bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical analysis of the resonance Raman spectra of diacetylene single crystals
Authors: Materny A. , Kiefer W.
Published: July 2002

Resonance Raman spectra of polymer chains in the single crystals of the diacetylenes FBS (2,4-hexadiynylene-di-p-fluorobenzene sulfonate), TS/FBS [6-(p-toluenesulfonyloxy)-2,4-p-fluorobenzene sulfonate] and TS6 (2,4-hexadiynylene-di-p-toluene sulfonate) are described theoretically by means of a Franck–Condon model, which considers a chain length dependence. The electronic transition energies and matrix elements are calculated by means of a linear-combination-of-atomic-orbitals–molecular-orbital method calculation in the Hückel approach. We are able to describe the Raman excitation profiles as well as the Raman band profiles for two Raman active modes, i.e., the C+C-, and the C 3/4 C-stretching vibrations of the polymer chains. The model also contains a description of a side group vibrational mode which is enhanced by Fermi resonance with the C=C-stretching vibration. Observed Raman excitation profiles can be well simulated by these calculations. An evaluation of the parameters shows a strong influence of defects, which is described by including inhomogeneous broadening in the theoretical model. An interaction between ensemble and intramolecular properties due to defects can be shown. Simulation of the Raman band profiles yields information on the influence of chain length distribution. Changes of position and Raman band profile as well as of the strength of the Fermi resonance can be explained by the enhancement of modes belonging to polymer chains having different chain lengths. A considerable influence of the substitutents results in remarkable changes of several parameters on which the model calculation is based.
Microwave spectrum, dipole moment, and dynamic structure of the ‘‘linear’’ HCCH–CO dimer
Authors: Germann T. C. , Tschopp S. L. , Gutowsky H. S.
Published: July 2002

Rotational spectra have been observed for several 13C/18O isotopic species of the pseudolinear acetylene–carbon monoxide dimer using the pulsed, Fourier transform Balle/Flygare microwave spectrometer. A quadrature detection system has been incorporated in the data processing system to avoid folding of the spectrum onto itself in taking the Fourier transform. Dipole moments of 0.3112(12) and 3.29(3) D were determined for the parent HCCH–CO species and for HCN–HCCH, respectively. Structural properties were derived from the various measurements, including a well depth of 300 cm−1 from DJ, and the results are compared with those reported earlier for HCN–HCCH. A substitution analysis is developed in which the rotational properties of a pseudolinear dimer are treated as a symmetric top. A simple method is described for determining effects of the off-axis bending of the monomers.
Further measurements of the H+3(v=0,1,2) dissociative recombination rate coefficient
Authors: Canosa A. , Gomet J. C. , Rowe B. R. , Mitchell J. B. A. , Queffelec J. L.
Published: July 2002

A new flowing afterglow apparatus that utilizes a Langmuir probe/mass spectrometer to monitor both electron and ion decay in a hydrogen plasma has been used to measure the dissociative recombination rate coefficient of H+3 at two different electron temperatures. At 300 K a rate coefficient of 1.5×10−7 cm3 s−1 was found for H+3 ions with a low degree of vibrational excitation (v≤2). The rate coefficient for ground state ions H+3(v=0) was measured as 1.1×10−7 cm3 s−1 at 650 K. A discussion is given of the excitation states of H+3 ions in the afterglow in the light of slow deexcitation rates for low vibrational states. A new model for the recombination of H+3 is presented.
Quantifying chaos and transient chaos in nonlinear chemically reacting systems
Authors: Coveney P. V. , Chaudry A. N.
Published: July 2002

In the chemical literature of recent years, there has been considerable interest in the study of deterministic chaos within the context of nonlinear kinetic schemes. However, dynamical systems theory admits a rather strict definition of ‘‘chaos’’ which has seldom been confirmed in the many cases where it is claimed that chaos exists in coupled chemical kinetic models. In this paper we carry out a systematic study of the dynamical properties of two such model systems, computing Lyapunov exponents, fractal dimensions, and power spectra from the (time) series arising from the associated differential equations. In both cases, the analyses presented here provide strong support for the existence of chaotic dynamics for certain values of the appropriate control parameters. In view of the potential difficulty of resolving stochastic fluctuations from chaotic temporal behavior in experimental situations, it is recommended that, wherever possible, authors report estimates of Lyapunov exponents and fractal dimensions of associated chaotic attractors.
Theoretical model for the dynamics of hydrogen recombination on the Si(100)-(2×1) surface
Authors: Sheng Jia , Zhang John Z. H.
Published: July 2002

We propose in this paper a quantitative theoretical model to describe the recombination dynamics of hydrogen on Si(100)-(2×1) surface. The desorption kinetics of hydrogen on Si(100) has been experimentally determined to obey a first-order rate law and the internal state distributions of desorbed hydrogen has recently been determined experimentally using the resonantly enhanced multiphoton ionization technique. In this theoretical model, which has the characteristic of preassociative desorption, the rate of desorption and the internal state distribution of H2 is given by a thermally averaged golden-rule expression. In particular, the desorption of H2 is supposed to result from a bound-free transition between an initially bound state composed of two H–Si dangling bonds on the same silicon dimer and a final continuum state consisting of H2 plus Si surface. In addition to explaining the first-order desorption kinetics, our model dictates that H2 will be vibrationally hot upon desorption, whereas rotation of H2 will generally be expected to be cold because of symmetry constraints. These conclusions about the dynamics are consistent with recent experiments [K. W. Kolasinski, S. F. Shane, R. N. Zare, J. Chem. Phys. 95, 5482 (1991); 96, 3995 (1992)] in which hydrogen is found to be vibrationally excited but rotationally cold. We show, in this paper, that essentially all recent experimental results on hydrogen desorption on Si(100) can be qualitatively explained based on the Franck–Condon factors in our model. A co-planar model calculation is carried out using our method, and the calculated rovibrational distribution of H2 is compared with the aforementioned experimental results of Kolasinski, Shane, and Zare.
A neural network approach to the study of internal energy flow in molecular systems
Authors: Sumpter Bobby G. , Getino Coral , Noid D. W.
Published: July 2002

Neural networks are used to develop a new technique for efficient analysis of data obtained from molecular-dynamics calculations and is applied to the study of mode energy flow in molecular systems. The methodology is based on teaching an appropriate neural network the relationship between phase-space points along a classical trajectory and mode energies for stretch, bend, and torsion vibrations. Results are discussed for reactive and nonreactive classical trajectories of hydrogen peroxide (H2O2) on a semiempirical potential-energy surface. The neural-network approach is shown to produce reasonably accurate values for the mode energies, with average errors between 1% and 12%, and is applicable to any region within the 24-dimensional phase space of H2O2. In addition, the generic knowledge learned by the neural network allows calculations to be made for other molecular systems. Results are discussed for a series of tetratomic molecules: H2X2, X=C, N, O, Si, S, or Se, and preliminary results are given for energy flow predictions in macromolecules.
The magic numbers of metal and metal alloy clusters
Authors: Yamada Y. , Castleman A. W.
Published: July 2002

Pure metal and metal alloy clusters including Cun, Agn, CunAgm, CunAlm, CunInm, AgnAlm, AgnInm, and CunPbm are produced by a gas aggregation source and investigated by time-of-flight mass spectrometry following ionization with a KrF excimer laser. In the case of pure metal clusters (Cun,Agn,Inn), as well as alloy clusters composed of these metals, magic numbers are observed in their cluster ions which correspond to jellium shell closings (counting the total valence electrons from the component metals). These findings are in good agreement with their expected free-electron behavior. Interestingly, the abundance of pure Pbn+ corresponds to species which are expected to be especially stable due to their geometric structure. A similar situation also arises for the Pb-rich alloy clusters. By contrast, the metal alloy clusters CunPbm+ show magic numbers at jellium shell closing in the series of Cu-rich clusters.
H–N2 interaction energies, transport cross sections, and collision integrals
Authors: Stallcop James R. , Partridge Harry , Walch Stephen P. , Levin Eugene
Published: July 2002

The energies for the interaction of a hydrogen atom with a nitrogen molecule have been calculated for large separation distances using a complete-active-space self-consistent-field/externally contracted configuration interaction method. H–N2 transport cross sections and collision integrals have been calculated using sudden approximations and a semiclassical description of the scattering. The values of these quantities are found to be close to the corresponding values determined from the average (isotropic) potential energy. The collision integrals are applied to determine diffusion and viscosity coefficients; the theoretical diffusion agrees well with the measured data available from experiments at low temperatures.
The length of intersection lines and the number of cusps in assemblies of interpenetrating spheres
Authors: Thompson Aidan P. , Glandt Eduardo D.
Published: July 2002

A previously derived expression for the specific surface area of a dispersion of mutually penetrable spheres or pores is generalized for the calculation of the length of sphere–sphere intersections (lines common to two pores) and the number of cusps (points common to three pores). The results are in the form of alternating series involving correlation functions of successively higher order. Closed-form expressions are obtained for the case of randomly centered spheres. For a porous material with number density ρ of spherical pores of radius a, the length of pore–pore intersections per unit volume is 2π3a4ρ2 exp(−4πa3ρ/3) and the number of cusps per unit volume is (4)/(3)  π4a6ρ3 exp(−4πa3ρ/3). The analysis is also extended to the general case of dispersions of hyperspheres in d dimensions.
Modified Donnan phenomena in polyaniline with poly(vinyl sulphonate) chains
Authors: Mafé Salvador , Manzanares José A. , Reiss Howard
Published: July 2002

We develop a physical model, based on the modified Donnan phenomena ideas introduced previously by the authors, to describe the acid doping of the conducting polymer polyaniline. The theory is motivated by the experimental work of Asturias et al. [Ber. Bunsenges. Phys. Chem. 95, 1381 (1991)]. Good agreement between theory and experiment is found.
Monte Carlo simulations at constant chemical potential and pressure
Authors: Ray John R. , Wolf Ralph J.
Published: July 2002

The usual way of carrying out Monte Carlo simulations of open systems is by using the grand canonical ensemble. In the grand canonical (TVμ) ensemble a system of fixed volume V is in contact with a temperature reservoir having temperature T, and a particle reservoir having chemical potential μ. In order to obtain values for thermodynamic functions for a given pressure in TVμ Monte Carlo simulations a series of simulations at fixed T and V and different chemical potentials is carried out to determine P(μ), and then an extrapolation to the desired pressure must be carried out. Here we discuss Monte Carlo simulations of open systems in the RPμ ensemble in which an energy R, the chemical potential μ, and the pressure are held fixed during the simulation. Thus each RPμ ensemble simulation replaces a set of TVμ simulations. Simulations of an embedded atom model of liquid palladium are discussed using the new method.
Collisional dynamics of the BrCl B 3Π(0+) state. II. Vibrational and rotational energy transfer
Authors: Perram Glen P. , Davis Steven J.
Published: July 2002

Vibrational and rotational energy transfer within the B 3Π(0+) state of bromine monochloride was studied using both pulsed and steady-state laser-induced fluorescence techniques. Rate coefficients for state-to-state vibrational transfer were determined from the observed BrCl(B–X) emission in the presence of Cl2, He, Ne, Ar, Kr, and Xe. Vibrational states v′=0 to 7 were probed, and resolved emission was observed from states v′=0 to 6. The vibrational transfer within BrCl(B) is rapid, as expected from the small vibrational spacing. The fundamental rate coefficient for vibrational transfer from v′=1 to v′=0 with chlorine as the collision partner is 1.3×10−11 cm3 molecules−1 s−1 and the vibrational transfer rate coefficient scales with vibrational quantum number as a power law with an exponent of 0.63. Vibrational transfer with the noble gases is less efficient with fundamental rate coefficients ranging from 4×10−12 cm3 molecules−1 s−1 for helium to 2×10−12 cm3 molecules−1 s−1 for krypton. The scaling of vibrational transfer rates with vibrational quantum number for the noble gases is consistent with the Landau–Teller theory. The Δv=−2 rate coefficients are estimated at 40% of the Δv=−1 rates. Methods of analyzing the temporal profiles of vibrationally resolved emission from laser-induced fluorescence experiments are described for conditions where vibrational transfer is rapid and the excited vibrational states are strongly coupled. Rotational transfer in BrCl(B) is very efficient with total removal rates ranging from 1.6×10−10 cm3 molecules−1 s−1 for chlorine collisions to 2.4×10−10 cm3 molecules−1 s−1 for argon.
Statistical basis for extended thermodynamics of diffusion in dense fluids
Authors: Nettleton R. E.
Published: July 2002

The distribution which maximizes the information-theoretic entropy functional has been used previously to calculate a Grad-type closure for the Grabert equation for the distribution of values of the diffusion flows. Kinetic equations for these flows, which are phenomenological equations of extended thermodynamics, can be obtained as previously from first moments of the Grabert equation. The phenomenological coefficients are time correlations of operators involving projection operators P̂ of a type introduced in the Grabert derivation. Here we use a method of Lee, expanding time-dependent operators in an orthogonal basis set, to evaluate the correlations. We can answer previously open questions of whether the correlations decay exponentially and whether the kinetic equations describe an irreversible approach to equilibrium. By suitably defining P̂ we can render as exact as we please results previously derived by making Markovian and Grad-type closure approximations.
Autoionization of excited Frenkel states in the anthracene crystal
Authors: Petelenz Piotr , Mucha Dariusz
Published: July 2002

Autoionization of Frenkel excitons is re-examined as a potential source of the anthracene photocurrent at photon energies higher than 4.4 eV. A crude estimate of the rate constant suggests autoionization as a feasible depletion channel, the resultant photocurrent quantum yield being in reasonable agreement with experiment. Bounds for the thermalization radius of charge carriers produced by autoionization are estimated. The estimates, based on a simple model of charge carrier transport and scattering, are found to agree, as to the order of magnitude, with experimental data. A fit of the experimental photocurrent spectrum indicates that while in the near-threshold region the photocurrent is dominated by charge pairs produced by direct optical excitation, at higher energies the autoionization mechanism generally prevails.
Perturbative treatments of pump–probe laser-molecule interactions with applications to azulene and trimethylazulene
Authors: Tran Phuc , Meath William J. , Wagner Brian D. , Steer Ronald P.
Published: July 2002

Semianalytic perturbative approaches for investigating the spectroscopy, and the underlying dynamics, associated with fast time-resolved, two-photon, two-color, pump–probe, low intensity laser-molecule interactions are developed and discussed. In particular the perturbation theory is developed with emphasis on molecular models associated with pump–probe experiments on the S0→S1→S2 two-photon transitions in azulene and trimethylazulene. The experiments are discussed and the theory is used to determine the lifetimes of the intermediate S1 states by analyzing the experimental two-photon fluorescence signals from the S2 states as a function of the time-delay between the pump and probe lasers. The advantages of using this approach relative to the traditional methods for determining the lifetime of the S1 states are discussed. The dependence of the two-photon fluorescence signals on the angle between the polarization vectors of the pump and probe lasers, for fixed time delay between the lasers, is also considered briefly.
A realistic molecular dynamics simulation of the plastic crystalline phase of neopentane. II. Individual orientational dynamics
Authors: Breymann Wolfgang , Pick Robert M.
Published: July 2002

For a ‘‘realistic’’ model of the ODIC phase of neopentane, the single particle orientational dynamics is analyzed; the data has been obtained previously by a molecular dynamics simulation. The orientational motion of the molecules from one potential well to another is studied by analyzing their orientational trajectories directly and through the temporal self correlation of an appropriate symmetrized rotator function. Self correlation functions of other rotators give information on the librational motion of the molecules at the bottom of the orientational potential wells. The mean orientational potential is shown to better represent this librational motion than the reorientational dynamics. The method for determining which rotator functions have to be used to describe the different types of motion makes use of the symmetry group of the orientational wells.
Infrared diode-laser spectra of the ν9 and ν11 N–O stretching bands of N2O4
Authors: Domenech J. L. , Andrews A. M. , Belov S. P. , Fraser G. T. , Lafferty W. J.
Published: July 2002

The rotationally resolved jet-cooled infrared spectra of the b-type ν9 (b2u) fundamental band at 1757 cm−1 and the a-type ν11 (b3u) fundamental band at 1261 cm−1 of the N–O stretches of N2O4 have been recorded with a diode laser. The ν9 band was found to be unperturbed, and it was possible to assign nearly 100% of the observed lines with a signal to noise greater than 2. In contrast, most of the Ka states of the ν11 band were found to be strongly perturbed. A large number of strong lines (≊20%) are unassigned and presumably arise from the perturbing state as well as torsional hot band transitions. The rotational analysis yields precise spectroscopic constants for the ground vibrational state which are interpreted in terms of a planar centrosymmetric dimer with a N–N bond length of 1.756(10) Å. The observed nuclear-spin statistical weights and near-zero inertial defect are consistent with the planar centrosymmetric structure determined in earlier electron-diffraction studies.
Response to ‘‘Comment on ‘Temperature-enhanced electron detachment from C6F6− negative ions’ ’’ [J. Chem. Phys. 100, 6981 (1994)]
Authors: Christophorou L. G. , Datskos P. G. , Carter J. G.
Published: July 2002

Thermally-induced electron detachment from collisionally stabilized C6F6 in a buffer gas of N2 has an activation energy of 0.477 eV. This value agreed well with the electron affinity (0.52 eV) reported by Chowdhury et al. and Knighton et al. However, it is lower than the value of 0.86 eV for the electron affinity of C6F6 reported by some authors. Based on the difference between these two values, Chen et al. suggested that the thermally induced electron detachment we observed for C6F6 in N2 may be occurring from a low-lying excited state of C6F6. If this is the case, collisional deactivation of this excited state of C6F6 to the ground state must be very inefficient.
Theoretical and experimental studies of the molecular second order nonlinear optical responses of heteroaromatic compounds
Authors: Wong K. Y. , Jen Alex K-Y. , Rao V. Pushkara , Drost K. J.
Published: July 2002

We present theoretical and experimental studies of the molecular second order nonlinear optical responses of donor–acceptor substituted conjugated compounds which consist of heteroaromatic moieties in place of the traditional benzenoid moieties. Electric-field-induced second harmonic generation measurement at a fundamental wavelength of 1.907 μm demonstrated a twofold increase of the off-resonant molecular hyperpolarizability of a heteroaromatic compound compared to its benzenoid counterpart. Results of theoretical calculations based on ab initio scheme are in good agreement with the experimental results.
O(N3) scaling of two-electron integrals during molecular geometry optimization
Authors: Ionova Irina V. , Carter Emily A.
Published: July 2002

We propose an optimization technique designed specifically for molecular structure optimization performed on an ab initio level. This gradient-based technique is a modification of quadratically convergent quasi-Newton method, and although it requires more energy evaluations than the conventional method, each of these energy evaluations is much cheaper due to O(N3) scaling of the two-electron integrals evaluation. Statistics obtained from numerous optimization runs with Lennard-Jones molecules shows that the number of energy and gradient evaluations for the proposed technique is only 1.5–5 times (for 3–27 atoms, respectively) larger than that for conventional method. Given the great advantage of O(N3) scaling of the two-electron integrals in the former, a substantial speedup of the overall computation can be achieved in certain cases. We consider the factors which affect the performance of the proposed technique and we also present timings and other details of several molecular structure optimization tests of the method on the ab initio level. Additionally, a novel approach to numerical Hessian evaluation during optimization is proposed, where the quality of the Hessian so obtained can be assessed.
Combining doubly charged cations and anions to form new species
Authors: Gonzales Nick , Simons Jack
Published: July 2002

In this study we predict, based on our ab initio electronic structure calculations, the existence and relative stabilities of several new species such as H3SMgF3, H2SMgF2, and HSMgF derived from combining the geometrically metastable and electronically stable parent doubly charged ions H4S+2 and MgF−24. Although this study focuses on compounds obtained from these particular double ions, we suggest that analogous observations will be obtained when other double ions are used. Stable structures of the HSMgF, and H2SMgF2 molecules, as well as a local energy minimum and two different second-order saddle point geometries for the H3SMgF3 molecule were found. The lowest-energy stable structure is that found for H3SMgF3, which is best described as a dative adduct of H2S, MgF2, and HF, and which lies ∼570 kcal/mol below the separated-ion starting materials and ∼140 kcal/mol below the (H4S+2)(MgF−24) contact ion pair.
Dynamics of sodium-doped polyacetylene
Authors: Dianoux A. J. , Kneller G. R. , Sauvajol J. L. , Smith J. C.
Published: July 2002

The low-frequency dynamics (<20 meV) of pure and sodium-doped trans polyacetylene are investigated using a combination of incoherent neutron scattering spectroscopy and molecular dynamics simulations. The simulations are performed using a molecular mechanics potential function and including explicitly the three-dimensional crystal environments of the molecules. Both the experiments and the simulations indicate that doping results in a marked change in the vibrational density of states of the polyene chains in the direction perpendicular to the chain axes, a broad minimum appearing at ∼16 meV. This spectral region is dominated by intramolecular torsional displacements. The results also suggest that the mean-square displacements of the polyacetylene atoms become more isotropic on doping. The contributions of various rigid-body motions to the simulation-derived mean-square displacements and vibrations are described.
Path integration of a general two time action involving local and nonlocal harmonic oscillator potentials
Authors: Chanana Namrata , Menon Vairelil J. , Singh Yashwant
Published: July 2002

Path integration of a general two-time quadratic action having local and nonlocal harmonic oscillator potentials is performed within the framework of Feynman’s polygonal path approach. The propagator (or the density matrix) thus obtained is diagonalized and the values of energies and eigenfunctions of a large number of states as a function of the strength of the nonlocal potential which simulates the effect of the medium on the solute represented by the local harmonic oscillator potential are derived. These values are used to calculate the effect of the solvent on the various properties of the solute. The possibility of representing the propagator in terms of an effective local action is also explored.
Linear viscoelasticity in dispersions of adhesive hard spheres
Authors: Woutersen A. T. J. M. , Mellema J. , Blom C. , de Kruif C. G.
Published: July 2002

The viscoelastic behavior of concentrated dispersions of adhesive hard spheres was investigated. By changing the temperature, the interaction potential of the particles was varied from a hard sphere repulsion to a strong attraction. Using torsion resonators and a nickel tube resonator the complex viscosity was measured in the frequency range 70–250 000 Hz. The results were described on the basis of a simple mechanical model in terms of a series of relaxation times and a single relaxation strength. The temperature dependence of the longest relaxation time and the relaxation strength indicated that the system undergoes a kinetic transition with decreasing temperature. The transition could be identified with the percolation threshold. Above the transition temperature where the dispersion is in a stable, fluid state, linear viscoelastic behavior is observed. Below the percolation threshold but still in the one-phase region, nonlinear viscoelasticity was measured, even at the smallest strains.
Spectroscopy of jet-cooled AlMn and trends in the electronic structure of the 3d transition metal aluminides
Authors: Behm Jane M. , Morse Michael D.
Published: July 2002

Jet-cooled diatomic AlMn has been spectroscopically investigated and is shown to possess a 5Πi ground state deriving from the Al (3s23p1,2P0)+Mn (3d54s2,6S) separated atom limit. This implies that the aluminum atom favors a 3pπ approach to the manganese atom, a result that is in agreement with previous studies on the related AlCa and AlZn molecules. The ground state bond length has been measured as 2.6384±0.0010 Å, a value which includes corrections due to spin–uncoupling effects in the X 5Πi state. It is suggested that the 5Πi state emerges as the ground state due to a particularly favorable configuration interaction with a low-lying 5Πi state that derives from the interaction of Al (3s23p1,2P0)+Mn [3d6(5D)4s1,6D]. Combination of the atoms in this excited state leads to a strong σ2 covalent bond via a 3pσAl–4sσMn interaction, which causes this excited electronic state to drop in energy so that it is expected to lie within a few thousand cm−1 of the ground electronic state. Following a discussion of the spectroscopic results on AlMn, an overall summary of the spectroscopic results on the 3d series of transition metal aluminides is presented, along with predictions of the ground electronic states of the as yet unobserved AlSc, AlTi, and AlFe molecules.
Energy-resolved angular distributions and the population partition of excited state Rh atoms ejected from ion bombarded Rh {001}
Authors: He Chun , Postawa Z. , El-Maazawi M. , Rosencrance S. , Garrison B. J. , Winograd N.
Published: July 2002

The energy-resolved angular distributions of Rh atoms ejected from Rh {001} by bombardment with 5.0 keV Ar+ ions have been measured for the ground state (a 4F9/2) and the two lowest lying excited state (a 4F7/2,a 4F5/2). Simultaneous measurements on these electronic states provide us an opportunity to examine the influence of electronic interactions on desorbed particles. The experimental results show that there is a sequential variation in the angular distributions as the excitation energy increases. These variations are attributed to the interaction between the substrate electrons and the excited state atom as it is being ejected from the surface. Since the measurements are performed using multiphoton ionization via a single intermediate state, the population partition among the three lowest states is obtained as well. The excitation probabilities of the a 4F7/2 and a 4F5/2 states are compared with those predicted from the expression exp(−A/av⊥) and with a recently proposed model involving interatomic collisions above the surface. Results suggest that atoms excited via this mechanism make a significant contribution to the population of atoms ejected with low ejection velocities for the first-excited a 4F7/2 state (0.19 eV), as reported previously. Moreover, we suggest that an even higher proportion of atoms in the a 4F5/2 state are produced via this mechanism.
Viscosity of gaseous 3He and 4He at low temperatures
Authors: Nacher P. J.
Published: July 2002

The viscosity coefficients of gaseous 3He and 4He have been computed for temperatures below 5 K. The results are compared to those of previous calculations and to recent experimental data. Measurements at lower temperatures should provide a better test of the accuracy of interatomic potentials.
Ion–molecule reactions of ArN+2 with simple aliphatic hydrocarbons at thermal energy
Authors: Tsuji Masaharu , Matsumura Ken-ichi , Kouno Hiroyuki , Aizawa Masato , Nishimura Yukio
Published: July 2002

The product ion distributions and rate constants are determined for ion–molecule reactions of ArN+2 with C2Hn (n=2,4,6) and C3Hn (n=6,8) by using a thermal ion–beam apparatus. Although charge-transfer channels leading to parent ions and/or fragment ions are found, no displacement reaction leading to ArCmH+n and N2CmH+n is detected. A comparison of the product ion distributions with breakdown patterns of the parent ions suggests that fragment ions, formed through cleavage of C–H and/or C–C bonds, are produced via near-resonant ionic states in the 13.1–13.4 eV range. The branching ratios of parent ions for C2H4 (68%) and C3H6 (20%) are larger than those for C2H6 (5%) and C3H8 (5%). The large branching ratios of the parent ions for the unsaturated hydrocarbons are explained as due to a strong interaction of a vacant orbital of ArN+2 with the highest occupied πC=C orbital of the unsaturated hydrocarbons which induces nonresonant charge transfer. The total rate constant for C2H2 is 6.8×10−10 cm3 s−1, while those for C2Hn (n=4,6) and C3Hn (n=6,8) are in the range (8.5–9.8)×10−10 cm3 s−1. The former and the latter values correspond to 69% and 77%–90% of the calculated values from Langevin or average dipole orientation (ADO) theory. The smaller kobs/kcalc ratio for C2H2 is attributed to the lack of near-resonant ionic states with favorable Franck–Condon factors for ionization.
Orientation-dependent electrical double-layer interactions. I. Rodlike macroions of finite length
Authors: Halle Bertil
Published: July 2002

The orientation-dependent electrical double-layer interaction between two thin rodlike macroions of finite length is considered. Starting from an expression for the interaction free energy as a double integral of the Green’s function for the linearized Poisson–Boltzmann equation, we obtain several useful analytical approximations. For certain rod configurations, we obtain simple analytical approximations to the double-layer interaction that are quantitatively accurate at physically relevant rod separations and electrolyte concentrations. For a general rod configuration, specified by the separation and three angles, we develop a generalized multipole expansion of the double-layer interaction energy, which converges rapidly if the rod length is not much larger than the center-of-mass rod separation or the Debye length. The orientation dependence of the double-layer repulsion can then be quantitatively accounted for in terms of the screened charge–quadrupole and quadrupole–quadrupole interactions. Furthermore, we show that the double-layer repulsion between finite rods depends on the rod orientations even at arbitrarily large separation.
Effects of temperature and optical heating on the optical spectra of metalloporphyrins in the vapor phase
Authors: Davis Wendy , Calongne Sally , Rodriguez Juan
Published: July 2002

The spectral trends associated with static and optical heating are studied for octaethylporphyrins in the gas phase near 600 K. Both the absorption and emission profiles are observed to red shift rapidly with increasing temperature. The 0–0 origin in the absorption spectrum of the Q bands all have a very diffuse character that is well described by a Lorentzian profile with a FWHM of about 20 nm. This linewidth cannot be accounted by collisional or population lifetime broadening. The impact of optical heating in the emission spectra is found to be comparable to that of static heating. The addition of 0.7 eV of excess energy results in a measured temperature rise in octaethylporphyrins of 70 K. The relevance of these results to ultrafast studies in the condensed phase, including heme proteins, is also discussed.
Correlation between the Madelung field and the reactivity of the MgO low-coordinated surface sites
Authors: Stefanovich Eugene V. , Truong Thanh. N.
Published: July 2002

We study correlations between Madelung constants, charges of surface ions, and chemical activity of low-coordinated sites of the MgO crystal surface with respect to dissociative adsorption of hydrogen. The ab initio Embedded Cluster model [Puchin et al., Phys. Rev. B 47, 6226 (1993)] employed in this study allowed us to reproduce correctly both short range and long range (Madelung) parts of the interaction between ions in a quantum cluster and the rest of the crystal. Our results show that sites having the same coordination numbers may have different properties, depending on values of Madelung constants. Lower Madelung constants correspond to lower ionicity, higher energy of H2 adsorption, stronger O–H and Mg–H bonds, and larger spatial separation of two adsorbed hydrogen atoms.
Generation of excited state potentials from photofragment spectral lines: Fano profiles in FNO
Authors: Shapiro Moshe , Reisler Hanna
Published: July 2002

We develop a procedure to determine excited states potentials from photofragmentation data using adiabatic energy curves. The adiabatic curves, obtained from empirically derived diabatic channel potentials, are inverted to yield a polyatomic potential energy surface which is expressed as a piecewise collection of diatomic cuts. The procedure is applied to obtaining a two-dimensional potential of the FNO molecule in the S1 excited state. The potential is more successful than available ab initio surfaces in reproducing the observed asymmetric (Fano) line shapes of the FNO S1←S0 state-specific photofragment yield spectra.
Ab initio calculation of nonadiabatic couplings using M
E
L
D
Authors: Castillo J. F. , Errea L. F. , Macías A. , Méndez L. , Riera A.
Published: July 2002

We present a numerical differentiation technique for the calculation of directional derivatives of electronic wave functions evaluated using a multireference configuration interaction method. The technique has been implemented in the set of programs MELD, and its application is illustrated by calculations of nonadiabatic couplings for the ArH+2 quasimolecule.
Response to ‘‘Comment on a proposed method for finding barrier height distributions’’ [J. Chem. Phys. 103, 1235 (1995)]
Authors: Straub John E. , Keyes T. , Thirumalai D.
Published: July 2002

A rebuttal is given to a criticism of an approximate method for the determination of the distribution of energy barrier heights in liquids and proteins. It is argued that the one-dimensional counterexample used in the critique violates clearly stated assumptions in the derivation, which also appear to hold in nature. The method is shown to be accurate in a more appropriate one-dimensional test.
Isotope effects and proton hyperfine interactions in the lowest 3nπ* state of substituted benzaldehydes
Authors: Neugebauer-Crawford S. M. , Tinti D. S.
Published: July 2002

The zero-field splittings, principal spin axes, kinetic parameters, and nuclear hyperfine interactions of the 3nπ* state of p-chloro- and p-methylbenzaldehyde and several of their deuterated derivatives are investigated by zero- and low-field optically detected magnetic resonance (ODMR) at 1.4 K in a p-dimethoxybenzene host. The zero-field splittings show large isotope effects. These are interpreted in terms of spin–orbit interaction with the nearby but higher lying 3ππ* state, yielding the energy gap between the two states in both benzaldehyde derivatives. The locations of the spin axes are approximately along the local symmetry axes of the carbonyl group and are insensitive to isotope. But, the spin axis most nearly normal to the plane of a host molecule deviates from the normal by an angle of 7°–13°. The kinetic parameters of the 3nπ* state also are relatively insensitive to isotope. The dominant hyperfine interactions are associated with the aldehyde hydrogen and indicate that the 3nπ* state is largely localized on the aldehyde moiety. Various properties of the 3nπ* and 3ππ* states are compared.
Detailed balance methods for chemical potential determination
Authors: Fay Patrick J. , Ray John R. , Wolf Ralph J.
Published: July 2002

In earlier work [J. Chem. Phys. 100, 2154 (1994)] we presented a new method of determining the chemical potential in Monte Carlo or molecular dynamics simulations which makes use of a detailed balance method (DBM). In the present paper we present results of a careful study of this method applied to liquid palladium down to and below the zero-pressure melting temperature. A new surface detailed balance method (SDBM) is introduced, which is much more efficient near and below the melting temperature where the original method becomes too inefficient to determine an accurate value of the chemical potential. We also present results where the new surface detailed balance method is used to determine the chemical potential of the solid phase at a number of different temperatures.
Photochemistry of NH3 on Pt(111): Ejection of thermalized atomic hydrogen from ammonia multilayers
Authors: Lanzendorf Eric , Masson Denis P. , Kummel Andrew C.
Published: July 2002

The dissociative 193 nm photochemistry of both multilayer and submonolayer NH3 on Pt(111) has been studied. Upon irradiation with 193 nm light, the submonolayer covered Pt surface yields only a small amount of NH3 desorption. In contrast, the desorption products from the 193 nm irradiation of a multilayer NH3 covered Pt surface are both NH3 and atomic H. The NH3 photodesorption yield from the multilayer is much greater than from the submonolayer covered surface. A nearly field-free resonantly enhanced multiphoton ionization technique was used to carefully distinguish between H and NH3 photoproducts. From the multilayer, NH3 desorbs with a distinct bimodal hyperthermal velocity distribution. The atomic H velocity distribution exhibits both a hyperthermal component and, surprisingly, a ∼150 K thermalized component. Our multilayer data is consistent with adsorbate absorption, and we have proposed a model based on multiple collisions of desorbing species to explain our results. In this model, NH3 molecules at the surface photodissociate and directly eject H atoms into the gas phase with a hyperthermal kinetic energy. NH3 molecules buried within the multilayer also undergo photodissociation but their ejected H atoms suffer multiple collisions, losing kinetic energy and becoming thermalized prior to desorption into the gas phase.
Dehydration of the cyclodextrins: A model system for the interactions of biomolecules with water
Authors: Marini Amedeo , Berbenni Vittorio , Bruni Giovanna , Massarotti Vincenzo , Mustarelli Piercarlo , Villa Marco
Published: July 2002

The thermodynamics of hydration of biomolecules is experimentally studied in the β-cyclodextrin (β-CD), which contains water molecules in a range of configurations and has been proposed as a model system for complex biomolecules. The thermal measurements point to the role of a structural transition from the hydrated β-CD (phase I) to a ‘‘dehydrated’’ form (phase II). We show that dehydration in phase I is assisted by a ‘‘compensation mechanism’’ for which β-CD contributes a constant amount of energy for each H2O mole. Despite the presence of different types of H2O’s, water losses in phase I are accurately described in terms of this energy and the isosteric molar enthalpy of dehydration. Moreover, in going from the fully hydrated to the fully dehydrated form, the contribution of β-CD to dehydration is over all equal to the enthalpy of transition from phase I to phase II. Our analysis yields the changes of an enthalpy associated with the biomolecule alone as a function of the water content. In the case of β-CD, we can sketch a qualitative phase diagram, which assists the interpretation of details of our thermal experiments. The role of kinetic factors in the attainment of the thermodynamic equilibrium is investigated with 2H-NMR in samples recrystallized from heavy water. We find that, over a wide range of hydration levels, water molecules have a liquidlike diffusion, which, together with the compensation mechanism, explains the fast and nearly reversible dehydration of the β-CD.
Comment on ‘‘Density functional calculation of nuclear magnetic resonance chemical shifts’’ [J. Chem. Phys. 102, 2806 (1995)]
Authors: Malkin Vladimir G. , Malkina Olga L. , Salahub Dennis R.
Published: July 2002

van Wüllen has correctly commented on the lack of gauge invariance of our sum-over-states density functional perturbation theory (SOS-DFPT) method for NMR chemical shifts. It is shown that, for practical basis sets, the improvement of the SOS-DFPT method over an uncoupled approach is greater than the difference in gauge dependence between these two methods.
Comment on ‘‘Size consistency and size extensivity of linear response properties using the perturbed electron propagator’’ [J. Chem. Phys. 102, 8967 (1995)]
Authors: Champagne Benoît , Mosley D. H. , Fripiat J. G. , André J. M. , Öhrn Yngve
Published: July 2002

In their recent work, Delueze and Pickup stated that in the random phase approximation (RPA) approach developed by Champagne et al. for evaluating the static and dynamic longitudinal polarizabilities per unit cell of stereoregular polymers (i) singularities due to the long-range character of the Coulomb forces were not completely addressed; and (ii) this deficiency might be possibly at the origin of some obvious discrepancies, of the order of several atomic units, between molecular and polymer calculations of polarizabilities per monomeric unit.
Density of states in the Drude–Lorentz model of a nonpolar fluid
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

The frequency-dependent self-susceptibility of a selected particle in the Drude–Lorentz model of a nonpolar fluid is shown to have a universal absorption spectrum in the limit of low density. We derive an expression for the universal lineshape and calculate the limit function by a scaling procedure, in a self-consistent ring approximation. The line shape is found to be an asymmetric function of frequency, as suggested by computer simulation data for systems of hard spheres and of atoms with Lennard-Jones interactions. A calculation of correction terms accounting for the effect of the microstructure at moderate density leads to fairly good agreement with existing computer simulation data.
Solvent effects on the potential energy surface of the 1:1 complex of water and formamide: Application of the polarizable continuum model to the study of nonadditive effects
Authors: Contador J. C. , Sanchez M. L. , Aguilar M. A. , Olivares del Valle F. J.
Published: July 2002

A study of the solvent effect on the potential energy surface of the 1:1 complex of water and formamide have been performed. In the description of the solvent we have employed the polarizable continuum model. The calculations were done at Hartree–Fock ab initio and Mo/ller–Plesset (MP) levels. We found that the geometry of the system is appreciably modified by the solvent. The most important changes are the inversion of the water molecule orientation and the increase of the O(formamide)–H(water) distance by about 0.2 Å. In the gas phase binding to the carbonyl is energetically equivalent to binding to the amino group. However, in solution, water binds better to the carbonyl oxygen that to the NH group. The nonadditive contributions are, in general, important and can be related to the change in the monomer energies when one passes from the monomeric to the dimeric reaction potential.
Adsorption-induced surface ordering on solutions of rigid-rodlike molecules
Authors: Matsuyama Akihiko , Umemoto Kazushi , Kato Tadaya
Published: July 2002

A simple model is developed to describe the adsorption and the surface ordering in solutions of rigid-rodlike molecules. We focus here on the planar adsorption of the rigid rods restricting the orientations to two mutually orthogonal directions which are both parallel to the surface. When the bulk concentration increases, the adsorbed amount is increased and the adsorption isotherm has a kink at a bulk concentration φc, where the second-order isotropic–nematic transition (INT) takes place in a surface. Further increasing concentration, the adsorbed amount jumps at a bulk critical concentration φIN, where the first-order INT takes place in the bulk phase. For various values of the axial ratio of the rigid rods, the solvent conditions, and the adsorption energy, we examine the adsorption isotherms, the surface tensions, and the surface ordering. We also discuss the co-occurrences between the adsorption phenomena and the phase separation of a surface phase.
Analytical description of gas transport across an interface with coupled diffusion in two phases
Authors: Huthwelker Thomas , Peter Thomas
Published: July 2002

A new analytical formulation of the absorption and desorption of a nonreactive gas across an infinite planar gas–liquid phase boundary is presented. The solution describes diffusive transport in the gas and liquid phases and the kinetic resistance at the phase boundary according to the Hertz–Knudsen equation. The expressions allow straightforward examination of various kinetic regimes and the calculation of their characteristic times. The violation of Henry’s law during the uptake is illustrated by derivation of simple formulas for the concentrations in both phases close to the surface. Also, expressions for the time-averaged uptake coefficient (γ̄) are presented.