bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic and nuclear effects in ion-induced desorption from NaCl{100}
Authors: Postawa Z. , Maboudian R. , El-Maazawi M. , Ervin M. H. , Wood M. C. , Winograd N.
Published: July 2002

Multiphoton resonance ionization (MPRI) spectroscopy has been employed to investigate the ejection mechanisms of neutral and ionic particles from an ion-bombarded NaCl{100} single crystal. The results are used to reveal the similarities and the differences between ion bombardment and electron irradiation of alkali halides. The mass spectra of neutral species and positive and negative ions have been measured. The yield of Na+ ions is found to be two orders of magnitude higher than in measurements with electron bombardment. It is suggested that the secondary ions are created by direct emission from the collision cascade. The ejection of neutral Na atoms is observed to be very sensitive to the temperature of the target, the angle of incidence, and the state of the surface as determined by the time-of-flight (TOF) measurements. In particular, it is found that most of the neutral Na atoms are emitted with thermal energies, which indicates that desorption via electronic transitions dominates over ejections from collision cascades. The relative yield of the collisional component to the thermal component is found to vary significantly as the surface structure is modified. This investigation emphasizes the importance of measurements with low incident-ion dose which allows one to decouple the single ion/surface interaction from the accumulative effect of ion-induced surface modifications.
The reactions of Si+ ions with CH3SiH3, CH3SiD3, C2H6, and CH3CHD2
Authors: Lim K. P. , Lampe F. W.
Published: July 2002

The reactions of Si+ with CH3SiH3, CH3SiD3, C2H6, and CH3CHD2 have been studied in a tandem mass spectrometric apparatus over the kinetic energy range of 1–10 eV laboratory-frame-of reference (LAB). In all systems, the major process is the formation of SiCH+3, as well as SiCH2D+ and SiCHD+2 in the case of the reaction with CH3CHD2. It is shown that in the reaction of Si+ with CH3SiH3 and CH3SiD3, the process is best described as a Walden inversion, while in the reaction with C2H6 and CH3CHD2, the process appears to approximate the spectator stripping model or modified spectator stripping (polarization-reflection model). In the reaction with CH3CHD2, the slight preference of Si+ to strip the CH3 radical rather than the CHD2 radical is shown to be in accord with a cross-sectional energy dependence of approximately E−1.
From bistability to temporal oscillations and spatial pattern formation
Authors: Li Ru-Sheng , Wu Xiaomao
Published: July 2002

The idea ‘‘from bistability to oscillations,’’ originally proposed by De Kepper and Boissonade for studying and designing chemical oscillations, is extended to ‘‘from bistability to spatial pattern formation.’’ To carry out such extension, the idea from bistability to oscillations is discussed again by intuitive arguments based on the analysis of the intersection of the nullclines of kinetic equations and the linear stability analysis. By analogy between the kinetic equations of homogeneous reactions and the stationary equations of reaction–diffusion processes, similar intuitive arguments and linear stability analysis are applied to the reaction–diffusion equations, which leads to the conclusion that the homogeneous steady state being on the middle branch of the nullcline of the bistable subsystem is a necessary condition for spatial pattern formation. This condition becomes sufficient if the diffusion coefficients of the species involved in the feedback subsystem are much larger than those of the species which constitute the bistable subsystem. To demonstrate the validity of these conclusions, an example of heterogeneous catalysis system is studied analytically and numerically, and various temporal-spatial patterns, including the coexistence of a homogeneous steady state and an oscillating state or a spatial pattern, are revealed. The potential applications of the idea from bistability to spatial pattern formation to the experimental study and design of spatial patterns are discussed.
Reaction dynamics from orbital alignment dependence and angular distributions of ions produced in collision of Ba(1P) with NO2 and O3
Authors: Suits Arthur G. , Hou Hongtao , Davis H. Floyd , Lee Yuan T.
Published: July 2002

Angular distributions, orbital alignment dependence, and energy dependence of the relative cross sections of various ions produced in crossed beams collisions of electronically excited barium with O3 and NO2 were used to explore the dynamics of these reactions. The Ba+ product from both reactions showed strong dependence on alignment of the Ba(1P) p orbital with respect to the relative velocity vector. The Ba+ was generally forward or forward-sideways scattered, with the latter favored by the dominant perpendicular orbital alignment. Similar results for Ba+ from both O3 and NO2 suggest that electron transfer is favored for large impact parameter collisions in which the Ba p orbital is directed toward the molecule at the critical configuration, regardless of the symmetry of the available orbital in the isolated molecule. BaO+2 from the O3 reaction exhibited angular distributions and energy dependences which were reminiscent of the Ba+, albeit 2 orders of magnitude lower in intensity, indicating a close relationship between these two channels. Important differences in the dynamics for the BaO+ channel were suggested in the angular distributions, which show a substantial backscattered component, as well as the alignment effects and energy dependence. These indicate that the BaO+ probably derives from neutral BaO* which is sufficiently internally excited to undergo vibrational autoionization.
A diffusion–reaction problem with adsorbate interactions
Authors: Datar A. S. , Prasad S. D.
Published: July 2002

Adsorbate interactions have been shown to widen the region of multiplicity for a model of Bragg–Williams and quasichemical approximation lattice gas.
The CO–CO interactions in transition-metal hexacarbonyls studied by Penning ionization electron spectroscopy: Cr(CO)6, Mo(CO)6, and W(CO)6
Authors: Masuda Shigeru , Harada Yoshiya
Published: July 2002

Penning ionization electron spectra resulting from thermal collisions of He*(2 3S) metastable atoms with gaseous Cr(CO)6, Mo(CO)6, and W(CO)6 were measured to probe the CO–CO interactions. The energy splitting of the CO 5σ-derived levels [I.P.(8a1g)−I.P.(8t1u)] was determined for the first time. Further, the spectra show that the through-space interaction among the CO 5σ orbitals not only stabilizes the orbital energy of the CO 5σ-derived 8a1g molecular orbital, but also gives rise to a diffuse electron distribution. The CO–CO interactions in the transition-metal (TM) carbonyls were compared to those in the CO overlayer formed on TM surfaces.
Coriolis interaction in polyatomic molecules: A quantum qualitative approach to a two mode model system
Authors: Martens Craig C.
Published: July 2002

A qualitative method for visualizing vibration–rotation eigenstates of polyatomic molecules is described and applied to a ‘‘coupled spin’’ model of Coriolis interaction of two nearly degenerate vibrations and molecular rotation. The method employs generalized coherent states of the Lie group SU(2)⊗SU(2) to define a quantum surface of section for coupled angular momentum problems. Stationary states of the vibration–rotation Hamiltonian are visualized on the rotational surface of section, and compared with the corresponding classical phase space structure. Striking classical-quantum correspondence is observed. The parallels between classical and quantum properties are investigated for several values of the total angular momentum, total number of vibrational quanta, and total energy. In addition, the evolution of classical and quantum mechanical phase space structure with variation of parameters in the Hamiltonian is studied.
Mechanism of thermal electron attachment to SO2
Authors: Shimamori Hiroshi , Nakatani Yoshitsugu
Published: July 2002

Rate constants for thermal electron attachment to SO2 in pure SO2 and SO2–M mixtures (M=N2, CO2, and n-C4H10) have been measured using the pulse radiolysis microwave cavity method in the pressure range from 0.4 to 700 Torr and at temperatures between 253 and 343 K. In all systems, the pressure dependence of the effective two-body attachment rate constants shows a combination of two saturation behaviors. The rate constants vary significantly with the nature of the medium gases, but very little temperature dependence has been observed. A new attachment mechanism has been proposed in which two different two-step, three-body processes (distinguished by I and II) occur simultaneously, and each involves different compound-negative-ion states of the SO−2 ion. The initial two-body attachment rate constants of (3.7±0.2)×10−12 and (5.0±0.5)×10−11 cm3 molecule−1 s−1 have been determined for processes I and II, respectively. Process II becomes dominant at higher gas pressures. The three-body rate constants range from 10−31–10−28 cm6 molecule−2 s−1 depending on the medium gas and the values for process I are relatively larger than those for process II. Lower limits of the autoionization lifetimes are estimated to be 4.7×10−8 and 1.2×10−10 s for the intermediate ions involved in processes I and II, respectively.
Photofragmentation of mass-selected (C6H6)+n clusters: Measurement of monomer–cluster binding energy for n=7–15
Authors: Beck S. M. , Hecht J. H.
Published: July 2002

Benzene cluster cations up to 15 molecular units in size, produced in the throat of a supersonic nozzle by laser-induced plasma formation, are studied by mass-selected photofragmentation spectroscopy. All of the clusters exhibit a strong, broad absorption which extends at least from 690 to 1064 nm, centered near 900 nm. This absorption is assigned to the dimer based intervalence transition observed in solid state spectra of the dimer ions. Excitation into this band produces prompt ejection of neutral benzene molecules from the cluster ion. At low fragmentation laser intensities, a single daughter ion species is observed. We observe a simple dependence of the number of neutral molecules ejected on the fragmentation laser wavelength, strongly suggesting a sequential evaporation process of neutral benzene molecules. For clusters larger than six, an average bond energy ranges from 0.37–0.34 eV/molecule, quite close to the heat of vaporization for liquid benzene. The 14 molecule cluster is found to have an anomalously high binding energy, providing strong evidence that it forms a stable icosahedral structure. The general trend of the monomer bond energy with cluster size is well accounted for by a simple model based on macroscopic constructs and liquid benzene dielectric constant, surface tension, and heat of vaporization values.
Periodic and random perturbation of a system exhibiting damped kinetic oscillations—Pt(100)/NO+CO
Authors: Dath J.-P. , Fink Th. , Imbihl R. , Ertl G.
Published: July 2002

The NO+CO reaction, which has been shown to exhibit damped kinetic oscillations on a Pt(100) surface after initial excitation, has been subjected to periodic and random forcing of the temperature and of the CO partial pressure. The experiments were conducted in the 10−7 mbar range and measurements of the CO2 production rate and of the work function were used to follow the response of the system. The response behavior is characterized by strong resonance effects and by the absence of quasiperiodic oscillations. The system is highly sensitive to temperature modulation, but rather insensitive to modulation of pCO with the latter requiring an amplitude of more than 5% of pCO for producing sustained oscillations. Random forcing experiments demonstrate that the response of the system can be described as a bandpass filter since only frequencies close to the natural frequency of the system are amplified. The results of the experiments led to the conclusion that the damping effect is due to the absence of an efficient synchronization mode under isothermal conditions at low pressures.
Slowing down near the critical point in optically bistable ZnSe
Authors: Wolff Anita N. , Ross J. , Harding Robert H.
Published: July 2002

We measure relaxation rates near the critical point and the left hysteresis limit of an optically bistable system, a ZnSe interference filter. Our ZnSe system has an inhomogeneous geometry where a long, narrow illuminated region has boundaries at a temperature near and below that of the lower state. We determine the critical angle, the angle of incidence of light at which the hysteresis limits coalesce to form a critical point, and perturb the system by changing the input power beyond the critical point. For incidence angles equal to or slightly greater than the critical angle, we find that relaxation rates increase exponentially as the critical point is approached. The critical exponents for perturbations which increase the input power beyond the critical point are greater than those for perturbations which decrease the input power. In either case, the critical exponents increase as the angle of incidence approaches the critical angle. When the hysteresis region is large, we find slowing down near the left hysteresis limit in accordance with our calculations based on a one-dimensional inhomogeneous model.
Observation of the laser induced fluorescence spectra of C2(d 3πg←a 3πu) from the infrared multiphoton dissociation of bis-trifluoromethyl peroxide
Authors: Li Zhuangjie , Francisco Joseph S.
Published: July 2002

C2 has been produced by infrared multiphoton dissociation of bis-trifluoromethyl peroxide. The (0,0), (1,0), and (2,0) transition of the Swan system for C2 have been detected using laser induced fluorescence method. The (0,0), (2,0) laser excitation spectra assignments agree with previous works. The newly observed (2,0) transition in the laser excitation spectrum of C2 has been tentatively assigned and agree with the assignment from the emission spectrum for the same band. The rotational constant for v′=2 of the d 3πg state of C2 has been determined to be 1.7163 cm−1.
Nonequilibrium statistical thermodynamics of nucleation
Authors: Bashkirov A. G.
Published: July 2002

A kinetic equation for nucleation of the Fokker–Planck type is derived with the use of a well-developed general method of nonequilibrium statistical mechanics. In deriving the equation the diffusion flow due to nonuniformity of the vapor concentration is taken into account. The nucleation process per se is realized because of some fluctuations of the diffusion vapor flow across the embryo surface. Note that this process is traditionally explained as a result of random condensation events, but this is true only in the specific case of a free molecular regime. In the more general case, an entropy production is a sum of two terms: the first term representing an entropy production due to phase transition and the second one being due to vapor diffusion in the gas mixture toward the embryo. In two limiting cases of very large and very small embryos, a nucleation coefficient expressed in terms of a diffusional flow fluctuation (Green–Kubo formula) provides one with two well-known expressions of the classical nucleation theory.
Kinetics in dynamically disordered systems: Time scale dependence of reaction patterns in condensed media
Authors: Plonka Andrzej , Paszkiewicz Adam
Published: July 2002

The dependence of the patterns of reactions in condensed media on the time scale is rationalized in terms of the renewal theory with a fractal set of renewal moments which results from the use of Kohlrausch (1863) function to describe the structural relaxations in the systems. When the rates of relaxations exceed markedly those of reactions the classical patterns of reaction kinetics are shown to be valid for disordered systems. The dispersive nature of kinetics becomes evident when the rates of reactions approach those of structural relaxations and in the limit, corresponding to suppressed internal rearrangements, the reaction patterns are shown to be those following from reaction modeling in statically disordered systems.
On the model of colloid aggregates and aggregating colloids
Authors: Potanin Andrew A.
Published: July 2002

Computer simulations were used to investigate the power law dependence of the elastic compression modulus and the yield stress (∝R−γ) of two-dimensional (2D) and three-dimensional (3D) aggregates of special type (fractal trees without loops on square and simple cubic lattices) on the radius R of the aggregates. The elastic forces between particles were described by the Born model. Scaling exponent γ was found to be slightly dependent on the geometrical parameters of the aggregates internal structure and the parameters of interaction (it ranges in the limits 1.3–1.7 in 2D and 1.8–2.6 in 3D). If the interparticle forces are purely central, the internal structure of aggregates is not able to transmit any elastic stress. In this case simulations of the disaggregation process were carried out in 2D in free draining approximation. It was shown that the initial aggregate is broken into two or more secondary aggregates. The mean radius of these aggregates decreases as the shear rate increases. Relevance of these data for the microrheological models of aggregating colloids in shear flow is demonstrated. In terms of the mean field model, the viscosity of aggregating colloid is calculated as a function of shear rate. In this model aggregates are characterized by two exponents: fractal dimension and γ. The values of γ, which were obtained in our simulations, are found to be close to those values which are needed to find the best fit between the mean field model and the computer simulations data for aggregating colloids previously obtained by other authors.
Electronic quenching of XeCl(B,C) and Xe2Cl*
Authors: Yu Y. C. , Wategaonkar S. J. , Setser D. W.
Published: July 2002

The two-photon, laser-assisted reaction between Xe and Cl2 has been used to generate XeCl(B,C) molecules for subsequent kinetic studies at 300 K. Two-body quenching rate constants for the collisionally coupled XeCl(B,C) molecules were determined from the pressure dependence of their decay rates on added molecular gases. The quenching constants for XeCl(B,C) by most molecules are large, even though acceptor electronic states are not available for quenching by an excitation-transfer mechanism. At higher pressure (>100 Torr) of Xe, Xe2Cl(4 2Γ) molecules were formed via three-body quenching of XeCl(B,C), and the quenching rate constants for Xe2Cl(4 2Γ) by added gases were determined and compared with those for XeCl(B,C). In all cases, the quenching rate constants for XeCl(B,C) are larger. The two-body quenching mechanisms for XeCl(B,C) and Xe2Cl(4 2Γ), which are ionic molecules, are discussed. Three-body quenching of XeCl(B,C) with formation of Xe2Cl(4 2Γ) was observed for Xe+N2, Xe+CF4, and Xe+SF6.
Time-resolved infrared-spectroscopic observation of relaxation and reaction processes during and after infrared-multiphoton excitation of 12CF3I and 13CF3I with shaped nanosecond pulses
Authors: Quack Martin , Schwarz René , Seyfang Georg
Published: July 2002

We have produced shaped infrared laser pulses of several kinds ranging from about 2–100 ns duration using a line tuned CO2 laser combined with intracavity absorbers and a CdTe electro-optical switch. The time-dependent infrared absorption of 12CF3I and 13CF3I during and after infrared-multiphoton excitation with these pulses was followed by means of a line tuned continuous wave-CO2 laser and a fast HgCdTe infrared detector (time resolution about 1 ns). The effective time-dependent absorption cross section shows fluence-dependent decay at large fluence with an effective exponential decay constant kI,σ≂1.12 cm2 J−1. This can be interpreted by first generation and then decay by further radiative pumping of highly excited levels of CF3I. The results have been analyzed by master equation modeling using a nonlinear case B/C master equation for multiphoton excitation and very simple models for the absorption properties of highly excited molecules. After nanosecond excitation to very high levels, one finds unimolecular decay CF3I→CF3+I with distinct rate constants (2±1)×108 and (5±4)×106 s−1, which corresponds to ensembles of molecules differing by one CO2 -laser quantum of energy, in agreement with unimolecular rate theory and master equation models. The most striking observation is a slow, collision-free intramolecular rovibrational redistribution process observed by real time spectroscopy on the nanosecond time scale for molecules excited by modest fluence corresponding to typical average energies of five CO2 laser quanta and somewhat more.
Theoretical study of the electronic structure of the BaH molecule
Authors: Allouche A. R. , Nicolas G. , Barthelat J. C. , Spiegelmann F.
Published: July 2002

The electronic structure of BaH is investigated using a 10-electron relativistic pseudopotential on Ba, frozen core configuration interaction (CI) with three active electrons and core-polarization potential. Fine structure is taken into account with a semiempirical spin–orbit operator. The electronic properties of all states dissociating into Ba(6s2,6s15d1,6s16p1) +H(1s) are obtained and generally found in agreement with experiment within 0.03 Å for equilibrium distances, 300 cm−1 for transition energies, 30 cm−1 for vibrational frequencies, except for the D 2Σ+ state. The methodology used in this work is discussed in the light of the results presented here.
A map-based algorithm for controlling low-dimensional chaos
Authors: Petrov Valery , Peng Bo , Showalter Kenneth
Published: July 2002

A simple proportional-feedback algorithm for controlling chaos is presented. The scheme is a map-based variation of a method recently proposed by Ott, Grebogi, and Yorke [Phys. Rev. Lett. 64, 1196 (1990)] in which unstable periodic orbits embedded within a strange attractor are stabilized through deliberate perturbations of a system constraint. The simplified method offers advantages for control of systems in which more complicated algorithms might not be feasible due to short time scales or limited computational resources. Applications to chemical and biological models are presented to demonstrate the utility and limitations of the method. Low-dimensional chaos can usually be stabilized through proportional feedback of one parameter; in some cases, however, a linear combination of several parameters must be utilized.
Polarized light scattering from large quantum systems with zero initial and final angular momentum
Authors: Harris R. A. , Shi Yaoming , McClain W. M.
Published: July 2002

We consider the scattering of light from a molecule or a cluster. The scattering may be elastic or inelastic, but we limit ourselves to the special case of transitions from initial states having total angular momentum number J=0 to final states also having J=0. In practice, this covers elastic scattering from clusters as the rotational temperature drops toward absolute zero. This hypothesis allows us to do the orientation average at the amplitude level, a method fundamentally incorrect in the general case. Without approximation, we are able to deduce the pattern of the observable Mueller matrix M of the scattering. We find a new symmetry that reflects the spherical nature of the J=0 states: M11+M33=M22+M44. This result includes all multipole orders of the scattered light, and is therefore valid for clusters large compared to wavelength, as well as for small clusters. In an Appendix, we discuss a new generating function for orientation averages, which allows one to make exact tensor averages weighted by two plane waves (for scattering, one incoming and one outgoing).
Electronic and chemical interactions between boron and carbon monoxide on Ru(0001)
Authors: Rodriguez José A. , Truong Charles M. , Kuhn W. Kevin , Goodman D. Wayne
Published: July 2002

The interaction between B and CO on Ru(0001) has been studied by means of thermal desorption mass spectroscopy, Fourier transform infrared reflection absorption spectroscopy, and x-ray photoelectron spectroscopy. Boron adatoms poison CO chemisorption approximately on a one-to-one basis. No reaction or direct bonding between B and CO was observed. The B⋅⋅⋅CO interaction is repulsive due to the electron-acceptor nature of both adsorbates. Boron adatoms modify the electronic and chemical properties of first and second nearest-neighbor metal atoms. In the presence of B, the CO desorption temperature decreases, whereas the O(1s) binding energy and CO stretching frequency increase. These trends are a consequence of (1) a reduction in 2π* back donation caused by competition for metal electrons between CO and B and (2) repulsive electrostatic interactions between the negative charges on CO and B. The infrared results indicate that metal atoms strongly affected by B are only occupied when no more unperturbed Ru sites are available on the surface, or when the high temperature of the system favors CO migration (due to entropic effects) onto these energetically less favorable sites.
Radiative transitions induced by short laser pulses in atomic collisions: Nonperturbative study of pulse shape effects
Authors: Nguyen-Dang T. T. , Abou-Rachid H.
Published: July 2002

A time-dependent adiabatic electronic representation is defined by solving the local N-level electronic time-dependent Schrödinger equations at each nuclear configuration of a general N-channel, laser-driven molecular system. These solutions are eigenstates of a time-dependent effective Hamiltonian with respect to which the exact time-evolution of the N-state system is adiabatic. For a two-channel system, the time-dependent adiabatic electronic representation depends on an effective area of the laser pulse and geometrical phases that are also functionals of the laser pulse shape. This adiabatic representation is used in constructing an algorithm for the generation and propagation of wavepackets in a two-channel system irradiated by a short laser pulse. The algorithm is applied to the study of the wavepacket dynamics in the Na–Ar collisional system excited by short laser pulses. The dynamics of the channel populations are analyzed as functions of the shape, duration, and intensity of the laser pulses.
Monte Carlo simulations of surface diffusion at high coverages
Authors: Rostkier-Edelstein D. , Efrima S.
Published: July 2002

Tracer diffusion on a surface is studied by performing Monte Carlo simulations in a two-dimensional noninteracting lattice gas with an emphasis on high coverages. Three different models were investigated: (1) The canonical model (CM), where the background particles on the surface can only diffuse in the parallel direction to the surface. (2) The grand canonical model (GM), where background particles are only allowed to desorb from or adsorb onto the surface (which means that the total number of particles on the surface fluctuates). (3) The combined grand canonic model (CGM), where background particles may both move in parallel (diffusion) or perpendicular (adsorption–desorption) to the surface. In all of these models the tracer is allowed to move only parallel to the surface. The results of the simulations were cast in the form of a correlation factor (the ratio between the calculated diffusion coefficient and the diffusion coefficient of a mean-field approximation). Correlation factors of the tracer were calculated for different surface coverages, and for different ratios between the jump rate of the tracer and the adsorption–desorption rate or/and jump rate of the background particles. In all of the cases, correlation factors showed an identical qualitative behavior: they decreased as surface coverage increases and as background particles became slower. This dependence of the correlation factors on surface coverage and on background particles rate becomes stronger for high surface coverage (above 70%), and for very slow background particles. Comparison to the theory of Harrison and Zwanzig (HZ) shows that it predicts reasonably well the general tendency of the correlation factors in different conditions and even agrees very well with the combined grand canonical simulations (Model 3) when the rates of adsorption–desorption and diffusion of background particles are of the same order.
Association of sidechain polymers: The self-consistent field approach
Authors: Hamad Esam , Qutubuddin Syed
Published: July 2002

The formation of micelles by low molecular weight, comblike polymers in aqueous solutions is studied. The polymers considered have both hydrophobic and hydrophilic side chains. A self-consistent field method is used to obtain the distribution of the backbone around the hydrophobic core. Spherical and lamellar micelles are considered in detail. A characteristic polymer molecule size is found, beyond which spherical micelles are not possible for strongly hydrophilic sidechains. The free energy of the micelle is calculated and used to determine the type of micelles that actually form. For uncharged polymers, lamellar micelles are formed almost exclusively.
Concerning analytical derivatives of kinetic and potential energies in the Hartree–Fock theory
Authors: Tokiwa Hiroaki , Ichikawa Hiroshi , Osamura Yoshihiro
Published: July 2002

Two analytical procedures for obtaining the derivatives of the kinetic and potential energies in the Hartree–Fock theory are presented. They are the derivation from direct differentiation of the expectation values of the kinetic and potential energies, and that from the virial theorem. Since the molecular orbital wave function in the linear combination of atomic orbitals, in which the scale factor is not optimized, does not satisfy the virial theorem, the results by those two methods are different. The derivatives are compared in relation to the adopted basis sets, incorporation of the derivatives of atomic orbitals in the basis set, universal scaling, and the internuclear coordinate.
Thermal variation of surface tension at liquid crystal–isotropic liquid interface
Authors: Mohandas K. P. , George A. K.
Published: July 2002

The surface tension of two nematic liquid crystals bounded by an isotropic liquid (glycerin) was measured as a function of temperature. A spinning drop technique was used for the measurements. Unlike monotonically decreasing dependence found in most isotropic liquids, the surface tension−temperature characteristic showed more than one region with positive slope in the nematic phase. A sharp increase in surface tension was observed near the nematic-isotropic transition temperature. The results obtained are in qualitative agreement with the theoretical predictions of Croxton and Chandrasekhar.
The x-ray excited Auger electron spectrum of NO and potential curves and photodissociation of the NO2+ ion
Authors: Pettersson L. G. M. , Karlsson L. , Keane M. P. , Naves de Brito A. , Correia N. , Larsson M. , Broström L. , Mannervik S. , Svensson S.
Published: July 2002

A study of the NO2+ ion by means of Auger spectroscopy, fast ion beam laser spectroscopy and ab initio calculations is reported. The photon induced Auger spectrum of NO was recorded. Potential curves for a number of electronic states of NO2+ were calculated by the complete active space SCF method in order to facilitate an analysis of the Auger spectrum. A photoabsorption spectrum of NO2+ was observed by means of photofragment kinetic energy spectroscopy and assigned to the A 2Π←X 2Σ+ transition. The two different experimental methods both give a value of 38.6 eV for the appearance energy of NO2+, which is entirely consistent with recent photoionization and double charge transfer results.
Phase behavior of parallel cut spheres. Monte Carlo and integral equation results
Authors: Azzouz H. , Caillol J. M. , Levesque D. , Weis J. J.
Published: July 2002

Monte Carlo simulations of a system of 2500 parallel cut spheres of thickness L/D=0.1 give evidence for a phase transition, at packing fraction η∼0.40, from the low density nematic phase to a smectic phase. No tendency to formation of columnar ordering is observed up to densities as high as η=0.60. This behavior contrasts with that of freely rotating cut spheres for which the sequence of stable phases has been identified as isotropic–nematic–columnar–solid [D. Frenkel, Liquid Cryst. 5, 929 (1989)]. For the present system of highly anisotropic molecules the slow convergence of the expansion of correlation functions on rotational invariants precludes quantitative comparison between integral equation theory and simulation results.
Self-consistent mean-field analysis of classical many-body systems at finite temperature
Authors: Logan L. R. , Murthy C. S. , Srinivasan G. R.
Published: July 2002

We present a methodology for calculating the thermodynamic and configurational properties of a classical system of N interacting particles. For solids we show how the method can be used to determine the structural properties in the equilibrium state. As an application we analyze the formation of misfit dislocations in a nickel-gold bicrystal with a (100) interface. We show that the dislocation exists in a state of minimum Helmholtz free energy. The equilibrium structure and energy are also determined through Monte Carlo calculations and they show good agreement with the results obtained from our method.
Numerical simulation of rodlike polymers in extensional and sink/source flows using the order-parameter version of Doi’s theory
Authors: Hu T. David , Ryskin G.
Published: July 2002

Doi’s kinetic equation for the order-parameter tensor is utilized to study the molecular orientation, stability, stress, and pressure distributions in sink/source flows of solutions of rodlike polymers. In these flows, the velocity field can be determined without having to solve the kinetic equation simultaneously with the fluid-dynamical equation of motion. The steady state solution is obtained by Newton’s procedure. The transient problem is solved by Runge–Kutta’s method. All of the bifurcated solutions are tested for their dynamical stability, and the results reveal that the most stable distribution of molecular orientation is axisymmetric (or nearly axisymmetric), with the most probable orientation being the axis of symmetry. For solutions which can be nematic at equilibrium, the simulation also predicts a sudden drop in stress, followed by an immediate recovery, if the initial molecular orientation in sink flow is not in the principal direction of stretching.
A modified parametrization scheme for the complete neglect of differential overlap method with relativistic correction of orbital exponents and energy levels with particular applications to rare earth compounds
Authors: Wu G. S. , Ma H. , Lin S. H.
Published: July 2002

A new unified parametrization scheme for the complete neglect of differential overlap method, applicable to heavy atom systems is formulated. The exponents ζμ and electronegativities X(μ) of atomic orbitals are corrected by the relativistic Dirac–Fock expectation values of atomic orbital radii and energy levels. This leads to the possibility of calibrating the β0 parameters directly by the equilibrium geometric configuration of molecules. The parametrization for hydrogen, halides, and rare earth series, as well as some test calculations for compounds of these elements, have been carried out to show the utility of the scheme. The results demonstrate that the present scheme can give satisfactory MO levels and ionization potentials as well as correct molecular geometry.
Order in thin films of a liquid crystalline polymer
Authors: Mensinger Hildegard , Stamm Manfred , Boeffel Christine
Published: July 2002

Ordering effects of a side group liquid crystalline polymer PMA64 have been observed by the x-ray reflectivity technique. Thin films on float glass substrate are investigated as a function of temperature and film thickness. At 100 °C, a phase transition from a smectic-A to a nematic phase is observed. The layers of the smectic-A phase are oriented parallel to the surface giving rise to a Bragg peak which is coherently superimposed on Kiessig fringes. The Kiessig fringes are caused by interference of beams reflected at the polymer surface and the polymer–glass interface, while the Bragg peak originates from the electron density modulation perpendicular to the surface. As a function of film thickness, we observe a significant deviation of the integrated intensity of the Bragg peak as compared to the integrated intensity of a Bragg peak due to perfect periodic one-dimensional density modulations. A model which includes the influence of the interfaces as well as distortions of the first and second kind is used to describe this thickness dependence of smectic order in thin films of liquid crystalline polymers. The Bragg peak is lost at the smectic–nematic phase transition where we observe a significant roughening of the free surface. This effect is discussed with respect to molecular order.
Far-infrared spectra and hindered pseudorotation of 1,3-oxathiolane
Authors: Leibowitz Sarah J. , Laane Jaan , Verastegui Ruben , Villarreal John R.
Published: July 2002

The far-infrared spectra arising from the two out-of-plane ring vibrations of 1,3-oxathiolane have been recorded and analyzed. Fifteen pseudorotational (ring-bending) transition frequencies were observed in the 25–125 cm−1 region and were fit with the one-dimensional potential function V(cm−1)=−541(1−cos 2φ)+139(1−cos 4φ). The molecule is twisted with a barrier to pseudorotation of 541±20 cm−1 (1.54±0.06 kcal/mol). The spectra recorded include transition frequencies occurring above the barrier, and this is the first molecule (other than a nearly free pseudorotor) for which such bands have been seen. Seven radial transitions arising from three different pseudorotational states have also been observed in the 297–317 cm−1 region. Those in the pseudorotational ground state can be fit with a one-dimensional double-minimum function with a barrier to planarity of 2720 cm−1. However, this value for the one-dimensional approximation is believed to be too high. Difference bands (180–240 cm−1), sum bands (398–426 cm−1), and double quantum transitions (144–220 cm−1) were also detected and used to confirm the assignments.
Low energy ion–molecule reaction dynamics: Complex and direct collisions of O− with NH3
Authors: Levandier D. J. , Varley D. F. , Farrar J. M.
Published: July 2002

Reactive and nonreactive collisions of O− with NH3 are studied at relative collision energies of 0.65 and 1.24 eV. We observed a significant contribution to the collision dynamics from nonreactive encounters between the reagents. In addition to elastic scattering, we observed a direct contribution to this nonreactive scattering with a very strong dependence of energy transfer on scattering angle. A third contribution to nonreactive scattering arose from O−⋅NH3 collision complexes that regenerate the reactants. In these collisions, ∼80% of the incident translational energy is transformed into vibrational–rotational excitation of the NH3 reagent. The kinetic energy distribution is in reasonable agreement with statistical phase space theory calculations. We also observed reactive collisions. The hydrogen atom transfer process to yield OH− is exothermic by 0.11 eV and exhibits direct dynamics at all collision energies. Proton transfer to form NH−2, endothermic by 0.9 eV, was studied as its deuterium analog and was observed only at the higher collision energy, and took place with very small cross section. The product kinetic energy distributions for the hydrogen atom transfer reaction approach a Gaussian form at the higher collision energy, and we ascribe that behavior to the impulsive nature of reactive collisions in which the ground state vibrational wave function of the N–H bond to be broken is reflected onto product translational energy states through the ‘‘corner’’ of the potential energy surface. Such a Franck–Condon picture of the reaction is a consequence of the highly skewed potential energy surface associated with the heavy–light–heavy mass combination.
The low-energy, charge-transfer excited states of 4-amino-4′-nitrodiphenyl sulfide
Authors: O’Connor Donald B. , Scott Gary W. , Tran Kim , Coulter Daniel R. , Miskowski Vincent M. , Stiegman Albert E. , Wnek Gary E.
Published: July 2002

Absorption and emission spectra of 4-amino-4′-nitrodiphenyl sulfide in polar and nonpolar solvents were used to characterize and assign the low-energy excited states of the molecule. Fluorescence-excitation anisotropy spectra, fluorescence and phosphorescence lifetimes, and fluorescence quantum yields were also used to characterize the photophysics of these states. The lowest-energy, fluorescent singlet state was determined to be an intramolecular charge transfer (ICT) state involving transfer of a full electron charge from the amino to the nitro group yielding a dipole moment of ∼50 D. A low-energy, intense absorption band is assigned as a transition to a different ICT state involving a partial electron charge transfer from sulfur to the nitro group.
Size dependence of the orientational disorder and melting transitions of finite N2 islands deposited on graphite
Authors: Kumar S. , Roth M. , Kuchta B. , Etters R. D.
Published: July 2002

The orientational order–disorder and melting transition temperatures for quasi two-dimensional islands of N2 molecules deposited on graphite are calculated for sizes 1≤N≤256 molecules, using a Monte Carlo procedure with free surface boundary conditions. These transition temperatures are strong functions of N, and the values for 256 molecules are close to those measured experimentally for fractional macroscopic monolayers. Calculated results show that the two sublattice herringbone orientational structure persists up to the orientational transition temperature, above which hindered planar rotation occurs. The registered √3×√3 mass center structure exists until melting, where the islands disassociate. These features are consistent with experiment. Below melting, the root mean square fluctuations of the mass centers away from their √3×√3 equilibrium values are substantially larger than for bulk N2 or for small three-dimensional rare gas clusters. The fluctuations are also substantially larger for edge molecules than for those in the center.
Absolute absorption cross sections of gas phase C60 at 600 °C
Authors: Brady Brian B. , Beiting Edward J.
Published: July 2002

The absorption cross section of gas phase C60 is measured between 300 and 400 nm at a temperature of 600 °C. The spectrum showed a peak blue shifted from the absorption spectrum of the solid phase.
Alignment effects in the photodissociation of Hg–Kr van der Waals molecules
Authors: Chen Xiushan , Burnett Keith
Published: July 2002

In this paper we present calculations of the yield and polarization of the fluorescence from the Hg λ253.7 nm transition following photodissociation of the Hg–Kr van der Waals molecule. We have used a coupled channel wave packet method to predict the alignment produced in photodissociation [J. Chem. Phys. 95, 8124 (1991)]. We show that the alignment effects depend sensitively on the repulsive part of potential near photodissociation threshold. We have made predictions for three different sets of interatomic potentials for Hg–Kr proposed by different authors [M. Findeisen and T. Grycuk, J. Phys. B 22, 1583 (1988); M. Findeisen, T. Grycuk, and J. Staby, The B and X State Potentials for Hg–Ar, –Kr, –Xe (to be published); M. Okunishi, H. Nakazawa, K. Yamanouchi, and S. Tsuchiya, J. Chem. Phys. 93, 7526 (1990); D. M. Segal and K. Burnett, J. Chem. Soc. Faraday Trans. 2 85, 925 (1989)].
Molecular-dynamics simulations on supercooled metallic liquids
Authors: Liu C. F. , Wang S. , Lu J.
Published: July 2002

A molecular-dynamics simulation, in which the mean atomic volume is a function of temperature as in experiments, is described and applied to examine the characteristics of metallic supercooled liquids. It appears that (1) the microstructure of a metallic system changes gradually in the liquid to glass transition, and (2) the thermodynamic and structural properties of a metallic supercooled liquid on heating differ more and more from those on cooling, when the heating rate becomes less and less than the cooling rate, lending support to an existing conclusion, i.e., the glass transition is essentially a kinetic phenomenon.
Origin of the blue bands in the luminescence spectrum of core excited O2
Authors: Mahalingam M. , Hanson D. M.
Published: July 2002

Molecular oxygen was excited by 1.5–3.0 keV electrons. Luminescence spectra from the decay products of K-shell excitation were obtained at high resolution (bandwidth of 0.3 Å). With this resolution it was possible to identify and assign the observed lines to transitions from highly-excited Rydberg states of O+ and O fragments. These states appear to result from inner valence two-hole states and three-hole, one-electron states produced by Auger decay of the core hole in molecular oxygen.
Fluids of linearly fused Lennard-Jones sites: Comparison between simulations and integral equation theories
Authors: Perera A. , Sokolić F. , Moreau M.
Published: July 2002

The structural and thermodynamic properties of model molecular fluids of linear aggregates of Lennard-Jones sites was investigated using molecular dynamics simulation, reference interaction site model (RISM), and hypernetted-chain theory (HNC). This paper presents molecules of two, three, and five sites, and of aspect ratios 1.5, 2, and 3, respectively. The emphasis is given here to the numerical solution of HNC closure for molecular fluids with nonspherical soft interactions. Appropriate expressions for excess internal energy and virial pressure are derived. It is shown that the excess internal energies as given by both theories are in relatively good agreement with the simulations, mainly in the low-medium density and high temperature limits, while for the virial equation of state RISM theory is generally better than HNC for the high densities. The density correlation functions as predicted by the HNC theory are in overall better agreement with the simulation data, for all aspect ratios, whereas RISM theory results show some marked discrepancies for large aspect ratios.
Rotational relaxation in moderately dense gases
Authors: Storozhev A. V.
Published: July 2002

A theory of rotational relaxation in the framework of the infinite order sudden approximation (IOSA) is developed for moderately dense gases. Semiclassical calculations with KDV potential for N2–He and N2–Ne mixtures provide deep understanding of temperature and density dependence of inelastic rotational cross sections. The deviation from binary collision approximation (the impact theory) is investigated.
A theoretical investigation of some low-lying electronic states of imidazole
Authors: Machado Francisco B. C. , Davidson Ernest R.
Published: July 2002

Results are presented from ab initio CI calculations on several low-lying singlet and triplet states of imidazole. The lowest triplet state is predicted to be the 1 3A′ state, having essentially valencelike π−π* character. The lowest singlet state is a Rydberg state representing the π−R3s excitation. Results are also presented for several states of the cation including some shake-up states.
Microwave measurements of the rotational spectrum of butadiene iron tricarbonyl
Authors: Kukolich S. G. , Roehrig M. A. , Henderson G. L. , Wallace D. W. , Chen Qi-Qi
Published: July 2002

The rotational spectrum of butadiene iron tricarbonyl was measured using a Flygare–Balle type microwave spectrometer. A total of 71 a-dipole and c-dipole transitions in the 5–17 GHz range were obtained for this asymmetric top transition metal complex. The ‘‘best fit’’ rotational constants are A=1005.4201(3) MHz, B=958.0408(2) MHz, and C=933.6865(3) MHz. Centrifugal distortion constants were also obtained in the least-squares fit to the measured transitions. The present measurements indicate an upper limit of 40 kHz for splittings due to internal rotation, which places a lower limit on the barrier height of V3 ≥ 1.2 THz. The measured rotational constants are used to determine CO bond angles and the distances between Fe and the butadiene C atoms.