bluuebunny/crossref_metadata_2025 huggingface dataset:
Inequivalent models of irreversible dimer filling: ‘‘Transition state’’ dependence
Authors: Nord R. S. , Evans J. W.
Published: July 2002

Irreversible adsorption of diatomics on crystalline surfaces is sometimes modeled as random dimer filling of adjacent pairs of sites on a lattice. We note that this process can be implemented in two distinct ways: (i) randomly pick adjacent pairs of sites,  jj′, and fill  jj′ only if both are empty (horizontal transition state); or (ii) randomly pick a single site,  j, and if  j and at least one neighbor are empty, then fill  j and a randomly chosen empty neighbor (vertical transition state). Here it is instructive to consider processes which also include competitive random monomer filling of single sites. We find that although saturation (partial) coverages differ little between the models for pure dimer filling, there is a significant difference for comparable monomer and dimer filling rates. We present exact results for saturation coverage behavior for a linear lattice, and estimates for a square lattice. Ramifications for simple models of CO oxidation on surfaces are indicated.
Layer growth mechanism and dipole–dipole interactions of CH3Cl on Pd(100): An infrared and thermal desorption study
Authors: Berkó A. , Erley W. , Sander D.
Published: July 2002

Fourier transform infrared spectroscopy (FTIRS) and temperature programmed desorption (TPD) have been used to study the layer growth mechanism of CH3Cl on a Pd(100) surface at 85 K. Methyl chloride adsorbs and desorbs in molecular form without decomposition. Three different adsorbed states are readily distinguishable by both methods. The chemisorbed state (first layer) saturates at Θ =0.5 coverage, with the characteristic C–H vibrations of 1011, 1330, 1426, 2927 cm−1. The desorption spectrum for this state suggests a coverage-dependent activation energy between 54.5 and 42 kJ/mol. Nearly the same amount of CH3Cl is bonded in the physisorbed state (second layer) with characteristic C–H vibrations at 1013, 1338, 1428, and 2925 cm−1. The activation energy of the desorption in this case is 40–35 kJ/mol. The condensation phase appears only after the completion of the first two layers and exhibits characteristic C–H vibrations at 1023, 1338/1346, 1444, and 2955 cm−1. In a model calculation {developed by Albano [J. Chem. Phys. 86, 1044 (1986)]} it is shown, that the intermolecular dipole–dipole interaction may explain correctly the coverage- dependent activation energy for desorption of methyl chloride.
Angular dependence of photofragments from K-shell excited N2 and O2: Photoabsorption anisotropy and excited state symmetry
Authors: Lee K. , Kim D. Y. , Ma C. I. , Lapiano-Smith D. A. , Hanson D. M.
Published: July 2002

The symmetry of excited states reached by photoexcitation of K-shell electrons of N2 and O2 is identified from the fragmentation anisotropy. For each molecule, ion yield excitation spectra were obtained at 90°, 54.7°, and 0° relative to the axis of soft x-ray polarization. The degree of polarization of the synchrotron radiation was determined experimentally. From this determination and the angular dependence of the ion yield, the photoabsorption anisotropy parameter β is obtained as a function of photon energy. Variations in the β spectra are discussed in terms of previously assigned spectral features and are compared with other experimental results. A comparison also is made with theoretical calculations, which are available for the case of N2.
A quasiclassical, surface hopping trajectory study of the reaction Na(2P)+HCl→NaCl+H(2S)
Authors: Eaker Charles W.
Published: July 2002

The reaction of electronically excited Na(2P) with HCl to produce NaCl and H has been studied using a quasiclassical surface hopping trajectory program. Two translational energies (5.6 and 16.3 kcal/mol) and two HCl vibrational states (v=0 and 1) were investigated. We find that the reaction cross section increases with vibrational excitation and decreases with increasing translational energy. The calculated reaction cross section of 3.02±0.25 Å2 at 5.6 kcal/mol and v=0 agrees with the results of recent molecular-beam experiments on this system. The ground and first excited state potential-energy surfaces and the nonadiabatic coupling between these surfaces were calculated using the diatomic-in-molecules (DIM) method. Reactive trajectories which occur on these surfaces remain in the interaction region for nearly 1 ps and must pass through a perpendicular geometry. This critical geometry corresponds to the avoided crossing seam region.
Cooper minima and circular dichroism in photoelectron angular distributions
Authors: Rudolph H. , Dubs Richard L. , McKoy V.
Published: July 2002

We demonstrate that circular dichroism in photoelectron angular distributions (CDAD), resulting from resonance enhanced multiphoton ionization (REMPI) of an aligned molecular Rydberg state, is a highly sensitive probe of the presence of a Cooper minimum near threshold. To illustrate this application of CDAD, we present the results of ab initio calculations for (1+1′) REMPI via the R21(5.5) branch of the D 2Σ+(3pσ) state of NO, where a Cooper minimum is found in l=2 (d wave) of the kπ continuum at a photoelectron kinetic energy of 3.2 eV. The CDAD signal is found to vary rapidly with photoelectron kinetic energy, go through zero, and change sign in the region of the Cooper minimum. This result is predicted by CDAD theory for photoionization from an aligned atomic p orbital.
(2+1′) rotationally resolved resonance enhanced multiphoton ionization via the E 2∑+(4s,3d) and H 2∑+(3d,4s) Rydberg states of NO
Authors: Rudolph H. , McKoy V.
Published: July 2002

The results of studies of ionic rotational branching ratios and photoelectron angular distributions resulting from (2+1′) resonance enhanced multiphoton ionization of NO via various high J (≊21.5) rotational branches of the E2∑+(4s,3d) and H2∑+(3d,4s) Rydberg states are presented. The rotational branching ratios show the expected ΔN=even rotational propensity rule with very small ΔN=odd signals. The branching ratios for the E2∑+ state are seen to be independent of photoelectron energy with the ΔN=+2 signals strongest and no appreciable higher rotational transfer peaks (‖ΔN‖≥3). The higher rotational transfer signal for ionization of the H2∑+ state are also negligible but the rotational branching ratios are strongly energy dependent due to a Cooper minimum in the l=3 partial wave of the kσ- and kπ-continua at a photoelectron kinetic energy of 2.6 eV and 2.9 eV, respectively. This leads to a strong rotational selectivity that can be exploited to produce ions in a specific rotational level. These consequences of Cooper minima close to threshold are quite general and their influence on rotational distributions should be readily observable in other molecular systems. The photoelectron angular distributions via both states show a strong energy dependence with a rapid change in the angular distributions around the Cooper minimum associated with the H2∑+ state.
Stochastic theory of stable and bistable biochemical systems: Effects of noise on enzymatic catalysis
Authors: Di Cera Enrico , Phillipson Paul E.
Published: July 2002

A theory is developed to analyze the effects of noise on the dynamics of stable and bistable biochemical systems where a substrate, continuously provided by a constant external flow, is enzymatically converted into product. Analytical expressions for the ensemble average concentration of substrate at steady state, 〈x〉, are derived. It is found that noise induces a bias in 〈x〉. The bias is always positive in the case of noncooperative enzymatic catalysis, but it can be positive, negative, or zero in the case of cooperative enzymatic catalysis. In the case of bistable systems, 〈x〉 is dramatically affected by noise. A first-order phase transition between two concentration states is approached as the noise becomes asymptotically small. The transition occurs at a critical value of the flow that can be arrived at by means of an equal-area rule. This rule is a generalized form of the Maxwell rule for the van der Waals gas.
Field-induced surface chemistry of NO
Authors: Kreuzer H. J. , Wang L. C.
Published: July 2002

A microscopic theory of field adsorption is used to study the adsorption and reaction of NO on a Pt(111) surface in high electrostatic fields. We find that below 0.4 V/Å only NO is adsorbed stably. Above this value dissociation sets in that leads around 1.2 V/Å to the formation of N2O. A molecular orbital analysis is given of the reaction pathway and to identify the stabilization mechanism.
Photoionization studies of niobium clusters: Ionization potentials for Nb2–Nb76
Authors: Knickelbein Mark B. , Yang Shihe
Published: July 2002

The photoionization efficiency (PIE) spectra for niobium clusters up to Nb76 are measured using laser ionization time-of-flight mass spectrometry. Ionization potentials (IPs) assigned from the PIE spectra evolve with the same general pattern observed for other transition metal clusters (Mn) studied to date: a rapid yet nonmonotonic decrease in IP to n≂15 followed thereafter by a relatively slow and smooth evolution. The measured IPs evolve with cluster radius R according to the predictions of the conducting spherical droplet model if the limiting (R→∞) IP is chosen to be 0.5 eV lower than the bulk polycrystalline work function. Kinetics experiments using D2 as the coreactant indicate the existence of two structural forms, one highly reactive, one relatively unreactive for clusters in the size range Nb9–Nb12, with the reactive forms predominant in each case. PIE spectra for Nb9–Nb12 were recorded using sufficient D2 added to the cluster source to selectively titrate the reactive forms of these clusters, leaving the unreactive forms as the predominant isomers. It is found that the unreactive forms of Nb9 and Nb12 possess higher IPs than those of the reactive forms, while both forms of Nb10 possess approximately the same IP.
Dynamical and spectroscopic studies of nonrigid molecules. Application to the visible spectrum of thioacetaldehyde
Authors: Smeyers Y. G. , Niño A. , Moule D. C.
Published: July 2002

The methyl torsion and aldehydic hydrogen wagging modes are studied theoretically in thioacetaldehyde, in both ground and first triplet excited states. For this purpose, the potential energy surfaces were determined by ab initio restricted Hartree Fock and unrestricted Hartree Fock calculations with 4–31 basis set +d orbitals on the sulphur atom. The two electronic states were found to have different preferred conformations. The singlet state exhibits a planar eclipsed conformation, whereas the structure of the triplet state is anti-eclipsed and pyramidal. It was found that a potential function, which was constructed from a symmetry adapted double Fourier expansion of the wagging and torsional coordinates, gave a reasonable fit to the energy points. The two-dimensional Schrödinger equations for the torsion and wagging motions were solved for both singlet and triplet states, taking into account the internal symmetry and the appropriate basis. The relative band locations and the intensities (Franck–Condon factors) were evaluated from the eigenvalues and the eigenvectors. Spectra were calculated for the thioacetaldehyde CH3CHS/CH3CDS/CD3CHS/CD3CDS isotopomers. New assignments for the higher bands in the CH3CHS spectrum are proposed.
Decay kinetics of XeCl(B,C) in Xe and in mixtures of Xe with Kr, Ar, Ne, and He
Authors: Quiñones E. , Yu Y. C. , Setser D. W. , Lo G.
Published: July 2002

The two-photon, laser-assisted reaction between Xe and Cl2 has been used to prepare XeCl (B,C) molecules in Xe and in Xe mixtures with lighter rare gases for kinetic studies of the coupled XeCl (B,C) states. The XeCl (B–X) spectra were used to assign the conditions for which the vibrational temperature was nearly 300 K. Relative intensity and coupled radiative lifetime measurements provide independent data which support an energy separation between the XeCl(B) and XeCl(C) states of ∼100 cm−1. The two-body quenching rate constant in Xe for XeCl (B,C) was measured as 5+3−2×10−12 cm3 s−1, and a three-body quenching rate constant of 13±4×10−31 cm6 s−1 is indicated. Experiments also were done with added Kr, Ar, Ne, and He to establish two-body and mixed (e.g., Xe+Ar) three-body quenching rate constants. The three-body quenching rate constants are nearly equal to the Xe2Cl* formation rate constants and three-body dark quenching of XeCl(B,C) is negligible.
Librational and tunneling frequencies in solid methane
Authors: Smith David
Published: July 2002

Librational frequencies derived from weak infrared and Raman vibration–libration bands have been used to determine tunneling frequencies of CH4 and CD4 in phases II and III of solid methane. Previously, these computations were usually carried out considering only the molecular field, disregarding the crystalline field. The computations in this paper have taken into account both the crystalline and molecular fields. The librational frequencies for solid methane in phase II have been computed using the atom–atom potentials of Bartell and those of Williams. For both sets of atom–atom potentials, there is good agreement between the computed and observed splittings of the librational frequencies in both CH4 (50 and 92 cm−1) and CD4 (40 and 66 cm−1). The tunneling frequency of CD4 is predicted to be about 0.03 cm−1 on the D2d sites and 3.5 cm−1 on the Oh sites in phase II. The computation of the librational and tunneling frequencies of solid methane in phase III is complicated by the uncertainty of the site symmetries on the three inequivalent sites in the solid. These frequencies have been computed assuming D2d site symmetry for the methane molecules in phase III.
Laser annealing of silicon clusters
Authors: Maruyama S. , Anderson L. R. , Smalley R. E.
Published: July 2002

Positive silicon cluster ions prepared by laser vaporization in a supersonic beam and trapped in an ion cyclotron resonance cell were probed in chemisorption reactions with ethylene and ammonia. Clusters in the 36–51 atom size range were effectively annealed to unique structural forms by excitation with a XeCl excimer laser followed by cooling through infrared radiation and collisions with argon.
Energy transfer and reaction dynamics of matrix-isolated 1,2-difluoroethane-d4
Authors: Raff Lionel M.
Published: July 2002

The molecular dynamics of vibrationally excited 1,2-difluoroethane-d4 isolated in Ar, Kr, and Xe matrices at 12 K are investigated using trajectory methods. The matrix model is an fcc crystal containing 125 unit cells with 666 atoms in a cubic (5×5×5) arrangement. It is assumed that 1,2-difluoroethane-d4 is held interstitially within the volume bounded by the innermost unit cell of the crystal. The transport effects of the bulk are simulated using the velocity reset method introduced by Riley, Coltrin, and Diestler [J. Chem. Phys. 88, 5934 (1988)]. The system potential is written as the separable sum of a lattice potential, a lattice–molecule interaction and a gas-phase potential for 1,2-difluoroethane. The first two of these are assumed to have pairwise form while the molecular potential is a modified form of the global potential previously developed for 1,2-difluoroethane [J. Phys. Chem. 91, 3266 (1987)]. Calculated sublimation energies for the pure crystals are in good accord with the experimental data.

The distribution of metastable-state energies for matrix-isolated 1,2-difluoroethane-d4 is Gaussian in form. In krypton, the full width at half maximum for the distribution is 0.37 eV. For a total excitation energy of 6.314 eV, the observed dynamic processes are vibrational relaxation, orientational exchange, and four-center DF elimination reactions. The first of these processes is characterized by a near linear, first-order decay curve with rate coefficients in the range 1.30–1.48×1011 s−1. The average rates in krypton and xenon are nearly equal. The process is slightly slower in argon. The decay curves exhibit characteristic high-frequency oscillations that are generally seen in energy transfer studies. It is demonstrated that these oscillations are associated with the frequencies for intramolecular energy transfer so that the entire frequency spectrum for such transfer processes can be obtained from the Fourier transform of the decay curve. Orientational exchange is shown to occur with much greater frequency as the unit cell spacing decreases. The occurrence of orientational exchange generally results in a very rapid dissipation of molecular rotational energy to the lattice which causes a characteristic break to occur in the decay curve. It is shown that 16% of the total energy transfer to the lattice in argon is a result of such rotational energy transfer. The propensity for four-center DF elimination is found to be greater in argon than in either krypton or xenon. The relaxation data show that this effect is not the result of different energy transfer rates but is probably associated with steric effects resulting from the smaller lattice dimensions in argon. Isotope effects upon the energy partitioning in unimolecular reactions of 1,2-difluoroethane and upon the energy transfer dynamics under matrix-isolation conditions are also reported.
A comparison of three time-dependent wave packet methods for calculating electron–atom elastic scattering cross sections
Authors: Judson Richard S. , McGarrah Dorothy B. , Sharafeddin Omar A. , Kouri Donald J. , Hoffman David K.
Published: July 2002

We compare three time-dependent wave packet methods for performing elastic scattering calculations from screened Coulomb potentials. The three methods are the time-dependent amplitude density method (TDADM), what we term a Cayley-transform method (CTM), and the Chebyshev propagation method of Tal-Ezer and Kosloff. Both the TDADM and the CTM are based on a time-dependent integral equation for the wave function. In the first, we propagate the time-dependent amplitude density, ‖ζ(t)〉=U‖ψ(t)〉, where U is the interaction potential and ‖ψ(t)〉 is the usual time-dependent wave function. In the other two, the wave function is propagated. As a numerical example, we calculate phase shifts and cross sections using a screened Coulomb, Yukawa type potential over the range 200–1000 eV. One of the major advantages of time-dependent methods such as these is that we get scattering information over this entire range of energies from one propagation. We find that in most cases, all three methods yield comparable accuracy and are about equally efficient computationally. However for l=0, where the Coulomb well is not screened by the centrifugal potential, the TDADM requires smaller grid spacings to maintain accuracy.
Potential energy surfaces of LaH+ and LaH+2
Authors: Das Kalyan K. , Balasubramanian K.
Published: July 2002

Using the complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by full second-order configuration interaction (SOCI) calculations, 16 electronic states of LaH+ and 8 electronic states of LaH+2 are investigated. The potential energy surface of these electronic states of LaH+2 and LaH+ are computed. These calculations show that the 3F(5d2) ground state of La+ ion forms a weak complex with H2. The La+(1D) excited state inserts into H2 with a small barrier (<8 kcal/mol) to form the 1A1 ground state of LaH+2 (re=2.057 Å, θe=106°). At the SOCI level of theory LaH+2 is found to be 11 kcal/mol more stable than La+(3F)+H2. Our calculations explain the experimental observations on La++H2→LaH++H reaction. The adiabatic ionization potential (IP) of LaH2 and LaH are calculated as 5.23 and 5.33 eV, respectively. The ground state of LaH+ was found to be a 2Δ state. We compute De(LaH+) and De(HLa–H+) as 2.54 eV in excellent agreement with the experimental De(LaH+)=2.57 eV measured by Armentrout and co-workers. The spin–orbit effects of LaH+ were also studied using the relativistic configuration interaction (RCI) method.
Unsymmetrical electrolytes with adhesive interactions
Authors: Zhu Jianjun , Rasaiah Jayendran C.
Published: July 2002

The sticky electrolyte mode for a weak unsymmetrical electrolyte is solved in the mean spherical approximation (MSA) when there are adhesive interactions between oppositely charged ions. The distribution functions at contact and the thermodynamic properties in this approximation are derived; the solutions reduce to those of corresponding symmetrical adhesive electrolyte studied by Rasaiah and Lee [J. Chem. Phys. 83, 6396 (1985)] when the sizes of the ions and the magnitudes of the charges are made the same and to those of adhesive nonelectrolytes when the charges are removed. When the stickiness is turned off the solutions of the primitive model electrolyte in the MSA are recovered.
Investigation of the mutual diffusion in poly(ethylene oxide)/poly(methyl methacrylate) blends by photon correlation spectroscopy
Authors: Xia J. L. , Wang C. H.
Published: July 2002

Photon correlation spectroscopic studies of the mutual diffusion of poly(ethylene oxide)/poly(methyl methacrylate) (PEO/PMMA) blends are carried out. The experimental investigation deals with the measurement of the mutual diffusion coefficient of the PEO/PMMA blend by varying the molecular weight of PEO but keeping the molecular weight of PMMA fixed. At 20% volume fraction of PEO, and high PMMA molecular weight, the mutual diffusion coefficient linearly decreases with M−1PEO. The temperature dependence of the mutual diffusion coefficient is non-Arrhenius. The Onsager coefficient of mutual diffusion is obtained as a function of temperature.
Nuclear magnetic resonance investigations in highly entangled polybutadiene chains. Temporary elasticity and flexibility
Authors: Labouriau A. , Cohen Addad J. P.
Published: July 2002

The property of temporary elasticity is studied from the relaxation of the transverse magnetization of protons attached to high molecular weight poly(butadiene) chains in concentrated solutions. The relaxation time τ measured from poly(butadiene) is found to be a function of the polymer concentration φ in cyclohexane, the glass transition temperature Tg(φ), and the inverse of the modulus of temporary elasticity G0N according to the formula τ=8×10−3[T−Tg(φ)−50](1+4.5/G0Nφ2.2); τ is expressed in millisecond units and G0N is in 106 dyn cm−2. The quantities G0N and Tg(φ) are varied by changing the chain microstructure. The front factor [T−Tg(φ)−50] is ascribed to an effect of chain flexibility disclosed from nuclear magnetic resonance. It is identified with a free volume term that also governs viscoelastic measurements. It is shown that the flexibility effect is associated to orientational correlations concerning three skeletal bonds along a given segment.
Modeling of lateral interactions in densely packed adsorbate overlayers: How far do chemisorbed C2N2 molecules tilt on a Ni(110) surface?
Authors: Rösch N. , Fox Th. , Netzer F. P. , Ramsey M. G. , Steinmüller D.
Published: July 2002

It is proposed to investigate geometric arrangements in chemisorption systems by standardized force fields. This ‘‘molecular modeling’’ technique is particularly simple to apply to densely packed adsorption layers with dominating nonbonding interactions where the substrate essentially acts as a template for interadsorbate distances. The approach is exemplified for Ni(110/c(2×2)C2N2. Tilting of the molecular axis by about 20–30 degrees with respect to the surface (the molecules oriented perpendicularly to the troughs) is deduced, supporting a corresponding interpretation of angular resolved photoemission data.
Interpretation of photoelectron spectra for ThF4, UF4, ThCl4, and UCl4 in terms of relativistic local-density molecular orbital calculations
Authors: Pierloot K. , Renders A. , Goodman G. L. , Devoghel D. , Görller-Walrand C. , Vanquickenborne L. G.
Published: July 2002

We describe fully relativistic calculations for the actinide tetrahalides UF4, ThF4, UCl4, and ThCl4 using the Dirac–Slater local-density model. We use an extended basis set of numerical atomic functions calculated in a potential well. A multipolar expansion technique is used to represent detailed features of the molecular charge density and the local exchange potential used is the Von Barth–Hedin–Lundqvist potential. By describing the molecules with physically reasonable wave functions and charge densities, we are able to calculate transition-state energies in good agreement with most features of the experimental photoelectron spectra. For the chlorine 3s levels we suggest revised ionization energies in the gas phase. The experimental values in the literature seem to have been incorrectly assigned, probably because they occur near the high energy limit for He(I) radiation.
Integral equation theory applied to a binary mixture under extreme conditions of density and temperature
Authors: Levesque D. , Weis J. J. , Chabrier G.
Published: July 2002

We apply integral equation theory to the calculation of the thermodynamics and the distribution functions of a mixture of hydrogen atoms and molecules at high densities and temperatures. The integral equations are found to tackle very efficiently the problem of the softness and the nonadditivity of the potentials, as encountered under these conditions, and give better results than hard sphere perturbation theory. Comparison with Monte Carlo calculations show that the so-called HMSA and modified hypernetted chain (MHNC) equations, give almost perfect results. This extends the range of validity of these schemes to domains of high pressures and temperatures.
On the optical response of Na2 and its relation to computational prediction
Authors: Pollack Stuart , Chris Wang C. R. , Kappes Manfred M.
Published: July 2002

We compare cluster size specific measurements of Na20 absorption response to the predictions of semiclassical electrostatic and quantum mean field treatments and discuss the implications for size trends in the optical properties of alkali microclusters.
Ultrafast rotational dynamics of electronically excited aniline molecules in solution from ultraviolet femtosecond fluorescence anisotropies
Authors: Pereira Marco A. , Share Paul E. , Sarisky Mark J. , Hochstrasser Robin M.
Published: July 2002

Fluorescence upconversion was used to measure the time evolution of the fluorescence anisotropy of aniline in three hydrocarbon, alcohol, and aprotic polar solvents. The deconvoluted anisotropy decay time of 0.94±0.05 ps in aniline/isopentane solution is only 54% longer than calculated for free rotation of collision-free molecules, and increasing the solvent viscosity by a factor of 15 by going from isopentane to hexadecane increases the rotational relaxation time by only another 70% to 1.6 ps. The extended diffusion theory applied to the aniline/isopentane anisotropy decay resulted in a collisional time of 205±15 fs, implying that electronically excited aniline in this hydrocarbon rotates on average 19.5±1.5 deg between collisions. These results indicate that the orientational motion of electronically excited aniline in isopentane is significantly nondiffusive. The anisotropy decayed much more slowly in methanol and acetonitrile than in the hydrocarbons, presumably due to dipole–dipole interactions in acetonitrile and/or hydrogen bonding interactions in the hydroxylic solvent. The anisotropy decay in methanol presented double exponential behavior, probably due either to the coexistence of two slowly interconverting aniline species (hydrogen and nonhydrogen bonded) in solution or to a fast contribution from the hydrogen-bonded aniline molecules rotating around their long axes. Free rotation, hydrodynamic models, extended diffusion, and Langevin calculations were used to evaluate the results and comparisons were made among the various approaches.
Nucleation in liquid crystals and liquid crystal polymers
Authors: Lansac Y. , ten Bosch A.
Published: July 2002

A nematic mixture that undergoes a first-order phase transition to an ordered state is considered. The dynamics of the phase separation in terms of a theory based on a time-dependent Landau–Ginzburg equation is described. The laws of growth and dissolution for isotropic and anisotropic droplets are discussed. An example is given for a bidisperse and a polydisperse (liquid crystal polymer) sample. The possibility of spinodal decomposition in these systems is also explored.
Preexponential-limited solid state chemistry: Ultrafast rebinding of a heme–ligand complex in a glass or protein matrix
Authors: Miers Jeffrey B. , Postlewaite Jay C. , Cowen Benjamin R. , Roemig Gary R. , Lee I-Yin Sandy , Dlott Dana D.
Published: July 2002

Ultrafast spectroscopy is used to investigate the temperature dependence of a bimolecular chemical reaction occurring at reaction centers embedded in a glycerol:water glass. The reaction centers consist of carbon monoxide bound to protoheme (PH–CO), or to myoglobin at pH=3 (Mb3–CO), a protein containing PH–CO with a broken proximal histidine–Fe bond. These systems have in common a small energetic barrier for rebinding of the photodissociated ligand. In the glass, the ligand is caged, so that only geminate rebinding is possible. Rebinding is not exponential in time. For t≳20 ps, the survival fraction of deligated heme N(t)∝t−n(n≥0). Below 100 K, rebinding is dominated by an inhomogeneous distribution of activation enthalpy P(ΔH‡) and n is temperature dependent. Inhomogeneous means that every site has a unique barrier. Above 150 K, n becomes independent of temperature. In this high temperature limit, the distribution of preexponential factors, attributed to a distribution of activation entropy P(ΔS‡), dominates rebinding. A picosecond two-pulse experiment demonstrates that the entropy distribution is also inhomogeneous. This work is the first study of heme–ligand rebinding in both low and high temperature limits, which allows a direct investigation of the nature of the activation entropy distribution in a glass. Because ligand rebinding in Mb3–CO and PH–CO is similar, despite the existence of a protein in Mb3–CO which provides a larger free volume for the ligand than does PH–CO, it is concluded that the low energetic barrier encourages immediate ligand rebinding and that the ligand does not diffuse far from the rebinding site at low temperature.
Structural study of a new Langmuir–Blodgett material designed for nonlinear optical applications
Authors: Geddes N. J. , Jurich M. C. , Swalen J. D. , Twieg R. , Rabolt J. F.
Published: July 2002

A new bifunctional material, [2-nitro-5-(N-methyl-N-octadecylamino)benzoic acid] (NMOB), has been synthesized and its film forming properties investigated by the Langmuir–Blodgett technique. Pressure-area isotherms indicate that NMOB forms stable monolayers below 40 mN/m which can be transferred to solid substrates. Multilayered samples were prepared on both transmission and reflection substrates and investigated by Fourier transform infrared (FTIR) spectroscopy. Polarization measurements indicate that the orientation of the NMOB molecules is definitely anisotropic with the alkyl tail tilted considerably relative to the surface while the dye chromophore has its plane oriented primarily parallel to the surface. Changes in this orientation upon interleaving these NMOB layers with deuterated cadmium arachidate spacer layers were also investigated both at room and elevated temperatures.
Resonances in the photodetachment cross section of Au−2
Authors: Gantefor G. F. , Cox D. M. , Kaldor A.
Published: July 2002

This paper reports the observation of autodetaching resonances in photoelectron spectroscopy of Au−2 according to the process Au−2 +hν■(Au−2)*■Au2 +e−. They appear as sharp features superimposed upon a nearly constant signal associated with direct photodetachment process from Au−2 :Au−2 +hν■Au2+e−. By altering the cluster expansion conditions, the dimer anion temperature can be varied. This leads to changes in the linewidth and relative intensities of the resonances and allows ground state and hot band transitions to be identified. The resonances correspond to electronic transitions to a long-lived excited state of Au−2 with a significantly lower vibrational frequency (128±2 cm−1) than that of the neutral ground state (190.9 cm−1), or even the anion ground state (149 cm−1). It decays via electronic autodetachment in a two-electron process similar to autoionization of neutral atoms.
Four-wave mixing spectroscopy of the NO B 2Π3/2(v=16) state
Authors: Tsukiyama Koichi , Tsukakoshi Motowo , Kasuya Takahiro
Published: July 2002

The four-wave mixing process has been utilized to probe the rotational levels of the predissociative B 2Π3/2(v=16) state of nitric oxide. The pump laser frequency ω1 was fixed for the A 2Σ+(v′=0)←X 2Π3/2(v″=0) two-photon transition, while the probe laser frequency ω2 remained variable to monitor the B 2Π3/2(v=16)←A 2Σ+(v′=0) transition. The intensity of the generated coherent vuv radiation was enhanced by tuning ω2 in the vicinity of the rotational levels of the B 2Π3/2(v=16) state. The spectral structures corresponding to the B 2Π–A 2Σ+ system significantly differed from those of a two-color resonant enhanced multiphoton ionization spectrum. Also discussed are the observed ΔJ constraints and line intensity patterns as related to the reduced expression of the third-order nonlinear susceptibility.
Fine structure and Zeeman effect in the b(1)3Πu state of 6Li2 by Doppler-free polarization spectroscopy
Authors: Jeng Wen-Haw , Xie Xingbin , Gold L. P. , Bernheim R. A.
Published: July 2002

The fine structure of the b(1)3Πu state of the lithium dimer was observed for the first time and studied using Doppler-free polarization spectroscopy. The triplet state was accessed by excitation of the A 1Σ+u(v=2,J=33)∼b(1)3Πu (v=9,N=32,F1,e) spin–orbit perturbed levels. The remaining two fine structure levels were excited by using the Zeeman interaction to mix levels of different J. Two polarization geometries, π pump and σ pump, were used to study separately different groups of MJ levels in magnetic fields up to 3 kG. An irreducible tensor derivation of the fine structure and Zeeman Hamiltonian matrix in Hund’s case (b) symmetrized basis functions was carried out and used to determine the perturbation parameters and the spin–spin and spin–rotation fine structure constants by a fit to the Zeeman splitting pattern. The spin–orbit contribution to the fine structure is too small in the high J states to be found accurately. The electronic spin–orbit coupling perturbation for the interaction between the singlet and triplet states was also determined to be Hel(A∼b)=0.1172±0.0008 cm−1.
Chemical turbulence and phase resetting dynamics
Authors: Wu Xiao-Guang , Kapral Raymond
Published: July 2002

The onset of chemical turbulence in the 2-D complex Ginzburg–Landau equation is studied for a class of finite-amplitude, inhomogeneous, random initial conditions. Numerical simulations on a coupled map lattice corresponding to this reaction–diffusion equation are carried out in order to characterize the nature of the turbulent state. The phase diagram giving the zone of chemical turbulence in the parameter space that specifies the initial condition is determined numerically and compared with an analytical estimate.
Cluster expansion of the Hubbard model energy of acyclic conjugated hydrocarbons
Authors: Schmalz T. G.
Published: July 2002

Regularities in the ground state energy of acyclic conjugated hydrocarbons have frequently been noted at the level of Hückel molecular orbital calculations. In this paper it is shown that the Hubbard model, which includes electron correlation, shows the same types of regularities even in the highly correlated limit. These regularities are rationalized by showing that the Hubbard energy can be fit to a (rapidly convergent) cluster expansion which embodies the physically appealing notion that a molecule consists principally of localized bonds which interact progressively more weakly with more distant parts of the molecule. Relations to previous expressions for the energy, derived within the Hückel approximation, are also given.
Electron affinities of small linear carbon clusters. Coupled cluster calculations with first-order correlation orbitals
Authors: Adamowicz Ludwik
Published: July 2002

Electron affinities of the C3, C4, C5, and C6 linear clusters are theoretically determined using an ab initio procedure which combines the first-order correlation orbital method and the coupled cluster method. The theoretical values 1.58, 3.41, 2.43, and 3.69 e.V. for C3, C4, C5, and C6, respectively, are systematically lower than the experimental estimates of Smalley et al. [Chem. Phys. Lett. 144, 431 (1988)] (1.95, 3.70, 2.80, and 4.10 e.V.); however, the trends corresponding to the odd–even numbered chain alternation and to the increase of the size of the cluster are very similar.
Scaling relations in hyperfine shifts of impurity atoms in rare gas crystals
Authors: Bucher Manfred
Published: July 2002

The hf interaction of impurity atoms in rare gas crystals is investiga The ratio of the orbital moments of the impurity atom and the rare gas atom serveas a measure of each constituent’s contribution to the net interaction energy. (AIP)
Scattering of excited molecules at a surface: The effect of van der Waals forces
Authors: Akulin V. M. , Rebentrost F.
Published: July 2002

The effect of the van der Waals potential on the processes of quenching and inelastic scattering of an excited quantum particle at a surface is considered. Regimes of high superelasticity at metallic surfaces and of capture at a dielectric surface covered with a layer of molecules of the same kind are apparent. Effective potentials due to multiple absorption-quenching steps are calculated for heavy and slow particles in laser fields. A consistent quasiclassical description of the translational motion of a two-level quantum particle interacting with a continuum is outlined and the dispersion of the trajectories after scattering is analyzed.
Methyl group torsional dynamics from rotationally resolved electronic spectra. 1- and 2-methylnaphthalene
Authors: Tan X.-Q. , Majewski W. A. , Plusquellic D. F. , Pratt D. W.
Published: July 2002

Rotationally resolved fluorescence excitation spectra of three vibronic bands in the S1←S0 transitions of 1- and 2-methylnaphthalene (1 and 2MN) have been obtained. Each band exhibits perturbations that are produced by an interaction between the restricted torsional motion of the attached methyl group and the overall rotational motion of the entire molecule. A complete analysis of these effects yields values of the torsional barrier heights, the rotational constants, and the torsion–rotation perturbation coefficients of all vibronic levels that participate in the transitions. These values depend significantly on the position of the methyl group attachment, on the electronic state of the naphthalene chromophore, and on its vibrational state, as well. For example, V3 (the threefold torsional barrier) decreases from 809 cm−1 in 00 1MN to 128 cm−1 in 00 2MN. D (the largest first-order torsion–rotation perturbation term) increases from 0.03 MHz in 00 1MN to 406 MHz in 00 2MN, a change of more than 4 orders of magnitude. The V3 values of 00 and 8̄1 1MN are 563 and ≤ 373 cm−1, respectively. A full discussion of these dynamically relevant effects and their dependence upon both electronic and geometric factors is given.
Vacuum ultraviolet emission spectra of the helium and neon alkali ions in the range between 60–80 nm
Authors: Petkau K. , Hammer J. W. , Herre G. , Mantel M. , Langhoff H.
Published: July 2002

Mixtures of neon or helium with alkali vapor were excited using an argon ion beam. The emission continua observed in the vacuum ultraviolet region between 60 and 80 nm are assigned to the decay of the ionic excimers He+K, He+Na, He+Li, Ne+Rb, Ne+K, Ne+Na, and Ne+Li into the ground state consisting of a rare gas atom and an alkali ion. The binding energies of these excimers range between 0.46 and 0.85 eV. The obtained decay energies and the fine structure splittings agree with the predictions by calculations using phenomenological potentials and by ab initio calculations. Intense emission due to the inner shell excitation of K and Rb is also observed.
Raman scattering evidence of the incommensurability influence on stretching modes in Rb2ZnCl4
Authors: Hedoux A. , Guinet Y. , Lefebvre J. , More M.
Published: July 2002

Temperature dependence of stretching modes are presented for two kinds of Rb2ZnCl4 crystals which exhibit different phase transition sequences. The comparison between both sets of results points out a special behavior of the line which appears below the normal–incommensurate phase transition. Appearance of the incommensurability is observed via the temperature dependence of certain stretching lines.
Laser-induced fluorescence measurements of rotationally resolved velocity distributions for CO+ drifted in He
Authors: Lauenstein Christian P. , Bastian Michael J. , Bierbaum Veronica M. , Penn Stephen M. , Leone Stephen R.
Published: July 2002

Measurements of ion-velocity distributions of CO+ in a He buffer gas are presented as a function of an applied electric field. The distributions are obtained by single frequency, laser-induced fluorescence from various initial rotational states with the laser beam propagating parallel and perpendicular to the drift velocity vector. All distributions are well represented by a Maxwellian for the observed E/N range of 0–13 Td. The reduced mobilities, calculated from the shift of the mean velocity as a function of electric field, increase from 18.7±1.0 cm2 V−1 s−1 at very low fields to 26.4±0.7 cm2 V−1 s−1 at 13 Td. From the width of the Doppler profiles, translational ‘‘temperatures’’ are calculated, which are compared to simple attractive and repulsive Maxwell models as a function of the field. The measured values disagree with the predictions, which are well established for atomic ion systems. The differences are discussed in terms of rotationally inelastic energy transfer in the collisions, which is predicted by kinetic theory models. This argument is strengthened by the fact that even though the rotational states rapidly equilibrate, measurements on different lines yield higher temperatures for higher rotational levels. Finally, the small influence of selective quenching of the electronically excited CO+ on the Doppler profiles is demonstrated by measuring effective lifetimes as a function of the applied drift field.
Relaxation dynamics in molecular alloys. II. Supercooled (C2F6)1−x (CClF3)x plastic crystals
Authors: Böhmer R. , Loidl A.
Published: July 2002

Dielectric measurements on quenched (C2F6)1−x (CClF3)x fluorocarbon alloys are reported. In a wide concentration range, rapid cooling prevents these samples from unmixing and the plastic phase, characterized by translational order and orientational disorder, can be supercooled. The relaxation dynamics of crystals with x=0.32 is examined in detail. For temperatures below 50 K it exhibits a dominant primary and indications of secondary relaxation, i.e., typical features of glasses. The α-relaxation process is parametrized in terms of a modified Vogel–Fulcher law. Using this representation, (C2F6)0.68(CClF3)0.32 can be characterized as fragile glass according to Angell’s classification scheme.
Isothermal measurements of NH3 and ND3 desorption from Cu(001)
Authors: Wu K. J. , Kevan S. D.
Published: July 2002

The kinetics of NH3 and ND3 desorption from Cu(001) surface have been measured by time-resolved electron energy loss spectroscopy. The desorption precess is observed to be characterized by pseudo-first-order kinetics. The desorption energy and the prefactor have been determined for both NH3 and ND3. The prefactors are unusually low as compared to those observed for simple diatomic molecules. Moreover, we observed an unusual behavior consistent with an isotope kinetic compensation effect. Both of these observations can be explained within the context of transition state theory by the interplay between isotopic masses and the shifts of frustrated vibrational modes of adsorbed molecules. Our results imply that the conventional Arrhenius form would not be precisely applicable to molecular desorption, if a large enough temperature range could be measured, particularly for hydrogen-containing molecules.
Hole burning as a probe of exciton bandwidths in amorphous solids
Authors: Reddy N. R. S. , Small G. J.
Published: July 2002

Hole burning experiments were performed on the B800-850 and B875 antenna complexes of Rhodobacter sphaeroides. A novel vibronic satellite hole narrowing phenomenon has been observed and shown to be the basis for determination of exciton bandwidths in amorphous solids.
A study of the reactions between Ba(1S) and N2O, O2, and CO2
Authors: Nien Chia-Fu , Plane John M. C.
Published: July 2002

A kinetic study is presented of the reactions between Ba(1S) atoms and N2O, O2, and CO2 over an extended temperature range. Ba atoms were produced in an excess of each oxidant and He bath gas by the pulsed 2-photon (193.3 nm) dissociation of BaO, produced in situ by the reaction between Ba vapor and the oxidant, and then monitored by time-resolved laser induced fluorescence spectroscopy at λ=553.55 nm (Ba(61P−61S)). The following results were obtained (at the 2σ level of uncertainty): k(Ba+N2O, 242<T<850 K) =(2.99±0.14)×10−10  exp[−(5.7±0.17) kJmol−1/RT]; k(Ba+O2, 254<T<566 K) =(4.89±0.38)×10−10 exp[−(7.06±0.23) kJmol−1/RT]; k(Ba+CO2, 257<T<849 K)=(3.97±0.21)×10−10 exp[−(7.66±0.18) kJmol−1/RT] cm3 molecule−1 s−1. These results are in good agreement with recent molecular beam studies. In the case of Ba+N2O, two line-of-centers collision models are employed to demonstrate that the effect of vibrational excitation of N2O is to enhance the chemiluminescence quantum yield as well as the reaction cross section. Comparison with the chemiluminescence cross section for the reaction Ba+N2O →BaO(A1Σ+ and A′1Π ), obtained in a beam-scattering experiment, indicates a branching ratio of (18.3±6.7)% into these states.
A theoretical study on the ionic states with analysis of vibrational levels of the photoelectron spectrum of formaldehyde (CH2O)
Authors: Takeshita Kouichi
Published: July 2002

A b initio calculations are performed to study the molecular equilibrium structure and the vibrational levels of the low-lying four ionic states, 1 2B2, 2B1, 2A1, and 2 2B2 of formaldehyde (CH2O). The theoretical intensity curve obtained by the Franck–Condon factors for the ionization transitions are also reported and compared with the photoelectron spectrum of CH2O. A number of new assignments of the vibrational levels of the photoelectron spectrum are proposed.
Intramolecular dynamics. I. Curvilinear normal modes, local modes, molecular anharmonic Hamiltonian, and application to benzene
Authors: Zhang Yongfeng , Klippenstein Stephen J. , Marcus R. A.
Published: July 2002

The Hamiltonian based on curvilinear normal modes and local modes (CNLM) is discussed using Wilson’s exact vibrational Hamiltonian as basis, the CNLM representation diagonalizing only the normal mode block of FG matrix in curvilinear internal coordinates. Using CNLM the kinetic and potential energy operators for benzene are given, including cubic and quartic anharmonicity in the potential energy and cubic and quartic terms in the kinetic energy expansion in curvilinear coordinates. Using symmetrized coordinates and cubic and higher force constants the number and identity of the independent symmetry allowed (A1g) such force constants are obtained. The relation to conventional anharmonic force constants is then given and the allowed contributions of the latter are obtained. The results are applied to CH overtone spectra and intramolecular vibrational dynamics in Part III of this series.