bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic spin-echo studies of triplet biacetyl under high pressure
Authors: Chan I. Y. , Qian X. Q.
Published: July 2002

Optically detected two-pulse and three-pulse echo experiments have been performed under pressure to 50 kbar using a diamond anvil cell. In this paper we report the results on excited triplet neat biacetyl (BA) and BA-d6. Three-pulse echoes are easier to perform, and coherence has been maintained for durations longer than one triplet lifetime. In contrast, the two-pulse echo technique is prone to have spurious contributions, and it is besieged by the envelope modulation effect for BA-d6. Our data point to the importance of spectral diffusion caused by the bulk nuclei. Their flip–flop motion creates a fluctuating magnetic field on the triplet spin, destroying its coherence. As pressure increases, an increase in number density of such nuclei shortens the phase memory time (TM). A plot of TM vs relative volume produces a linear correlation. Our three-pulse echo lifetime shows a 40% increase from BA-h6 to BA-d6. This very small deuteration effect might suggest that electronic flip–flop, rather than nuclear spin flip–flop, is the dominant relaxing mode. However, a study of TM on different exciting light levels disproves this mechanism.
Structure and decomposition path of the HIF radical
Authors: Kucharski Stanislaw A. , Bartlett Rodney J.
Published: July 2002

The potential-energy surface for the inorganic free-radical, HIF, known only from molecular-beam studies, is investigated using fourth-order many-body perturbation theory [MBPT(4)]. All-electron calculations with three different basis sets are compared with results using relativistic and nonrelativistic effective potentials on I. Two local minima are found with an H–I–F angle α equal to 137.5° and 82°, with the energy difference ranging from 2.3 to 5.1 kcal/mol depending upon the calculation. The barrier to internal conversion is 1.1 kcal/mol. From the lowest minimum our MBPT(4) calculations predict HIF to be bound to dissociation (De) to IF+H by 27.0 to 32.6 kcal/mol, and to HI+F dissociation by 22.1 to 30.9 kcal/mol, in good agreement with experiment. However, very large superposition errors are shown to be possible in all-electron calculations because of inevitable basis-set deficiencies for the I atom. Explanations for the double minima in HIF are presented.
Constrained Newton approach adequate to direct self-consistent field calculations in closed- and open-shell configurations
Authors: Sano Tatsuji , I’Haya Yasumasa J.
Published: July 2002

The quadratically convergent approach to solve self-consistent field (SCF) solutions for closed- and open-shell configurations is derived from the Lagrange–Newton method with constraints of the molecular orbital orthonormality. The resulting Lagrangian supermatrix is expressed in a simple manner in terms of atomic orbital (AO) integrals. The present scheme realizes a decrease in computational cost when used with secondary memory to save the Lagrangian supermatrix. Furthermore, the direct SCF approach of the proposed scheme may be efficient to cut down computational cost when used with a preconditioner useful to improve slow convergence in each Lanczos iteration process. The present scheme converges nearly quadratically to a SCF solution when once a good starting point is chosen. The initial estimates of the Lagrange multipliers play a relatively minor role in the convergence rate. As an illustration of the proposed procedure ab initio calculations for the ground and excited states of CO and the ground states of BO and HCN are carried out. The results are compared with those of the Newton–Raphson SCF approach based on the exponential parametrization of orbital rotations.
Dynamic light scattering by aqueous solutions of rodlike fd-virus particles
Authors: Graf C. , Deggelmann M. , Hagenbüchle M. , Kramer H. , Krause R. , Martin C. , Weber R.
Published: July 2002

Dynamic light scattering (DLS) measurements on aqueous solutions of rodlike fd-virus particles (length=880 nm, diameter=9 nm) below and above the overlap concentration c*=1 particle/length3 are reported. In samples with screened Coulomb interaction the short-time behavior of the time correlation functions does not show a significant concentration dependence, in contrast to samples at very low ionic strength (strong Coulomb interaction), where the short-time behavior is dominated by the static structure factor S(q). In this case S(q) derived by DLS methods, using an equation that has been proved for interacting spherical particles and weakly interacting rods, is in excellent agreement with S(q) determined by static light scattering (SLS) up to a particle concentration of about 5c*. Above significant deviations are found. At very low ionic strength the existence of a distinct very long tail in the time correlation function as previously reported can be confirmed, but the new experiments suggest that this is due to dust particles and aggregates and not to the interaction between the fd particles.
Remarks on nonequilibrium contributions to the rate of chemical reaction in the Lorentz gas
Authors: Cukrowski A. S. , Popielawski J. , Stiller W. , Schmidt R.
Published: July 2002

The conception of nonequilibrium Shizgal–Karplus temperature is analyzed for a chemical reaction in three component Lorentz gas. The results for nonequilibrium contributions to the rate of chemical reaction obtained by this conception are similar to those obtained from perturbation method.
Experimental determination and calculation of the collision relaxation rates in the 5 2P1/2 and 5 2P3/2 levels of atomic iodine
Authors: Cerny D. , Bacis R. , Bussery B. , Nota M. , Vergès J.
Published: July 2002

Pressure broadening of the hyperfine lines of the 2P1/2 –2P3/2 atomic iodine transition is analyzed from the emission line shapes. Collisions with Ar and O2 in the 0–100 Torr range were examined as well as pressure broadening due to I2 in the 0–15 Torr range. Pressure broadening coefficients α were determined: α (O2)=4.8 (3) MHz/Torr and α (Ar)=3.6 (3) MHz/Torr. In the case of I2, α (I2) is estimated to be (35±10) MHz/Torr in the 0–5 Torr range. In addition, pressure broadening coefficients were calculated using the impact approximation. In this approximation relaxation cross sections are calculated between hyperfine levels of the I atom. Estimates of the relaxation rates between the hyperfine levels of I(2P3/2) and I(2P1/2) are deduced from these results, for both elastic and inelastic processes. Experimental and theoretical results are in reasonable agreement.
Bound→free emission spectra and photoassociation of 114Cd2 and 64Zn2
Authors: Rodriguez G. , Eden J. G.
Published: July 2002

Structured bound-free emission spectra of 64Zn2 and 114Cd2 have been observed by photoassociating pairs of ground state atoms (ns2 1S0, n=4 and 5 for Zn and Cd, respectively) at several discrete wavelengths in the 222–277 nm region. The observed fluorescence is attributed to B 1Σ+u→X 1Σ+g transitions of the molecules where the bound excited state is correlated with np 1P1+ns2 1S0 in the separated atom limit. The pressure dependence of the emission intensity demonstrates that the dominant 1Σ+u production mechanism is photoassociation. Numerical, quantum mechanical simulations of the experimental spectra yield 1Σ+u spectroscopic constants that are generally ∼10% larger than ab initio values in the literature.
Density and correlation functions for the two-component plasma at Γ=2 near a metal wall
Authors: Forrester P. J.
Published: July 2002

The density profiles and correlation functions are computed exactly, at the special coupling Γ=2, for the two-dimensional two-component plasma separated by an impermeable membrane of width ε from a metal wall. In a grand canonical formalism, the Hamiltonian used includes the image forces due to the metal wall. This provides a direct calculation of results obtained previously as a limiting case of the ideally polarizable interface [F. Cornu and B. Jancovici, J. Chem. Phys. 90, 2444 (1989)], and extends those results.
Laser induced crossed-beam charge transfer: Collision energy effects of the Na(3 2P3/2,1/2)+I2→Na++I−2 system
Authors: Bañares L. , Velarde M. G. , González Ureña A.
Published: July 2002

A crossed-beam charge transfer study was carried out for the Na(3 2P3/2,1/2)+I2→Na++I−2 system using laser excitation of the Na atom into its two spin–orbit states. The collision energy effects including the energy threshold and post-threshold laws were measured for both spin–orbit states.
Isotopic mixing in surface reactions
Authors: Dumont Martine , Sente B. , Dufour P. , Dagonnier R.
Published: July 2002

The isotopic exchange surface reaction A2+B2→2nAB+(1−n)A2+(1−n)B2 is investigated within (i) a model with direct dissociative adsorption and (ii) models with one-site (or bridged) molecular adsorption precursor states. These three kinetic schemes involve the various associative desorption mechanisms as well as the diffusion of the adatoms and, possibly, of the admolecules at the surface. The effect of these diffusions on the equilibration coefficients is discussed within the pair-mean-field approximation and, when possible, by means of Monte Carlo simulations.
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o theoretical study of arsine and trimethylgallium: The formation of GaAs by a stable adduct
Authors: Graves Richard M. , Scuseria Gustavo E.
Published: July 2002

Theoretical calculations for the closed-shell ground state of arsine (AsH3), trimethylgallium (TMG) [Ga(CH3)3], and the arsine–TMG adduct [H3As⋅Ga(CH3)3] are carried out at the self-consistent field (SCF) Hartree–Fock level of theory. In addition, the SCF results are compared with theoretical predictions obtained at the coupled cluster level of theory including all single and double excitations (CCSD). The equilibrium structure of arsine is found to be of C3v symmetry, while TMG and the adduct are found to be of C3 symmetry. Ab initio calculations are also performed on D3h gallane (GaH3) and the gallane–arsine adduct (H3Ga⋅AsH3) of C3v symmetry. The adduct binding energies and vibrational frequencies (SCF) are also obtained resulting in two stable bonded adduct species—arsine⋅TMG and arsine⋅gallane. Finally, our theoretical predictions support a slightly exothermic gas-phase reaction yielding GaAs through a TMG⋅AsH3 adduct which is formed without an activation barrier.
Negative differential capacities in electrical double layers
Authors: Torrie G. M.
Published: July 2002

Monte Carlo simulations of an electrical double layer in a primitive model of a 0.05 M 1:2 electrolyte are reported in which the total potential drop across the interface decreases as the surface charge is increased. This violates an alleged exact condition for such systems which has, however, recently been called into question by Attard et al. [J. Chem. Phys. 96, xxxx (1992)] and confirms their analysis that the surface potential need not be a monotone function of the surface charge.
Critical fluctuations and molecular dynamics at liquid-crystalline phase transitions. I. Theoretical aspects of the nematic–smectic-A transition
Authors: Freed Jack H.
Published: July 2002

A theoretical model is developed for treating molecular dynamics at the nematic–smectic-A (N–SA) phase transition, which is frequently second order. This model is motivated by electron-spin-resonance (ESR) spin-relaxation studies of molecular probes. The critical dynamics of the hydrodynamic modes is described in accordance with dynamic scaling arguments of Brochard. Following Zager and Freed, the molecular dynamics of a probe molecule (governed by the molecular orientation and/or rotational diffusion) is assumed to couple to fluctuations in the smectic order parameter, because these molecular properties are a function of the precise location of the probe within the transient smecticlike layer. Two limiting cases of (1) (nearly) free translational diffusion of the probe across the smecticlike layer; and (2) expulsion of the probe to the aliphatic chains with highly hindered diffusion (i.e., jump diffusion) across the smecticlike layer are considered. The relevant spectral density shows critical types of divergence, where the exponent depends strongly on the details of the model. It is found that only the (near) zero-frequency spectral densities can show such divergences. It is pointed out that spectral densities available for spin relaxation do not truly diverge as the N–SA transition is approached arbitrarily closely, because ultimately motional-narrowing theory will no longer be valid, and fluctuations begin to be frozen on the ESR time scale. This matter is briefly analyzed. Also considered briefly are the effects of anisotropies in the smectic phase and of fluctuations in nematic director near the N–SA transition.
The infrared photofragmentation of Ar+2. Evidence of excited state population from dimer and cluster ionization
Authors: Woodward C. A. , Whitaker B. J. , Knowles P. J. , Stace A. J.
Published: July 2002

Argon dimer ions have been generated via three different techniques: (1) autoionization; (2) vertical ionization of neutral Ar2; (3) ionization and subsequent fragmentation of argon cluster ions. In experiments (2) and (3) the dimers and clusters are formed via the adiabatic expansion of argon in a supersonic beam. In each case Ar+2 ions have been mass selected and subjected to single-photon infrared excitation (912–1094 cm−1) using a line-tunable carbon dioxide laser in a crossed-beam arrangement. Only those Ar+2 ions with internal energies within 1000 cm−1 of a dissociation limit yield Ar+ photofragments, the kinetic energy spread of which has been measured using an electrostatic analyzer. The photofragment kinetic energy spectra of dimer ions formed by autoionization do not exhibit any dependence on the angle of laser polarization; it is proposed that such behavior is due to the presence of a high thermal rotational temperature (500 K). In contrast, the corresponding spectra of Ar+2 formed via vertical ionization, exhibit two quite distinct features, one of which shows a strong dependence on laser polarization angle. Calculations show that the latter behavior is most probably due to photodissociation out of an excited spin–orbit state of Ar+2. A very pronounced increase in Ar+2 infrared photodissociation signal is observed as a function of increasing nozzle stagnation pressure. To account for such behavior it is proposed that, following ionization, argon cluster ions fragment to give dimer ions in excited vibrational/rotational levels both in the electronic ground and an excited spin–orbit state.
The optimization of single mode basis functions for polyatomic vibrational problems with application to the water molecule
Authors: Schwenke David W.
Published: July 2002

We consider the optimization of the wave functions for coupled vibrations represented by linear combinations of products of functions depending only on a single vibrational coordinate. The functions themselves are optimized as well as the configuration list. For the H2O molecule, highly accurate results are obtained for the lowest 15 levels using significantly shorter expansions than would otherwise be possible.
Negative and conventional Poisson’s ratios of polymeric networks with special microstructures
Authors: Wei Gaoyuan
Published: July 2002

A theoretical model for evaluating effective Poisson’s ratios of polymeric networks with special microstructures has been developed, which takes into account both stretching and bending deformation mechanisms. It is a complete generalization of the formalism first presented by Warren and Kraynik [Mech. Mater. 6, 27 (1987) and J. Appl. Mech. 55, 341 (1988)] for analytically calculating effective elastic properties of certain types of polymeric foams. Structural anisotropy in two special microstructures considered is found to have very significant effects on both magnitude and sign of the effective Poisson’s ratios of the macroscopically isotropic network materials containing them. As two limiting cases, the effective Poisson’s ratios of random assemblies of 2D and 3D bars or rods are shown to be 1/3 and 1/4, respectively, the latter of which recovers the well-known Poisson’s result.
Resonance Raman spectroscopy of the B1u region of benzene: Analysis in terms of pseudo-Jahn–Teller distortion
Authors: Sension Roseanne J. , Brudzynski Richard J. , Li Shijian , Hudson Bruce S. , Zerbetto Francesco , Zgierski Marek Z.
Published: July 2002

Raman spectra of benzene and benzene-d6 vapor obtained with excitation in the range from 208 to 188 nm are presented and discussed in terms of vibronic coupling of the resonant B1u state and the nearby E1u electronic state. The Raman spectra show strong activity in the binary overtones and combinations of the e2g modes ν8, ν9, and ν6 with the ν8 activity being dominant. (The Wilson numbering scheme for the modes of vibration is used throughout.) These bands, plus a progression in ν1 alone or in combination transitions involving two quanta of e2g modes, constitute the major intensity bands in the spectra. A simple first-order vibronic coupling model can be constructed which accounts adequately for most of the intensity in the Raman spectra observed in resonance with the B1u state, but this model results in a calculated absorption spectrum that is qualitatively different from that observed. The nature of the discrepancy indicates a significant Duschinsky mode rotation in the B1u state relative to the ground state. An analysis of the vibronic coupling of the B1u state with the nearby E1u state is performed using semiempirical calculation methods to provide excited state geometries, vibrational frequencies, and vibronic coupling parameters.

This analysis confirms that there is strong vibronic coupling between these states resulting in a pseudo-Jahn–Teller effect. This results in a highly distorted potential surface for the B1u state with three D2h symmetry antiquinoidal minima lower in energy by ∼1000 cm−1 than the D6h symmetry geometry. The three minima of the threefold symmetric potential surface are separated by local maxima corresponding to the quinoidal geometry with a height of ∼300 cm−1. Three of the four e2g modes are calculated to have significant vibronic activity with mode 8 much stronger than modes 6 or 9. A small Jahn–Teller coupling is calculated. A strong Duschinsky rotation results from the vibronic activity of more than one mode. The absorption spectrum and the resonance Raman spectra calculated from this model are in good agreement with the experimental results. This analysis provides the first evidence for the expected pseudo-Jahn–Teller instability of the isolated 1B1u state of benzene.
A theory of electron baths: One-electron system dynamics
Authors: McDowell H. Keith
Published: July 2002

The second-quantized, many-electron, atomic, and molecular Hamiltonian is partitioned both by the identity or labeling of the spin orbitals and by the dynamics of the spin orbitals into a system coupled to a bath. The electron bath is treated by a molecular time scale generalized Langevin equation approach designed to include one-electron dynamics in the system dynamics. The bath is formulated as an equivalent chain of spin orbitals through the introduction of equivalent-chain annihilation and creation operators. Both the dynamics and the quantum grand canonical statistical properties of the electron bath are examined. Two versions for the statistical properties of the bath are pursued. Using a weak bath assumption, a bath statistical average is defined which allows one to achieve a reduced dynamics description of the electron system which is coupled to the electron bath. In a strong bath assumption effective Hamiltonians are obtained which reproduce the dynamics of the bath and which lead to the same results as found in the weak bath assumption. The effective (but exact) Hamiltonian is found to be a one-electron Hamiltonian. A reduced dynamics equation of motion for the system population matrix is derived and found to agree with a previous version. This equation of motion is useful for studying electron transfer in the system when coupled to an electron bath.
The spectroscopic and photophysical effects of the position of methyl substitution. II. 2-methylpyrimidine
Authors: Bandy Ralph E. , Garrett Aaron W. , Lee H. D. , Zwier Timothy S.
Published: July 2002

Laser-induced fluorescence excitation and dispersed fluorescence spectra of the first n–π* transition of jet-cooled 2-methylpyrimidine have been recorded and analyzed. This work extends our earlier study of the spectroscopic and photophysical effects of methyl substitution in 4- and 5-methylpyrimidine. An unusual Fermi resonance involving the 6an0 progression forms the focus of the present study. The 6a10 vibronic transition is observed to be split into a triad of transitions. Dispersed fluorescence spectra are used to identify the dark background state responsible for the Fermi resonance coupling as the 16b1(3a′′2) vibration/internal rotation combination level. This level is selectively coupled by symmetry constraints to 6a1(0a1), leaving the 6a1(1e″) level unperturbed. The positions and intensities of the triad of peaks in the excitation spectrum allow a quantitative determination of the 6a1(0a′1)↔16b1(3a2) coupling matrix element of V=4.1 cm−1. This vibration/internal rotation Fermi resonance is thus typical of the new types of routes to vibrational state mixing which are opened by methyl substitution. Higher members of the 6an0 progression are also involved in Fermi resonance mixing. However, in addition, these levels experience weaker, less state-specific coupling to a bath of same-symmetry states at that energy. The excitation spectrum provides an estimate of the average coupling matrix element of this second tier coupling of ∼1 cm−1.
A rigid-ion model of the structure, vibrational, and elastic properties of the tetragonal alkali-azide crystals KN3, RbN3, and CsN3
Authors: Zieliński Piotr , Marzluf Christian
Published: July 2002

Known variants of the Born model have been used to calculate all the vibrational frequencies at the Brillouin zone center in the tetragonal, orientationally ordered phase I4/mcm of KN3, RbN3, and CsN3. The model with the steric potential of the azide ion represented by two ellipsoidal repulsive centers has given the best agreement with the experimental structure and frequencies. Full tensor of the static effective elastic constants has been evaluated for these materials. A very good agreement has been found between the calculated and experimental pressure dependence of the Raman active vibrational frequencies. A softening of the translational Eg mode has been found in KN3 above the pressure of 30 kbar.
Band structures, density of states, and Fermi surfaces of KxC60, x=1,2,3,6
Authors: Huang Ming-Zhu , Xu Yong-Nian , Ching W. Y.
Published: July 2002

The first-principles orthogonalized linear combination of atomic orbitals method (OLCAO) in the local density approximation is used to study the electronic structure-density of states (DOS), and Fermi surface (FS) of superconducting potassium carbides. (AIP)
A guided-ion beam study of the reactions of N+4 with H2, HD, and D2: An evaluation of pseudo-Arrhenius analyses of ion–molecule reaction systems
Authors: Schultz Richard H. , Armentrout P. B.
Published: July 2002

Reactions of N+4 ions with H2, HD, and D2 are studied from thermal to 5 eV kinetic energy under single-collision conditions in a guided-ion beam mass spectrometer. Reactant ions are formed in a flow tube source to ensure thermalization. Despite being exothermic by 1.5 eV, formation of N2H+ (N2D+) is observed to proceed with an activation barrier that we measure to be 0.09±0.03 eV at 0 K, independent of the hydrogen isotopomer used. Possible reaction mechanisms are discussed, and the present results are compared to previous flow and drift tube measurements. Effects of collisional reheating on the derivation of thermochemistry in drift-tube experiments are discussed. We also discuss the difficulty of deriving reliable thermochemistry from pseudo-Arrhenius plots when the mathematical form of the cross section excitation function is unknown.
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o study of the structure, vibrational frequencies and intensities of the chloroformyl van der Waals radical
Authors: Francisco J. S. , Abersold N. J.
Published: July 2002

The structure, infrared vibrational frequencies and intensities were calculated for the chloroformyl van der Waals radical using ab initio molecular orbital theory. Calculations were carried out using split valance basis sets augmented with diffuse functions and several sets of polarization functions. The most elaborate optimized geometry was calculated using the Mo/ller–Plesset perturbation theory (UMP2) with a 6-31+G(3df) basis set. The CCl and CO bond lengths are 2.865 Å and 1.135 Å, respectively. The Hartree–Fock limit of the dipole moment calculated at the UHF/6-31+G(3df) level is 0.06 D. At the UMP 2/6-31+G(3df) level the dipole moment is 0.30 D. The most reliable harmonic vibrational frequencies were calculated at the UMP2/6-31+G(3d) level. The vibrational frequencies were more sensitive to correlation corrections than to basis set effects, while the infrared intensities are much more sensitive to both basis set and electron correlation effects.
Phase separation in solutions of binary polymer mixtures
Authors: Cherayil Binny J.
Published: July 2002

An analytical model of equilibrium phase separation in a mixture of two polymers in a common good solvent is developed, and its predictions compared with the results of a lattice simulation of the same system. The model combines the Landau–Ginzburg–Wilson formalism of two earlier papers (avoiding certain ad hoc assumptions introduced there) with the known scaling results of renormalization group calculations on critical fluids to derive expressions for the exponents that characterize the amplitudes of the singular temperature functions of various thermodynamic properties of the solution. The values so obtained for these exponents compare favorably with the data from the simulation, and considerably improve upon the predictions of simple scaling analyses.
A three-dimensional potential energy surface for the reaction N+(3P)+H2(1 Σ+g) ⇔ NH+ (X 2Π)+H(2S)
Authors: Wilhelmsson Ulla , Siegbahn Per E. M. , Schinke Reinhard
Published: July 2002

Extensive ab initio calculations have been performed on the reaction N+3(P)+H21(Σ+g)⇔NH+ (X 2Π)+H(2S), using complete-active-space self-consistent-field and multireference contracted configuration interaction methods. The 752 calculated energy points belonging to the 1 3A″ surface were fitted to an analytical potential energy surface, with an average least-squares deviation of only 0.026 eV (2.5 kJ/mol). A key to this accurate fit of the surface, which does include minima as deep as 6 eV, is the use of R−n polynomials in the asymptotic region, and Rn polynomials in the deep minima regions. The calculated reaction endothermicity is 0.03 eV, after correcting for the differences in zero point vibrational energies. This is in agreement with the essentially thermoneutral reaction enthalpy, obtained in recent experiments. An important feature of the potential energy surface is the anisotropy in the reactant channel, arising from the ion–quadrupole interaction.
Theoretical investigation of asymmetric methyl rotor dynamics in partially deuterated acetophenones
Authors: Siebrand Willem , Zgierski Marek Z. , Zerbetto Francesco , Lim Edward C.
Published: July 2002

A b initio molecular orbital calculations at the HF/4-21G level, are reported of the structure, the molecular force field, and the methyl torsional potential energy curve of acetophenone in an attempt to interpret the observed tunneling and rotamer splittings in the S0→T(nπ*) excitation spectra of methyl-deuterated acetophenone-d1 and -d2. It is found that the angular dependence of the torsional kinetic energy operator does not contribute significantly to the rotamer splittings, which instead are mainly due to the angular dependence of the overall zero-point energy. The calculated zero-point energy differences between the two rotamers in the ground state are 14 cm−1 for acetophenone-d1 and 12 cm−1 for -d2. To account for the observed rotamer splittings of 7.5 and 4 cm−1, respectively, the corresponding zero-point energy difference in the triplet state must be about 9 cm−1 for both isotopomers. Comparison with experimental results for isotopomers of acetaldehyde indicates that these values are reasonable and support the notion that the level of theory used yields relevant results in the case at hand. The similarity of the zero-point difference between the rotamers in the ground and excited state contrasts strongly with the large difference in torsional barrier: 700–800 cm−1 for the ground state and 95 cm−1 for the triplet state of acetophenone-d0; the latter value is derived from the tunneling splitting observed in the excitation spectrum. These contrasting results are due to the fact that the barrier is an electronic effect whereas the rotamer splitting is a purely vibrational effect.
Multiphoton ionization studies of C6H6–(CH3OH)n clusters. II. Intracluster ion–molecule reactions
Authors: Garrett Aaron W. , Zwier Timothy S.
Published: July 2002

The neutral C6H6–(CH3OH)n clusters, which have been spectroscopically characterized in Paper I, serve here as precursors for the study of intracluster ion chemistry initiated by resonant two-photon ionization. Resonant enhancement allows ion chemistry product yields to be determined as a function of cluster size by selective excitation of a single size cluster with the laser. Most of the work presented here uses one-color resonant ionization via the 610 transition of the C6H6 chromophore in the cluster. No ion chemistry is observed for the 1:n clusters with n≤2. At n=3, dissociative electron transfer (DET) to form C6H6+M+3 (M=CH3OH) is observed with a product yield of 6%. The remaining 94% of ionic products result from fragmentation of the 1:3 cluster by loss of a single CH3OH molecule. The unprotonated M+3 product ion is unusual in that electron bombardment or photoionization of pure methanol clusters yields exclusively protonated methanol cluster ions. The attachment of a C6H6 molecule to the methanol cluster provides an extremely gentle photoionization mechanism which produces M+3 with little enough internal energy to preclude its breakup to M2H++CH2OH (or CH3O ). The opening of this product channel at n=3 is consistent with estimates of the ionization potential of Mn clusters which predict an endothermic ET reaction at n=2 which becomes exothermic at n=3.

Despite the increasingly exothermic DET and dissociative proton transfer product channels, larger clusters (n≥4) continue to predominantly undergo unreactive fragmentation. For the 1:4 and 1:5 clusters, in addition to DET products, dissociative proton transfer (DPT) products are also observed. The MnH+ product arises from proton transfer from C6H+6, while Mn−1H+ probably occurs by DPT within the M+n cluster following loss of C6H6 in DET. Scans of the 1:2–1:5 clusters through their 610110 transitions yield a broader set of products which reflect the 5 kcal/mol increase in the reactant (1:n)+ cluster internal energies.
Reorientation differential cross sections at high energy
Authors: Sisak Michael , Secrest Don
Published: July 2002

The angular momentum decoupling approximations, used extensively in approximate calculations for complex systems, are evaluated here in extensive comparisons with exact calculations for a heteronuclear system with a moderately large dipole moment at high enough energy that there are many open rotational states. Orientation-dependent differential cross sections for He–SiO rigid-rotor scattering calculations at high translational energy are presented. Coupled states and recoupled states results are compared with those of close coupling at a total energy of 27 meV. Although the coupled states (CS) method approximates exact calculations by ignoring m coupling in the body frame, the l-averaged CS results are good for orientation-dependent differential cross sections. The l-averaged form of recoupled states makes significant improvements over the l-averaged form of coupled states for orientationally elastic transitions and for large-j transitions. As in past studies, the l-initial and l-final forms of coupled and recoupled states are found to be inferior to the l-averaged form. Orientation-dependent integral and degeneracy-averaged differential and integral cross sections are also discussed.
Gas phase atomic association reactions of Ar+, Kr+, and Xe+ at temperatures near 1 K
Authors: Hawley Michael , Smith Mark A.
Published: July 2002

We report the measurement of gas phase three-body atomic ion–atom association reaction rate coefficients at temperatures near 1 K using a free jet flow reactor. The association rate coefficients for the reactions of Ar+ with Ar and Ne, Kr+ with Kr and Ar, and the reactions of Xe+ with Xe, Kr, and Ar have been measured. In all these cases except Ar++Ne, the rate at 1 K is in excess of 9×10−29 cm3/s and a strong negative temperature dependence is observed. These observations are discussed with respect to several microscopic descriptions of the association process.
A model for repulsive hard spheres with surface adhesion
Authors: Koper G. J. M. , Bedeaux D.
Published: July 2002

Baxter’s sticky hard sphere model has been extended so that it adequately describes systems where the attractive region of the interparticle potential is preceded by an energy barrier. The new feature of the model is that the fraction of aggregated particles may increase with temperature.
Fourier transform electron spin echo envelope modulation of a S=1/2, I=5/2 spin system: An exact analysis and a second order perturbation approach
Authors: Matar Khalid , Goldfarb Daniella
Published: July 2002

Electron spin echo envelope modulation (ESEEM) induced by 27Al nuclei can be used to characterize the interactions of paramagnetic transition metal cations with the framework of various aluminosilicates. The quantitative analysis of such systems is complex due to the 27Al nuclear quadrupole interaction which is often large and unknown. In order to obtain a better understanding of the various spectral features of the 27Al modulation, the ESEEM of a S= (1)/(2) I= (5)/(2) spin system in orientationally disordered systems was investigated in the frequency domain. The relative contributions of the various electron-nuclear double resonance (ENDOR) frequencies to the Fourier transform (FT) ESEEM spectrum were studied as a function of the size of the nuclear quadrupole coupling constant and of the relative orientation of the nuclear quadrupole tensor with respect to the g-tensor. The parameters range investigated is that expected from Cu2+ interacting with framework Al in zeolites. Two approaches were employed in the calculations, exact diagonalization of the nuclear Hamiltonian and a second order perturbation treatment of the quadrupole interaction. It is found that the second order perturbation approach applies up to e2qQ/h<7 MHz. The conditions under which the FT-ESEEM spectrum is dominated by the ‖ (1)/(2) 〉−‖− (1)/(2) 〉 ENDOR transitions were explored as well. This occurs when e2qQ/h is relatively large and it strongly depends on the orientation of the quadrupole tensor. When the FT-ESEEM can be described only by the ‖ (1)/(2) 〉−‖− (1)/(2) 〉 ENDOR transitions, the simulations are significantly simplified since these can be calculated using the analytical expressions obtained by perturbation theory also for a quadrupole coupling constant as large as 10 MHz.
Comment on the tilting transition in Langmuir monolayers
Authors: Cai Zhong-hou , Rice Stuart A.
Published: July 2002

We report a calculation of the free energy difference between a reference monolayer, in which the amphiphile molecules are vertical and arranged in a hexagonal lattice, and the derivative monolayer, in which the amphiphile molecules are tilted and arranged in a distorted hexagonal lattice. For the case that the interaction between amphiphile molecules, and that between an amphiphile molecule and the surface, are described by a Lennard-Jones interaction per unit length, it is found that a tilted state of the monolayer is stable only if the ratio of the amphiphile chain–surface to amphiphile chain–amphiphile chain interactions exceeds a critical value. When the area per molecule and temperature are the same, a monolayer with molecules tilting toward their nearest neighbors has a larger tilt angle and lower free energy than a monolayer with molecules tilting between the nearest neighbors.
Bifurcation analysis of the three-variable model for the NO+CO reaction on Pt surfaces
Authors: Imbihl R. , Fink Th. , Krischer K.
Published: July 2002

Under isothermal conditions at low pressure (10−6 mbar range), the NO+CO reaction exhibits oscillatory behavior on a Pt(100) surface. Based on the results of in situ low-energy electron diffraction (LEED) measurements which showed that the 1×1⇄hex phase transition is not essential for producing oscillations, a three-variable model of coupled differential equations was developed which instead relies solely on the autocatalysis provided by the stoichiometry of the individual reaction steps. This model has been analyzed with the help of bifurcation theory using realistic values for the constants in the equations. The results demonstrate that the model reproduces, quite well, the existence range for oscillations on Pt(100). Two oscillatory regions exist with a large one located above the stoichiometry ratio pNO: pCO=1 and a very small one which is found just below pNO: pCO=1. Only the former one has a counterpart in the experiment. At low temperature two isolated branches of the reaction exist which merge at higher T in a transcritical bifurcation thus creating a peculiar hysteresis loop in the shape of a mushroom. Bifurcation analysis has also been applied to investigate the role of the internal parameters, e.g., the role of the constants in the differential equations. The most critical constants were those which control the dissociation of NO and, therefore, are decisive whether ignition or extinction in the reaction takes place. The high degree of qualitative and quantitative agreement which could be achieved with the three-variable model demonstrates that the model provides a reasonable description of the experiments.
Dynamic properties of concentrated solutions above the glass transition temperature
Authors: Brown Wyn , Johnsen Robert M. , Konak Cestmir , Dvoranek Ladislav
Published: July 2002

Concentrated solutions of polybutylacrylate (PBA) in dioxane and polybutadiene (PBD) in cyclohexane and carbon tetrachloride have been studied at 50 °C in the concentration range 0.1–0.9 g/ml using Rayleigh–Brillouin scattering (RBS) and static and dynamic light scattering. The purpose was to extend measurements to concentrated solutions of ‘‘soft’’ polymers in a temperature range well above the glass transition temperature. The high frequency modulus from RBS is much larger than the low frequency osmotic modulus, but they have identical power law behavior. Contrary to the pronounced maximum in Dc observed as a function of Φp in concentrated polystyrene/toluene solutions [Brown et al., J. Chem. Phys. 95, 8568 (1991); Macromolecules 24, 5484 (1991)], the concentration dependence of Dc increases strongly above Φp≊0.56 in the PBA/dioxane system. This change is attributed to a reduction in the effective solvent viscosity caused by the presence of the polymer and analogous to the effect noted by Lodge and co-workers in polybutadiene solutions using oscillatory electric birefringence and pulsed-field-gradient nuclear magnetic resonance (NMR) measurements.
Heisenberg Hamiltonian for charge-transfer organometallic ferromagnets
Authors: Tchougreeff A. L.
Published: July 2002

A highly anisotropic Heisenberg spin one-half Hamiltonian is derived for the organometallic charge-transfer ferromagnet DMeFc-TCNE and its effective exchange parameters are estimated. Its relationship to the generally adopted McConnell picture of ferromagnetic interaction in such systems is established and particular charge-transfer states responsible for the ferromagnetic sign of the effective spin–spin interaction are discussed. The model proposed is valid for a number of charge-transfer magnets. Possible effects of high anisotropy on critical temperature in the DMeFc-TCNE ferromagnet are discussed briefly.
Spatial variation of a short-lived intermediate chemical species in a Couette reactor
Authors: Vigil R. Dennis , Ouyang Q. , Swinney Harry L.
Published: July 2002

We have conducted experiments and simulations of the spatial variation of a short-lived intermediate species (triiodide) in the autocatalytic oxidation of arsenite by iodate in a reactor that is essentially one dimensional—the Couette reactor. (This reactor consists of two concentric cylinders with the inner one rotating and the outer one at rest; reagents are continuously fed and removed at each end in such a way that there is no net axial flux and there are opposing arsenite and iodate gradients.) The predictions of a one-dimensional reaction–diffusion model, which has no adjustable parameters, are in good qualitative (and, in some cases, quantitative) agreement with experiments. Thus, the Couette reactor, which is used to deliberately create spatial inhomogeneities, can be exploited to enhance the recovery of short-lived intermediate species relative to that which can be obtained with either a batch or continuous-flow stirred-tank reactor.
On the use of semiphenomenological closures in integral equations for classical fluids
Authors: Lomba E. , Martín C. , Lombardero M. , Anta J. A.
Published: July 2002

We have studied two choices of semiphenomenological closures for the Ornstein–Zernike equation, both for a monoatomic Lennard-Jones fluid and a dipolar homonuclear hard diatomic fluid. One of the closures was originally proposed by Verlet for hard-sphere systems, for which is known to yield good results. A second closure is proposed by us in the frame of the reference hypernetted chain (RHNC) theory. We have described the reference systems in this closure by means of Verlet’s approximation and its recent extension to systems of nonspherical particles. This second approach, which we denote by RHNC-VM (Verlet’s modified), turns out to give an excellent description of the structure and thermodynamics of the Lennard-Jones fluid and very accurate predictions for the structure of the dipolar diatomic system. In this latter case the apparent superiority of hypernetted chain results for configurational energies is found to stem from fortuitous cancellation of errors in the integration of the components of the pair correlation function. Nonetheless, an approximation for the bridge function capable of accounting for the particular dielectric behavior of the dipolar diatomic fluid is still lacking.
Comment on: Dielectric constant of a hard-sphere fluid with induced dipoles and quadrupoles
Authors: Logan David E. , Madden Paul A.
Published: July 2002

Our previously published virial expansion describing deviations from Clausius–Mossotti behavior in the dielectric constant of a hard-sphere fluid within the quadrupole approximation [Mol. Phys. 46, 715 (1982)] is shown to account well for recent simulation results.
Algebraic determination of the vibrational quantum numbers of a diatomic molecule
Authors: Bautista Manuel A. , Ladera Celso L.
Published: July 2002

The quantum numbers of electronic-vibrational levels of a diatomic molecule are assigned using a new and simple algebraic procedure which is applied to data from a single absorption spectrum of the molecule. This procedure represents a considerable simplification in the solution to the problem of quantum numbering of a molecule since the method does not need additional spectra of other isotopic species, nor does it use advanced, yet more complicated, molecular potentials different from the usual Morse potential. In this paper the new method, applied to the absorption bands from the lowest X 1Σ+g state of I2 to the vibrational manifold of its B 3Π+0u excited electronic state, allows the assignation of the quantum numbers of the vibrational levels and, in addition, the evaluation of the main spectroscopic constants with reasonable accuracy even when a low-resolution spectrum is used as the input data.
Many-particle quantum dynamics: An exact algorithm for correlated motion on lattices
Authors: Zhang Qing , Whaley K. Birgitta
Published: July 2002

The time dependent many-body dynamics of strongly interacting particles on a two-dimensional lattice are studied with the recently developed checkerboard propagator. Application is made to the two-dimensional infinite U Hubbard model at high concentrations, for both fermion and boson systems. The concentration, initial state, and boundary condition dependence of vacancy correlation functions are analyzed, and related to the local densities of states. Quasidissipative behavior of the correlation functions is seen beyond a short initial time scale during which single-particlelike motion is seen.
Metastable fragmentation of (C4H1)nC4H+7 cluster ions induced by delayed isomerization reactions of C4H+7
Authors: Foltin M. , Grill V. , Rauth T. , Märk T. D.
Published: July 2002

The metastable fragmentation of the cluster ions (C4H10)nC4H+q (q=7 to 10) produced by the electron impact ionization of a neutral n-butane cluster beam has been studied with a double focusing sector field mass spectrometer. For q>7, the usual statistical single-monomer evaporation is the only observable metastable fragmentation channel. Surprisingly, the (C4H10)nC4H+7 cluster ions were found to exhibit a more complex metastable decay pattern, involving not only the single-monomer evaporation, but also two additional fragmentation channels leading to evaporation of 2 to 4 and 4 to 7 (C4H10) monomers, respectively. After studying the dependence of the parent and the fragment ion currents on the electron energy, we concluded that the C4H+7 ionic chromophore of the (C4H10)nC4H+7 cluster ion is produced after internal H− transfer between n-C4H10 and some primary fragment ion (e.g., C2H+5, C2H+3, C3H+3, CH+3, C3H+4 ) of n-butane within the cluster. A mechanism based on the energy storage in metastable isomers of the C4H+7 chromophore ion is suggested to explain the unusual metastable fragmentation pattern of (C4H10)nC4H+7 cluster ions.
Time dependent integral equation approaches to quantum scattering: Comparative application to atom–rigid rotor multichannel scattering
Authors: Sharafeddin Omar A. , Kouri Donald J. , Judson Richard S. , Hoffman David K.
Published: July 2002

In this paper we generalize earlier work on potential scattering to atom–rigid rotor scattering. We compare six approaches including the interaction picture, modified Cayley, amplitude density, and symmetric split operator methods. All methods derive from the integral equation form of the time-dependent Schrödinger equation. The methods were tested using the standard Lester–Bernstein model potential. All methods were found to perform well with the same parameters. Fast Fourier transforms were not used in these methods, and an average execution time for a 16 channel problem on CRAY YMP supercomputer was about 45 s. This single calculation yields results at any energy significantly contained in the initial packet. In the present study, the S matrix was computed at a total of 42 energies, but results could have been obtained at many more energies without a large increase in computing time. Timing results for one of the methods are reported for 25, 64, 144, and 256 coupled channels.