liquid lattice model
Authors: Meijer Paul H. E. , Keskin Mustafa , Pegg Ian L.
Published: July 2002

The critical properties of several compressible binary gas–liquid models are described: the three state lattice gas, the Tompa model for polymer solutions, the van der Waals equation for binary mixtures, and an intermediate model. The critical lines are expressed as functions of x1 and x2, the density of type 1 molecules and the density of type 2 molecules, instead of using the pressure and temperature; representative figures are given for each of the models. The general conditions for criticality, stability, and tricriticality are given as functions of x1 and x2 through the intermediary of the spinodal temperature function T(x1,x2). A closed form solution is given for the Berthelot case (geometrical-mean combining rule). All the models exhibit a characteristic intersection of two critical lines, and the behavior near this point is investigated. In the van der Waals case we confirm the coordinates given by van Laar.
Beyond Poisson–Boltzmann: Images and correlations in the electric double layer. I. Counterions only
Authors: Attard Phil , Mitchell D. John , Ninham Barry W.
Published: July 2002

The general solution to the zero size mean spherical model is found for an inhomogeneous electrolyte (with specified profile) between charged planar surfaces. The analysis includes the effects of images and correlations. It provides the primary correction to the classical mean field theory of the double layer and allows the error in that theory to be easily estimated over experimental regimes. The relationship between Lifshitz theory and primitive model theories of electrolytes is made explicit. Results obtained for the one component double layer are accurate for systems with low coupling.
Determination of transition moment directions in molecules of low symmetry using polarized fluorescence. I. Theory
Authors: van Gurp M. , Levine Y. K.
Published: July 2002

A theoretical description of angle-resolved fluorescence depolarization experiments of dye molecules in stretched polymer films is presented. It is shown how the technique can be used for the determination of transition moment directions in organic molecules of low symmetry. The application of these angle-resolved measurements to the determination of the direction of the transition moments in a number of dye molecules is discussed in the following paper. The theoretical treatment implicitly assumes that the motions of the fluorescent molecules are slow on the time scale of their excited state lifetime (in the order of ns) so that their orientation is identical at the times of absorption and emission. Furthermore the molecules are taken to be uniaxially distributed around the stretch direction of the polymer film and it is shown how this can be checked experimentally. The macroscopic orientational distribution is assumed to be invariant for a rotation of 180° of the molecules around a ‘‘mechanical’’ symmetry axis chosen in the molecular frame as well as to a reflection in the molecular plane (C2v D2h symmetry). The treatment presented makes use of the Wigner rotation matrix formalism in the description of the fluorescence intensities in terms of molecular orientational properties (order parameters and angles between transition moments and the mechanical symmetry axis). The advantge of this formalism is the ease with which rotational transformations are carried out between molecular and laboratory frames and further that the symmetry properties of both the molecules and the system are treated in a simple and general way.
Test of the accuracy of small-curvature and minimum-energy reference paths for parametrizing the search for least-action tunneling paths: (H,D)+H′Br→(H,D)Br+H′
Authors: Lynch Gillian C. , Truhlar Donald G. , Garrett Bruce C.
Published: July 2002

The least-action semiclassical algorithm for multidimensional tunneling probabilities [B. C. Garrett and D. G. Truhlar, J. Chem. Phys. 79, 4931 (1983)] has usually been employed by interpolating tunneling paths between two limits, a least-motion limit appropriate for large reaction-path curvature and a minimum-energy limit appropriate for small reaction-path curvature. In the present study we test whether, when the reaction-path curvature is small, more accurate results might be obtained by using a general small-curvature reference path. Least-action algorithms with both types of reference paths are compared to each other, to five other semiclassical approximations, and to accurate quantal dynamical rate constants for one three-dimensional and two collinear reactions with the mass combination ℒ+ℋ ℒ′→ℒ ℋ+ℒ′ where ℒ and ℒ′ denote light atoms (H or D) and ℋ denotes a heavy atom (Br). We find, perhaps surprisingly, that the usual least-action method works best. This is encouraging because the minimum-energy reference is easier than the small-curvature reference to extend to polyatomic reactions.
Molecular orientation near liquid–vapor interface of methanol: Simulational study
Authors: Matsumoto Mitsuhiro , Kataoka Yosuke
Published: July 2002

Computer simulations with molecular dynamics technique were carried out to investigate properties of liquid–vapor interface of methanol for a wide temperature range of 160–350 K. The estimated surface excess thermodynamic quantities, especially surface entropy, are characteristic of strongly hydrogen-bonding liquids, but the density profile resembles that of simple fluids. Orientational structuring near the surface was also studied, and one apparent tendency, much stronger than that of water, was observed; the methanol molecule projects its methyl group toward the vapor phase. This orientational ordering can be understood as a result of putting a hydrophobic methyl group outside of liquid phase to maximize the number of hydrogen bonds near the surface for energetical stabilization. A considerable part of the thermodynamic anomaly is due to this orientational ordering. Since the electric dipole is almost parallel to the surface in this orientation, the calculated surface potential is quite small, about −30 mV at room temperature.
Kinetics and thermochemistry of ClCO formation from the Cl+CO association reaction
Authors: Nicovich J. M. , Kreutter K. D. , Wine P. H.
Published: July 2002

Laser flash photolysis of Cl2/CO/M mixtures (M=N2, CO, Ar, CO2) has been employed in conjunction with Cl(2PJ) detection by time-resolved resonance fluorescence spectroscopy to investigate equilibration kinetics in the reactions Cl(2PJ) +CO↔ClCO as a function of temperature (185–260 K) and pressure (14–200 Torr). The association and dissociation reactions are found to be in the low-pressure limit over the range of experimental conditions investigated. In N2 and/or CO buffer gases, the temperature dependences of the ClCO formation and dissociation reaction rate constants are described by the Arrhenius expressions k1=(1.05±0.36)×10−34 exp[(810±70)/T] cm6 molecule−2 s−1 and k−1=(4.1±3.1)×10−10 exp[(−2960±160)/T] cm3 molecule−1 s−1 (errors are 2σ). Second- and third-law analyses of the temperature dependence of the equilibrium constant (k1/k−1) lead to the following thermodynamic parameters for the association reaction: ΔH0298 =−7.7±0.6 kcal mol−1, ΔH00 =−6.9±0.7 kcal mol−1, ΔS0298 =−23.8±2.0 cal mole−1 K−1, ΔH0f,298 (ClCO)=5.2±0.6 kcal mol−1 (errors are 2σ). The results reported in this study significantly reduce the uncertainties in all reported kinetic and thermodynamic parameters.
Aspects of dissipative electronic and vibrational dynamics of strongly vibronically coupled systems
Authors: Schneider R. , Domcke W. , Köppel H.
Published: July 2002

The intramolecular non–Born–Oppenheimer quantum dynamics on conically intersecting potential-energy surfaces is analyzed on the basis of exact (numerical) time-dependent quantum calculations for two representative two-state three-mode vibronic-coupling models. A compact description of the time-dependent dynamics in terms of reduced density matrices of the electronic and vibrational subsystems is introduced. Results are presented for the time evolution of electronic and vibrational coherences, populations, as well as subsystem entropies. It is found that such simple two-state three-mode vibronic coupling models exhibit a rich variety of dissipative phenomena on femtosecond time scales. The numerical results reveal an interesting interplay of driven electronic surface-hopping processes and dephasing of coherent vibrational motion which is presumably a generic feature of ultrafast internal conversion processes in polyatomic molecules.
Cross section and product time-of-flight measurements of the reaction of N+2 with H2O and D2O at suprathermal energies
Authors: Dressler Rainer A. , Gardner James A. , Salter Richard H. , Wodarczyk Francis J. , Murad Edmond
Published: July 2002

Charge exchange and hydrogen atom pickup cross sections, and product ion time-of-flight measurements are reported for N+2 –H2O(D2O) collisions at center-of-mass collision energies ranging between 1 and 15 eV. No isotope effect is detected for the charge exchange branch, while a significant isotope effect is observed for the atom pickup reaction. Throughout the measured energy range, the time-of-flight measurements show that the H2O+(D2O+) charge exchange product is produced with near-thermal energy in the laboratory frame, implying little or no momentum transfer. The charge exchange reaction products are therefore formed with internal energy comparable to the exothermicity of the reaction (2.96 eV). The atom pickup ion product velocity distributions and the atom pickup isotope effect are consistent with a spectator stripping mechanism.
A symmetry-based model for selective rotational energy transfer in collisions of spherical top molecules
Authors: Parson Robert
Published: July 2002

Recent state-resolved experiments have shown that rotational energy transfer in collisions of vibrationally excited spherical top molecules is remarkably selective with respect to the fine structure components of the rovibrational states. It is shown that this selectivity can be explained by means of symmetry arguments and the Harter–Patterson theory of spectral clustering. A new propensity rule, which extends the well-known symmetric top selection rule to perturbed spherical tops, is derived and shown to account well for the experimental results of Steinfeld and co-workers on 13CD4 and SiH4.
Multiconfigurational self-consistent field–multireference configuration interaction study of the spectroscopy of CuI including spin–orbit couplings
Authors: Ramírez Solís A. , Daudey J. P. , Teichteil C.
Published: July 2002

Multiconfigurational self-consistent field–multireference configuration interaction calculations followed by the inclusion of spin–orbit corrections were performed to study the 18 lowest (Ω) fine structure states arising from the 9 lowest purely electronic ΛSΣ states of the CuI molecule. An important difference with previous studies on the first excited states of CuF and CuCl is found regarding the relative position of the first neutral states [with the Cu(d10s1)I(s2p2σp3π) configuration] even before taking into account the spin–orbit effects. We have been able to assess the nature of the observed transitions and verified that the first (A) and third (D) systems arise both from a Σ←Π-type transition while the second (C) and fourth (E) systems arise from a Σ←Σ type. The adiabatic potential energy curves for all the Ω states are given as well as the calculated spectroscopic constants of the states having a minimum in the studied range of interatomic distance. We also confirm the hypothesis put forward by Delaval et al.[Chem. Phys. Lett. 139, 212 (1987)] stating that the X←A and X←C transitions are spin forbidden. We explain this through the mixing of singlet and triplet states owing to the spin–orbit coupling of iodine (for the A 1,3Πn state) and that due to copper (for the C 3,1Σ+ state). Finally, an avoided crossing is predicted between the third and fourth Ω=1 fine structure states and a dramatic change of character for the D(1Π) ionic state towards a neutral Π configuration at long interatomic distance.
Comparison of deterministic and stochastic kinetics for nonlinear systems
Authors: Zheng Qiang , Ross John
Published: July 2002

The deterministic kinetics of chemical reactions is compared with a stochastic description for the cubic Schlögl model with a single stable steady state, which has a nonlinear reaction mechanism. We solve numerically the birth-death master equation for this system for various numbers of particles (N=20–160). For small systems with tens of particles the deviation of the first moment of the stochastic distribution from the deterministic temporal variation of concentration can be substantial in the initial relaxation towards a stationary state. The relaxation of the master equation is faster than that of the deterministic equation. The maximum deviation in trajectories decreases as the parameters in the kinetic model are altered towards a linear mechanism. The maximum deviation differs from N1/2 as N decreases, but approaches N1/2 as N increases. Deviations from deterministic temporal evolution due to fluctuations depend on the extent of nonlinearity of the reaction. The variance of a stationary distribution of the master equation is shown to be significantly larger than the average for a nonlinear system.
Photoelectron spectroscopy of CH2N−
Authors: Cowles Daniel C. , Travers Michael J. , Frueh Jennifer L. , Ellison G. Barney
Published: July 2002

We have measured the negative ion photoelectron spectra of CH2N− and CD2N− and find the electron affinities: EA(CH2N)=0.511±0.008 eV and EA(CD2N)=0.498±0.011 eV. Franck–Condon simulations of these spectra are carried out and we estimate the CH2N− and CH2N geometry differences; we fit our spectra with the following [constrained] molecular structures. We combine our EA(CH2N) with the results of previous gas phase ion studies to extract a number of thermochemical parameters (kcal/mol): Do0(CH2N–H)=85±5, Do0(H–HCN)=23±6, Do0(H2C–N)=144±6, and the isomerization enthalpy of H2CN+→HCNH+ is ΔHisom(C2v→C∞v)=−51±7. Attempts to calculate the geometry and vibrational frequencies of the H2CN radical are disappointing. Unrestricted Hartree–Fock and second-order Mo/ller–Plesset ab initio calculations in a 6-31++G** basis produce badly spin-contaminated wave functions which do not reproduce the experimental findings.
Molecular dynamics algorithm for multiple time scales: Systems with disparate masses
Authors: Tuckerman Mark E. , Berne Bruce J. , Rossi Angelo
Published: July 2002

A frequently encountered problem in molecular dynamics is how to treat the long times that are required to simulate condensed systems consisting of mixtures of light and heavy particles. Standard methods require the choice of time step sufficiently small to guarantee stable solution for the low mass component with the consequence that these simulations require a very large number of central processing unit cycles to treat the relaxation of the heavier component. In this note, we present a new method that allows one to use a time step appropriate for the heavy particles. This method uses a similar idea to numerical analytical propogator algorithm, an algorithm we invented to treat high frequency oscillators interacting with low frequency baths and is based on a choice of a reference system for the light particle motions. The method is applied to the case of a liquid containing 864 Lennard-Jones spheres, 824 of these particles having a mass, M=100 and 40 spheres picked at random have a mass m=1. It is shown that molecular dynamics using the new algorithm runs seven to ten times faster than standard methods and this approach as well as suitable generalizations should be very useful for future simulations of quantum and classical condensed matter systems.
Thermodynamics of solid and liquid embedded-atom-method metals: A variational study
Authors: LeSar R. , Najafabadi R. , Srolovitz D. J.
Published: July 2002

We present results of variational calculations of the Helmholtz free energy and the thermodynamic properties of a series of metallic liquids and solids (Ag, Au, Cu, Ni, Pd, Pt) described by embedded-atom-method potentials. For the solids, we use a variational procedure based on an Einstein-model reference state. The free energies of liquids are calculated with an approximate variational method proposed by Ross. At the respective melting points, the present results for the Helmholtz free energy are within about 1% of the results of accurate Monte Carlo (MC) calculations with the same interaction potentials, both for the fluid and the solid. The average error in the melting points calculated with the present procedure relative to Monte Carlo results is about 7.5%. The internal energies and entropies are compared to MC results, and show, in general, good agreement.
Determination of the order parameters of liquid crystals from carbon-13 chemical shifts
Authors: Guo Wen , Fung B. M.
Published: July 2002

The orientational ordering of four 4-n-alkyl-4′-cyanobiphenyls (kCB’s: 5CB, 6CB, 7CB, 8CB) and four 4-n-alkoxy-4′-cyanobiphenyls (kOCB’s: 5OCB, 6OCB, 7OCB, 8OCB) in their nematic phase was studied at different temperatures by C-13 NMR. The technique used was a combination of variable-angle spinning and a two-dimensional method called separated local-field spectroscopy. The C-13 chemical shifts for each carbon nucleus were measured with the sample spinning parallel to the magnetic field. For the carbon atoms in the aromatic rings and the cyano group as well as the carbon atoms in the in α and β positions of the aliphatic chain, the order parameters for all four compounds in each homologous series show a linear dependence on the corresponding chemical shifts. For the ω-CH3 group and the (ω−1)-CH2 group, a simple linear dependence of the C–H order parameter on the chemical shift was found for all eight compounds in the two homologous series. The slopes and intercepts of all the results are tabulated. The results offer a simple way to determine the order parameters of liquid crystals directly from C-13 chemical shifts.
Relaxation and dynamics of coupled spin systems subjected to continuous radio frequency fields
Authors: Ravikumar Muppirala , Shukla Rajesh , Bothner-By Aksel A.
Published: July 2002

A computational method for the prediction of spin trajectories in systems of two or more spins evolving under the influence of the static field, an applied radio frequency field, chemical shifts, spin–spin couplings, and magnetic dipolar relaxation has been described. The program has been tested to establish its correct operation by calculation of limiting cases with known solutions. It is shown that it is necessary to retain the nonsecular elements of the relaxation matrix in order to obtain correct results, primarily elements controlling cross relaxation between transverse components of magnetization of different spin states belonging to the same quantum number of the total magnetization along the axis of the static field. As a result of the retention of these terms, two unexpected features are predicted: frequency and phase shifts in spectra of two non-spin-coupled nuclei with relaxation rate comparable to frequency separation and unusually intense magnetizations in the locked state of two spins with the radio frequency applied at the center of the spectrum.
Wetting in water–oil–nonionic amphiphile mixtures
Authors: Aratono M. , Kahlweit M.
Published: July 2002

In a three-phase equilibrium of H2O –oil–amphiphile mixtures, the middle amphiphile rich phase may or may not wet the water/oil interface. For nonwetting middle phases, theory predicts a nonwetting→wetting transition upon approaching either one of the two critical endpoints. With respect to an experimental confirmation of this prediction, the situation appears to be controversial. In this paper, we have, therefore, studied the wetting behavior of the middle phase as it depends on the amphiphilicity of nonionic amphiphiles. We find that in mixtures with short-chain amphiphiles, the middle phase wets the water/oil interface in the entire three-phase interval, whereas with long-chain amphiphiles it (apparently) never wets. For medium-chain amphiphiles, however, one does find a nonwetting→wetting transition. On the basis of this result, we suggest that there exist four cases for the wetting behavior as a consequence of the dependence of the relations between the three interfacial tensions on amphiphilicity. The wetting behavior can be correlated with the evolution of the three-phase bodies from a tricritical point. Upon increasing amphiphilicity, their characteristic properties pass through maxima in the range of medium-chain amphiphiles, coinciding with the transition from always wetting to never wetting.
Electron states of the HCl dimer
Authors: Pradeep T. , Rao C. N. R.
Published: July 2002

The He i photoelectron spectrum of HCl dimer has been recorded and the observed ionizations at 12.3, 13.6, 15.1 and 17.0 eV have been assigned to different orbitals on the basis of molecular orbital calculations. The observed first adiabatic IE (11.9 eV) agree well with the value of 11.91 eV from a photoionization study of the dimer. The two HCl units in the dimer are nonequivalent and there is significant interaction between the σ orbitals. Both the calculations and the experiment show that the geometry undergoes considerable distortion on ionization.
Torsionally inelastic collisions between a near-symmetric top molecule and a structureless atom
Authors: Davis Stephen L.
Published: July 2002

The close-coupling formulation is presented for collisions of a structureless atom with a near-symmetric top exhibiting internal rotation, such as methanol. The molecule is approximated as a symmetric top whose internal rotation axis coincides with the symmetry axis. The K doubling arising from the asymmetry is taken into account only to first order. Both rotational and torsional inelasticity are considered, but the molecule is considered to be rigid with respect to all other vibrational degrees of freedom. Expressions are given for the matrix elements of the interaction potential between any two rotational–torsional states. It is shown that A↔E and E1↔E2 collisional excitation is forbidden. The infinite order sudden (IOS) approximation is extended to systems exhibiting internal rotation by applying the sudden limit to the torsional motion as well as to the overall rotation. Based on the expressions for the IOS cross sections, it is shown that a propensity rule holds for transitions elastic in J or in K between two A± doublets similar to that governing transitions between K doublets in asymmetric top molecules. It is also shown that in the low-barrier limit, torsionally inelastic cross sections depend only on Δn, the change in the angular momentum of the internal rotor, and not on the particular initial and final n. The IOS factorization expression shows that in the low-barrier limit, all of the dynamical information for E↔E transitions is contained in the cross sections for excitation out of the J=0, K=0, υ=0 ground state.
Variational optimizations in the Rice–Ramsperger–Kassel–Marcus theory calculations for unimolecular dissociations with no reverse barrier
Authors: Klippenstein Stephen J.
Published: July 2002

A method is described for variationally optimizing not only the value of the reaction coordinate but also its definition in transition state theory calculations for reactions without a barrier. In this method the reaction coordinate is assumed to be described by the distance from a point fixed in one of the fragments to another point fixed in the other fragment. For linear fragments the fixed points are chosen along the fragment axes whereas for nonlinear fragments each fixed point may be chosen anywhere within a three-dimensional fragment-fixed coordinate system. Results of the variational optimization of the distance and the choices for the fixed points are reported for the dissociation of NCNO into CN and NO. The optimized reaction coordinate is seen in this case to correspond to each of the fixed points being outside of the actual fragment towards the overall center-of-mass. Comparison is made with previous calculations based on bond length and center-of-mass separation distance reaction coordinates, these latter two reaction coordinates being specific cases of the present general reaction coordinate.
Energy- and angle-resolved measurements of the Rh(4F9/2) and Rh(4F7/2) populations from ion bombarded Rh{100}
Authors: Winograd N. , El-Maazawi M. , Maboudian R. , Postawa Z. , Bernardo D. N. , Garrison B. J.
Published: July 2002

The first energy- and angle-resolved measurements are presented for ground and excited state atoms ejected from a single crystal metal surface due to keV ion bombardment. These results show that at high velocities the ratio of the excited state to ground state intensity varies as exp(−A/av⊥) but at lower velocities the ratio is almost independent of velocity. We use a collisional excitation model to show conclusively that the details of the atomic motions are necessary to explain the experimental data.
Quantum free-energy calculations: A three-dimensional test case
Authors: Topper Robert Q. , Tawa Gregory J. , Truhlar Donald G.
Published: July 2002

An optimized integration scheme for calculating vibrational–rotational partition functions by the Fourier path-integral method, as presented in the previous paper [R. Q. Topper and D. G. Truhlar, J. Chem. Phys. XX, ▪▪▪▪ (19▪▪)] is applied to a three-dimensional test case involving the coupled vibrational and rotational motions of a diatomic HCl molecule in Cartesian coordinates. Converged partition functions are calculated by the new Fourier path-integral Monte Carlo scheme and by standard variational methods, and the two sets of results are compared. We obtain good agreement (∼2%) between the two methods over a range of a factor of 20 in temperature.
Calculation of the polarizability and hyperpolarizability tensors, at imaginary frequency, for H, He, and H2 and the dispersion polarizability coefficients for interactions between them
Authors: Bishop David M. , Pipin Janusz
Published: July 2002

Accurate calculations of the polarizability and hyperpolarizability tensors at imaginary frequency, α(−iω;iω) and γ(−iω;iω,0,0), for H, He, and H2 are reported for a range of frequencies (ω) useful for Gauss–Legendre quadrature. They have been used to evaluate the dispersion polarizability coefficients which govern the nonclassical contribution to the change in electronic polarizability due to long-range interactions between the aforementioned species. Previously, these coefficients have only been found by more approximate methods. The basis of the calculations of α(−iω;iω) and γ(−iω;iω,0,0) was the sum-over-states method and, for He and H2, electron correlation was explicitly taken into account. With respect to γ(−iω;iω,0,0), we believe these to be the first calculations of any kind.
Core correlation and the binding energy of Sc2
Authors: Åkeby Hans , Pettersson Lars G. M. , Siegbahn Per E. M.
Published: July 2002

The dissociation energy of the Sc2 dimer has been computed using a multireference CI treatment and including the effects of core(3p) correlation. Special emphasis has been put on the contraction of the basis set to minimize atomic correlation energy loss and molecular superposition errors. Core(3p) correlation gives a contraction of the 4s orbital resulting in a reduced ratio between the radial extents of the 4s and 3d orbitals. This results in a decreased bond distance (by 0.20 a0) and improved 3d–3d overlap. The binding energy is increased by 0.11 eV through core correlation. The finally obtained De is 0.77 eV (1.1±0.2 eV expt.). An extension of the core-polarization potential formalism to include field-gradients and quadrupole core polarizabilities is presented and compared with the results of explicit core correlation for both the atom and Sc2.
Electron paramagnetic resonance line shapes of photoexcited triplets with rotational diffusion
Authors: Gamliel Dan , Levanon Haim
Published: July 2002

A new line shape formula is developed for magnetic resonance on systems which are initially far from thermal equilibrium. A formalism for treating electron paramagnetic resonance (EPR) of rotationally diffusing molecules is extended to include typical triplet states, in which intramolecular interactions are not very small compared with the Zeeman interaction. The two developments are combined to a new formalism for calculating line shapes of photoexcited triplet states undergoing rotational diffusion. Approximations are made for parts of the calculation, and their range of validity is examined. Simulations based on this theory for randomly oriented samples of chlorophyll a (Chl a) demonstrate the usefulness of the theory in calculating line shapes for both equilibrated and polarized photoexcited EPR triplets undergoing rotational diffusion. In particular we study the effect of anisotropy in the molecular diffusion tensor on the line shapes. A comparison is also made between simulated and experimental line shapes for two expanded porphyrinoids, cadmium–texaphyrin complex (TxP–Cd), and zincoctaethylporphycene (ZnOEPC). Finally, we present a series of dynamic line shapes for the photoexcited triplet state of C60 (buckminsterfullerene), discussing their correspondence with available experimental results.
Far-infrared spectra of diatomic polar molecules in simple liquids: Accounting for the anisotropic interaction
Authors: Roco J. M. M. , Hernández A. Calvo , Velasco S.
Published: July 2002

A theoretical expression giving the far-infrared spectrum of a dilute solution of diatomic polar molecules in a nonpolar liquid solvent is derived in order to analyze the influence on this spectrum from the J=1 and J=2 components of the Legendre polynomial expansion of the anisotropic solute–solvent potential. A non-Markovian spectral theory incorporating finite correlation of the interaction, mixing effects between rotational lines and quantum intermolecular potential correlation functions is used. This theory allows one to analyze the influence of each anisotropic part of the interaction in terms of a very reduced set of parameters: the strength and the width of its time correlation function. The zero and finite frequency components of the J=2 contribution are also studied. In particular, we show that the use of quantum potential correlation functions gives a zero frequency component not only in the width, but also in the shift of the different rotational lines. Numerical results for DCl, HCl, and HF in SF6 liquid at 273 K are obtained.
Rydberg∼valence perturbations in matrix-isolated NO
Authors: Chergui Majed , Schwentner Nikolaus
Published: July 2002

The absorption spectrum of NO in rare gas matrices has been reinvestigated. The valence B 2Π(v′,0) bands which, in the gas phase, show perturbations for v′≥7 due to homogeneous configuration mixing with the Rydberg C 2Π state, appear unperturbed up to v′=10 in Xe, 12 in Kr, 14 in Ar, and 19 in Ne matrices. The perturbations are shifted to higher energies due to the large blue matrix shifts experienced by the Rydberg states. They appear as irregular vibrational spacings, anomalous intensities, as well as modifications of the B 2Π(v′,0) band profiles. The intensity patterns are qualitatively explained in terms of quantum-mechanical interference effects. When resonant with a multiphonon Rydberg band, Fano line shapes appear in place of the sharp B(v′,0) valence lines. The line shapes have been simulated by means of the Fano theory assuming that only part of the Rydberg multiphonon quasicontinuum interferes with the sharp valence band. Good agreement is obtained with the experimental spectrum and the fitted Fano indices lie close to the calculated ones.
Crossed beam studies of four-atom reactions: The dynamics of OH+D2
Authors: Alagia Michele , Balucani Nadia , Casavecchia Piergiorgio , Stranges Domenico , Volpi Gian Gualberto
Published: July 2002

The reaction OH+D2→HOD+D was studied in a crossed beams experiment at a collision energy of 6.3 kcal/mol. Center-of-mass translational energy and angular distributions were determined. The HOD product shows a very pronounced backward scattering. Thirty-two percent of the available energy is released as product translational energy.
Third- and fourth-order analysis of orbit- and spin-forbidden transitions in two-photon spectroscopy of lanthanide compounds
Authors: Ceulemans A. , Vandenberghe G. M.
Published: July 2002

The role of spin–orbit and crystal field interactions in the intensity mechanism for orbit- and spin-forbidden transitions in two-photon absorption spectroscopy of lanthanide compounds is investigated. The existing third-order analysis of the crystal field contribution to the transition moment has been restated in a more compact way using a 9j symbol. New general expressions for the fourth-order combination of spin–orbit and crystal field interactions are developed. The highest recoupling is found to require a 12j symbol. The associated selection rules allow a distinction between the various coupling mechanisms.
Boundary effects in a surface reaction model for CO oxidation
Authors: Brosilow Benjamin J. , Gulari Erdogan , Ziff Robert M.
Published: July 2002

The surface reaction model of Ziff, Gulari, and Barshad (ZGB) is investigated on finite systems with ‘‘hard’’ oxygen boundary conditions. The rate of production of CO2 is calculated as a function of y and system size. When the rate of CO adsorption y is above the first-order transition value y2, the reactive region is found to extend into the system a distance ξ which scales as (y−y2)−0.40 when y→y2.
A quantitative electron-spin resonance line shape study of the local motion in the micellar and liquid crystalline lamellar phases of the oleoyllysolecithin water system
Authors: Liang Zhichun , Westlund Per-Olof , Wikander Göran
Published: July 2002

The experimental electron-spin resonance (ESR) line shapes of the micellar (L1) and lamellar (Lα) phases of the binary system 1-oleoyl-sn-glycero-3-phosphocholine (OlLPC)/water have been interpreted quantitatively using a two-dynamic (TD) model. This dynamic model comprises the local dynamics of Freed’s model [J. Chem. Phys. 55, 5270 (1971); 58, 3185 (1973); 77, 3915 (1982)] and a second Brownian motion referring to the combined effect of the micellar reorientation and the translational motion of the amphiphile molecule along the micellar surface. With this model, we focus our attention on a comparison of the local dynamics between the L1 and Lα phases. It is found from the TD model that the local ordering (λ) and dynamics (τ⊥) of the L1 phase change compared with those of the Lα phase according to λ(L1)≳λ(Lα) and τ⊥(L1)<τ⊥(Lα). The observed lower local ordering in the lamellar phase compared with that in the micellar phase may be interpreted in terms of the different packing constraints of the OlLPC molecule in the two phases.
The scaling exponents of polyacrylamide and acrylamide-sodium acrylate copolymer gels
Authors: Hu Zhibing , Li Chunfang , Li Yong
Published: July 2002

The ratio of bulk modulus to shear modulus, Ke/Ge, of polyacrylamide (PAAM) gels was studied. This ratio is directly related to a scaling exponent, δ, which is related to the concentration dependence of both bulk modulus and shear modulus. It was found that δ depends on the details of chemical ingredients and process. It increases as the crosslinker or monomer concentration increases. The behavior of acrylamide/sodium acrylate gel was found to be dramatically different from the nonionic gels. As sodium acrylate (SA) concentration increases from zero to 2 mol %, δ decreases from 2.09 to 1.08. Further increase of SA results in the formation of surface pattern and the value of δ increases slightly, eventually reaching a plateau around 1.25.
Ultraviolet Raman scattering from simple alcohols and a related diol as a study of low-lying Rydberg-valence state characteristics
Authors: Bonang C. C. , Anderson D. J. , Cameron S. M. , Kelly P. B. , Getty J. D.
Published: July 2002

Relative Raman scattering cross sections of totally symmetric vibrational modes of liquid methanol, ethanol, and ethylene glycol are measured as a function of excitation wavelength in the preresonant spectral region 37 593–47 393 cm−1. Raman spectra are also obtained with 50 043 and 54 198 cm−1 excitation, the latter energy being directly resonant with the first excited state of the alcohols. The data indicate that the first three excited electronic states (with transition energies of approximately 54 600, 62 500, and 66 000 cm−1, respectively) make no significant contribution to the Raman scattered intensities and are nearly entirely Rydberg in character. The low-lying electronic structure of ethylene glycol appears to be primarily Rydberg in character as well. The dominant C–H and O–H stretching modes gain intensity largely from valence state(s) with transition energies near 70 000 cm−1, while the C–O stretching and CH2,3 bending modes gain intensity from even higher energy states (∼85 000 cm−1).
Investigation of the β–γ phase transition in single crystal malononitrile by polarized Raman spectroscopy
Authors: Bonafede S. J. , Eckhardt C. J.
Published: July 2002

Temperature dependent, polarized Raman spectra in the lattice mode region for sublimed, single crystal malononitrile through the β–γ phase transition (Tc=294.7 K) are reported. Symmetry assignments and frequencies of vibrational modes are made and compared to previous studies. Nine out of 12 possible lattice modes are observed in each phase with three of the observed modes being affected by the phase transition. The temperature dependence of these three affected modes is attributed to cyano group and molecular dipole interactions between molecules in the crystal. The behavior of malononitrile through the β–γ phase transition is analogous to the smectic A↔nematic phase transition reported for cyano-based, liquid crystal systems.
Chain dimensions near the critical point
Authors: Biswas Parbati , Cherayil Binny J.
Published: July 2002

We calculate the average end-to-end distance 〈R2〉 of a polymer in a semidilute solution that is near the temperature Tc at which phase separation occurs. The calculation is carried out within the usual canonical partition function formalism, the Hamiltonian of the system being taken to comprise a reference term, in which the chains are represented as collapsed coils, and a perturbation, which originates in repulsive excluded volume interactions between different monomers. The description of the reference state employs the fractional Brownian walk approach developed in an earlier paper, while the perturbation is modeled by delta function pseudopotentials. The treatment of excluded volume follows the methods developed by Edwards, Singh, and Jeffers, which make use of the equations derived for an effective step length and an effective monomer–monomer potential to determine various polymer properties. In this way, we find that near Tc, R scales with chain length N as N0.462.
Variational principles for the time-independent wave-packet-Schrödinger and wave-packet-Lippmann–Schwinger equations
Authors: Kouri Donald J. , Huang Youhong , Zhu Wei , Hoffman David K.
Published: July 2002

Several variational principles, whose Euler equations are the recently derived time-independent wave-packet-Schrödinger or wave-packet-Lippmann–Schwinger equations, are presented. A particularly attractive wave-packet-Kohn variational principle for either the T- or S-matrix is given which yields inhomogeneous algebraic equations whose ‘‘universal inhomogeneity’’ does not depend explicitly on the collision energy. The validity of the approach is demonstrated with calculations for two simple one dimensional scattering problems and for the collinear H+H2 reactive scattering problem.
Radial dimensions of starburst polymers
Authors: Biswas Parbati , Cherayil Binny J.
Published: July 2002

Radial properties of starburst polymers are calculated by renormalization group techniques starting from the Edwards model of the chain. The calculations are carried out for a polymer in good solvents grown out to an arbitrary number of generations g and having an arbitrary branch functionality f. Excluded volume effects are modeled by delta function pseudopotentials. Only pair interactions are included in the calculations, which specifically determine the amplitude of the average center-to-end distance R of the starburst for definite values of f and g. Our first order in ε estimates of the exponents for R and the number of configurations C coincide with results obtained earlier by direct methods for networks of arbitrary topology in specific limits.
Time-resolved multiphoton ionization study of the 102 nm state of NO
Authors: Owrutsky J. C. , Baronavski A. P.
Published: July 2002

We have used subpicosecond deep UV laser pulses (near 205 nm) in a time-resolved (2+1) multiphoton ionization study of the two-photon state of NO at 102 nm via a one-photon resonance with the A 2Σ+(3sσ) (ν=2) state. The two-photon excited state lifetime exhibits a strong pressure dependence from which the collisionless lifetime and the bimolecular depopulation or quenching rate constant were obtained. The large quenching rate constant strongly suggests that the state is a medium (10–12) n Rydberg level in the series converging to the ground electronic state of NO+. The subnanosecond collisionless lifetime implies that this state is subject to predissociation and/or autoionization.
Excited potential energy surfaces of CH3SH from theab initioeffective valence shell Hamiltonian method
Authors: Stevens Jonathan E. , Freed Karl F. , Arendt Michael F. , Graham Richard L.
Published: July 2002

The ground and first and second 1A″ potential surfaces of methyl mercaptan (CH3SH) are calculated as a function of the C–S and S–H bond coordinates using the ab initio effective valence shell Hamiltonian (Hν) method. The computations for this highly nontrivial system provide the first serious tests for choosing restricted valence spaces and for computing global potential energy surfaces with the Hν methods. The quasidegeneracy constraints on the Hν method suggest choosing a valence space which consists of the two (3a″ and 10a′) highest energy occupied orbitals in the ground state and the three lowest a′ unoccupied orbitals. The global potential surfaces are computed with a modest basis, but larger basis set computations at selected geometries test convergence for vertical excitation energies, ionization potentials, and C–S and S–H bond energies. The calculations are compared to both experiment and other calculations for this system. The computations assist in the interpretation of CH3SH photodissociation dynamics observed by Butler and co-workers.
Off-specular reflection from flat interfaces
Authors: Wu Wen-li
Published: July 2002

The reflection of x ray or neutron from a flat interface with density or compositional fluctuations was studied. An integral equation relating the reflection intensities to the interfacial fluctuations was derived in the Fraunhofer limit. At the specular condition this integral equation, although derived for the off-specular intensities, has an expression identical to a well-established one for the specular component. This integral equation was applied to a flat surface onto which gold spheres of uniform size were randomly adhered. The out-of-plane components for both the real wave and the time reversed wave were approximated by the solution from the specular component. Based on the calculated off-specular intensities near the specular region, an approximation scheme was proposed to estimate the in-plane correlation length.
Variational upper and lower bounds on quantum free energy and energy differences via path integral Monte Carlo
Authors: Hogenson Gordon J. , Reinhardt William P.
Published: July 2002

A newly developed variational method for obtaining upper and lower bounds on free energy changes is extended to quantum systems. The convergence of the upper and lower bounds is demonstrated for simple test cases in one and three dimensions, using various path integral Monte Carlo methods, including the ‘‘bead’’ method, the Fourier component method, and multigrid techniques.