bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure of high-density amorphous water. II. Neutron scattering study
Authors: Bellissent-Funel M.-C. , Teixeira J. , Bosio L.
Published: July 2002

High-density amorphous water is studied by neutron scattering in a Q range extending to 16 Å−1. The low density form of amorphous water is also analyzed and compared with previous results. There are very important differences in the composite pair correlation functions of the two forms of amorphous ice, in particular beyond the first nearest-neighbors distance. We conclude that the hydrogen bond network is strongly deformed in a manner analogous to that found in water at high temperature. This is in contrast with the behavior of the pair correlation function of low-density amorphous water, which is closer to that of supercooled water.
Detection of HBNH by infrared diode laser spectroscopy
Authors: Kawashima Yoshiyuki , Kawaguchi Kentarou , Hirota Eizi
Published: July 2002

The infrared spectrum of a transient species HBNH was observed in the gas phase using a diode laser spectrometer. The HBNH molecule is formed in an ac discharge plasma of a diborane/ammonia or diborane/nitric oxide mixture. The ν3 band origin and the rotational constants of H11BNH were determined to be ν0=1786.193 08(72), B0=1.099 335(81), and B3=1.093 481(72) cm−1, with three standard deviations in parentheses. The ν3 band of H10BNH was also observed and analyzed.
Direct observation of tetrahedral hydrogen jumps in ice Ih
Authors: Fujara F. , Wefing S. , Kuhs W. F.
Published: July 2002

Using deuteron NMR, spin alignment tetrahedral jumps of 2H in deuterated polycrystalline hexagonal ice (Ih) have been observed. Both geometry and time scale of the process are determined directly. At T=230 K the correlation time of these jumps is 0.21(3) ms. The tetrahedral hydrogen jumps turn out to scatter—most characteristically for the disorder within the hydrogen subsystem—randomly about the tetrahedral angles by about ±3° with a correlation time of 0.5(1) ms. With additional experiments on specifically doped ices it is shown that these tetrahedral jumps are due to diffusing Bjerrum vacancies.
Two-dimensional exchange NMR of powder samples. I. Two-time distribution functions
Authors: Wefing S. , Spiess H. W.
Published: July 2002

A systematic description of the two-dimensional (2D) NMR exchange experiment for studying molecular motions in static powder samples is presented in terms of two-time distribution functions. Various angular distributions and their transformation to the NMR frequency domain yielding 2D absorption spectra are discussed. The concept of jump angle distributions is introduced. General and isotropic reorientation as well as different symmetries of the tensorial interaction between spin and lattice are distinguished in the analysis. Special attention is directed to the question, to what extent can angular information be reextracted from a 2D exchange spectrum without referencing any model of reorientation. For tensorial interactions of axial symmetry, projections of the 2D spectrum are also calculated and their usefulness is compared to that of the complete 2D spectra. The close relationship between the dynamic exchange experiment and several static NMR experiments like 2D separated local field spectroscopy of powders and NMR of oriented samples is pointed out.
Extended measurements of the ν2 band of CD3 and the determination of the vibrational potential function for methyl
Authors: Sears Trevor J. , Frye Joan M. , Spirko V. , Kraemer W. P.
Published: July 2002

Measurements in the ν2 (umbrella) vibrational spectrum of CD3 are reported. The data include transitions up to and including the v2=4–3 bands and cover a wider range of rotational excitation than has been previously reported [Frye, Sears, and Leitner, J. Chem. Phys. 88, 5300 (1988)]. The spectroscopic data have been combined with all previously published data pertaining to the methyl radical and an estimate of the vibrational potential function for the species has been made. From this, we predict the positions of so far unobserved vibrational bands for all the various symmetric isotopic modifications and also extract an equilibrium structure. Preliminary results on the population distribution of v2 levels produced on the photofragmentation of acetone-d6 at 193 nm are reported and it is suggested that the methyl v3=1 and v3=0 levels may be produced in this reaction with a population inversion.
A fast intrinsic localization procedure applicable for a
b i
n
i
t
i
o and semiempirical linear combination of atomic orbital wave functions
Authors: Pipek János , Mezey Paul G.
Published: July 2002

A new intrinsic localization algorithm is suggested based on a recently developed mathematical measure of localization. No external criteria are used to define a priori bonds, lone pairs, and core orbitals. It is shown that the method similarly to Edmiston–Ruedenberg’s localization prefers the well established chemical concept of σ–π separation, while on the other hand, works as economically as Boys’ procedure. For the application of the new localization algorithm, no additional quantities are to be calculated, the knowledge of atomic overlap intergrals is sufficient. This feature allows a unique formulation of the theory, adaptable for both ab initio and semiempirical methods, even in those cases where the exact form of the atomic basis functions is not defined (like in the EHT and PPP calculations). The implementation of the procedure in already existing program systems is particularly easy. For illustrative examples, we compare the Edmiston–Ruedenberg and Boys localized orbitals with those calculated by the method suggested here, within both the CNDO/2 and ab initio frameworks (using STO-3G and 6-31G** basis sets) for several molecules (CO, H2CO, B2H6, and N2O4). Some similarities concerning the localization procedures of von Niessen as well as Magnasco and Perico are also discussed.
Extreme mode selectivity in a dissociating van der Waals molecule: Complexes of aminotetrazine and argon
Authors: Alfano Joseph C. , Martinez Selso J. , Levy Donald H.
Published: July 2002

The photodissociation of aminotetrazine-Ar and aminotetrazine-Ar2 was studied using supersonic jet spectroscopy. Excitation of aminotetrazine-Ar to the levels 16b2, 16a2, and 6a1 (429, 505, and 606 cm−1 vibrational energy, respectively) produced photodissociation lifetimes that differed by more than two orders of magnitude. Excitation of these three levels in aminotetrazine-Ar2 caused large changes in the branching ratio between the two dissociation products, aminotetrazine-Ar and aminotetrazine.
Molecular dynamics study of the structural and thermodynamic properties of MgO crystal with quantum correction
Authors: Matsui Masanori
Published: July 2002

We have applied the constant temperature and pressure molecular dynamics technique to the simulation of structural and thermodynamic properties of the MgO crystal in the temperature range between 300 and 2000 K, using empirical pair potential functions consisted of the Coulomb, dispersion, and repulsion interactions. Quantum corrections are included, based on the Wigner–Kirkwood expansion of the free energy in powers of Planck constant h. In the present study, only the first nonvanishing term with the order h2 is considered. As expected, quantum effects on the properties are found to become increasingly important with decreasing temperature. In view of the simplicity of the potential used, our simulation is quite successful in reproducing a wide range of measured properties of MgO, including the crystal structure, bulk modulus, thermal expansion coefficient, heat capacity, and mean-square atomic displacements over a wide temperature range above 500 K. However, at lower temperatures, both the heat capacity and thermal expansion coefficient of MgO are computed to decrease too sharply with decreasing temperature when compared with experiment. This indicates the necessity of including higher-order quantum correction terms at low temperatures for a more accurate description.
Two-dimensional exchange nuclear magnetic resonance of powder samples. III. Transition to motional averaging and application to the glass transition
Authors: Kaufmann S. , Wefing S. , Schaefer D. , Spiess H. W.
Published: July 2002

The two-dimensional (2D) exchange nuclear magnetic resonance (NMR) experiment is applied to study ultraslow as well as faster motions in powdered solids. The theoretical framework required for the simulation of 2D exchange of the faster motions, and for the evaluation of the experimental data, is developed. Calculations are presented for two standard models: two-site jump and isotropic rotational diffusion. For discrete jump motion, anisotropic spin-lattice relaxation during the mixing time is also considered. The resulting, simulated 2D line shapes show new characteristics in the intermediate dynamic range. Experimental data are presented for two-site exchange in the model compound polycrystalline dimethylsulphone. The technique is then applied to study the chain dynamics of linear polystyrene in the glass transition range. Close to Tg the correlation times extracted from 2D exchange NMR exhibit strong non-Arrhenius behavior. This data together with correlation times obtained at higher temperatures from conventional T1 data follows the WLF equation over 11 orders of magnitude, from 10−6 to 1000 s. It is shown that 2D exchange NMR and spin-lattice relaxation probe the α and the β process, respectively, of the chain dynamics in the glass transition region.
Structure and quadrupole coupling measurements on ClF3
Authors: Haubrich S. T. , Roehrig M. A. , Kukolich S. G.
Published: July 2002

Seventy-nine new microwave transitions for 35ClF3 and 37ClF3 in the 6–18 GHz range were measured using a Flygare–Balle-type spectrometer. Rotational transition frequencies were used to obtain ‘‘effective’’ structure parameters for the ground vibrational state zCl–F (along C2 axis)=1.5985(4) Å, rCl–F =1.700 73(5) Å and ΘF–Cl–F =87.48(4)°. Analysis of hyperfine structure due to chlorine quadrupole coupling and observed transition frequencies yield the following molecular parameters for 35ClF3: A=13 748.25(1) MHz, B=4611.719(2) MHz, C=3448.629(3) MHz, eQqaa=82.03(3) MHz, and eQqbb=65.35(2) MHz. Molecular parameters obtained for 37ClF3 are: A=13 653.54(1) MHz, B=4611.866(2) MHz, C=3442.719(4) MHz, eQqaa=64.66(4) MHz, and eQqbb=51.53(3) MHz.
Interaction of inert-gas atoms with some closed-shell cations and formation of cluster molecules
Authors: Patil S. H.
Published: July 2002

We have analyzed the interaction of inert-gas atoms with some closed-shell ions such as alkali and alkaline-earth ions. A perturbative approach leading to exchange-Coulomb and asymptotic terms allows us to calculate the equilibrium properties of the cluster molecules formed by these systems.
Theory of the resonance line shape in velocity-modulation spectroscopy
Authors: Farley John W.
Published: July 2002

Velocity-modulation spectroscopy is often used to obtain absorption spectra of ions in discharges. However, there have been few studies of the line shape. The resonance line shape in velocity-modulation spectroscopy is derived in the present work. Both sinusoidal modulation and square-wave modulation are treated, and the differences in the resulting line shapes are emphasized. The results are relevant to measurements of ion mobilities, resolved with respect to the internal state of the ion.
Dissociative chemisorption of CH4 on Ni(111)
Authors: Yang Hong , Whitten Jerry L.
Published: July 2002

The dissociative chemisorption of methane at an atop-atom site on a (111) surface of nickel is treated using a many-electron embedding theory to describe bonding, modeling the lattice as a 41-atom, three layer cluster. Ab initio valence orbital configuration interaction (multiple parent) calculations carried out on a local surface region permit an accurate description of bonding at the surface. Ni 3d orbitals are explicitly included on seven nickel atoms on the surface. The calculated activation energy of CH4 adsorbed at an atop Ni site to produce CH3 and H coadsorbed at separated threefold sites is 17 kcal/mol. The dissociation of CH4 to CH3(ads)+H (ads) is predicted to be 2.8 kcal/mol exothermic. The Ni 3d orbitals contribute to the bonding by directly mixing with methane C–H orbitals during the dissociation process and through a direct interaction of 3d9 and 3d10 configurations at the transition state. The dissociation pathway and the bonding properties of adsorbed CH4 and coadsorbed CH3 and H are discussed.
A local reaction field method for fast evaluation of long-range electrostatic interactions in molecular simulations
Authors: Lee Frederick S. , Warshel Arieh
Published: July 2002

One of the major problems in molecular-dynamics simulations of polar fluids or macromolecular systems is the evaluation of electrostatic interactions. A system of N atoms demands an amount of work proportional to N2 for such calculations. Truncation procedures that neglect a significant part of the long-range effects are often necessary for computational feasibility, though such procedures may introduce serious errors in the simulations. This work introduces a simple and very effective approach for treating the long-range electrostatic forces. The present method, which is referred to as the local reaction field method, follows some of the ideas of the previously developed generalized Ewald method but then develops into a much simpler method. This is done by dividing the system into M groups of atoms and evaluating separately the short- and long-range contributions to the potential of each group. The short-range potential is evaluated explicitly as in any standard truncation method, while the long-range potential is approximated by the first four terms in a multipole expansion. In addition, the long-range potential is updated only once in every L time steps leading to a method of the order of N×M×q/L+N×P where q is related to the number of expansion terms and P is the average number of atoms inside the cutoff range. The speed, accuracy, and precision of the present method is assessed by evaluating the self-energy of a sodium ion in water and the self-energies of the acidic residues of bovine pancreatic trypsin inhibitor using an adiabatic charging free-energy perturbation approach. It is found that the present method reproduces accurately the corresponding results obtained without any cutoff but an order of magnitude faster. Furthermore, at the limit of very large systems, the speed of this method can be 2 orders of magnitude faster than that of the no-cutoff method even when each group contains only a small number of atoms. It is also found that the method gives much better results for electrostatic energies in proteins than those obtained by truncation methods. The stability and speed of the local reaction field method provides a powerful tool for the microscopic evaluation of electrostatic energies in macromolecules.
Surface plasmon enhanced photochemistry: Mo(CO)6–Al–quartz
Authors: Wolf M. , Zhu X.-Y. , White J. M. , Koschmieder T. H. , Thompson J. C.
Published: July 2002

We demonstrate that surface plasmon oscillations excited at an adsorbate covered metal–vacuum interface can effectively couple to the electronic system of the adsorbed molecule. Using p-polarized light (hν=3.5 eV) incident at the surface plasmon resonant angle in Kretschmann’s attenuated-total-reflection (ATR) configuration, we observe a strong enhancement of the photodissociation rate of Mo(CO)6 from a 180 Å Al film, evaporated on a quartz prism in UHV.
Diffusion reaction in restricted spaces of spherical symmetry: Surface quenching of luminescence
Authors: Yekta Ahmad , Duhamel Jean , Winnik Mitchell A.
Published: July 2002

The dynamics of bimolecular diffusion reaction in spherical microsystems is considered by solving Smoluchowski’s equation. The results are presented in the format of luminescence quenching experiments. The model allows an inner reactant, a lumophore, to diffuse about inside the microsphere while an outer reactant, a quencher, is allowed only on the surface of the sphere where the reaction, not necessarily diffusion controlled, takes place. Two dimensionless variables describe the system’s behavior completely. Simple analytical relations are derived for I0/I, the relative luminescence yield, and F(t), the luminescence intensity decay function. I0/I shows a simple dependence on the quencher concentration [Q], typically observed in systems where a certain fraction of lumophores are protected from quenching. F(t) shows multiexponential decay, which practically attains a single-exponential form [F(t)∼exp(−t/τlong)] when only less than 50% of the initial intensity has disappeared. An explicit expression for [Q] dependence of the long-time decay constant τlong shows the system has a finite lifetime even in the presence of an infinite quantity of quenchers. The experimentally observed behavior of luminescence quenching in surfactant micellar systems is contrasted with the theoretical results of this work, and the consequences of a Poissonian distribution of quenchers among micelles are discussed. Guidelines are presented for choosing the right lumophore in a given experimental system.
Electron-impact excitation of [1.1.1] propellane
Authors: Winstead Carl , Sun Qiyan , McKoy Vincent
Published: July 2002

We have calculated cross sections for low-energy electron impact excitation of the first triplet state (5a1′→3a2″) of [1.1.1] propellane. Our results support the assignment of a recently observed [O. Schafer, M. Allan, G. Szeimies, and M. Sanktjohansen, J. Am. Chem. Soc. (submitted)] near-threshold peak as a (5a1′)(3a2″)2 core-excited shape resonance.
van der Waals functional forms for molecular simulations
Authors: Hart J. R. , Rappé A. K.
Published: July 2002

We derive the general functional form for the van der Waals interaction of two 1s orbitals without resorting to approximating the interaction Hamiltonian by a power series expansion. We find the functional form to be the difference between two Extended Rydberg functions. The Morse function offers a good approximation for it. Reference potentials for several small systems are fit with the Morse, exponential-6, and Lennard-Jones functional forms. The Morse functional form describes nonbonded interactions quite well, while the more conventional functional forms possess the wrong shape.
A Fourier transform-infrared reflection absorption spectroscopy study of the formation and decomposition of chemisorbed formate on clean and potassium-modified Ru(001)
Authors: Weisel M. D. , Chen J. G. , Hoffmann F. M. , Sun Y.-K. , Weinberg W. H.
Published: July 2002

The formation and decomposition of formate species on the clean and on potassium-modified Ru(001) surfaces have been investigated with time-resolved vibrational spectroscopy and thermal desorption mass spectrometry (TDMS). Utilizing Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) we have characterized chemisorbed formate produced by the decomposition of formic acid on clean Ru(001), Ru(001)–(√3×√3)R30° K and on a K-multilayer adsorbed on Ru(001). The vibrational spectra show that formate is adsorbed on both clean Ru(001) and Ru(001)–(√3×√3)R30° K with C2v symmetry indicative of a bridged or bidentate species. There are, however, characteristic differences in the vibrational spectra, which indicate that for the Ru(001)–(√3×√3)R30° K surface the formate is directly bound to potassium. The vibrational spectrum of the latter species is found to be in good agreement with that of bulk potassium formate adsorbed on Ru(001).

Based on the agreement with literature data for bulk formate, we propose a bonding model for the potassium formate monolayer, which also accounts for the observed contraction of the potassium monolayer resulting from the compound formation. The thermal decomposition of the various formate overlayers has been monitored by simultaneous thermal desorption mass spectrometry and time-resolved FT-IRAS. This combination allows us to correlate the desorbing gas-phase products with the appearance and disappearance of surface intermediates. In the case of formate adsorbed on the clean Ru(001), the C–H and C–O bond cleavage reactions occur simultaneously, leading to the production of equal amounts of CO and CO2. The simultaneous observation of desorbing CO2 (TDMS) and of adsorbed CO (IR) confirms earlier work, which postulated a mechanism involving a coupling of the C–H and C–O bond cleavage reaction channels of two neighboring formates. The presence of potassium changes dramatically the reaction pathway of the formate as it suppresses the C–H bond cleavage channel, leaving CO and OH as the main decomposition products. Compound formation with potassium also leads to thermal stabilization of the formate in comparison to formate adsorbed on the clean surface. However, formate adsorbed on the potassium-modified ruthenium substrate is found to be thermally less stable than formate adsorbed on clean Ru(001).
Calculations of absorption and emission spectra: A study of cis-stilbene
Authors: Todd David C. , Fleming Graham R. , Jean John M.
Published: July 2002

Absorption and emission spectra are calculated by modeling cis-stilbene as a system of 12 displaced harmonic oscillators. We are able to obtain good agreement with the peaks of both the room temperature absorption and low temperature emission spectra using parameters from resonance Raman experiments by adjusting the position of the zero–zero transition energy (E0,0) and slightly adjusting the displacements along the normal modes. The best fit value for E0,0 is 29 000 cm−1. Using the displacements along the 12 degrees of freedom, and a normal mode description based on semiempirical quantum force field calculations (QCFF/PI), we determine a ‘‘relaxed’’ excited state geometry which is twisted a maximum of approximately 37 deg along the ethylenic torsional coordinate relative to the relaxed ground state geometry. An extension of the spectral calculations is described which allows for one or more of the modes to be anharmonic and vibrationally unrelaxed. We apply this extension to show that cis-stilbene emission can not be originating from a geometry with a 90 deg twist along the ethylenic coordinate. Comparison of our results with a recently obtained room temperature solution phase emission spectrum suggest that this emission originates from vibrationally unrelaxed molecules.
Collisional electronic quenching of NO A 2Σ+ by N2 from 300 to 4500 K
Authors: Thoman J. W. , Gray J. A. , Durant J. L. , Paul P. H.
Published: July 2002

We report rate coefficients for electronic quenching of NO A 2Σ+ v′=0 by N2 measured at room temperature and at high temperatures behind reproducible shock waves. The quenching cross section increases from 0.007 Å2 at 300 K to 7 Å2 at 4500 K, and the temperature dependence in the range 300–2300 K is best understood using a charge-transfer (harpoon) collision model. We also record dispersed fluorescence spectra and time-resolved fluorescence from both the directly pumped level (v′=0) and a collisionally populated level (v′=1); from these results we obtain rate coefficients for vibrational energy transfer in NO (A) and electronic quenching of v′=1 at 1900 K.
Ab initio calculations of the vibronically averaged hyperfine coupling constants for the 1 2Πu electronic state of CH2+
Authors: Engels B. , Perić M.
Published: July 2002

The results of pure ab initio calculations of the hyperfine coupling constants for the 1 2Πu electronic state for various isotopomers of CH2+ in the energy range between 0 and 20 000 cm−1 are presented. Effects of vibronic and spin–orbit coupling are discussed.
Production of excited Rh atoms via keV particle bombardment of Rh{100}: Simulation of excitations due to collisions above the surface
Authors: Bernardo Dan N. , Garrison Barbara J.
Published: July 2002

The production of atoms in the first excited state (4F7/2) of Rh via collisions 1.5–20 Å above the surface is studied. A method for efficiently simulating the contribution of excited atoms produced by this mechanism to the total yield of excited atoms is developed. Resulting velocity- and angle-resolved distributions are in good agreement with experimental results from previous studies.
Translational energy distributions and angular difference Doppler profiles of the excited hydrogen atom produced in e-C2H4 collisions: Dissociation dynamics of ethylene
Authors: Yonekura Nobuaki , Nakashima Keiji , Ogawa Teiichiro
Published: July 2002

Formation of an excited hydrogen atom (H*) through electron-impact dissociation of ethylene has been investigated by measuring Doppler profiles of the Balmer-β line and their angular dependence at an optical resolution of 0.007 nm. The Doppler profiles show a clear anisotropy. The translational energy distribution (TED) and the angular difference Doppler profile were obtained. There are four major dissociation processes for the formation of H*(n=4). Component 1 has a peak of TED at 1 eV, is produced in a perpendicular distribution, and should be produced by predissociation through the Rydberg states converging to the (1b1u)−1 state. Component 2 has a peak of TED at 1.8 eV, is produced in a parallel distribution, and should be produced through the Rydberg states converging to the (2ag)−1 state. Component 3 has a peak of TED at 2–6 eV and is produced in a parallel distribution. Component 4 has a peak of TED at 5–10 eV. Molecular orientation at the time of excitation was estimated; the molecular plane is perpendicular to the electron beam for component 1, and the C=C bond is perpendicular and the molecular plane is parallel to the electron beam for components 2 and 3. The asymmetry parameters of components 2 and 3 were ≤0.2; these values were much smaller than anticipated due largely to molecular rotation and deformation at the time of dissociation.
Vibrational deactivation in Kr/O2+ collisions: Role of complex formation and potential anisotropy
Authors: Ramachandran Gomathi , Ezra Gregory S.
Published: July 2002

We report a theoretical study of vibrational deactivation of the diatomic ion O2+(v=1) by low-energy (≤1 eV) collisions with Kr. It is shown that one-dimensional collision models are not able to reproduce the experimentally observed minimum in the deactivation rate as a function of collision energy, even when attractive interactions are included. Classical trajectory calculations on an empirical potential surface lead to good agreement with experiment, confirming the essential role of rotational degrees of freedom in the deactivation process. We find that the upturn in the deactivation rate at low energies is due to the formation of orbiting complexes, as suggested by Ferguson. Our results show that the energy dependence of the association (complex formation) rate is an important factor in determining that of the deactivation rate at low collision energies, whereas orbiting complex lifetimes show relatively little variation over the energy range studied. At very low collision energies, our computed deactivation rate becomes independent of energy, in accord with recent experiments of Hawley and Smith . We also investigate the role of potential anisotropy in the deactivation process. It is suggested that the decrease in deactivation rate with increased potential well width (i.e., decreasing anisotropy) is due to the elimination of a transient resonance between hindered rotation and diatom vibration.
The temperature and concentration dependencies of diffusion coefficients of seven helium–fluoroethane systems
Authors: Dunlop Peter J. , Bignell C. M.
Published: July 2002

Binary diffusion coefficients are reported as functions of concentration and temperature for seven helium–fluoroethane systems. An empirical correlation is made between the temperature-dependent effective cross sections for diffusion and the number of electrons in the heavy species interacting with the light helium atoms. Some of the experiments suggest a possible ‘‘isomer effect’’ due to the differences in molecular configuration of the fluoroethane species.
Temperature-jump measurements on the kinetics of association and dissociation in weakly bound systems: N2O4+M=NO2+NO2+M
Authors: Markwalder B. , Gozel P. , van den Bergh H.
Published: July 2002

Kinetic measurements have been made on the gas-phase reaction N2O4+M=2NO2+M by applying the temperature-jump relaxation method. Equilibrium mixtures containing nitrogen dioxide, dinitrogen tetroxide, a small quantity of an IR absorber, and an excess of a third-body bath gas, have been subjected to a short CO2 laser pulse. The induced temperature jump, of the order of 1 K, displaces the equilibrium towards NO2 formation. After the fast heating of the mixture, the time dependence of the NO2 and N2O4 concentrations has been monitored at 420 and 250 nm, respectively. For small perturbations, the relaxation to the new equilibrium concentrations at the higher temperature follows first-order kinetics, and the thermal rate constant can be deduced from the measured relaxation rate constant. Measurements have been made at 255 K in the pressure range between 0.3 and 200 bars He, which corresponds to the falloff range for this reaction. The high- and low-pressure limiting rate constants, krec,∞=(7.0±0.7)×1011 cm3 mol−1 s−1 and krec,0/[He]=(2.1±0.2)×1014 cm3 mol−1 s−1, respectively, extrapolated from the experimental data at 255 K, are in agreement with those calculated with a simplified statistic adiabatic channel model. The temperature dependence of the high- and low-pressure recombination rate constant, determined between 255 and 273 K, is given by the relations krec,∞=(2.2±0.2)×106×T(2.3±0.2) cm3 mol−1 s−1 and krec,0/[He]=(7.5±0.8)×1035×T(−9.0±0.9) cm3 mol−1 s−1. The corresponding high- and low-pressure dissociation rate constants are kdiss,∞= (2.8±0.3)×1013×T(1.3±0.2) exp{−(6790±700)/T} s−1 and kdiss,0/[He]=(9.6±0.9)×1042T(−10.0±1.0) ×exp{−(6790±700)/T} s−1, respectively.
Simulation of polymer chains in elongational flow. Kinetics of chain fracture and fragment distribution
Authors: López Cascales J. J. , García de la Torre J.
Published: July 2002

In this paper the Brownian dynamics technique for computer simulation of the fracture of polymer chains in a dilute solution subjected to elongational flow was used. Individual randomly coiled chains are placed at the stagnation point of a steady, uniaxial flow, and their evolution with time is monitored for chain fracture. The chains are modeled either as Rouse chains with a cutoff spring length, or as chains of springs obeying a Morse potential. Kinetic parameters are the first-order kinetic constant and the half-life time. The dependence of these parameters with elongational rate and chain length is expressed in the form of scaling laws, and the results are compared for the two models. Also studied was the size distribution of the resulting fracture fragments, finding that it may vary from sharply peaked to uniform, depending on the strength of the flow.
Solvation effects on the molecular 3s Rydberg state: AZAB/CYCLO octanes clustered with argon
Authors: Shang Q. Y. , Moreno P. O. , Li S. , Bernstein E. R.
Published: July 2002

Two color, 1+1, mass resolved excitation spectroscopy (MRES) is used to obtain molecular Rydberg (3s←n) spectra of azabicyclo[2.2.2]octane (ABCO) and diazabicyclo[2.2.2]octane (DABCO) clustered with argon. Nozzle/laser timing delay studies are employed together with time-of-flight mass spectroscopy to identify cluster composition. Population depletion techniques are used to differentiate between clusters with the same mass, but different geometries. A Lennard-Jones 6–12 potential is used to model the intermolecular interactions and predict minimum energy cluster geometries and cluster binding energies. The experimental results are combined with the cluster geometry calculations to assign spectral features to specific cluster geometries. Three different excited state interactions are required to model the experimentally observed line shapes, spectral shifts, and cluster dissociation. The relationship between these model potentials and the cluster binding sites suggests that the form of the cluster intermolecular potential in the Rydberg excited state is dictated by the distance between the argon and chromophore atoms. A comparison of results for ABCO(Ar)1 and DABCO(Ar)1 leads to the conclusion that the nitrogen 3s Rydberg orbital in clusters of DABCO is delocalized.
Radiative lifetimes of CH2(b 1B1)
Authors: Garcia-Moreno I. , Lovejoy E. R. , Moore C. Bradley , Duxbury G.
Published: July 2002

Radiative lifetimes are reported for CH2(b 1B1) in a wide range of rotational states in the (0,14,0), (0,15,0), (0,16,0), and (0,17,0) vibrational levels. Laser photolysis of ketene in a supersonic-jet expansion produced CH2(a 1A1) which was excited to single rovibrational levels of the (b 1B1) state by a second laser. Analysis of the temporal evolution of the fluorescence (b 1B1)–(a 1A1) yielded collision-free radiative lifetimes for the (b 1B1) state. The measured lifetimes range from about 4 to 10 μs and decrease with increasing vibrational energy. For the (0,14,0) overtone, the lifetimes increase slightly as a function of J (Ka=0). However, the lifetimes of rotational levels with Ka=0 in the (0,16,0) vibrational state are found to be independent of the rotational state. Calculations of the vibronic lifetimes show that the considerable lifetime lengthening when Ka≥1 is due to Renner–Teller coupling to the ã 1A1 state. The random lifetime fluctuation observed in some vibronic bands is probably due to spin–orbit coupling to the X̃ 3B1 state, e.g., 413 and 414 of (0,15,0). Here the radiative lifetime of the singlet component is shorter than that of the corresponding triplet component.
Concentration fluctuations of Brownian particles in a viscoelastic solvent
Authors: Stasiak Wojciech , Cohen Claude
Published: July 2002

We examine the dynamic light scattering time correlation for a dilute solution of Brownian particles in a solvent with internal relaxation. We model the solvent as a non-Newtonian, Maxwell-type fluid, with a single relaxation time, τm. We find that scattering from the concentration fluctuations of the particles, resulting from their translational Brownian motion, is modified due to the relaxing nature of the solvent. For a quiescent solution, the decay of the photon correlation function is given by exp[−Dk2t/(1+τmDk2)], where k is the scattering wave vector, D is the diffusion coefficient of the particles, and t is time. The effect of a time-dependent shear flow imposed on the solution is also considered. Unlike the case of particles in a solvent with no internal relaxation under simple shear oscillations of frequency ω, the time correlation exhibits oscillations even for large wave vectors when ω/Dk2 is less than unity.
Mechanisms and rate constants of the collisional deactivation of CO+(A 2Π,v) by He and Ar
Authors: Imamura Takashi , Imajo Takashi , Suzuki Shinzo , Koyano Inosuke
Published: July 2002

Collisional deactivation processes of CO+(A  2Π,v) by He and Ar are investigated in detail using both a time-resolved laser induced fluorescence and a threshold electron-secondary ion coincidence technique. Three deactivation mechanisms, pure vibrational relaxation, internal conversion, and charge transfer reaction, are identified. The time-resolved fluorescence from both the initially photoexcited and vibrationally relaxed levels of CO+ A 2Π makes clear the two distinct vibrational relaxation mechanisms: indirect two-step population of the lower levels in the CO+ (A)+He system and direct transition to the lower levels in the CO+(A)–Ar system. Threshold electron-secondary ion coincidence (TESICO) experiments demonstrate that the charge transfer reactions CO+ (A,v)+Ar→Ar++CO contribute significantly to the overall fluorescence quenching by Ar.
Inversion of the Bloch equation
Authors: Shinnar Meir , Leigh John S.
Published: July 2002

A method for the inversion of the Bloch equation without relaxation is described. It converts the continuous Bloch equation to a discrete analog, which is solved explicitly. The theory enables us to determine the space of physically realizable excitation profiles. Given an arbitrary desired result, it is approximated by a physically realizable profile. A pulse is then generated that will yield that profile.
The 480 nm system of Cs2 studied in a very cold molecular beam: Direct observation of a new E″ and the ion-pair states
Authors: Kim Bongsoo , Yoshihara Keitaro
Published: July 2002

The 480 nm absorption system of Cs2 is studied in a molecular beam using the resonance enhanced two photon ionization method. The ion-pair state (0u+) and a new E″ state are directly observed for the first time.
Comment on: Cation vibrational spectroscopy of trans and gauche n-propylbenzene rotational isomers. Two-color threshold photoelectron study and ab initio calculations
Authors: Liu Ruifeng , Zhou Xuefeng
Published: July 2002

It is concluded in this comment that the vibrational assignment of n-propylbenzene cation given in the above-mentioned paper should not be considered conclusive, because the level of their ab initio calculation is too low and there is no sufficient experimental information.
Structure and thermochemistry of ClO2 radicals
Authors: Francisco J. S. , Sander S. P.
Published: July 2002

The structure of ClO2 has been calculated for the X 2A″ ground state using unrestricted Hartree–Fock (UHF), unrestricted second-order Mo/ller–Plesset perturbation (UMP2), configuration interaction using single and double excitation (CISD), and quadratic configuration interaction (QCI) ab initio molecular orbital methods. Calculations using UMP2 and CISD wave functions predict a ClO bond length of 1.728±0.01 Å. The single-configuration based QCI in the singles and doubles space with perturbation inclusion of triple substitutions, denoted QCISD(T), yield a ClO bond length of 2.205 Å. The QCI results are consistent with results of Jensen who showed that the ClO bond length is 2.181 Å using annihilated self-consistent methods (AUMP2). The thermochemistry of ClO2 radical has been calculated using MP2 and QCI methods using an isodesmic scheme. Our scheme predicts the heat of formation for ClO2 at 0 K to be 24.6±2 kcal mol−1.
Computational studies of protein adsorption at bilayer interfaces
Authors: Gersappe Dilip , Li Weixiong , Balazs Anna C.
Published: July 2002

We used a Monte Carlo computer simulation to determine the behavior of proteins near and within a bilayer. The bilayer is represented by a hydrophobic slab, which is bounded above and below by hydrophilic regions. The protein is modeled as a neutral, self-avoiding chain, which contains both hydrophobic and hydrophilic sites. Through these simulations, we examined the effect of sequence distribution and the strength of the interaction energies on the interfacial activity of the proteins. The findings reveal both structural and energetic conditions under which proteins will remain localized at the surface of the bilayer or, penetrate and traverse the membrane. The results provide design criteria for fabricating proteins or biopolymers that display the desired interactions with bilayers.
Time-of-flight investigation of infrared laser-induced multilayer desorption of benzene
Authors: Braun R. , Hess P.
Published: July 2002

Infrared (IR) laser-induced resonant desorption/ablation of benzene layers condensed on liquid-nitrogen-cooled substrates has been investigated using the time-of-flight (TOF) technique. Adsorbates of up to 5 μm thickness were irradiated with a line-tuned TEA CO2 laser. Time-resolved detection of desorbed particles was performed with a quadrupole mass spectrometer (QMS). Measured TOF spectra can usually be well fitted with a stream-modified Maxwell–Boltzmann distribution. Systematic deviations from these fits were observed depending on film thickness, laser fluence, and angle of detection. Spectroscopic data of the adsorbate–substrate system needed for model calculations were measured in situ. The absorption coefficients were found to be fluence dependent. Measurements of the desorption yield and kinetic energy of the desorbed particles were performed with the P932 and P934-CO2 laser lines as a function of film thickness and laser fluence. The solid–liquid phase transition could clearly be observed in these experiments. Average kinetic energies of up to 0.8 eV were measured in the TOF spectra, where the adsorbate temperature could be estimated to be less than 600 K. IR multilayer laser-induced desorption (LID) exhibits features very similar to those reported for UV multilayer LID. A mechanism based on the sudden rise of the adsorbate vapor pressure due to fast energy deposition by the laser pulse is discussed.
Copolymer adsorption onto regular surfaces
Authors: Issaevitch T. A. , Jasnow David , Balazs Anna C.
Published: July 2002

We study the transition from infinite to finite-sized loops of Gaussian chains adsorbed onto a regular patterned surface as a function of sticking energy, E, sticker concentration, C, and surface lattice constant, m. Three cases are studied in the limit of infinite chain length. In the first case, which corresponds to a finite substrate, the ratio of the maximum surface dimension to the chain length, R/N, is zero as N→∞. In the second and third cases, the number of possible sticking sites is infinite in one and two dimensions, respectively. In the first case, we find that there is a first order transition from infinite to finite loop sizes, and there exists a range of average loop size that is inaccessible. In the second and third cases no such gap exists, and the transition is continuous.
Charge storage states in polyenes
Authors: Stafström S. , Brédas J. L. , Lögdlund M. , Salaneck W. R.
Published: July 2002

Changes in the geometric and electronic structure of polyenes upon sodium doping have been studied in detail. A short polyene containing three double bonds, hexatriene, forms a charge transfer complex with a maximum of one sodium ion. The interaction results in the formation of a polaron (radical anion) on the polyene chain. This type of charge storage state is shown to be unstable relative to the formation of a hexatriene dimer when interaction between two polyenes is allowed for; the two polyenes are then bound to each other via a carbon–carbon single bond. The charge storage states in this case correspond to two singly charged, spinless defects, one on each polyene segment of the dimer molecule. Decapentaene, which contains five carbon–carbon double bonds, is shown to be able to accept two electrons from a pair of sodium atoms. Similar to the case of doped trans-polyacetylene, the charges are stored as a soliton–antisoliton pair. However, when there is an excess of the number of polyene molecules relative to the number of sodium atoms, i.e., below saturation doping, this type of charge storage state is also unstable towards the formation of a decapentaene dimer of the same type as in hexatriene.
On the theory of two-photon induced fluorescence anisotropy with application to indoles
Authors: Callis Patrik R.
Published: July 2002

Useful relationships are derived from the general work by McClain [J. Chem. Phys. 57, 2264 (1972)] to connect the traditional two-photon polarization ratio of absorptivities using circularly and linearly polarized light (Ω) in fluid media with the expected anisotropy of fluorescence induced by two-photon excitation in viscous media or shortly after pulsed excitation. Emphasis is on ππ* transitions of planar systems with small out-of-plane two-photon tensor elements. Large ranges of anisotropy are found for most of the tensor space. The maximum and minimum anisotropies for ππ* excitations and emissions appear to be 0.6123 and −0.3277, somewhat larger than the previously supposed values of 4/7 (0.5714) and −2/7 (−0.2857). Application is made to indole and three-substituted indoles for which two-photon tensor shapes and orientations are deduced. Some predictions from semiempirical molecular orbital theory—including simulated aqueous solvent effects—are presented which agree qualitatively with experimental two-photon anisotropies reported in the literature.
Quantum resolved rotational energy transfer in the B 3Π(0+u) state of Br2
Authors: Perram Glen P. , Massman Dean A. , Davis Steven J.
Published: July 2002

Steady-state spectrally resolved laser-induced fluorescence techniques have been used to study rotational energy transfer within the B 3Π(0u+) state of molecular bromine. Rate coefficients for electronic quenching and state-to-state rotational energy transfer within vibrational state v′=11 were determined for Br2, He, Ar, and Xe collision partners. The individual, state selective rotational transfer rate coefficients were an order of magnitude slower than electronic quenching rates. Electronic quenching is attributed to both collisional predissociation and energy transfer followed by rapid spontaneous predissociation. For Br2(B, v′=11, J′=35) collisions with helium, population in the states J′=19–47 were observed and the resulting state-to-state rotational transfer rate coefficients ranged from 6.2×10−11 to 6.5×10−12 cm3/molecule s. Both exponential energy gap and statistical power gap laws for the scaling of rotational energy transfer rate coefficients with rotational energy are compared to the observed data.
Method for accelerating chain folding and mixing
Authors: Liu Zhihua , Berne B. J.
Published: July 2002

An algorithm based on allowing the form of the intermolecular potential to fluctuate is introduced into molecular dynamics and Monte Carlo simulations. It is shown that this method accelerates the approach to equilibrium in frustrated systems. It provides a very useful method for determining all of the local minima in complex systems. Examples are mixing in dense binary liquids and folding of long polymer chains. When this method is combined with umbrella sampling techniques it accelerates the simulations of a wide class of chemically important systems.
Direct observation of the (2) 3Πu state of Cs2 by resonance enhanced two photon ionization spectroscopy in a very cold molecular beam
Authors: Kim Bongsoo
Published: July 2002

A very cold molecular beam of Cs2 is generated using a high temperature pulsed nozzle. We observe new absorption bands of Cs2 with an origin near 648 nm by resonance enhanced two photon ionization. From the simulation of the rotational band contours, the bands are assigned to the (2) 3Πu(1u)–X 1Σg+(0g+) and (2) 3Πu(0u+)–X 1Σg+(0g+) transitions. The electronic term values and the dissociation energies are determined as Te=15 456.9±0.1 cm−1 and De=2692.1±0.9 cm−1 for the 1u state, and Te=15 432.3±0.1 cm−1 and De=2716.7±0.9 cm−1 for the 0u+ state. These values as well as the vibrational frequency and rotational constant show good agreement with those from the relativistic quantum mechanical calculations. We show that the potential minimum of the (2) 3Πu state lies lower than those of the C 1Πu and D 1Σu+ state. The potential energy curve of the (2) 3Πu state crosses that of the C 1Πu state on the inner branch.