bluuebunny/crossref_metadata_2025 huggingface dataset:
Nematic ordering of 4-n-alkyl-4′-cyanobiphenyls studied by carbon-13 NMR with off-magic-angle spinning
Authors: Fung B. M. , Afzal Jalees , Foss Tina L. , Chau Mei-Hing
Published: July 2002

A method called RHODIUM (removal of homonuclear dipolar coupling and the use of off-magic-angle spinning) can be applied effectively to study the carbon-13 NMR of liquid crystals in natural abundance. In this method, the sample is spun rapidly (∼1 kHz) at an angle (θ) slightly less than the magic angle, so that the nematic director aligns along the spinning axis and the dipolar couplings are reduced by a factor of (3 cos2 θ−1)/2. Proton–proton dipolar couplings are removed by a special decoupling sequence (BLEW-48) so that carbon–proton dipolar couplings can be readily identified. The coupling constants (DC–H) of individual carbons are obtained by using a two-dimensional technique of separated local field spectroscopy. The RHODIUM method has been used to study the molecular ordering in the nematic phase of 4-n-alkyl-4′-cyanobiphenyls (kCB). Three homologs (5CB, 6CB, and 7CB) were investigated at TNI−T =4 K. Values of DC–H of the aliphatic carbons give rich information on the energetics of chain conformations and those of the aromatic carbons can be used to calculate order parameters of the phenyl rings. It was found that the values of DC–H for the α, β, and γ carbons in 5CB, 6CB, and 7CB are almost the same, and so are the values of Szz and Sxx−Syy of the rings. The results for 5CB are in good agreement with those obtained from deuterium and proton NMR studies of specially deuterated compounds.
A calorimetric investigation of the low-temperature phase transition in NaOD
Authors: White Mary Anne , Moore Stanley A.
Published: July 2002

The heat capacity of NaOD from T=20 to 350 K is reported. An anomaly centered at T=153.2±0.1 K (ΔH=131.1±0.7 J mol−1, ΔS=0.864±0.005 J K−1  mol−1) corresponds to the onset of a low-temperature antiferroelectric phase. This transformation, which likely involves tunneling of the deuterons, does not appear to have an atmospheric pressure analog in NaOH above T=6 K.
Physics of reacting particle swarms in gases
Authors: Robson R. E.
Published: July 2002

A wide variety of reactive phenomena in gaseous swarms is studied using an extension of momentum-transfer flight theory. Empirical formulas, such as the generalized Einstein relations and the Wannier energy relation are derived, with modifications due to reactive effects made explicit. The emphasis throughout is on simple mathematics and succinct physical reasoning.
Tricritical phenomena in quasi-binary mixtures. V. New measurements on ternary methane systems
Authors: Fernandez-Fassnacht Enrique , Williamson Arthur G. , Sivaraman Alwarappa , Scott Robert L. , Knobler Charles M.
Published: July 2002

Phase equilibria have been studied in ternary mixtures of methane with 2,2- and 2,3-dimethylbutane near the tricritical point. The quasi-binary approximation allows the path to the tricritical point to be defined experimentally, making it possible to study the details of the phase behavior as the tricritical point is approached. The decrease in the differences in temperature and pressure between the upper and lower critical end points is consistent with the asymptotic classical theory of tricritical phenomena; an extended classical theory is necessary to account for the shape of the coexistence curves.
Water-like particles at surfaces. I. The uncharged, unpolarized surface
Authors: Valleau John P. , Gardner Anne A.
Published: July 2002

Monte Carlo computations have been used to study the behavior of the TIPS2 model of water next to various surfaces. This paper reports on the model next to inert plane surfaces at 399 K. This study serves as a preliminary to studies of the model next to other surfaces especially interesting for electrochemistry; these are reported in an accompanying paper. Meanwhile the surface structure and polarization are examined and compared with those previously reported for other water models. There is a mild tendency to form an oriented ice structure near the surface, as had been found [C. Y. Lee, J. A. McCammon, and P. J. Rossky, J. Chem. Phys. 80, 4448 (1984)] for the ST2 model. Extremely vexing convergence problems were encountered, and as well an unexpected anisotropy of the angular distributions. The latter problem turns out to involve questions of the sample shape and boundary conditions and seems to have worrying implications even for simulations of homogeneous fluids of nonspherical particles.
Maximal coverage in random sequential absorption
Authors: Meakin P. , Cardy John L. , Loh E. , Scalapino D. J.
Published: July 2002

Random sequential absorption (RSA) represents an important kinetic limit for structural formation which leads to significantly different properties from that obtained under thermal equilibrium adsorption. Here we present results for the maximal coverage θJ obtained from numerical simulation of RSA on various lattices and discuss their relationship in terms of a soluble 1D model.
Anomalous isotope dependence of hydrogen diffusion rates on tungsten (110) surfaces: Implications for lattice–hydrogen interactions
Authors: Auerbach Assa , Freed Karl F. , Gomer R.
Published: July 2002

A theoretical analysis is provided of the anomalous isotope effects observed for the diffusion of hydrogen on the W(110) surface in the limit of zero coverage. Low temperature tunneling diffusion shows an isotope effect several orders of magnitude smaller than predicted by simple rigid lattice models, while the higher temperature activated diffusion displays an inverse isotope effect several orders of magnitude larger than the rigid lattice predictions. It is shown here that both effects can be explained consistently by a single model of hydrogen–tungsten interactions in which there is a large separation in time scales between the hydrogen and tungsten motions. Tunneling is described with a small polaron model. Large phonon overlap factors are found to diminish the role of the tunneling matrix element and thereby to decrease the isotope effect. Activated diffusion is described as a many-phonon process in which the vibron is thermally excited as a result of phonon–vibron coupling. The same coupling parameter explains both the tunneling and activated diffusion results. This coupling parameter is shown to be dependent on adsorbate mass.
Rotational Raman excitation profiles of symmetric tops: Subpicosecond rotation dependent lifetimes in the Ã state of ammonia
Authors: Ziegler L. D.
Published: July 2002

The analysis of Raman excitation profiles of rotational scattering transitions is shown to be a sensitive measure of rotationally specific intramolecular dynamics in the subpicosecond time regime. O, P, Q, R, and S Raman REPs of ammonia are reported and theoretically treated using previously derived resonance rotational scattering intensity expressions. Resonance rovibrational Raman spectra are obtained with UV Raman excitation continuously resonant with the v′2 =2 vibronic band (207–210 nm) of the NH3 X̃ → Ã absorption spectrum. The resulting REP analysis reveals that the rates of photodissociation on the Ã state surface have a significant rotational quantum dependence in the 22 band. Rovibronic lifetimes decrease from 140 to 70 fs as J increases from 2 to 8 in this resonant Ã state vibrational level. These lifetime effects are largely ascribed to centrifugal effects.
Ultraviolet photoelectron spectroscopy of solid iodine
Authors: Yamamoto Hiromichi , Seki Kazuhiko , Mori Takehiko , Inokuchi Hiroo
Published: July 2002

The ultraviolet photoelectron spectra of solid iodine have been measured. The observed spectral features show a large deviation from a one-to-one correspondence with the gas phase spectrum. These results are examined by an extended Hückel band calculation, which indicates that this deviation is caused by the effect of a strong intermolecular interaction on the electronic structure, namely the formation of broad bands (0.7 to 2.0 eV). The characters of the observed spectral features are assigned by the calculated density of states. The energy gap is estimated to be 1.6 eV from the observed energy difference between the Fermi level and the top of valence bands (5.52 and 6.34 eV relative to the vacuum level, respectively), by assuming that iodine is an intrinsic semiconductor. The large decrease of ionization threshold energy (2.92 eV) from the gas to the solid state is also discussed.
Symmetry and geometry of the first two excited singlet states of dimethylnitrosoamine studied by vector correlations
Authors: Lavi R. , Bar I. , Rosenwaks S.
Published: July 2002

A study of the geometry and symmetry of two different excited potential surfaces of the same molecule is demonstrated for the first time by analyzing the alignment and Λ doubling of the NO fragment resulting from the photodissociation of dimethylnitrosoamine irradiated at 363.5 nm, [S1(nπ*)←S0], and 250 nm, [S2(ππ*)←S0]. The values obtained for the alignment and Λ doubling indicate that (1) the fragmentation process is fast in both cases; (2) there is some planarity in the fragmentation process in both cases; (3) at 250 nm the transition dipole moment lies in the frame of the molecule, while at 363.5 nm it is perpendicular to the frame of the molecule; (4) the first excited state has an A″ symmetry and the second excited state has an A′ symmetry.
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o calculation of harmonic force fields and vibrational spectra for the methyl, silyl, germyl, and stannyl halides
Authors: Schneider Winfried , Thiel Walter
Published: July 2002

Theoretical harmonic force fields are reported for 16 symmetric tops H3MX (M=C, Si, Ge, Sn; X=F, Cl, Br, I). Based on the evaluation of systematic test calculations for H3CF, H3CCl, H3SiF, and H3SiCl, all molecules are treated uniformly at the Hartree–Fock level using effective core potentials and polarized double-zeta basis sets for the valence electrons. For each molecule the calculated geometries, rotational constants, frequencies, centrifugal distortion constants, Coriolis coupling constants, and infrared band intensities are compared with the available experimental data. The agreement is satisfactory and generally of similar quality as in analogous all-electron Hartree–Fock calculations of molecules with first-row atoms. It is stressed that theory may reliably provide data which are hard to derive from experiment, e.g., the off-diagonal symmetry force constants and the signs of the dipole moment derivatives. The effects of scaling the theoretical force fields are investigated carefully including the transferability of scaling factors between related molecules, and the vibrational spectrum of the as yet unknown H3SnF molecule is predicted. Our present results indicate that the chosen theoretical approach is generally suitable for studying the harmonic force fields of inorganic molecules with heavy main-group elements.
Rotational optical activity
Authors: Polavarapu P. L.
Published: July 2002

Optical activity generated by the rotational normal coordinates is investigated. The theoretical formulation is based on classical principles and uses the definition of the rotational normal coordinates and the changes in electric and magnetic dipole moment components along the principal axes of inertia due to rotations represented by these coordinates. It is assumed that the effective atomic charges and polarizabilities are uneffected during the rotational motions of the molecule.
Electron-impact spectroscopy of various diketone compounds
Authors: Walzl K. N. , Xavier I. M. , Kuppermann A.
Published: July 2002

The spectra of the diketone compounds biacetyl, acetylacetone, acetonylacetone, 1,2-cyclohexanedione, and 1,4-cyclohexanedione have been investigated by the technique of low-energy variable-angle electron energy-loss spectroscopy. With this method low-lying, spin-forbidden transitions have been observed. The energy difference between the lowest spin-allowed and spin-forbidden n→π* excitations in the acyclic diketones is found to be 0.35 eV, on average, which is nearly the same as that of comparable acyclic monoketone compounds; in 1,2-cyclohexanedione, however, this energy difference is 0.84 eV, more than twice as large. This discrepancy in the magnitude of the n→π* singlet–triplet splittings may be attributed to differing amounts of overlap between the initial and final orbitals.
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o synthesis of the ozone ultraviolet continuum
Authors: Lohr Lawrence L. , Helman A. J.
Published: July 2002

Potential energy surfaces for the ground and excited electronic states responsible for the Hartley continuum of ozone are used to obtain quadratic, cubic, and quartic force constants. Vibrational dependence of rotational constants to sixth order is calculated by perturbation theory. The spectroscopic constants enable computation of rovibronic energy levels. Overlap of ground state and excited state perturbed vibrational wave functions yield Franck–Condon factors. Electric dipole allowed rovibronic transitions are generated under the Ir representation. The entire set of results generate the ultraviolet absorption spectrum. It is shown that inclusion of anharmonic terms in the vibrational Hamiltonian has a small effect upon the final spectrum, whereas rotational broadening plays a greater role in achieving agreement with experiment.
Vibrational relaxation of the ν2=1 level of ortho and para NH3
Authors: Danagher D. J. , Reid J.
Published: July 2002

A lead salt diode laser operating in the 12 μm region is used in conjunction with a Q-switched CO2 laser to measure relaxation rates on many different transitions in the ν2 bands of 14NH3 and 15NH3. The measured lifetime of the ν2=1 level is found to differ for ortho and para NH3 transitions. This difference is shown to be caused by a relatively slow (V–V) energy transfer between the ortho and para species. (V–V) and (V–T,R) relaxation rates are reported for NH3–NH3 and NH3–N2 collisions at 200 and 300 K. The rate of vibrational energy transfer between 14NH3 and 15NH3 is also determined.
Group theoretical approach to semiclassical dynamics: Single mode case
Authors: Gilmore Robert , Yuan Jian-Min
Published: July 2002

The algebraic structure of a truncated semiclassical Hamiltonian describing the translational–vibrational interaction has been determined and exploited to compute the S matrix for the system. The equations of motion have been integrated in a 4×4 faithful nonunitary representation of the group, and the final results lifted to the infinite dimensional unitary representation of the group. Computations of transition probabilities are facilitated by using powerful disentangling theorems to write the S matrix in convenient operator-ordered form. These methods are in quantitative agreement with results of Gazdy and Micha, and generalize easily to the multimode case. The structure of the Ehrenfest equations of motion for the operators of the algebra is identical to the structure of the dynamical algebra itself. The S matrix is also reconstructed from these equations of motion. Finally, a simple geometric picture is presented to describe the two extreme limits of the parametric oscillator problem involving only single quantum transitions or double quantum transitions.
Lifetimes of local and hyperspherical vibrational resonances of ABA molecules
Authors: Bisseling R. H. , Kosloff R. , Manz J. , Mrugal/a F. , Römelt J. , Weichselbaumer G.
Published: July 2002

The complete spectrum of vibrationally excited ABA* molecular resonance lifetimes is evaluated using the simple Rosen–Thiele–Wilson model of coupled Morse oscillators. Two complementary methods are used: First, unimolecular dissociative resonance wave functions are propagated in time by the Fourier method, where the initial wave functions are obtained as an approximation by linear combinations of symmetry-adapted products of Morse functions. Second, bimolecular reaction S matrices are propagated along the hyperspherical radius of the system giving the diagonalized lifetime matrix, which is analyzed for resonance lifetimes and energies. The resulting uni- and bimolecular resonance energies agree within ±0.002 eV and the lifetimes within ±30%. Uni- and bimolecular assignments of gerade (+) and ungerade (−) ABA* symmetries agree perfectly. On the average, the unimolecular decay times decrease as the resonance energies increase from the ABA*→A+BA to about 3/4 of the A+B+A dissociation threshold; even more highly excited resonances tend to be slightly more stabilized. Superimposed on this overall nonmonotonous energy dependence is a strong, 1–2 orders of magnitude variation of lifetimes, indicating substantial mode selectivity for the decay of individual resonances, irrespective of the excitation energy. The mode selectivity is investigated for hyperspherical mode resonances with lobes extending across the potential valleys, in contrast with local mode resonances with frontier lobes pointing towards the valleys. On the average, hyperspherical mode resonances decay at a slower rate than local mode resonances. This conclusion agrees with our previous analysis of low energy ABA* resonances, and with Hose and Taylor’s analysis of the Hénon–Heiles system. However, these correlations are also violated by several important exceptions: the ABA* system has many slowly, but also a few rapidly, decaying hyperspherical resonances.
Effects of nonzero correlation time of system–reservoir interaction on the excitation profiles of second-order optical processes in β-carotene
Authors: Watanabe Junji , Kinoshita Shuichi , Kushida Takashi
Published: July 2002

Excitation spectra for narrow Raman lines and a broad luminescence band have been measured in β-carotene in isopentane at 177 K over a wide spectral range on the low-energy side of the 1Ag–1Bu absorption band. The result has been found to be well explained by a stochastic theory that takes into account the finiteness of the correlation time for the intermediate-state interaction. Various parameters, such as the magnitude and the rate of the energy modulation in the intermediate state, have been determined from the analysis of the excitation profiles. The parameter values obtained indicate that the intermediate-state interaction is far from the motional-narrowing limit in this system.
Internal molecular motions and activated chemisorption: CH4 on rhodium
Authors: Brass Stephen G. , Ehrlich Gert
Published: July 2002

If activated chemisorption occurs directly from the gas phase and also from a precursor state on the surface, then the importance of internal molecular excitations as compared with translational motion in promoting chemisorption can be determined by rate measurements under two different conditions: (a) When the surface and the gas are kept isothermal, the net activation energy is the same whether translational or internal motions contribute to activation, and is given by the difference Ec−Ed between the barrier Ec to the conversion of the precursor into the chemisorbed state and the barrier Ed to the desorption of the precursor back into the gas phase. (b) When the surface is kept cold and only the gas is heated, the activation energy for chemisorption directly from the gas phase is measured. The result will be equal to that in (a) only if the translational energy of the incident molecules can be utilized effectively in passing over the barrier. If internal motions must be excited, however, a higher activation energy, approaching in value Ec, will be measured in experiments with the gas heated. A special apparatus has been built for doing adsorption studies of the type (b), on freshly evaporated metal films maintained at a temperature different from that of the surrounding gas. Measurements of chemisorption on rhodium films at 0 °C, with CH4 at temmperatures from 35 to 91 °C, yield an activation energy of 11.1 kcal/mol, as compared with a value of 5.0 kcal/mol determined in previous isothermal experiments. The difference between the activation energies found for CH4 in experiments of type (a) and type (b) is in excellent agreement with the desorption energy Ed of precursor molecules established previously. This is expected if internal molecular motions, such as vibrations, rather than translation are most effective in promoting the passage of methane over the barrier to chemisorption.
Comparative study of theory and simulation calculations for excess electrons in simple fluids
Authors: Laria Daniel , Chandler David
Published: July 2002

RISM-polaron theory and simulation results of the primitive hard sphere model for an excess electron in simple fluids are used to interpret the recent path integral quantum Monte Carlo studies of an electron in supercritical helium and in xenon by Coker, Berne, and Thirumalai. It is shown that the different behaviors of the excess electron in these two different fluids are due primarily to differences in excluded volume effects. For xenon, due to the nature of the electron–solvent pseudopotential, this volume is relatively small and the excess electron remains extended for all fluid densities. In contrast, for helium, the random excluded volume is high leading to self-trapping or localization of the electron.
On the entropy of mixing of a binary mixture of hard spheres
Authors: Meroni A. , Pimpinelli A. , Reatto L.
Published: July 2002

The entropy of mixing of a binary mixture of hard spheres at constant pressure is computed as a function of the diameter ratio on the basis of the Mansoori–Carnahan–Starling equation of state. The Flory–Huggins form for the entropy of mixing is a poor approximation and becomes worse for increasing values of the diameter ratio. The ideal gas form is closer to the hard sphere result.
Polarized fluorescence spectroscopy of O+2
Authors: Keller J. W. , Hill W. T. , Ederer D. L. , Gil T. J. , Langhoff P. W.
Published: July 2002

Polarized fluorescence from the A 2Πu and b 4Σ−g states of O+2 following photoionization of O2(X 3Σ−g) in the 17–21 eV region has been employed to determine population alignment in the O+2 photoion. The measured A-channel polarization exhibits pronounced structure which correlates well with the known positions of (autoionizing) Rydberg states, whereas polarization signal in the b channel has broad and less prominent features. The measured b-channel polarization values (ranging from −0.02 to −0.05) are consistent with single-channel calculations, while the A-channel polarization measurements (which range from 0 to 0.09) are of opposite sign to the previously reported single-channel calculations. The results of more involved calculations show that this sign reversal in the A channel can be attributed to the presence of intravalent 3Σ−u states which autoionize into the A channel in the near-threshold region.
The multiconfigurational spin-tensor electron propagator method for determining vertical principal and shake-up ionization potentials for open shell and highly correlated atoms and molecules
Authors: Golab Joseph T. , Yeager Danny L.
Published: July 2002

We propose and develop the multiconfigurational spin-tensor electron propagator (MCSTEP) technique for the theoretical determination of vertical ionization potentials (IPs) and electron affinities (EAs) for general open-shell and highly correlated atoms and molecules. We obtain these equations from a Green’s function or electron propagator approach where we properly couple electron removal and addition tensor operators to a multiconfigurational tensor state. To account for important shake-up effects and to achieve a ‘‘balance’’ in initial and final state correlation corrections, we include in MCSTEP ionization and electron affinity operators analogous to the ‖c〉〈0‖ state transfer operators necessary in multiconfigurational linear response. In repartitioned MCSTEP (RMCSTEP) we augment the MCSTEP operator manifold with operators of the form a+iajak by first employing partitioning theory to estimate their contributions and then repartitioning only the important operators into the primary space. In this way, important shake-up processes to diffuse orbitals are accurately and reliably handled with RMCSTEP at the same level of approximation, i.e., as part of the primary space operator manifold . Initial application of these methods is extremely encouraging for both principal and shake-up IPs. Using a 〈5s5pld〉 contracted Gaussian valence basis set augmented with two diffuse s, two diffuse p, and two diffuse d functions, the RMCSTEP ionization potentials to the low-lying (<∼24 eV) 2S and 2P bound ionic states (including diffuse states) for Be are calculated within ±0.07 eV of experiment. The IP to the lowest 2D state is calculated 0.14 eV from experiment.
Hard-sphere theory for correlation of tracer diffusion of gases and liquids in alkanes
Authors: Matthews Michael A. , Akgerman Aydin
Published: July 2002

The rough hard-sphere (RHS) theory for transport properties has substantially improved understanding of diffusion, viscosity, and thermal conductivity. At present, however, quantitative predictions for polyatomic species are hampered by uncertainties in assigning the hard-sphere diameter and the roughness factor, and by lack of molecular dynamics calculations. Because the qualitative features of the theory have proven correct for a variety of chemical systems, methods have been proposed for using rough hard-sphere theory even when the exact parameters needed to apply the theory are unknown. One such approach is examined for tracer diffusion in alkane solvents for solutes ranging in size from hydrogen to hexadecane. Useful relations are shown which are simple, yet entirely consistent with hard-sphere theory. The approach circumvents the difficulties in assigning roughness factor, diameter, and molecular dynamics results.
Study of the spectrum and decay of the doubly charged water ion using spin-coupled valence bond theory
Authors: Cooper David L. , Gerratt Joseph , Raimondi Mario , Sironi Maurizio
Published: July 2002

Potential energy surfaces are computed for several low-lying singlet and triplet states of H2O++ using spin-coupled valence bond theory. Wide regions of the surfaces are investigated including all the dissociation processes to OH++H+ in different states, to O+H++H+ and to O++H++H. The correlation of all these states is very straightforward. The final wave functions are very compact (187 spatial configurations) and the electronic structure of the various states is interpreted directly in terms of the spin-coupled orbitals. This makes possible a completely new description of the ion–ion coincidence (PIPICO), Auger, and double charge transfer spectra of doubly ionized water.
Theory of line shapes for zero field NMR in the presence of molecular motion
Authors: Meier P. , Kothe G. , Jonsen P. , Trecoske M. , Pines A.
Published: July 2002

Dynamic molecular processes modulate dipolar and quadrupolar interactions in nuclear spin systems. Zero field NMR measures evolution arising purely from these interactions, and thus can be used to characterize molecular dynamics in disordered samples. A theory is presented which numerically simulates the effect of molecular reorientation in zero field NMR for a spin-1 nucleus or pairs of dipolar coupled spin-1/2 nuclei. Representative zero field NMR simulations are compared to their high field NMR analogs to demonstrate features which make zero field NMR a potentially useful tool for the study of molecular dynamics.
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o study of hydrated chloride ion complexes: Evidence of polarization effects and nonadditivity
Authors: Foresman James B. , Brooks Charles L.
Published: July 2002

An ab initio study of complexes involving a chloride ion with one through six water molecules is presented. Calculations are performed using an RHF/6-31+G(D) theoretical model and partially optimized geometries. Chloride–water interaction energies are computed with a correction for basis set superposition. Polarization of the chloride ion by increasing numbers of water molecules causes a decrease from linearity among the energies. For the six water case, deviation from two-body additivity is approximately 10% of the stabilization energy.
Picosecond photofragment spectroscopy. I. Microcanonical state-to-state rates of the reaction NCNO→CN+NO
Authors: Khundkar Lutfur R. , Knee Joseph L. , Zewail Ahmed H.
Published: July 2002

This paper, the first in a series of three papers, gives a detailed account of our studies on picosecond photofragment spectroscopy. The unimolecular reaction NCNO→CN+NO is examined in detail here. Microcanonical state-to-state rates are measured in molecular beams at different energies in the reagent NCNO using pump–probe techniques: one picosecond pulse initiates the reaction from an initial (v,J) state and a second pulse, delayed in time, monitors the CN radical product in a specific rovibrational state, or the reagent NCNO (transient absorption). The threshold energy for reaction is determined to be 17 083 cm−1 (bond energy=48.8 kcal/mol). Measured rates are found to be sharply dependent on the total energy of the reagent, but independent of the rotational quantum state of product CN. Results of transient absorption measurements are used to argue that the ground state potential energy surface dominates the reaction in the range of excess energies studied. The energy dependence of the rates, kMC(E), is compared with that predicted by statistical theories. Both standard RRKM (tight transition state) and phase space theory (loose transition state) fail to reproduce the data over the full range of energies studied, even though nascent product state distributions are known to be in accord with PST at these energies. Furthermore, kMC(E) is not a strictly monotonically increasing function of energy but exhibits some structure which cannot be explained by simple statistical theories. We advance some explanations for this structure and deviations from statistical theories.
Multiple steady states, complex oscillations, and the devil’s staircase in the peroxidase–oxidase reaction
Authors: Larter Raima , Bush Christopher L. , Lonis Timothy R. , Aguda Baltazar D.
Published: July 2002

The steady and oscillatory states of the Olsen and Degn model for the peroxidase–oxidase oscillator are found. The stability of the steady states is determined, and a bifurcation diagram for the oscillatory regime is found. A complex sequence of multiply periodic oscillations is observed and found to follow an orderly pattern when described via the firing number, the number of small oscillations divided by the total number of large and small oscillations per period. A plot of the firing number vs one of the system parameters is found to have a stairstep relationship and it is shown that this stairstep relationship is a complete devil’s staircase, an infinite self-similar staircase with chaotic properties and a fractal dimension.
Nearest neighbor order in an aligned solution of interacting rod-like micelles
Authors: Kalus J. , Hoffman H.
Published: July 2002

Rod-like micelles of hexadecylpyridiniumsalicylate in aqueous solution were aligned by a shear gradient Γ of up to 40 s−1. Small angle neutron diffraction patterns of a solution of 20 mM/ℓ show a pronounced peak in scattering intensity at a momentum transfer of 0.15 nm−1 due to a correlation of the charged micelles. This correlation peak becomes deformed substantially with increasing Γ. A simple model is presented, taking into account an anisotropic change of the pair correlation function. It turns out that the order is reduced in the direction of the vector of velocity v of the sheared liquid and increases perpendicular to this vector.
Electromagnetic field of a two-dimensional molecular aggregate
Authors: Young Ralph H.
Published: July 2002

Fourier-series expressions are derived for the electric and magnetic fields of a regular planar array of molecular current and charge distributions, static or oscillatory, with or without a plane-wave modulation. The point-dipole approximation is found to be quantitatively unreliable at moderate-to-large distances from the array. The complete electric field comprises a slowly varying ‘‘long-range’’ part and a remainder that falls off exponentially with increasing distance from the array. The former is well approximated by the field of a suitably polarized dielectric slab. A rapidly converging real-space lattice summation procedure is introduced for the electrostatic case.
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e Rydberg states of acetylene studied by multiphoton ionization and photoelectron spectroscopy
Authors: Ashfold M. N. R. , Tutcher B. , Yang B. , Jin Z. K. , Anderson S. L.
Published: July 2002

The lowest few gerade Rydberg states of C2H2, C2HD, and C2D2 have been observed as two-photon resonances in the multiphoton ionization (MPI) spectra of these isotopic species. Analysis of the two-photon excitation spectra of these molecules, and of the accompanying MPI-photoelectron spectra for C2H2, enable characterization of the 1Σ+g and 1Δg Rydberg states arising from the electronic promotion π3u3pπ1u←π4u and reveal extensive vibronic interaction in this energy region. A search was made for higher gerade Rydberg states at energies up to 90 000 cm−1. Weak, diffuse signal is observed for the 4p complex. No ionization signal was observed in the energy range where the np (n=5–7) Rydberg states are expected to appear. This is taken as evidence for opening of a facile nonradiative decay channel at high excitation energies.
Removing chemical bonding ambiguities in condensed media by steepest-descent quenching
Authors: Weber Thomas A. , Stillinger Frank H.
Published: July 2002

Chemical bonds between neighboring pairs of atoms tend to be significantly shorter than distances between nonbonded pairs that are in ‘‘van der Waals’’ contact. Consequently there is usually no theoretical uncertainty in identifying chemical bonds in cold dense media from the atomic pair correlation functions: covalently bonded pairs appear as isolated narrow peaks at small separation. Under elevated temperature and pressure conditions, however, this peak isolation disappears, leading to considerable apparent bonding ambiguity. Using molecular dynamics simulation for sulfur as a testing device, we show that removal of thermal distortion even in extremely hot and dense media by steepest-descent quenching on the potential energy hypersurface completely restores peak isolation and removes bonding ambiguity.
Monte Carlo methods for real-time quantum dynamics of dissipative systems
Authors: Cline Raymond E. , Wolynes Peter G.
Published: July 2002

A numerical path integral technique based upon a quasiclassical Langevin equation is presented for the calculation of quantum mechanical properties of a system coupled to a dissipative bath. Fully quantum mechanical results are obtained by decorating quasiclassical paths with quantum fluctuations. The use of diophantine integration has been compared to Monte Carlo sampling of the fluctuation variables. Results are presented for several model systems.
Structural characterization of Langmuir–Blodgett multilayer assemblies by plasmon surface polariton field-enhanced Raman spectroscopy
Authors: Duschl Claus , Knoll Wolfgang
Published: July 2002

Plasmon surface polariton field-enhanced Raman spectroscopy has been performed with multilayer assemblies of cadmium arachidate deposited by the Langmuir–Blodgett (LB) dipping technique onto silver gratings. The gain in sensitivity can be used to record the Raman spectrum of even a single monolayer. The quantitative analysis of the C-H stretching region allows for the estimate of the tilt angle of the fatty salt molecules relative to the substrate normal. The extent of disorder in the LB films has been determined as a function of layer thickness, substrate conditions and temperature. By the investigation of monolayers of normal protonated CdA embedded into a matrix of multilayers of fully deuterated molecules we were able to monitor orientation and disorder locally resolved. Thus for the first time, it was possible to study the influence of the deposition process on the structural properties of underlying monolayers.
Efficient microcanonical sampling for triatomic molecular systems: Exact distributions verified
Authors: Nyman Gunnar , Rynefors Kjell , Holmlid Leif
Published: July 2002

Statistical distributions of several quantities, such as linear and angular momenta, for a triatomic molecular system similar to an excited H2O molecule were obtained with an efficient microcanonical sampling method, previously described by Severin et al. The distributions were recorded as a function of the total angular momentum. Using this sampling procedure to obtain initial values for classical trajectory calculations and comparing with the trajectory quantities after variable times in the range 0.7 fs to 0.2 ps, it was verified that the sampling method gives exact distributions. This may open a way of reducing computer time in future trajectory calculations of unimolecular reactions, particularly important at low total energies. We also illustrate the breakdown of equipartition of energy between the various degrees of freedom in highly excited complexes. In some cases equipartition is not even valid for potential energy surfaces built solely from harmonic terms.
Intramolecular energy transfer rates in photoexcited cluster ions: The photodissociation dynamics of CO−3⋅H2O and CO−3⋅CO2
Authors: Snodgrass Joseph T. , Kim Hyun-Sook , Bowers Michael T.
Published: July 2002

The photodissociation dynamics of CO−3⋅H2O and CO−3⋅CO2 have been investigated at photon energies of 2.13, 2.41, 2.54, and 2.71 eV. Experiments were conducted by crossing a mass-selected, 8 kV ion beam with a linearly polarized laser beam, and measuring the kinetic energy distributions of the charged photodissociation products. By varying the angle between the ion beam and the laser polarization vector, product angular distributions were obtained. The only ionic product observed from both systems was CO−3. The average energy partitioned into relative translation of the photofragments was determined to be ∼0.1 eV for CO−3⋅H2O and ∼0.07 eV for CO−3⋅CO2. In both cases, these kinetic energy release values were found to be nearly independent of photon energy. The small fraction of the available energy partitioned into kinetic energy of the photofragments indicates that the upper states of the transitions leading to photodissociation are bound, and that a substantial fraction of the available energy must be channeled into internal energy of the dissociating fragments. The angular distributions of CO−3 photoproducts from both CO−3⋅H2O and CO−3⋅CO2 were found to be extremely isotropic. Modeling the experimental data using statistical phase space theory shows that dissociation occurs prior to complete

energy randomization, and provides a measure of the extent of energy randomization prior to dissociation. Comparison of theory and experiment indicates the photodissociation processes proceed by the following mechanism: (1) Photon absorption occurs via a transition localized on CO−3 moiety: CO−3 (2B1)⋅X+hν→CO−3 (2A1)⋅X, where X=CO2 or H2O. (2) CO−3(2A1)⋅X internally converts to CO−3(2B1)⋅X, with a high degree of vibrational excitation being localized on the CO−3 moiety. (3) The vibrational excitation localized on the CO−3 moiety begins to slowly randomize throughout the cluster ion. (4) Before complete energy randomization has occurred, CO−3(2B1)⋅X dissociates to CO−3 and X, with the nascent CO−3 product containing a significant amount of internal energy (∼1.4 eV). The time required for approximately 1.0 eV of vibrational energy localized in CO−3(2B1) to randomize throughout the CO−3⋅X cluster is at least 10−9±1 s.
Observation of strong Rydberg–valence mixing in the E 3Σ−u state of O2 by 3+1 MPI photoelectron spectroscopy
Authors: Miller Paul J. , Li Leping , Chupka William A. , Colson Steven D.
Published: July 2002

The photoelectron spectrum (PES) of the E 3Σ−u state of O2 has been measured with 3+1 multiphoton ionization (MPI). The observed vibrational progression in the PE spectrum is interpreted as due to ionization from a (3pπ)3Σ−u Rydberg state which is strongly mixed with the (πu 3π3g)B 3Σ− valence state. The observation of this mixing is in near agreement with the calculation by Buenker et al.
Electronic excitation and quenching of atoms at insulator surfaces
Authors: Swaminathan P. K. , Garrett Bruce C. , Murthy C. S.
Published: July 2002

A semiclassical method is employed for dynamical calculations of electronic transitions in collisions of gas atoms with insulator surfaces. The theory is based upon combining Micha’s self-consistent eikonal method (SCEM) with a stochastic reduction of the equations of motion for the condensed phase as represented in a generalized Langevin equation (GLE). The merged theory provides a framework that manifests the attractive computational advantages of both the SCEM and GLE modeling methods and can be readily applied to many modern problems involving electronically inelastic gas/surface collisions. The theoretical approach is numerically illustrated for a simple two-electronic-state curve crossing problem, where the effects of model parameters, surface temperature, and collision energy upon transition probabilities and energy accommodation are examined. For the model system studied the loss of energy of the gas atom into the surface is appreciable with pronounced effects depending upon the electronic transition probabilities. In collisions with a rigid model of the surface the transition probabilities exhibit Stückelberg oscillations as a function of the translational energy. These oscillations are ‘‘washed out’’ by the thermal effects of the surface at finite temperatures. The effect of electronic inelasticity can, however, be seen in the magnitudes of finite temperature energy transfers.
Optical spectra and energy level analysis of Dy3+:LaCl3
Authors: Rana R. S. , Shertzer J. , Kaseta F. W. , Garvey R. , Rana D. , Feng S. Y.
Published: July 2002

The absorption spectrum of Dy3+:LaCl3 at 4 K has been photographed and measured from 20 000 to 38 000 cm−1. Based on this and previous data, an empirical energy level scheme consisting of 151 observed crystal levels from 0 to 34 130 cm−1 has been determined for the 4f9 ground configuration of trivalent dysprosium in LaCl3 crystals. An extended Hamiltonian with 20 adjustable parameters is used to fit by least squares the observed levels with a mean error of 6.9 cm−1.
Gaussian functions in hylleraas-CI calculations. I. Ground state energies for H2, HeH+, and H+3
Authors: Urdaneta C. , Largo-Cabrerizo A. , Lievin J. , Lie G. C. , Clementi E.
Published: July 2002

Gaussian basis functions have been used in explicitly correlated configuration interaction calculations for the ground states of H2, HeH+, and H+3. Due to the use of rij, the polarization functions are found to be much less important in comparison with their effectiveness in the convectional CI methods. Our lowest ground state energies for H2 and HeH+ are 1.8 and 8 cm−1, respectively, above the best literature values obtained by using generalized James–Coolidge expansion in terms of confocal elliptical coordinates. With 10s and 2p contracted functions, the calculated ground state energy for H+3 is −1.343 500 hartrees, which is believed to be the lowest variational energy in the literature.
Reference-hypernetted chain equation with anisotropic bridge function for fluids of diatomic molecules
Authors: Lado F.
Published: July 2002

Modeling the bridge function in the reference-hypernetted chain equation with that of a hard-sphere fluid has proven to be highly successful for simple liquids, particularly when the reference-hard-sphere diameter is treated as an adjustable parameter. In this paper, we examine the generalization of this technique to liquids of diatomic molecules, with the reference bridge function taken from that of the corresponding hard-diatomic-molecule fluid. Specifically, the diatomic molecules of the system of interest interact through an atom–atom Lennard-Jones potential while the hard-diatomic-reference model is solved in Percus–Yevick approximation. We find that variation of the sphere diameter of the hard-diatomic molecule makes possible excellent agreement with simulation results for the two-center Lennard-Jones fluid, just as it does for simple fluids.
Model polarizabilities and multipoles for ionic compounds. Alkali halides
Authors: Davis Stephen L.
Published: July 2002

Various dipole interaction models were compared with one another and with literature values for dissociation energies, multipole moments (u, Q, and Ω), and polarizabilities α for the alkali halide molecules. A displaced-shell (D-shell) model, which takes account of the overlap repulsion as in the shell model but also uses the effective electron shell displacements in calculating the electrostatic interactions, gives slightly better dissociation energies and as good or better dipole and quadrupole moments than the other models. All of the models considered here give mean polarizabilities in fairly good agreement with published SCF values; however, none gives consistently good polarizability anisotropies. It is found that both the overlap repulsion (as treated in the shell model) and the effective displacements of the electron shells (as treated in the D-shell model) have large effects on the calculated polarizability anisotropies relative to the DID model.
A generalization of Widom’s model of microemulsions
Authors: Hofsäss T. , Kleinert H.
Published: July 2002

We construct a generalization of Widom’s model of microemulsions which permits the contact of polar and apolar material. This is necessary to explain the behavior of certain short chain amphiphilic systems. The same interactions have previously been proposed by Robledo, but the partition function which he derived from these interactions and discussed in detail in his paper (a partition function of a spin 1 Ising system) does not quite represent these interactions. We arrive at a spin 3/2 Ising model with nearest neighbor interactions. Robledo’s spin 1 form is recovered as an approximation whose range of validity is discussed. We particularly investigate the limit towards Widom’s model, the progression of Winsor microemulsion, and the question of retrograde solubility in the binary water–amphiphile subsystem, which has been advocated by Robledo.
Structure, vibrational spectra, and IR intensities of polyenes from a
b i
n
i
t
i
o SCF calculations
Authors: Villar H. O. , Dupuis M. , Watts J. D. , Hurst G. J. B. , Clementi E.
Published: July 2002

A b initio SCF calculations are presented which deal with the structure of polyenes, as model systems for polyacetylene (PA). We used a double zeta quality basis set, and determined the structure of clusters up to C22H24 as well as the vibrational spectra of clusters up to C12H14, including infrared intensities. All of these data are analyzed by comparing with experimental information available on polyacetylene and previous theoretical studies. Specifically, we identified the vibrational modes corresponding to the observed absorption bands of PA. We comment on controversial points such as the relative contributions of internal coordinates to these modes, on the magnitude of interaction force constants, and also on the effect of light polarization on the absorption of some vibrational modes. We establish that the inclusion of polarization function in the basis set does not lead to significant changes in structural or energetic properties. Convergence of the cluster properties toward the 1D polymer properties was also tested with ab initio crystal orbital calculations.
On the calculation of model potential parameters from molecular spectra
Authors: Fairen Victor , Lopez Vicente
Published: July 2002

In this paper we propound a procedure for inversion of an energy spectrum in order to obtain the original Hamiltonian function, applicable to nonresonant multidimensional systems. We use the Lie transform method to find the appropriate EBK-quantizable Hamiltonian, explicitly dependent on the unknown potential parameters. Comparison with the observed spectra allows the determination of explicit expressions for those parameters in terms of the spectral parameters. We first illustrate these concepts with the construction of power series and perturbed Morse oscillator model potentials for diatomics, and its extension to a simple model of triatomics including bending motion. We finally apply the procedure to the construction of a Morse potential model for linear polyatomic molecules.