bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiple steady states in coupled flow tank reactors
Authors: Hunt Katharine L. C. , Kottalam J. , Hatlee Michael D. , Ross John
Published: July 2002

Coupling between continuous-flow, stirred tank reactors (CSTR’s), each having multiple steady states, can produce new steady states with different concentrations of the chemical species in each of the coupled tanks. In this work, we identify a kinetic potential ψ that governs the deterministic time evolution of coupled tank reactors, when the reaction mechanism permits a single-variable description of the states of the individual tanks; examples include the iodate-arsenous acid reaction, a cubic model suggested by Noyes, and two quintic models. Stable steady states correspond to minima of ψ, and unstable steady states to maxima or saddle points; marginally stable states typically correspond to saddle-node points. We illustrate the variation in ψ due to changes in the rate constant for external material intake (k0) and for exchange between tanks (kx). For fixed k0 values, we analyze the changes in numbers and types of steady states as kx increases from zero. We show that steady states disappear by pairwise coalescence; we also show that new steady states may appear with increasing kx, when the reaction mechanism is sufficiently complex. For fixed initial conditions, the steady state ultimately reached in a mixing experiment may depend on the exchange rate constant as a function of time, kx(t) : Adiabatic mixing is obtained in the limit of slow changes in kx(t) and instantaneous mixing in the limit as kx(t)→∞ while t remains small. Analyses based on the potential ψ predict the outcome of mixing experiments for arbitrary kx(t). We show by explicit counterexamples that a prior theory developed by Noyes does not correctly predict the instability points or the transitions between steady states of coupled tanks, to be expected in mixing experiments. We further show that the outcome of such experiments is not connected to the relative stability of steady states in individual tank reactors. We find that coupling may effectively stabilize the tanks. We provide examples in which coupled CSTR’s can be operated stably with one of the tanks at or beyond the single-tank marginal stability point.
Infinite dilution activity coefficients of nitrotoluene isomers in liquid crystal PBHpB
Authors: Zhenggang Wang , Zhiyong Liang , Yunxiao Zhong
Published: July 2002

Liquid crystal PBHpB(H15C7O–C6H4COO C6H4COO C6H4OC7H15) is used to separate m,o,p-nitrotoluene in gas chromatography experiments, and the infinite dilution activity coefficients of the three isomers are determined at various temperatures. A lattice model of mixed liquid crystal system is applied to calculate these coefficients. The results are in agreement with experimental data with properly chosen parameters.
Melting of colloidal crystals: A Monte Carlo study
Authors: Zahorchak James C. , Kesavamoorthy R. , Coalson Rob D. , Asher Sanford A.
Published: July 2002

Electrostatically stabilized colloidal crystals show phase transitions into liquid and gaslike states as the ionic impurity concentration increases. Using Monte Carlo simulations we theoretically investigate the melting of four colloidal crystals (two fcc crystals and two bcc crystals) which have also been examined experimentally. We calculate the pair correlation function g(r), the total potential energy Ut, and the mean square displacement of a particle 〈u2〉 for the colloidal suspensions at various ionic impurity concentrations ni. We calculate the structure factor S(Q) by Fourier transforming g(r). We find that the parameters gmax [the maximum of the first peak in g(r)], Smax [the maximum of the first peak in S(Q)], Δr [the half width at half maximum of the first peak in g(r)], and Ut (the total potential energy) all show discontinuous behavior on melting. We relate the calculated values of Smax, gmax, and the mean square displacement at the point of melting of our colloidal crystals to that of atomic crystals. We find that the ratio of the rate of change of the Wendt–Abraham parameter, gmin/gmax [gmin : the minimum value of g(r) after the first peak], with respect to ni, in colloidal crystal to that in liquid is constant but specific to the crystal structure (bcc or fcc). We calculate the latent heat of melting of colloidal crystals.
Energy distribution of low-energy electrons and free-ion yields in irradiated liquid hydrocarbons
Authors: Schiller Robert
Published: July 2002

Pairs of ions and low-energy electrons, below about several eV, are regarded as H-like atoms embedded in a fluctuating medium. In contrast to the polar medium case, where dielectric relaxation plays a central role, electrons lose energy via spontaneous deexcitation and undergo constant energy exchange with local fluctuations. By comparing the parameters of the pair-correlation and autocorrelation functions of the medium fluctuations with the radius and lifetime of the Rydberg states, respectively, it is strongly suggested that fluctuations interact only with the three lowest-lying states. This makes the n=4 state more populous by orders of magnitude than any other state below or above, since it is populated by fast fluctuations and depopulated by the slow spontaneous process. Hence electron escape, effected by local energy fluctuations, can be regarded as a transfer from the E(n=4) to the V0 state, the latter being the energy of the quasifree electrons. Computing free-ion yields as a function of V0, one finds good agreement with experimental data without making use of any adjustable parameter.
Useful results for the determination of excited state geometries from resonance Raman spectra: Application to inorganic complexes
Authors: Lee Soo-Y. , Lee S. C.
Published: July 2002

The time-dependent formulation of Raman scattering is used to derive simple expressions for fundamental and overtone intensities that depend on potential energy features in the Franck–Condon region and the homogeneous damping constant due to the bath modes. From the Raman excitation profiles, the dependence of the full width at half-maximum on the damping constant is calculated. The results are applied to the rich resonance Raman spectra and Raman excitation profiles of transition metal complexes, in particular, Cs3[Re2OCl10] and Cs4[W2OCl10], to determine the magnitude of the geometric changes occurring upon excitation of the molecule from the ground to the excited electronic state. For each compound, the multidimensional harmonic potential surface and damping constant derived for the excited electronic state are then used to simulate the observed Raman excitation profiles and resonance Raman spectrum.
SUq(2) quantum group analysis of rotational spectra of diatomic molecules
Authors: Esteve J. G. , Tejel C. , Villarroya B. E.
Published: July 2002

The spectra of some diatomic molecules are analyzed in terms of a SUq(2) symmetry. Particular emphasis is put on those molecules with relatively high values of Dn and Hn. The excellent agreement found between the observed and calculated wave numbers can be seen as an indication that an up to now hidden symmetry SUq(2) is present.
New singlet Rydberg states of the NH(ND) radical in the energy range 92 000–100 000 cm−1 characterized by resonance enhanced multiphoton ionization-photoelectron spectroscopy
Authors: Clement S. G. , Ashfold M. N. R. , Western C. M. , de Beer E. , de Lange C. A. , Westwood N. P. C.
Published: July 2002

Two new singlet excited electronic states of the imidogen radical have been characterized through (i) analysis of the two photon resonance enhancements they provide in the wavelength resolved multiphoton ionization spectrum of a 1Δ state NH(ND) radicals and (ii) measurements of the kinetic energy of the photoelectrons accompanying the multiphoton ionization process. The i 1Π state is shown to involve a ...3σ21π1 ion core and a Rydberg electron of mixed 3pσ/3dσ character. The zero-point level of this state in ND exhibits an anomalously large Λ-doubling. Photoelectron kinetic energy release measurements reveal that 2+1 resonance enhanced multiphoton ionization via the j 1Δ state of NH leads to population of a range of v+ vibrational levels in the ground state ion and, importantly, to a substantial population of electronically excited B 2Δ state ions. We conclude that the j 1Δ state of NH(ND) also involves a mixture of configurations, including ...3σ21π13dπ1 and the core excited configuration ...3σ11π23sσ1. All observed vibrational levels of this state in both NH and ND show perturbations, some of which have proved amenable to analysis.
Electronic spectroscopy and relaxation dynamics of OH–Ne and OD–Ne
Authors: Lin Yaomin , Fei Suli , Zheng Xiaonan , Heaven Michael C.
Published: July 2002

The structure and dynamics of OH/D–Ne complexes have been probed via studies of the A–X electronic transition. Bands associated with the OH/D 0–0, 1–0, and 2–1 transitions have been rotationally resolved and analyzed. Closely similar progressions of van der Waals vibrational levels were seen in conjunction with each parent transition. In the A state, the observed levels were assigned to the zero point, the–Ne stretch fundamental, and internal rotor-stretch combinations. From this data, the barrier to internal rotation was estimated to be 43 cm−1 and a lower limit of D′0>68 cm−1 was established for the OH/D(A)–Ne bond. Predissociation of OH/D(A,v=1,2)–Ne has been characterized by time- and wavelength-resolved fluorescence measurements. Vibrational predissociation rates were found to be in the range of (2–6)×105 s−1. Fragment OH/D(A,v=0) rotational distributions indicated that vibration–rotation transfer was the primary decay channel. Electronic predissociation of OH(A,v=2)–Ne was observed. The rate for this process was found to be dependent on the average position of the Ne atom.
A stochastic golden-rule treatment for thermal desorption of gases from solid surfaces
Authors: Zhang John Z. H.
Published: July 2002

The thermal desorption rate of a gas molecule from a solid surface has been derived using a stochastic time-dependent perturbation method. In this approach the solid surface is modeled by a fluctuating thermal bath which exerts a random force on the gas molecule. The rate of desorption is derived using a first order time-dependent perturbation method and an ensemble averaging over the distribution of random force. We use a generalized Langevin equation to model the stochastic process. The final operator expression of the rate constant contains two parameters pertaining to the solid surface, i.e., temperature T and the friction kernel γ(t). A simple analytical expression is given when the molecule–surface bond is approximated by a truncated-harmonic potential, and both the high temperature and low temperature limits of the rate constant are given in this approximation. Simplified expression of rate constant is also obtained in the limit of Markovian approximation.
Thermally stimulated depolarization currents: A new analysis and significance of the compensation law
Authors: Marchal Eve
Published: July 2002

The model introduced by Ngai to predict the relaxation behavior of condensed matter is used here to derive the expression of thermally stimulated depolarization currents (TSDC). This is done via Ngai’s second universality equation and by assuming that the stretching index β is temperature dependent. The four parameters involved in the model can, in principle, be determined from TSDC data. Two parameters are determined for polyisoprene below its glass transition temperature Tg. Furthermore, expressing that a compensation law holds for the α relaxation of polymers around Tg, the variation of β with T is given. This relation leads to a Williams–Landel–Ferry-type expression of the macroscopic relaxation time near and above Tg, a well-known experimental observation, and gives a strong support in favor of the model.
Quantum dynamics via mobile basis sets: The Dirac variational principle
Authors: Hsu D. , Coker D. F.
Published: July 2002

We propose a variation on the wave-packet interpretation of quantum mechanics introduced by Heller, here using the Dirac variational principle to optimize the time evolution of all the parameters that enter our basis set. We solve the general one-dimensional problem, and test our method on the Morse oscillator and the symmetric quartic double well.
Trajectory studies of collisional relaxation of highly excited CS2 by H2, CO, HCl, CS2, and CH4
Authors: Lendvay György , Schatz George C.
Published: July 2002

The collisional relaxation of vibrationally highly excited CS2 by thermal ensembles of H2, CO, HCl, CS2, and CH4 is investigated theoretically using classical trajectory calculations with empirical potentials that include van der Waals and electrostatic interactions. The successive collisions method suggested by Bruehl and Schatz is shown to be an effective way to study energy transfer from vibrationally hot polyatomic molecules to molecules of a thermal bath. The average energy transferred per collision to H2 and CO matches experimental measurements within a factor of 2, while that to CH4 and CS2 matches experiment within a factor of 4. These differences are consistent with combined theoretical and experimental uncertainties. Energy transfer to HCl is, however, off by as much as a factor of 7, despite our use of a potential which is realistic at long range. The energy transferred is partitioned mostly to relative translation when the collider is H2 or CO and partly to rotation for CH4. The energy dependence of the energy transfer and the interrelations among energy-transfer moments, including 〈ΔEup〉, 〈ΔEdown〉, and 〈ΔE2〉, are also reported.
An experimental and theoretical study of the doubly charged ion O2+2
Authors: Fournier J. , Fournier P. G. , Langford M. L. , Mousselmal M. , Robbe J. M. , Gandara G.
Published: July 2002

The energy levels of triplet states of doubly charge oxygen, obtained using double charge transfer spectroscopy, have been examined in the light of ab initio calculations. Four peaks were found in the double charge transfer spectrum with energies of 41.1±0.2, 43.2±0.2, 48.2±0.3, and 51.3±0.3 eV, respectively. The first and third peaks have been assigned to the 3Σ+u and the 3Πu excited states, respectively. The second peak has been assigned to the population of the 3Πg states. The configuration of the state at 51.3 eV is unknown. The effect of the symmetry rule Σ+↔/Σ− is discussed and the validity of the spin conservation rule is demonstrated when a proton is used as a projectile in this experimental technique.
Site-selective resonance fluorescence of Eu+3-exchanged sodium β″-alumina
Authors: Laberge M. , Simkin D. J. , Dunn B.
Published: July 2002

Site-selective excitation of Eu+3 β″-alumina into the 5D0 level (568–579 nm) shows that the europium ion occupies at least two types of site distributions. The first distribution occurs at the higher energy of the tuning range and has site symmetry C3v or C3. It corresponds to the crystallographic Beevers–Ross position. The other site distribution is observed at lower energy and is associated with the mid-oxygen crystallographic position (site symmetry: C2h ). We also show that interpretation of the site-selective spectroscopy of Eu+3 β″-alumina must take into account the occurrence of site-to-site energy transfer linked to vibronic coupling phenomena.
Electron paramagnetic resonance study of Cu(II) in the Cu(II) doped (NH4)2[Zn(NH3)2(CrO4)2] and (NH4)2[Cd(NH3)2(CrO4)2]: Orientational disorder of NH3 and dynamically induced structural change
Authors: Wang D. M. , Kovacik I. , Reijerse E. J. , de Boer E.
Published: July 2002

Electron paramagnetic resonance (EPR) spectra recorded at X band on Cu(II) in the Cu(II) doped single crystals of (NH4)2[Zn(NH3)2(CrO4)2] and (NH4)2[Cd(NH3)2(CrO4)2] showed an unusual temperature dependence in the temperature range between 4.2 and 370 K. This unusual temperature dependence is attributed to the dynamics of NH3 molecules. It is concluded that NH3 undergoes an order–disorder transition. This transition occurs at 282±2 K (T*) for (NH4)2[Zn(NH3)2(CrO4)2] and at 300±5 K for (NH4)2[Cd(NH3)2(CrO4)2]. For temperatures above T* the EPR spectra are found independent of temperature. This observation indicates that the NH3 molecules behave as free rotors rotating about their threefold symmetry axis. As the temperature is lowered, the free rotation gradually becomes hindered which leads at T* to site doubling in the EPR spectra. The hindered rotation simultaneously induces a structural change at T* reducing the local symmetry at the Cu(II) site from C2/m to approximately Cs. In the temperature range between T* and about 80 K, the EPR spectra with the magnetic field perpendicular to the threefold axis of NH3 are temperature dependent, but those with the magnetic field parallel to the threefold axis remain unchanged. This behavior is qualitatively interpreted as being due to the torsional oscillation of NH3. At temperatures lower than 80 K, the NH3 molecules exhibit the torsional oscillation of the lowest state and accordingly, all the EPR spectra become temperature independent.
The equilibrium state of solid–liquid interfaces of aliphatic compounds
Authors: Liu Xiang-Yang , Bennema P.
Published: July 2002

The ordering and orientation of fluid units at interfaces of the aliphatic crystal solution system are the theme of this paper. Based on the idea that the bulk phase and the interfacial phase are two different phases, appropriate relations between the interfacial bond energies and the bulk bond energies are developed by introducing a surface scaling factor Cl. As a description of the interfacial structure of aliphatic compounds, it is assumed that a high degree of ordering is stored in the interfacial fluid phase. Then the so-called α molecule approach is used to estimate the values of bond energies in the interfacial phase. This leads to the result of Cl≤1, meaning that the the interfaces of n-paraffin crystals and the solutions will be extra wetting. This is consistent with the experimental data obtained from the roughening transitions occurring in several n-paraffin–n-hexane solution systems. Finally, a roughening phase transition diagram of the n-C23H48–n-hexane system is presented, which indicates that the roughening temperature of a surface on a crystal is concentration dependent.
Monte Carlo studies of grain boundary segregation and ordering
Authors: Alba William L. , Whaley K. Birgitta
Published: July 2002

This paper describes a systematic microscopic study of solute segregation and ordering at a grain boundary. We develop for this inhomogeneous system several Monte Carlo techniques and apply these to analyze the distribution of substitutional impurities near a symmetric coincident-site-lattice tilt boundary. The calculations demonstrate the importance of ensemble and boundary condition for a Monte Carlo simulation, especially one with an inhomogeneous lattice and with ordering, as opposed to segregating, bulk interactions. The resulting concentration profiles exhibit segregation to the boundary at high temperatures and bulk ordering at low temperature. Based on our results, we propose a mechanism for a solid–solid interfacial ordering phase transition previously suggested by experiment. We also compare these simulations to our earlier one-dimensional mean-field work and find that the three-dimensional simulations confirm the essential mean-field predictions.
Nuclear modulation effects in high-spin electron systems with small zero-field splittings
Authors: Coffino Alan R. , Peisach Jack
Published: July 2002

An analytic theory has been developed to simulate the nuclear modulations arising from the application of the two-pulse electron-spin-echo envelope modulation (ESEEM) technique to a high-spin-electron system with S=5/2, coupled both to its own nuclear spin, I′=5/2, and a distant nuclear spin, I=1/2. This theory differs from that previously used by Mims [Electron Paramagnetic Resonance, edited by S. Geschwind (Plenum, New York, 1972), pp. 263–351] to describe the ESEEM results of a simpler system (S=1/2, I=1/2) by the inclusion of a zero-field splitting (ZFS) term in the spin Hamiltonian. The major assumption made in the derivation is that only transitions that do not share a common electron energy level are excited. This and other assumptions are discussed. The resulting analytic form suggests that each of the five possible single-quantum electron transitions in the S=5/2 system make a contribution to the ESEEM spectrum. The contribution from each electron transition is similar to that described by Mims for the simpler system, i.e., two nuclear frequencies, a sum frequency and a difference frequency. The frequencies and intensities, however, differ from those predicted for the simpler spin system. The nuclear frequencies are shown to be a function of the particular electron transition excited by the microwave field. Whether or not a specific electron transition makes a contribution to the ESEEM spectrum is shown to be a function of the magnitudes and relative orientations of the ZFS and superhyperfine tensors. This results in an orientation selective property of the ESEEM experiment imparted by the anisotropy of the ZFS term. This is shown to affect the predicted powder-pattern-averaged spectra. Although there is a specific application to the case of Mn(II), the result is general and is applicable to all-electron high-spin systems with S≥1/2.
Pure absorption-mode chemical exchange nuclear magnetic resonance spectroscopy with suppression of spinning sidebands in a slowly rotating solid
Authors: Geen Helen , Bodenhausen Geoffrey
Published: July 2002

A new technique is presented for studying chemical exchange in solids under conditions of magic angle spinning (MAS); it is based on the original two-dimensional exchange experiment but uses the combination of a TOSS (total suppression of sidebands) sequence with its time-reversed counterpart in the first dimension to effect evolution under the isotropic chemical shift alone. Pure absorption-mode spectra can then be obtained in the usual way. One version of this experiment also uses TOSS at the start of acquisition, producing an exchange spectrum that has isotropic chemical shifts in both frequency dimensions. This is useful for isotropic powder samples with anisotropies that are too large to be averaged completely by sample spinning but are still within the regime where TOSS can be applied. A second experiment allows free MAS evolution during acquisition, giving a spectrum with isotropic shifts in the first dimension and both shifts and sidebands in the second dimension. This version has two important features: (i) it does not suffer from the intensity losses normally inherent in TOSS when the anisotropy is large compared to the spinning speed; and (ii) it is applicable even to samples that have an anisotropic distribution of crystallites. The tautomeric hydrogen shift in solid tropolone is used to illustrate how chemical exchange can be readily monitored, irrespective of the number of spinning sidebands present in the one-dimensional MAS spectrum. This method of obtaining ‘‘isotropic evolution’’ can be used in any two- or three-dimensional MAS experiment and provides a practical alternative to high-speed MAS.
A molecular beam study of ethane on Si(111)7×7: Energy accommodation and trapping
Authors: Thoms B. D. , Lorraine P. W. , Ho W.
Published: July 2002

The interaction of a molecular beam of ethane with the Si(111)7×7 surface has been studied using electron energy loss spectroscopy (EELS), low energy electron reflectivity (LEER), and temperature programmed desorption (TPD). Dissociative adsorption of ethane is not observed on this surface at <90 K for incident energies up to 1.5 eV and nozzle temperature of 1000 K. Ethane adsorbs molecularly on Si(111)7×7 at <80 K in submonolayer coverages and forms multilayers at temperatures below 66 K. The monolayer is found to desorb with first order kinetics with an activation energy of 0.25 eV and pre-exponential factor of 1014±1 s−1. At normal incidence, the probability of trapping is found to decrease with increasing translational energy, falling by a factor of 10 as the incident energy is increased from 0.1 to 1.5 eV. The data are fit by a hard cube model with an effective surface mass of 100 amu, which is between three and four times the mass of a silicon atom. For impact energies ≳0.8 eV, trapping is observed to be higher than would be expected from the hard cube model. Possible explanations for this deviation are discussed. The trapping probability is found to obey total energy scaling. Changes in the internal energy of the ethane due to variations in the nozzle temperature from 300 to 1000 K are observed to produce negligible effects on the trapping probability.
The complex coordinate scattering theory: Broken inversion symmetry of corrugated surfaces in helium diffraction from Cu(115)
Authors: Peskin Uri , Moiseyev Nimrod
Published: July 2002

The complex coordinate scattering theory is reformulated and applied to He/Cu(115) scattering. The weakly asymmetric corrugated surface leads to strong dependence of the diffraction intensities on the direction of the incident He beam. The calculated transition probabilities are in excellent agreement with the experimental values measured by Perreau and Lapujoulade. We show that additional information about the atom/surface interaction potential can be obtained if another experiment would be carried out when the Cu crystal will be rotated by 180°.
Electron–ion recombination rate constants in gaseous and liquid krypton
Authors: Honda Kenji , Endou Kentaro , Yamada Hitoshi , Shinsaka Kyoji , Ukai Masatoshi , Kouchi Noriyuki , Hatano Yoshihiko
Published: July 2002

Electron drift mobilities μe and electron–ion recombination rate constants kr have been measured by the analysis of transient current induced with the irradiation of an x-ray pulse on gaseous or liquid krypton in a cell as a function of external electric field strength. The effects of an external dc electric field on kr up to 12 mTd (1 Td=10−17 V cm2/molecule) in liquid and 63 mTd in gas have been examined. The observed kr values in both liquid and gas phases have been found to be much smaller than those calculated by the reduced Debye equation. The deviation has been compared with recent theoretical studies, leading to the conclusion that recombination in liquid and gaseous krypton is not the usual diffusion-controlled reaction. The kr values in the gas phase increase with an external dc electric field up to a critical electric field strength below which μe values are nearly constant, and above which both kr and μe values descrease with further increase in the electric field for both gaseous and liquid phases.
Intersecting resonances and chaos in a three-oscillator model. I. Classical studies
Authors: Atkins Keith M. , Logan David E.
Published: July 2002

We consider a model system of three weakly coupled, weakly anharmonic oscillators, containing two near 2:1 Fermi resonances. This system is initially reduced via a resonance approximation to one of two coupled pendula, each pendulum representing one of the interoscillator resonances. This leads to a natural description of the system in terms of analytically predictable resonance zones and their intersection in action space. A full numerical analysis of the classical dynamics of the two-pendulum system is given, with respect to both the appearance of stochasticity and the development of periodic orbits in the resulting two-dimensional area-preserving map. The success of the resonance approximation in describing adequately the behavior of the three-oscillator model is shown by comparison with a representative study of the classical dynamics of the full three-oscillator model.
Absorption, luminescence, resonance Raman, and resonance coherent anti-Stokes Raman spectroscopy on unsymmetrically substituted diacetylene single crystals with color zones
Authors: Materny A. , Kiefer W. , Schwoerer M.
Published: July 2002

We have analyzed the color zones in single crystals of the diacetylene derivative 6-(p-toluenesulfonyloxy)-2,4-hexadiynyl-p-fluorobenzenesulfonate by means of various optical spectroscopical methods in order to derive a plausible model for the origin of this type of chromism. By applying absorption, luminescence, resonance Raman, and coherent anti-Stokes Raman spectroscopy we obtained sufficient information for an understanding of the nature of the color zones. With the help of a nearly free-electron model calculation and a theoretical model for side group torsions, we found that a disturbance of the π-electron conjugation by a defect hindrance on side groups is the proper reason for the observed chromism. Growth defects in {111} crystal-growth sectors with a Burgers vector in the [010] direction cause the shape of the color zones. We found, particularly, that resonance coherent anti-Stokes Raman spectroscopy proves to be a valuable tool for the investigation of electronic and structural properties of the diacetylene crystals.
Wave packet dynamics of vibrational quenching in collisions of Kr and O2+
Authors: Goldfield Evelyn M.
Published: July 2002

Collisional quenching of vibrationally excited O+2 by Kr atoms is studied using time-dependent quantum mechanics with an emphasis on exploring the underlying mechanisms. Three-dimensional solutions to the time-dependent Schrödinger equation with zero total angular momentum are obtained at two values of the average collision energy, 0.5 and 0.1 eV. At 0.5 eV, the 1→0 quenching probability is computed to be 0.091. The rotational distributions of the vibrationally quenched O+2 are highly bimodal. An analysis of ‘‘nascent’’ v=0 probability density reveals the origin of this bimodality and yields insight into the mechanisms of vibrational deexcitation. Comparisons are made to a corresponding classical study. Scattering at 0.1 eV is complicated by the existence of overlapping resonances in addition to the direct scattering. Due to these resonances, the quenching probability is an extremely sensitive function of energy. An averaged quenching probability of 0.078 at 0.1 eV is computed. Average resonance lifetimes of 2.5–3.5 ps are also obtained from an analysis of the temporal behavior of the dissociating wave packet. The influence of the potential anisotropy on the quenching probabilities is discussed.
Electron spin resonance and electron nuclear double resonance studies in single crystals of N-methylurea oxalic acid (2:1)
Authors: Kumar M. Ravi , Lingam K. V. , Rao T. K. Gundu
Published: July 2002

Radicals trapped in a γ-irradiated single crystal of N-methylurea oxalic acid have been investigated using the techniques of electron spin resonance (ESR) and electron nuclear double resonance (ENDOR). Detailed ENDOR studies have been carried out on the two initially observed radicals. One of these radicals has been assigned to a protonated anion. Two protons, belonging to neighboring molecules, have been identified as transferred protons. The other radical is formed at the urea molecule by hydrogen abstraction from the methyl group. The observed completely transferred proton exhibits very small isotropic coupling. The third radical seen, only in aged crystals, after prolonged duration has been assigned to a R–ĊHOH type of radical. These results are compared to those from the urea oxalic acid systems.
Oxygen induced added-row reconstruction of the Ni{110} surface
Authors: Bu H. , Roux C. D. , Rabalais J. W.
Published: July 2002

The oxygen induced reconstructed phases of the Ni{110} surface have been studied by time-of-flight scattering and recoiling spectrometry (TOF–SARS). The substrate structures are determined from experimental measurements of azimuthal angle (δ) and polar incident angle (α) anisotropies in the scattered Ne intensities coupled with classical trajectory simulations for shadow cone analysis. By monitoring features in the TOF–SARS scans that are unique to specific phases, it is possible to follow the migration of the first-layer Ni atoms as a function of O2 exposure. The results show that upon increasing exposures of the clean Ni{110}–(1×1) surface to O2, a series of LEED patterns [initial p(3×1), p(2×1), and final p(3×1)] is produced corresponding to three surface phases which differ only in the density of the first-layer Ni 〈001〉 rows. These nascent ‘‘added rows’’ are stabilized by bonding to oxygen atoms which reside in the long-bridge positions along the 〈001〉 rows. Structural models for the three phases are confirmed from the TOF–SARS data and the inhibiting effects of carbon and sulfur impurities on the reconstruction is considered.
Brownian dynamics simulation of uniform comb polymers in two dimensions
Authors: Bishop Marvin , Saltiel Craig J.
Published: July 2002

Brownian dynamics simulations are made for continuum, excluded, and nonexcluded volume, uniform H-comb polymers in two dimensions. It is found that the end-to-end distance and radius of gyration of the inner branches are expanded more than those of the outer branches for excluded volume combs. The ratio of the end-to-end distance scaling amplitudes appears to be a universal quantity in agreement with the conjecture of Gaunt et al.1 Moreover, the ratio of the radii of gyration scaling amplitudes seems to be universal.
Energy and momentum distributions and projections in the scattering of CO from Ag(111)
Authors: Hanisco Thomas F. , Yan Chun , Kummel Andrew C.
Published: July 2002

We have studied the cross correlations between the rotational state distributions, angular momentum alignment, and velocity distributions of monoenergetic, rotationally cold CO and N2 scattered at 0.75 eV from Ag(111). Measurements were made for both normal incidence and glancing incidence beams and for specular and off-specular detection. The comparison between N2 and CO is most dramatic. (1) For N2 the rotational state selected velocity distributions are very narrow while for CO the rotational state selected velocity distributions are wide. (2) For glancing incidence beams, N2 exhibits a higher degree of parallel momentum conservation than CO. (3) The velocity resolved rotational rainbows for N2 are more prominent for glancing incidence while the velocity resolved rotational rainbows for CO are more prominent for normal incidence. (4) There is 100% cartwheeling type alignment for N2 in medium and high exit rotational states while for CO the alignment is weak except at the very highest rotational states where it is still <100% cartwheeling. Our data can be interpreted as showing that the N2 molecules at these relatively high energies collide with a slightly corrugated surface and have nearly linear trajectories. Conversely, the CO scattering data are consistent with scattering from a more corrugated surface. The CO molecules have a permanent dipole moment, therefore the gradient for the CO–Ag(111) gas surface potential with respect to molecular orientation is larger. In addition, CO has a deeper physisorption well on Ag(111). Thus, the CO molecules probe deeper into the corrugated repulsive portion of the potential and have a more inelastic collision that results in greater rotational and phonon excitation but lower exit translational energy. The lower alignment for CO scattering into high J states is consistent with the CO molecules having curved exit trajectories and/or multiple collisions with the surface. For both N2 and CO, rotational excitation into high J states scales with the normal component of incident translational energy, but the phonons can be excited by both the parallel and normal components of the incident translational energy.
Time-dependent Monte Carlo studies of diffusion with surface traps
Authors: Bowler Ann M. , Hood Eric S.
Published: July 2002

Surface adsorbate diffusion is examined using a numerical algorithm which incorporates a kinetic treatment in conjunction with a time-dependent Monte Carlo formalism. The method is based upon a probabilistic description of adparticle jump events. The diffusion rate is determined by the energetics of interactions among adparticles, the surface substrate atoms, and surface defects. The rare event problem associated with other theoretical treatments of diffusion is overcome by our highly efficient algorithms. Consequently, we are able to observe events, including adsorbate overlayer ordering and island formation, which occur over time scales which are longer by orders of magnitude than those for simple adsorbate diffusion. Our initial investigations of perfect surfaces indicated that a variety of diffusion mechanisms were operative depending on the adparticle interactions and other controlling parameters in the system. We have now examined the effects of surface traps on adsorbate diffusion dynamics. We find that, at low attractive adparticle interactions, the surface traps become titrated after which diffusion proceeds as it might otherwise. At higher adparticle interactions, diffusion is found to be strongly affected by the surface traps, even beyond the range of direct influence of the trap sites. In fact, we find that traps may act as nucleation sites for both cluster formation and long range island formation. We conclude that no comprehensive analysis of surface diffusion can fail to address the effects of imperfections such as these on the diffusion rate and mechanism.
The role of curvature and pulse width for transition to unstable wave fronts in the Belousov–Zhabotinsky reaction
Authors: Nagy-Ungvarai Zsuzsanna , Ungvarai Janos , Müller Stefan C. , Hess Benno
Published: July 2002

Systematic measurements of the stability of chemical waves were carried out over a wide range of reagent concentrations in the cerium-catalyzed Belousov–Zhabotinsky reaction. The critical radius and the width of the autocatalyst band in the wave fronts were measured in order to verify the computed prediction for the region of parameters where lateral instabilities of the wave ends appear. The data yield an experimental parameter set for the onset of unstable behavior and allow a discussion about different theoretical descriptions of the critical curvature.
CO product distributions from the visible photodissociation of HCO
Authors: Neyer D. W. , Kable S. H. , Loison J-C. , Houston P. L. , Burak I. , Goldfield E. M.
Published: July 2002

The final state distribution of carbon monoxide produced in the photodissociation of the formyl (HCO) radical has been studied both experimentally and theoretically. Renner–Teller coupling between the excited HCO Ã state and the ground state leads to dissociation and yields H and CO. Vibrational and rotational distributions have been measured for CO produced after excitation to specific vibrational levels on the Ã←X̃ transition of HCO cooled in a supersonic expansion. The strongest transitions are for excitation to vibrational states with six to 16 quanta in the bending mode, and dissociation from these states produces inverted CO rotational distributions with average rotational quantum numbers <J≳ in the 22–33 range. The value of <J≳ increases monotonically with the vibrational quantum number describing the bend of the excited triatomic. Experiments involving excitation of one quantum of the C–H stretching motion have revealed that this vibration results in increased rotational excitation of the product CO with values of <J≳ as high as 41. In contrast, experiments indicate that the C–O stretching mode of HCO acts nearly as a spectator during the dissociation process. Excitation of HCO states with one quantum of C–O stretch yields vibrationally excited CO as the dominant dissociation product, but with a rotational distribution similar to that for CO(ν=0) produced following the excitation of HCO states without the quantum of C–O stretch. Classical trajectory calculations on an ab initio potential energy surface have modeled many of the experimental features and trends of the CO product distributions. There are, however, some discrepancies in the positions of rotational maxima and in the efficiency of the coupling of the C–O vibration of HCO to the dissociation coordinate. It is not clear whether these are due to approximations made in the modeling or inaccuracies in the potential energy surface.
Spin–orbit effects in the luminescent charge-transfer reaction F++CO
Authors: Ottinger Ch. , Reichmuth J. , Zimmermann S. , Kusunoki I. , Shiraishi Y.
Published: July 2002

In an ion beam study of the luminescent charge-transfer reaction F+(3P0,1,2)+CO(X 1Σ+)→F(2P1/2,3/2) +CO+(A 2Π1/2,3/2), the product CO+(A) emission spectra have been examined at a collision energy of 11.9 eVCM, using high (∼1 Å FWHM) optical resolution. For the near-resonant vibrational level v′=5, the 2Π3/2 S/O component of CO+(A) was found to be formed with an excess population of ∼40% relative to the 2Π1/2 component. Lower-resolution data show a similar enhancement of 2Π3/2 also for the v′=6 level, but not for v′=4 and 3. This can be rationalized in terms of recent ab initio calculations. Also in accordance with these calculations, the product rotational excitation was found to be very low in the v′=5 level. The best-fit rotational ‘‘temperature’’ is for this level actually less than the target gas temperature, a common artifact in collisional processes with small angular momentum transfer.
Chemical potentials based on the molecular distribution functions. An exact diagrammatical representation and the star function
Authors: Lee Lloyd L.
Published: July 2002

A closed form for the chemical potentials of a fluid is presented that involves only integrals of the molecular distribution functions at the given state, (e.g., temperature and density). Thus no Kirkwood charging or thermodynamic integration is needed. An exact formula from a previous study is reanalyzed and a diagrammatical representation of the correlation functions involved is given. This representation involves, in addition to the usual total correlations, direct correlations, and the bridge function, B(r), a new star function, S(r). Analysis shows that the integral of the star function is the primitive of the bridge function, i.e., its functional derivative yields B(r). It is also related to the free-energy functional F[ρ] in density-functional theories for nonuniform systems. Methods for estimating the star function are given. Tests on uniform hard-sphere fluid are carried out to demonstrate the new formulas. We have examined several current closures: the Percus–Yevick, Martynov–Sarkisov, Ballone–Pastore–Galli–Gazzillo, and a Verlet-modified (VM) closure. The VM approach gives the best reproduction of the bridge function. Much improved results are obtained for the chemical potentials of hard spheres at densities ρd3 ranging from 0.3 to 0.85.
State-to-state rotational energy transfer in highly vibrationally excited acetylene
Authors: Tobiason J. D. , Utz A. L. , Crim F. F.
Published: July 2002

Vibrational overtone excitation of single rovibrational eigenstates in acetylene, followed by state-resolved, laser-induced fluorescence (LIF) interrogation of the collisionally populated quantum states, permits a direct determination of both the pathways and rates of state-to-state rotational energy transfer in a polyatomic molecule containing about 10 000 cm−1 of internal energy. The data, which we acquire under single-collision conditions, demonstrate the importance of rotational energy transfer, even at high levels of vibrational excitation. The observed state-to-state rotational energy transfer pathways populate a wide range of angular momentum states and account for about 70% of the total relaxation rate. About one-third of the total relaxation occurs by ‖ΔJ‖=2 transitions, which are the smallest allowed, but there are also single-collision energy transfer pathways with ‖ΔJ‖ as large as 20 and ‖ΔE‖ as large as 600 cm−1 (≊3kT). The state-resolved rate constants for rotational energy transfer decrease monotonically as the energy difference between the initial and final states increases. Empirical exponential energy gap and combined power-exponential gap fitting relations recover the energy dependence of the state-to-state rate constants, but a simple power gap law does not. The discrepancy between the total observed rotational energy transfer rate and the total collisional relaxation rate suggests that rapid vibrational energy transfer, perhaps enhanced by Coriolis or anharmonic coupling, occurs as well.
Spectra of the rare gas hydrides. IV. Three new bands of argon deuteride involving a low-lying ‘‘p’’ Rydberg state
Authors: Dabrowski I. , DiLonardo G. , Herzberg G. , Johns J. W. C. , Sadovskií D. A. , Vervloet M.
Published: July 2002

Three fairly strong emission bands of ArD at 6100, 6900, and 10 200 cm−1 have been studied at high resolution. They show very different structures: the first exhibits only Q-form branches, the second P, R, O, and S-form branches, and the third Q, S, and O-form branches. The rotational analysis of each of these bands is not trivial, mainly because the upper state of the first which proved to be also the lower state of the second and third is strongly affected by l uncoupling [i.e., transition to Hund’s case (d)] and as a result the usual method of combination differences cannot be applied in their analysis. However, more sophisticated combination differences have been found between the three bands which lead to unambiguous assignments of rotational quantum numbers and thus to rotational constants. It was found that the upper state of the 6100 cm−1 band can be most conveniently represented as a p(2Σ,2Π) complex. This assumption, together with the theoretical selection rules for such a case, led to a full understanding of the structure of the three bands.
Quasiclassical integral cross sections for H+H2(0,j=0,2)→H2(1,j′=1,3,5)+H
Authors: Mandy M. E. , Martin P. G.
Published: July 2002

This paper presents state-to-state integral cross sections for the exchange reaction H+H2(0,j=0,2)→H2(1,j′=1,3,5)+H calculated from quasiclassical trajectories (QCT) on the Liu–Siegbahn–Truhlar–Horowitz (LSTH) potential energy surface in the relative energy range 0.4 to 1.2 eV. These are compared to results of exact quantum calculations at relatively low total energy (∼1 eV) [J. Z. H. Zhang and W. H. Miller, Chem. Phys. Lett. 153, 465 (1988)] and to those of the recent experiments of Kliner et al. [J. Chem. Phys. 94, 1069 (1991)]. Cross sections were calculated directly from trajectories starting in the (0,0) and (0,2) states and indirectly, using microscopic reversibility, from trajectories starting in (1,j′). The resulting QCT cross sections are in reasonable agreement with the quantum calculations, the indirect QCT results being the more satisfactory near threshold. The QCT cross sections behave smoothly with energy and, as expected, do not exhibit mild undulating structure such as is seen in the quantum calculations for (0,0)→(1,1); this is a quantum effect. Also included are QCT cross sections at translational energies near 1.28 and 1.90 eV which are relevant to the prompt reaction used in the experiments of Kliner et al. and for which quantum results are unavailable. The quasiclassical result was found to be somewhat higher than the experimental product ratio (1,3)/(1,1).
Intramolecular and dissociation dynamics of the CF2Br radical
Authors: Bintz Karen L. , Thompson Donald L. , Gosnell T. R. , Hay P. Jeffrey
Published: July 2002

Classical trajectory methods were used to investigate the nature of the intramolecular dynamics (quasiperiodic vs chaotic) of the CF2Br radical. The potential energy surface is based on empirical and ab initio results. Power spectra show that the Br–C–F bend exhibits quasiperiodic dynamics while the other modes are chaotic. Despite the presence of quasiperiodic dynamics, the dissociation rates for mode-specific excitations of the normal modes are essentially the same as those for equipartitioning of the excitation energy among all the normal modes.
Cross relaxation in magnetic resonance: An extension of the Solomon equations for a consistent description of saturation
Authors: Boulat Benoit , Bodenhausen Geoffrey
Published: July 2002

In magnetic resonance studies of cross relaxation or Overhauser effects by steady-state continuous irradiation, it is shown that the Solomon equations do not allow one to obtain a consistent picture of the expectation values of various magnetization components. The contradictions can be resolved if, in addition to the Zeeman polarizations, one takes into account a transverse magnetization component that is associated with the spin which is being irradiated.