bluuebunny/crossref_metadata_2025 huggingface dataset:
Decay of local stretching overtones for terminal groups attached to a hydrocarbon chain: A theoretical investigation based on a Morse-harmonic model
Authors: Lami Alessandro , Villani Giovanni
Published: July 2002

The decay of excited stretching overtones of some local modes (C–H, C 3/4 C,C=C, C=O) attached to a hybrocarbon chain is theoretically investigated. The assumptions used in a previous paper [A. Lami and G. Villani, J. Chem. Phys. 88, 5186 (1988)] are critically examined and the model is improved by allowing the transfer of up to three quanta from the local mode (simulated by a Morse oscillator) to the harmonic bath of the chain modes. Use is made of a Lanczos tridiagonalization procedure. It is shown that the population transfer from the local mode to an aliphatic carbon chain is rapid but not complete due to the fact that the local mode frequency lies above the top of the band of chain modes and to the particular structure of the Hamiltonian for kinetically coupled stretching modes. It is argued that the complete decay should involve other low frequency modes and should require a longer time.
Variation with temperature of the electron attachment to SO2F2
Authors: Datskos P. G. , Christophorou L. G.
Published: July 2002

The total electron attachment rate constant ka(〈ε〉,T) for SO2F2 has been measured, in a buffer gas of N2, as a function of the mean electron energy 〈ε〉 (0.046–0.911 eV) and temperature T (300–700 K) using an electron swarm technique. From the measured ka(〈ε〉,T), the total electron attachment cross sections σa(ε,T) were determined. At 300 K the σa(ε,T) exhibits a maximum at ∼0.22 eV which is due to dissociative electron attachment and an increase below ∼0.1 eV which is due to the formation of parent negative ions SO2F−2 at near zero energy. At T=400 K, σa(ε,T) has only one main peak at ∼0.13 eV which is due only to dissociative electron attachment reflecting the depletion of the parent anions and the prevalence of the fragment negative ions as T increases. The main peak of σa(ε,T) shifts to lower electron energies with increasing T so that at 700 K the peak is located at ∼0.03 eV. The value, σda(εmax), of the total electron attachment cross section at the peak energy εmax increases by a factor of ∼32 as T increases from 300 to 700 K. The analysis of these results—and similar earlier work—leads to the conclusion that the increase in σda(ε,T) for the dissociative electron attachment processes in molecules, with increasing T results mainly from an increase with T of the internal energy (principally vibrational) of the molecule.
Exciton diffusion at finite frequency: Luminescence observables for anisotropic percolating solids
Authors: Parris P. E.
Published: July 2002

A study is made of the luminescence intensities associated with exciton diffusion and trapping on a three-dimensional anisotropic percolating lattice. The calculation is based upon a relationship that exists between the frequency dependent diffusion tensor at frequencies comparable to the inverse excitation lifetime, and luminescence observables such as the host and trap luminescence intensities for conditions of constant illumination. The present approach allows the study of crossover behavior in percolative systems that are of intermediate transport dimensionality 2<dt<3. Our results suggest that curvature seen in luminescence observables near the transition need not always be a direct reflection of the critical indices associated with classical isotropic percolation. We have identified three possible sources of deviation from the classical behavior: (1) the radiative time scale of the luminescence measurements, (2) the functional dependence of the luminescence yields on the diffusion tensor, and (3) the demands of dimensional crossover in the critical region arising from the anisotropy of the medium.
Hierarchical fitting and scaling models for rotationally inelastic cross sections
Authors: Kreutz Thomas G. , Rabitz Herschel
Published: July 2002

A new method of scaling inelastic cross sections is presented which uses an intermediate-level parametrization of the collision dynamics. By expressing the scattering matrix as exp[−iB] and approximating B with a parametrized functional form, this procedure yields a unitary scaled S matrix and incorporates infinite-order rotational coupling into the scaled results. The inclusion of high order collision dynamics enables the scaled cross sections to exhibit physical behavior which is substantially more realistic and flexible than that allowed by traditional fitting methods. The intermediate level scaling procedure accurately produces complete matrices of rotationally inelastic cross sections for He–HD scattering generated from subsets of input cross sections.
Nonlocal and quasilocal potentials in the spontaneous emission of molecular exciplexes coupled to the phonon bath of a solid matrix
Authors: Lam Kai-Shue , George Thomas F.
Published: July 2002

Molecular exciplexes coupled to the phonon bath of a solid matrix are treated by a two-state model allowing for radiative relaxation from the excited to ground state and thermal relaxation from each of the states to the phonon bath. Molecular dynamics is considerably simplified by a canonical (Duke–Soules) transformation on the model Hamiltonian. This transformation (i) renormalizes the bath-free exciplex potential surfaces to phonon-dressed surfaces, and (ii) combines the radiative and phonon couplings into a single effective coupling. This leads to an effective nonlocal potential whose kernel is a product of radiative and nonradiative parts, and which can be interpreted pictorially in an intuitive manner when individual terms in the Born series solution of the corresponding Schrödinger equation are linked to special ‘‘Feynman diagrams.’’ Under the small phonon energy and the weak correlation (between phonon and nuclear degrees of freedom) approximations, a thermally averaged wave function for nuclear motion can be constructed, and the nonlocal potential reduces to a quasilocal potential in which the phonon bath ‘‘filters out’’ certain nonlocal transitions between the exciplex states.
The surface reaction of SO2 with adsorbed NO−3 on Ag metal microstructures: A surface enhanced Raman scattering experiment
Authors: Bates Jennifer L. , Dorain Paul B.
Published: July 2002

Surface enhanced Raman scattering has been used in conjunction with mass spectrometry to study the in situ replacement reaction of adsorbed NO−3 on Ag metal powder with SO2 gas to form adsorbed SO=3 and SO=4 and gaseous NO2. Analysis of the intensity changes of the observed peaks lead to a model for surface reaction at low coverage. The time dependent background intensity, proportional to the adatom concentration which changes during the chemical reaction, is used to normalize the surface enhanced Raman scattering (SERS) peaks intensities. The resulting spectrum is proportional to the time dependent coverage and, if the reaction heat is small, the time independent electromagnetic enhancement. Examples are given demonstrating this effect.
Carbon monoxide chemisorption on Cu covered Ni(111) surfaces
Authors: Feng X. H. , Yu M. R. , Yang S. , Meigs G. , Garfunkel E.
Published: July 2002

The adsorption of carbon monoxide on copper covered Ni (111) surfaces has been studied by high resolution electron energy loss spectroscopy (HREELS). CO is used to titrate the various adsorption sites present, yielding information about the structure and composition of the two metal species at the surface. The results show that CO adsorbs at locations which resemble the top and bridge sites of pure Cu(111) and Ni(111) surfaces, and on a new mixed Cu–Ni bridge site. For a Cu coverage greater than one monolayer, the CO HREEL spectra are very similar to those observed on the clean Cu(111) surface. For Cu coverages less than one monolayer, and deposition at 80 K, small Cu clusters and Cu islands are observed. For submonolayer Cu deposition at temperatures >300 K, a mixed Cu–Ni layer can be formed. The HREELS results for the mixed surface with low Ni concentration show that CO adsorbs on the Ni top sites first (at T<400 K), Cu top sites next (at T<150 K), and then a mixed Cu–Ni bridge site (at T<120 K). Strong evidence suggests that the mixed CuNi bridge site is occupied by depopulating the Ni–CO top-site species.
Pathologic inversion of core orbitals in approximate Hartree–Fock calculations
Authors: Koga Toshikatsu , Ohta Katsuhisa , Nitta Kiyoharu
Published: July 2002

The common sense in quantum chemistry that each bonding orbital is filled before the corresponding antibonding orbital is shown to be often violated for the core orbitals of diatomic systems in approximate Hartree–Fock calculations. Through theoretical analysis of the core orbital energies and numerical analysis of some specific diatomic systems, the origin of this pathologic ordering is suggested to be unusual mixing of valence orbitals into core orbitals induced by the use of basis sets with unbalanced core–valence quality.
Probability tables for small clusters of impurity atoms in sc, bcc, and fcc lattices assuming long-range interaction
Authors: Pohl U. W. , Busse W.
Published: July 2002

Powers and coefficients of polynomials describing the concentration dependent probabilities of small clusters of impurities are presented. For singles, pairs, and triples of impurities randomly distributed in simple cubic, body-centered-cubic, or face-centered-cubic host lattices, all interaction ranges between first next neighbor (1NN) and 8NN interaction are considered (singles and pairs up to 16 NN). The change of the probability functions with increasing interaction range is described. It is noted that an additional consideration of clusters with more impurities than a triple does not essentially improve the statistical error in the case of long-range interaction. For the three lattices, empty shells and inequivalent lattice vectors are given.
Molecular-dynamic studies of monatomic glasses: How is glass transition simulated by computer?
Authors: Shinjo Kazumasa
Published: July 2002

The correspondence between glass transitions predicted by computer simulations and those observed in the laboratory is studied, reexamining results obtained previously, by computer simulation. Computer simulations have reported an explicit appearance of sharp or broad break in the volume (V)–temperature (T) curve; Here, broad means that the volume changes linearly at each side of higher and lower temperatures although it changes continuously at intermediate temperature region. The sharp or broad break, reminding of the second-order-like thermodynamic phase transition, has been ascribed to the evidence of glass transition because of its resemblance to results in the laboratories. However, the V–T curve depends crucially on the cooling procedure and less on the cooling rate: (1) Stepwise cooling yields the sharp or broad break in the V–T curve, while continuous cooling yields only a mild V–T curve over whole temperature region. The mild V–T curve no longer reminds one of a transition. This means that the appearance of a sharp or broad break in the V–T curve should not be used as evidence for a glass transition in computer simulations.
Interaction of hydrocarbon monolayer surfaces across n-alkanes: A steric repulsion
Authors: Herder Christina E. , Ninham Barry W. , Christenson Hugo K.
Published: July 2002

We present results of force measurements between hydrocarbon monolayer surfaces across n-alkanes (hexane, decane, and tetradecane). The interaction is qualitatively different from that of any previously studied system and, in particular, bears no resemblance to an oscillatory solvation force. Instead, the force is repulsive from about 2.5 nm, with the exception of a shallow minimum just outside a force maximum at 0.8–0.9 nm. At smaller separations the force becomes attractive and there is a weak adhesion at contact. We suggest that the force law is due to a steric effect—a repulsive interaction originating in restrictions on chain conformations of the alkanes at small surface separations. This interaction is accessible via simple mean-field theories. The similarity of the liquid–liquid and liquid–surface interactions allows this to dominate over solvation effects. The results are of significance for interaggregate interactions in lamellar liquid crystals, microemulsions, and surfactant-stabilized dispersions.
Energy partitioning of the self-consistent field interaction energy of ScCO
Authors: Frey Regina F. , Davidson Ernest R.
Published: July 2002

The SCF interaction energy for ScCO was partitioned into the electrostatic, polarization, exchange, and charge-transfer terms via the Morokuma analysis. This definition of polarization does not satisfy the Pauli exclusion principle and therefore, there is nothing to prevent the valence electrons of one fragment from collapsing into the core orbitals of the other fragment. To correct this problem, the polarization and charge-transfer terms were calculated in the presence of exchange. The dominant interactions were exchange repulsion, electrostatic attraction, and distortion of the wave function by polarization and charge transfer. Pi backbonding was the dominant distortion for the 4 Σ− 4s1 3dπ2 state, while sigma distortions dominated the 4s2 3d 1 states.
An exactly solvable model three-component solution with a double critical point
Authors: Huckaby Dale A. , Shinmi Masato
Published: July 2002

A lattice model of a three-component solution is considered in which the bonds of a lattice are covered by molecules of types AA, BB, and AB. Unlike molecular ends near a common site have orientation-dependent interactions. For small but constant AB concentrations, the system exhibits a closed-loop phase diagram. The closed loop shrinks as the concentration of AB molecules increases and vanishes at a double critical point. Exact results are presented for the model on a three-coordinate Bethe lattice.
Hyperfine and Zeeman quantum beats of the Na(3 2P3/2–3 2S1/2) transition, and the decay of coherence
Authors: Baba Masaaki , Namba Kazumasa , Katô Hajime
Published: July 2002

Quantum beats of the Na(3 2P3/2–3 2S1/2) transition and their change with the magnetic field were observed in pure Na vapor and Na+He gas by using a picosecond laser system. The energies of the hyperfine and Zeeman sublevels of the 3 2P3/2 and 3 2S1/2 states were calculated by using the reported hyperfine constants and g factors, and the theoretical curve of the quantum beats was calculated. The results were in good agreement with the observed profile of the quantum beats in phase of modulation, but the difference of the amplitudes increased with time. By comparing the amplitude of the theoretical curve of the quantum beats with that of the experimental curve, we determined the relaxation time of coherence. The decay of amplitude of the modulation was found to become faster as the pressure of the foreign gas (He) was increased. The decay of amplitude, i.e., the decay of coherence arises from the dephasing, which occurs from the fluctuation of the level energies induced by the interatomic interaction.
Thermal electron attachment to NO. II. Temperature dependence and implication of the attachment to van der Waals molecules
Authors: Shimamori H. , Nakatani Y. , Hotta H.
Published: July 2002

Thermal electron attachment in pure NO, NO–Ar, and NO–(n-C4 H10 ) mixtures has been studied by using a microwave conductivity technique with pulse radiolysis at temperatures between 250 and 350 K. The three-body attachment rate constants for NO and n-C4 H10 as the third bodies have negative activation energies, −0.035 and −0.038 eV, respectively. The results are well explained by assuming that the initial three-body electron attachment is associated with dimers or van der Waals molecules.
Crystal–melt surface widths of simple liquids and an empirical law of freezing. II
Authors: Zeng X. C. , Stroud D.
Published: July 2002

We propose a relation among the crystal–melt surface tension τsl, the compressibility KT, and the thickness wsl of the crystal–melt interface for simple liquids, based on the density-functional approach of Moore and Raveche.1 Combining this with a similar relation in note I by Mon and Stroud, we obtain a formula connecting the surface tension ratio τsl/τlv, and ratio of surface width wsl/wlv. A new empirical law of freezing is conjectured.
Gas–surface interactions on InP monitored by changes in substrate electronic properties
Authors: Weiss P. S. , Trevor P. L. , Cardillo M. J.
Published: July 2002

The electronic properties of compensated InP crystals can be used to sensitively monitor gas–surface interactions. When a gas is adsorbed on these low carrier density semiconductors both the conductance and minority carrier lifetime exhibit large changes which we interpret in terms of band bending. Changes of greater than 50% in the bulk-averaged conductance of Fe-compensated semi-insulating InP crystals have been measured for adsorption of ∼0.5 monolayers of Cl2. Using modulated NO2 molecular beams the conductance changes are demonstrated to be fast (<1 ms to steady state) so as to be capable of yielding quantitative rate information about the gas–surface interaction.
Relationships between electric field shielding tensors and infrared or Raman intensities: An explanation based on nonlocal polarizability densities
Authors: Hunt K. L. C.
Published: July 2002

New equations for the derivatives of molecular dipole moments and polarizabilities with respect to nuclear coordinates are derived in terms of nonlocal polarizability densities, linear and nonlinear. New equations are also derived for the electric field shielding tensors at nuclei of molecules in static external fields of arbitrary spatial variation. Both involve integrals of the dipole propagator and the polarizability densities. This analysis explains the relationship between the linear electric field shielding tensors and the infrared intensity for a vibrational mode; it also accounts for the relationship between the quadratic electric field shielding tensors and the Raman intensity, as well as relations connecting higher-order shielding tensors to hyper-Raman intensities. When a nucleus moves infinitesimally, the electronic charge distribution responds via its nonlocal polarizability density to the change in the Coulomb field due to that nucleus, and this produces the change in the electronic dipole moment. All of the quantum mechanical effects are contained within the polarizability density. Analogously, the change in the Coulomb field and response via the hyperpolarizability densities determine the change in electronic polarizability when a nuclear position shifts.
Magnetic circular dichroism of atomic rhenium in a krypton matrix: Large ground state crystal field splitting
Authors: Pellow Robert , Eyring Marc , Vala Martin
Published: July 2002

The absorption and magnetic circular dichroism (MCD) spectra of the rhenium atom isolated in a krypton matrix at 10 K are presented. Assignment of all optical bands observed in the 510–230 nm range is made. Expressions are developed to determine the ground state crystal (matrix) field splitting using the MCD saturation technique on nonoverlapped optical bands. A large crystal field (CF) splitting of 7.5±3.0 cm−1 is found and attributed to two sources, both of which must be present for the splitting to occur: (1) the spin–orbit mixing of the low-lying 4P5/2 state (11 600 cm−1) into the 6S5/2 ground state and (2) the crystal (matrix) field effect on this mixed state by the tetradecahedral substitutional matrix site. Band moment expressions are extended to account for a large ground state CF splitting and are used to determine the excited z 6PJ state spin–orbit splitting parameter (−102 cm−1). Finally, evidence for Jahn–Teller activity in the z 6PJ state is presented from an analysis of the modified MCD/absorption band moment expressions.
Basis set additivity in calculation of ionization potentials of AHn compounds
Authors: Curtiss Larry A. , Pople John A.
Published: July 2002

The use of a basis set additivity approximation for the calculation of ionization potentials of AHn compounds has been tested for NH2 and found to be valid. The calculated ionization potentials for NH2→NH+2(3B1) and NH2→NH(1A1) remain in good agreement with experiment.
Investigation of the spin diffusion in (dicyclopentaheptalene)2SbF6 by pulsed electron spin resonance
Authors: Maresch G. G. , Mehring M. , von Schütz J. U. , Werner H.-P. , Gökelmann K. , Enkelmann V. , Müllen K. , Klabunde K. U.
Published: July 2002

We have determined the electron spin diffusion constant in (dicyclopentaheptalene)2SbF6 by means of a magnetic field gradient electron spin resonance spin echo technique. Parallel to the chain axis we obtain a value of D∥≊0.064 cm2 s−1. From the comparison of temperature dependent conductivity and susceptibility measurements it is concluded, that the charge carriers are localized in this compound. These localized charge carriers can be described as a system of 1d-Heisenberg spin chains. The measured D∥ value therefore corresponds to an effective exchange coupling constant of J≊4.5 meV (J/kB≊51 K).
Analysis of experimental nucleation data for silver and SiO using scaled nucleation theory
Authors: Hale Barbara N. , Kemper Paul , Nuth Joseph A.
Published: July 2002

The experimental vapor phase nucleation data of Nuth et al., for silver [J. A. Nuth, K. A. Donnelly, B. Donn, and L. U. Lilleleht, J. Chem. Phys. 77, 2639 (1982)] and SiO [J. A. Nuth and B. Donn, J. Chem. Phys. 85, 1116 (1986)] are reanalyzed using a scaled model for homogeneous nucleation [B. N. Hale, Phys. Rev. A 33, 4156 (1986)]. The approximation is made that the vapor pressure at the nucleation site is not diminished significantly from that at the source (crucible). It is found that the data for ln S have a temperature dependence consistent with the scaled theory ln S≊ΓΩ3/2 [Tc/T−1]3/2, and predict critical temperatures 3800±200 K for silver and 3700±200 K for SiO. One can also extract an effective excess surface entropy per atom Ω=2.1±0.1 and an effective surface tension σ≊1500−0.45T ergs/cm2 for the small silver clusters (assuming a range of nucleation rates from 105 to 1011 cm−3 s−1). The corresponding values for SiO are Ω≊1.7±0.1 and σ≊820−0.22T ergs/cm2 (assuming a range of nucleation rates from 109 to 1012 cm−3 s−1).
Torsional damping and solvent friction in liquid n-butane: Experimental estimates from Raman spectroscopy
Authors: MacPhail Richard A. , Snyder Robert G.
Published: July 2002

The isotropic Raman linewidths of the in-phase CCC bending modes of liquid n-butane are analyzed in terms of dephasing by torsional oscillations. A simple, effective Hamiltonian is developed to calculate the coupling between this bending mode and the torsion. For the gauche conformer the coupling is linear in the torsional coordinate and quite strong, but for the trans conformer the coupling is weak. This coupling is used to relate the linewidths of the bending modes to the torsional dynamics, which are modeled by a damped, harmonic oscillator. The damping constant and a related torsional correlation time are then extracted from the experimental linewidths. The resulting correlation times are compared with those calculated assuming either hydrodynamic or collisional (Enskog) friction on the torsional coordinate. Both theoretical models give values that lie below the experimental upper bound, but the Enskog friction compares somewhat better with our best experimental estimates of the torsional damping.
Energy-dependent vibrational frame transformation for electron–molecule scattering with simplified models
Authors: Gao Hong , Greene Chris H.
Published: July 2002

A recent extension of the vibrational frame transformation method to allow for energy dependence of its parameters is explored through applications to model systems. Assuming the Born–Oppenheimer approximation when the incident electron moves inside the target, this method accounts for the energy dependence of body-frame scattering information by defining potential curves of the electron–molecule compound which depend on an eigenphase parameter. Escape of the scattered electron to large distances is treated by quantum defect methods. Calculations for some simple models show many realistic aspects encountered in electron–molecule scattering.
The Hamiltonian formulation of configurationally constrained bimolecular kinetics: A test against simulations
Authors: Cotes N. J. , Sceats Mark G.
Published: July 2002

The Hamiltonian-based model developed by the authors [N. J. Cotes and M. G. Sceats, J. Chem. Phys. 89, 2816 (1988)] for bimolecular reactions which exhibit severe configurational restrictions, such as orientational requirements for reaction at surface sites, provides an analytical expression for the reaction rate which involves the evaluation of the potential of mean force along the radial coordinate that describes the binding. The expression for the reaction rate is evaluated for the case of a charged molecule interacting with a binding site on a dipolar particle. The results of the model are compared with the multidimensional Brownian-dynamics simulations of Northrup et al. [J. Chem. Phys. 84, 5536 (1986)] and excellent agreement is obtained in the diffusion limit.
Phase transitions in ZnTiF6⋅6H2O observed by the electron paramagnetic resonance of Mn2+
Authors: Rubins R. S. , Drumheller John E. , Hutton Stuart L.
Published: July 2002

The temperature dependence and hysteresis of the trigonal–monoclinic phase transition occurring near 180 K in ZnTiF6⋅6H2O were studied as functions of Mn concentration, and spin–Hamiltonian parameters were determined in both the trigonal and monoclinic phases. Changes in the 9 GHz electron paramagnetic resonance (EPR) spectrum between 30 and 10 K indicated a first order phase transition on cooling to a more disordered structure. Additional lines showing ligand hyperfine structure, with an even number of components, are attributed to paramagnetic impurities occupying Ti4+ sites with single F− vacancies.
Optical–optical double resonance spectroscopy of Br2 through the A 3Π(1u) state: Analyses of the 1u and 0−u(3P1) ion-pair states
Authors: Ishiwata Takashi , Obi Kinichi , Tanaka Ikuzo
Published: July 2002

The technique of optical–optical double resonance is applied to study the ion-pair states of Br2 correlating to Br−(1S)+Br+(3P1). The Br2 molecules in the ground state are pumped to the A3Π(1u) state as an intermediate, and subsequently excited into the ion-pair states by a coherent two-photon transition. The double resonance transition is detected through the dispersed fluorescence terminating on the lower valence states. The 1u(3P1) ion-pair state is observed for the first time and the energy level analyses are made on the v=0 to v=10 levels in detail. The 0−u (3P1) state is also identified through the heterogeneous interaction with the 1u(3P1) state, which is interpreted in terms of the pure precession approximation. The molecular parameters thus obtained in the Dunham expansion are Y00=53 257.605(10), Y10=164.1571(69), Y20=−0.7588(15), Y30=5.650(98)×10−3, Y01=0.040 011 6(40), Y11=−1.7488(42)×10−4, and Y02=−8.40(53)×10−9 for the 1u(3P1) state of 79Br2 isotope species (all in cm−1 with 3σ in parentheses). The main parameters of the 0−u(3P1) state are Y00=53 479.72(25), Y10=148.620(54), and Y01=0.038 021 5(79).
Dynamic light scattering observation of droplet aggregation in a Winsor type W/O microemulsion system
Authors: Waaler Dag , Strand Knut Arne , Stro/mme Gunvald , Sikkeland Torbjo/rn
Published: July 2002

We have performed scattered light intensity autocorrelation measurements on a Winsor type microemulsion system composed of brine, cyclohexane, SDS and a mixture of 1-butanol and 1-pentanol. At high cosurfactant concentration, where the microemulsion phase was considered to consist of individual, spherical water-in-oil droplets of relatively low droplet volume fraction, the autocorrelation functions were observed to be essentially single exponential, as expected. Above a certain droplet volume fraction, however, additional decay modes were observed to enter the correlation data. These modes were interpreted to be due to rotation and/or internal motion of droplet aggregates.
Chemiluminescent reactions of ground and metastable states of the group IV A elements with halogens
Authors: Wang J. H. , Cheong B. S. , Parson J. M.
Published: July 2002

Earlier work on chemiluminescent beam-gas reactions of the ground states (3 PJ ) of Si, Ge, and Sn with F2 and of both ground and metastable states (1 D, 1 S) of Sn with Cl2 , Br2, and I2 is extended to include reactions of both ground and metastable states of Si, Ge, and Sn with F2 , Cl2 , Br2 , I2 , and ICl. Chemiluminescent products of metal halides and/or dihalides were observed in all of these reactions, although attempts to observe that in the similar reactions of Pb were unsuccessful. In ICl reactions with Si, Ge, and Sn, both monochloride and dichloride emission occurs, confirming the previously proposed mechanism for creating dihalide emission in Sn reactions with Br2 and I2 via a monohalide intermediate.
Laser-induced explosive desorption
Authors: Fain Benjamin , Lin Sheng H.
Published: July 2002

A theory of the explosive desorption, observed by Chuang and Domen [Phys. Rev. Lett. 59, 1484 (1987); J. Vac. Sci. Technol. A 5, 473 (1987); J. Chem. Phys. (90, 3318 1989; Chem. Phys. Lett. (with A. Mödl). (154, 187 (1989)] is developed. When adsorbates are subjected to UV laser action, an explosive, molecular nonselective desorption takes place. This phenomenon is explained as follows: adsorbate molecules are excited by an UV laser to the higher electronic states. The electronic energy is delivered to the low energy vibronic states. The energy differences of these states are resonant to the corresponding phonon modes. This leads to the excitation of these phonons. When the energy input to the phonon modes exceeds their decay, phonon numbers start to increase exponentially. The phonon avalanche takes place. This in turn causes molecular nonselective desorption. The main result of the theory is the finding of the fluence and coverage dependent threshold conditions.
Thermal diffusion at 300 K in three binary gaseous systems containing Ne2
Authors: Dunlop Peter J. , Bignell C. M. , Taylor W. L.
Published: July 2002

Thermal diffusion factors were obtained at 300 K for the systems He−Ne, Ne−Ar, and Ne−SF6 using Ne with mass 20 with astated isotopic purity of 99.98%. (AIP)
Doubly charged clusters of 1,1-difluoroethylene: Observation of an anomalous critical size
Authors: Peifer William R. , Garvey James F.
Published: July 2002

We present in this Communication the first observation of a critical size (n2) of 9 for doubly charged clusters of 1,1-difluoroethylene. This is a highly unusual result in that it is the lowest critical size measured so far (the previous record was n2=21 for SO2). In addition, this is the first example of a doubly charged cluster where its critical size is not predicted by the Echt scaling law (that the critical size of the doubly charged cluster is proportional to the inverse of the product of Tc and v1/3, where Tc is the critical temperature and v is the molecular volume). This suggests that the doubly charged CH2CF2 clusters are packed in a nonspherical arrangement (i.e., chains, rings, or stacking of the ethylene planes).
Low frequency modes of surface adsorbed dye molecules studied by optical high resolution techniques
Authors: Basché Th. , Bräuchle Ch.
Published: July 2002

We observed for the first time in an optical high resolution experiment (fluorescence line narrowing) the coupling of low frequency adsorbate motions to vibronic bands of the electronic transition of quinizarin adsorbed on γ-alumina. These motions may provide an important dephasing mechanism for the electronic excitation of absorbed molecules. The increased hole width of adsorbed dyes which is several times larger than in amorphous hosts is thought to be caused by these low frequency modes.
Two-dimensional experiments with collision-induced transfer of populations in microwave Fourier transform spectroscopy
Authors: Vogelsanger B. , Bauder A. , Mäder H.
Published: July 2002

The two-dimensional (2D) technique in microwave Fourier transform (MWFT) spectroscopy has been extended to four-level double resonance. By applying a three-pulse sequence with a particularly selected phase cycle, it was possible to correlate the spectra excited by pump and signal radiation in two dimensions. Cross peaks in such 2D spectra appeared only when collision-induced transitions took place between the levels of the irradiated transitions. Also the dynamics of such collision processes could be investigated. The power of the method is greatly enhanced by the high sensitivity of the MWFT technique. Experiments were performed on HC15N in order to investigate the collision-induced transitions between different pairs of l-type doublets. The one-dimensional analog of this 2D method was tested on acetaldehyde.
On the enumeration of the end-to-end distance distribution in lattice polymers
Authors: Poland Douglas
Published: July 2002

A simple matrix method is given that generates the probability distribution for the coordinates of the end of a polymer chain (modeled as a random walk on a regular lattice with a finite range of intrachain correlation). From this information the exact end-to-end distance distribution can be constructed. The method can be applied to specific sequence copolymers and to the distance distribution for arbitrary points within a polymer chain.
Conformational stability and barriers to internal rotation of 2-methylpropanal by far infrared and microwave spectroscopy
Authors: Durig J. R. , Guirgis G. A. , Little T. S. , Stiefvater O. L.
Published: July 2002

From the far infrared spectrum of 2-methylpropanal (isobutyraldehyde), (CH3)2CHCHO, in the gaseous state, the asymmetric torsion for the gauche conformer was observed as a series of Q branches at 75.0, 80.9, 85.8, and 90.3 cm−1 with similar transitions observed for (CD3)2CDCHO. Also from an investigation of the Raman spectrum of the gas a transition at ∼110 cm−1 has been tentatively assigned as the ‘‘double jump’’ of the asymmetric torsion for the trans conformer. The microwave spectrum of the excited vibrational states for the light molecule has been investigated in the region from 12.5 to 40 GHz and, from relative intensity measurements, the frequencies of the asymmetric torsions were determined to be 72±7 and 50±20 cm−1 for the gauche and trans rotamers, respectively. Additionally, ‘‘tunneling’’ splitting of 2.0±0.4 and 60.0±0.3 MHz has been observed for the second and third excited states, respectively, of the asymmetric torsion of the gauche conformer. From these data the asymmetric potential function has been calculated and the following potential constants have been evaluated: V1=−131±35, V2=−225±21, V3=555±10, V4=−70±5, V5=95±5, and V6=−140±10 cm−1 where the 1←0 transition of the gauche has been assigned at 75.0 cm−1. This potential function leads to an enthalpy difference of 250±66 cm−1 (715±189 cal/mol) which is in good agreement with the value of 248±35 cm−1 determined from variable temperature studies of the Raman spectrum of the gas. Analysis of the transitions observed between 250 and 150 cm−1 in the far infrared spectrum of the gas indicated that the two methyl rotors are coupled for the gauche conformer and the barriers to internal rotation of the methyl tops have been calculated to be 987 cm−1 (2.82 kcal/mol) and 1132 cm−1 (3.24 kcal/mol).
Projection operator formalism for the characterization of molecular eigenstates: Application to a 3:4 resonant system
Authors: Neshyba S. P. , De Leon N.
Published: July 2002

A projection operator formalism is used to quantify the normal/resonant character of eigenstates of a vibrational Hamiltonian with two degrees of freedom. We show how the application of the formalism quantifies the resonant character of these eigenstates, in the form of a probability, and how this probability is related to the width of the resonance in classical phase space. Normal and resonant quantum numbers shared by a given eigenstate are found to be canonically related.
Photoionization spectra of CsnOm⋅(CO)x clusters
Authors: Malinowski N. , Martin T. P.
Published: July 2002

One-photon ionization spectra are reported for 82 clusters with the composition CsnOm⋅CO and CsnOm⋅2CO. These spectra are presented in the form of an energy distribution of ionizing transitions. Carbon monoxide apprears to react with CsnO clusters to form CO2−2.
Tunneling versus sequential long-range electron transfer: Analogy with pump–probe spectroscopy
Authors: Hu Yuming , Mukamel Shaul
Published: July 2002

The interplay between the sequential and the superexchange (tunneling) mechanisms for electron transfer in condensed phases is studied by formulating the problem using the density matrix. The sequential mechanism proceeds via populations of intermediate electronic states (diagonal density matrix elements) whereas the superexchange proceeds through coherences (off diagonal density matrix elements). The present formulation establishes a complete formal analogy between these mechanisms and the incoherent and the coherent pathways in nonlinear optical measurements, in particular, pump–probe spectroscopy.
The Griffiths theory of tricritical phase behavior and its application to a model microemulsion system
Authors: de Boer W. P. H.
Published: July 2002

The Griffiths theory is used to model the three-phase behavior of a microemulsion system composed of heptane, water, sodium dodecyl sulphate, butanol, and salt. It is shown that this particular system requires a Griffiths theory extended to (at least) the eighth power in the order parameter when that parameter is taken proportional to the oil-in-water concentration.
Electronic structure of Cr3+ in Cs2NaYCl6 and Cs2NaYBr6 lattices. Electron-paramagnetic resonance and electron-nuclear double resonance measurements and multiple scattering Xα calculations
Authors: Monnier A. , Chambaz D. , Bill H. , Güdel H. U. , Weber J.
Published: July 2002

The electronic ground state of isolated Cr3+ introduced into the title compounds has been investigated with electron spin resonance and electron-nuclear double resonance spectroscopy. Simultaneously a multiple scattering (MS) Xα study of the (CrCl6)3− cluster has been performed. The experimental results agree with a cubic Cr site. They further show evidence for strong quadrupole interaction at the anion neighbor nuclei and for observably different covalency in the two hosts. Rather good agreement is found between the predictions of the MS Xα model and the experimental superhyperfine interaction constants but not with the Cr-hyperfine structure constant. It is suspected that the second neighboring Cs play a non-negligible role in the electronic structure of the cluster.
Statistical thermodynamics of flexible-chain surfactants in monolayer films: Mixtures of different chain lengths
Authors: Cantor Robert S.
Published: July 2002

Recently, a simple mean-field theory was developed to describe structural and thermodynamic properties of fluid phases in monolayer films of flexible-chain surfactants [J. Chem. Phys. 91, 4423, 4431 (1989)]. This approach is extended here to examine films of mixed chain length. The high-density (LC-LE) phase transition predicted for the single component monolayers is found to exhibit nonideal mixing behavior, ranging from positive azeotropy to phase separation (in the LC phase) as the difference in chain lengths is increased. In contrast, the phase diagram in the LE-G transition region is predicted to exhibit almost ideal behavior, with a nearly linear bubble-point curve.
A new approach to sputtering simulations from rare gas solids
Authors: Krallafa A. M. , Clarke J. H. R.
Published: July 2002

Classical molecular dynamics simulations of sputtering using large samples of up to 15 500 particles are demonstrated for Lennard-Jones solids. Two features are introduced to represent the effect of the surrounding solid substrate. First, the outermost particles on each side of the sample are subjected to an additional harmonic potential restraining them close to the equilibrium position. Second, these outermost particles are maintained at a preset temperature by assigning velocities at each time step randomly from the corresponding Gaussian distribution. This boundary thermostat produces a ‘‘sandbag’’ effect and provides a means for excess kinetic energy to dissipate out of the sample. Preliminary results were obtained for single trajectories at 45° incidence and these illustrate that the sputtering yield from these model rare gas solids is strongly dependent on both the length of the simulation and the size of the sample. Simulations were continued to times in excess of 25 ps after which time for the largest sample there was some recoalescence of particles remaining in the surface region of the substrate. The long time sputtering yield was about 500 atoms which is comparable with experimental data on rare gas solids. It is demonstrated that particles within a cluster are just as likely to arise from nonadjacent sites as from near neighbors in the original lattice.
Heteronuclear dipolar dephasing of deuterium quadrupole echoes
Authors: Heaton Nicholas J. , Vold Regitze R. , Vold Robert L.
Published: July 2002

Analytical expressions are derived for the NMR signal from isolated dipolar coupled 2H–14N spin pairs following a quadrupole echo pulse sequence. The expressions are used to calculate the pulse spacing dependence of the deuterium quadrupole echo intensity and the powder line shape. The theoretical predictions are in good agreement with experimental results obtained for urea-d4 which show that the rigid lattice powder spectra depend significantly on the pulse spacing.
On some less familiar properties of anharmonic oscillators
Authors: Ali M. K. , Snider W. P.
Published: July 2002

By considering asymmetric anharmonic oscillators with polynomial potentials of degrees 4, 6, and 8, it is shown that the periods and actions in different potential wells are closely related. The Einstein–Brillouin–Keller (EBK) semiclassical energies are compared against the corresponding quantum energies. It is shown that when all the EBK quantum levels are taken into account, the counting of the levels follow a pattern that is analogous to that for a single minimum oscillator.
Ligand-field model for the spin–lattice coupling coefficients of Mn in ZnS and ZnSe
Authors: Boulanger D. , Parrot R.
Published: July 2002

A ligand-field model is used to compute the spin–lattice coupling coefficient C11 to strains of symmetry EΘ of the fundamental state 6A1 of Mn in cubic ZnS and ZnSe. It is shown that the proposed ligand-field model in which C11 is given by a second-order perturbation scheme involving the molecular spin–orbit interaction between the fundamental state 6A1 and the three orbital triplet states 4T1 at lower energy gives theoretical values which correctly account for the experimental results. A detailed study is made of the strain-induced variations of the crystal electrostatic field, the metal–ligand, and ligand–ligand group overlap integrals and the molecular spin–orbit interaction. It is shown that in ZnS the strain-induced variation of the crystal electrostatic field is almost completely compensated by the strain-induced variations of the group overlap integrals and the molecular spin–orbit interaction thus explaining the small experimental value for C11 in this compound. In ZnSe the contributions of the strain-induced variation of the group overlaps and of the molecular spin–orbit interaction are preponderant.
A permutation-group direct configuration interaction procedure for second order molecular properties. Program implementation with application to the A 2Σ+←X 2Π two-photon transition of the OH radical
Authors: Bendazzoli Gian Luigi , Palmieri Paolo , Rettrup Sten
Published: July 2002

A permutation group direct configuration interaction program, using graphical techniques, has been extended to second order properties. Using this program, absolute and relative two-photon cross sections have been evaluated for the rotational lines of three vibronic bands 0–0, 1–0, 1–1 of the A 2Σ+←X 2Π transition of OH. The computed absorption coefficients are compared with the available experimental data.
The bifurcation structure of waves in excitable chemical systems
Authors: McCarty P. , Horsthemke W.
Published: July 2002

We report the results of a numerical study of how traveling wave branches originate and terminate in a prototypical excitable chemical system, viz. the Kernevez model. These results furnish a qualitative understanding of recent experimental observations on sustained spiral waves in a continuously fed unstirred reactor [W. Y. Tam et al., J. Chem. Phys. 88, 3395 (1988)], in particular of the hysteretic nature of the transition between the wave state and the homogeneous steady state.
Velocity correlations in the molecular dynamics ensemble: Computation of the distinct diffusion coefficients
Authors: Raineri Fernando O. , Friedman Harold L.
Published: July 2002

The theory of the correction for the difference in the fluctuations among the different ensembles is applied to derive the time correlation function formula required to compute the distinct diffusion coefficients in the barycentric reference frame by molecular dynamics. In the process it is found that the dynamical variables in the time correlation function of any mass transport coefficient of any reference frame are the same in the canonical as in the molecular dynamics ensemble. The result agrees with the accepted formulas for computing the kinetic part of the mutual diffusion coefficient in a binary mixture of nonelectrolytes and for computing the electrical conductivity in molten salts and electrolyte solutions. The static velocity correlations in the molecular dynamics ensemble are analyzed without recourse to the other ensembles. In sharp contrast with the ensembles which are not constrained with respect to the total momentum, the laboratory-frame velocities of two different particles at the same time are found to be correlated. The result obtained is equivalent to the initial value of the canonical time correlation function of the barycentric distinct diffusion coefficients derived in the preceding paper.
Computer simulation of depolarized light scattering from diatoms with hard core and square well interactions at low temperatures
Authors: Gray Mark G. , Schieve W. C.
Published: July 2002

We show that the depolarized light scattered from two interacting atoms is qualitatively different for a purely repulsive hard core potential and partially attractive square well potential. This difference is due to the bound states possible with attractive potentials and is used to obtain the spectra from these dimers. The spectra were calculated from the autocorrelation of the scattered electric field found by an effective field model. Values were phase space averaged using Gaussian integration. Molecular dynamics routines evolved the phase space points in time for the autocorrelation. The hard core spectra showed an exponential decay characterized by the mean passage time. The square well spectra consisted of a hard core-type term, plus a harmonic component due to dimer contributions. Subtraction of the hard core spectra from the square well spectra gave the dimer component. The dimer component of the spectra was proportional to the square of the number of dimers and was characterized by the thermal rotational energy for temperatures below the saturation temperature.
Calculation of the rotational cooperative and self-friction coefficients in a dilute suspension of spheres: Two-body effects
Authors: Urdaneta Cristina , Jones Dumont M. , Muthukumar M.
Published: July 2002

By employing the computational methods of Jones and Muthukumar, and the hydrodynamic scattering formalism of Muthukumar and Freed, we have computed the rotational friction coefficients for a dilute suspension of spheres to first order in concentration. These results are compared with previous calculations.