bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of the H bond and cooperative effects in normal and supercooled water studied by anisotropic low frequency light scattering
Authors: Aliotta F. , Vasi C. , Maisano G. , Majolino D. , Mallamace F. , Migliardo P.
Published: July 2002

Depolarized Rayleigh and low frequency Raman scattering data in bulk supercooled water are presented. The measurements, performed from the normal region down to −26.95 °C for the first time, allow a quantitative data analysis and reveal the existence of a fine structure of the central contribution. We have identified a ‘‘fast’’ and ‘‘slow’’ contribution whose intensities show an opposite behavior as a function of temperature. Such a behavior is understood as a decay of local order due to the H bond. The spectral density function, as derived from the depolarized Raman data, shows the presence of acoustical and intermolecular optical contributions characteristic of many structured disordered materials. A critical comparison with molecular dynamics (MD) and neutron scattering results is also presented.
Diffusion anisotropy of oxygen and of tungsten on the tungsten (211) plane
Authors: Tringides M. , Gomer R.
Published: July 2002

The diffusion of oxygen and also of W on W(211) has been measured along (∥) and across (⊥) channels which exist on this plane. For W E∥ =13.2 kcal, D0(∥)=7.1×10−7 cm2 s−1; E⊥ =6.2 kcal, D0(⊥) =1.8×10−9 cm2 s−1. The cross channel values suggest diffusion by displacement of a channel wall atom; this is also supported by prediffusive flip–flop measurements, which show almost the same activation energy and frequency. For O/W (211) complicated low activation energy, low D0 regimes are seen at temperatures below ‘‘normal’’ diffusion and are tentatively attributed to low probability cooperative motions of substrate and adsorbate atoms. In the normal regimes E∥ ≊E⊥ but D0(∥)>D0(⊥). Activation energies are very coverage dependent, and this can be accounted for semiquantitatively in terms of repulsive nearest neighbor and attractive next nearest neighbor interactions in a channel, as well as attractive interactions between atoms in equivalent positions in adjacent channels, if the energies used by Wang and Lu to account for the phase diagram are used. With these assumptions values of E∥ ≂E⊥ =10–14 kcal at θ=0 are deduced. In addition to diffusion prediffusive flip–flop is also seen. This is activated on the clean W surface but shows virtually no activation for oxygen. Anisotropies in mean square fluctuations are seen not only in the diffusion but also in the flip–flop temperature ranges, suggesting that flip–flop may be a cooperative long wavelength process, akin to phonon modes of the adsorbate, in which the displacements of more than a single atom from the stablest to less stable binding sites and back occur in phase.
Translational diffusion of chain polymers. I. Improved variational bounds
Authors: Fixman Marshall
Published: July 2002

Variational estimates of an upper bound to the diffusion constant have been obtained from equilibrium simulations of Gaussian and cubic lattice chains. The trial functions that represent the deformation of the chain due to the external force have been inferred from the previous dynamical simulations of short Gaussian chains, which implied that local rather than large scale deformations were important. The variational results are in excellent agreement with the dynamical results for short Gaussian chains, and with exact results for rigid, planar, polygonal polymers. The variational results for long Gaussian chains (up to 896 beads) and cubic lattice chains (up to 448 beads), give an extrapolated reduction of the diffusion constant due to fluctuating hydrodynamic interaction of about 8% below the Kirkwood formula. The exact amount of the decrease may depend on friction constants and local chain structure.
Theory of homogeneous nucleation: A chemical kinetic view
Authors: Yang C. H. , Qiu H.
Published: July 2002

In classical theory of homogeneous nucleation the flux is derived initially as a function of rate constants for the condensation and vaporization reactions of clusters in the kinetic path. The evaluation of the vaporization rate for clusters of different size is provided by the Thomson–Gibbs relation. In view of discrepancies still existing between measurements and theoretical predictions and the continuing reservations on applying TG relation to microscopic size clusters a different approach is proposed in the present work. A simple function with two undetermined parameters has been used in place of the TR relation to relate the activation energy of the vaporization reaction to cluster size. The parameters are iterated to assume optimum values in numerical computation so experimental data may be correlated. Calculations show this approach closely predicts and correlates available data for water, benzene, and ethanol. The nucleation formulism is redeveloped with an emphasis on the chemical kinetic view. Surface tension of the liquid and free energy of droplet formation are not used in its derivation.
Frequency dependence of spin-lattice relaxation times of lipid bilayers
Authors: Brown Michael F. , Ellena Jeffrey F. , Trindle Carl , Williams Gerald D.
Published: July 2002

2H and 13C spin-lattice (T1) relaxation time studies of 1,2-dipalmitoyl-sn-glycero-3-phospho- choline (DPPC) in the lamellar, liquid crystalline (Lα) phase are discussed. It is shown that the T−11 results as a function of Larmor frequency ω0 are statistically better described by an ω−1/20 dependence than by an ω−10 or ω−20 dependence.
Raman spectral studies of the dynamics of ions in molten LiNO3–RbNO3 mixtures. II. Vibrational dephasing: Roles of fluctuations of coordination number and concentration
Authors: Katō Toshiko
Published: July 2002

The isotropic Raman spectra of the ν1 (A′1) mode of NO−3 ions are measured in molten LiNO3–RbNO3 mixtures at various concentrations and at temperatures from 478 to 750 K. The isotropic Raman bandwidth Γiso is found to increase almost linearly with the mole fraction c(Li+) of LiNO3, from 9–13 cm−1 for molten RbNO3 to 26–27 cm−1 for molten LiNO3. Vibrational correlation functions are analyzed on the basis of a model of simultaneous homogeneous and inhomogeneous vibrational dephasing. The contribution of inhomogeneous dephasing was found to be negligibly small in mixtures of c(Li+)≲0.33. While in mixtures of concentrations c(Li+)≳0.5, homogeneous and inhomogeneous processes are found to give comparable contributions to the total vibrational widths, where the latter contribution increases as the temperature decreases. The observed isotropic Raman bands are asymmetric. The asymmetry parameter α=Γ1/Γh , where Γ1 and Γh are the half-widths at half-maximum height of the low and high frequency sides of the band, is larger than one in molten LiNO3 and RbNO3, while α is smaller than one in molten mixtures of concentrations c(Li+)=0.33–0.67. This concentration dependent asymmetry is found to arise predominantly from coordination number fluctuation, i.e., fluctuation of the environmental cation number of a reference NO−3 ion, while concentration fluctuation in the coordination sphere plays a minor role. A model of local environmental states in a molten mixture, which assumes a linear change of vibrational frequency with neighboring cation numbers, is presented. The observed spectral asymmetry can be interpreted by assuming that the equilibrium distribution has a peak at the small coordination number side in pure molten LiNO3 and RbNO3, while the distribution peak shifts to the high coordination number side in molten mixtures.
Vibrational analysis of polyacetylene and copoly(acetylene+acetylene-d2): In-plane vibrations of t
r
a
n
s-polyene chains
Authors: Takeuchi Hideo , Furukawa Yukio , Harada Issei , Shirakawa Hideki
Published: July 2002

Normal coordinate calculations have been performed for the in-plane vibrations of several trans-polyene chains from butadiene to polyacetylene in order to obtain a set of force constants commonly applicable to short and long polyenes. The force field finally obtained consists of short range forces, which are primarily determined by the frequency data on short polyenes, and long range interactions which are important in explaining the data on polyacetylene. The long range interactions between the C=C stretching coordinates as far as the fifth neighbor have significant effects on the dynamical properties of the infinite chain. Frequencies of some Raman active vibrations are greatly lowered by the long range terms to become comparable with the experimental frequencies of polyacetylene. These vibrations shift to higher frequencies steeply as the phase difference between adjacent HC=CH units increases from zero. Such steep dispersion as well as the other features of calculated dispersion curves are fully consistent with the expectation based on the Raman spectra of copoly(acetylene+acetylene-d2). Further support for the dispersion curves is given by a general agreement between the experimental Raman spectra and theoretical spectra calculated for model copolymer chains using an approximate equation of resonance Raman intensity.
The ligand dependence of lanthanide 4f → 4f magnetic dipole transition moments
Authors: Berry Mary T. , Reid Michael F. , Richardson F. S.
Published: July 2002

The magnetic-dipole moments associated with 4f → 4f radiative transitions in lanthanide complexes are usually calculated on the basis of a static crystal-field model in which the ligands’ influence is confined to mixing (and splitting) the 4fN eigenstates of the ‘‘free’’ lanthanide ion. In this model the ligand charge distributions are assumed to be passive to the radiation field, with the latter acting only on the ligand-perturbed 4fN states of the lanthanide ion. Here we consider another model in which the ligand charge distributions are assumed to be polarized by the radiation field and then coupled to 4f → 4f electric-multipolar transition moments to effect a resonant magnetic-dipole interaction with the radiation field. This model (or mechanism) is exactly analogous to the dynamic-coupling (or ligand-polarization) model described previously for lanthanide 4f → 4f electric-dipole transition moments [see, for example, J. Phys. Chem. 88, 3579 (1984)]. It is shown that the dynamic-coupling (DC) mechanism for 4f → 4f magnetic-dipole transition moments will be competitive with the static-coupling (SC) mechanism only for transitions with strong ‖ΔJ‖>1 character, and even for these transitions the DC contributions to total magnetic-dipole strength are predicted to be small (<10−7 D2). However, it is also shown that the DC contributions to the electric- and magnetic-dipole transition moments of 4f → 4f transitions in chiral lanthanide systems may be crucial to explaining (or rationalizing) the rotatory strengths of these transitions. The latter suggestion is especially applicable to transitions whose intensities are known to be hypersensitive to the ligand environment.
Some additional information on the dynamics of the reaction O+2+CH4
Authors: Böhringer H. , Arnold F.
Published: July 2002

The reaction O+2+CH4 was studied in a selected-ion drift tube in a He buffer at various temperatures. At sufficiently high buffer pressure the reaction product O+2 ⋅ CH4 was observed beside the well known third order kinetics. From a study of the reaction equilibrium O+2 ⋅ O2+CH4 ⋅ O+2 ⋅ CH4+O2 the bond energy of O+2 ⋅ CH4 was determined. Values of ΔH=−37.7(±1.5) kJ/mol and ΔS=−42(±15) J/mol ⋅ K were found for the association of O+2 and CH4. These values are very close to those for the O+2–O2 bond.
Changes of thermodynamic quantities at the glass transition
Authors: Bengtzelius U. , Sjögren L.
Published: July 2002

Based on a microscopic approach we present results for the changes in thermodynamic derivatives at the glass transition. These changes are expressed in terms of correlation functions of the frozen out microscopic pressure and energy fluctuations. Quantitative results for the discontinuities as well as the Prigogine–Defay ratio are obtained within a simple model and agrees quite well with experimental results. A generalized order parameter is identified in terms of a correlation function, describing the frozen in local structure of the glass.
Influence of restricted geometries on the direct energy transfer
Authors: Blumen A. , Klafter J. , Zumofen G.
Published: July 2002

We investigate the direct energy transfer from a donor to acceptors embedded in spaces with restricted geometries. The decay of the donor is expressed in terms of a site density function ρ0(r) which depends on the position of the donor within the structure. For randomly distributed donors one has to average over their locations. We focus on spherical and cylindrical shapes. The geometrical restrictions manifest themselves in deviations from the known Förster direct transfer behavior.
Unified large basis set diatomics-in-molecules models for ground and excited states of H3
Authors: Roach A. C. , Kuntz P. J.
Published: July 2002

A recently developed systematic diatomics-in-molecules (DIM) procedure has been applied to the system H+H2 in order to generate large basis set models capable of approximating both the ground and low-lying excited state potential energy surfaces in a unified manner. The procedure, based exclusively on an analysis of diatomic ab initio wave functions, suggests that a 20-structure model including structures with not more than one excited H atom (2s or 2p) should suffice for the H3 (2A′) states. An 80-structure model including up to two excited H atoms yielded potential energy surfaces in close agreement with the smaller model. The ground state surface shows a greatly improved behavior in D3h configurations when compared to the simplest, two-structure DIM model for H3 but is otherwise very similar to that surface. This result exemplifies the stability of our systematic DIM methodology to increases in the size of the basis set. A number of excited state surfaces, including the lower 2A″ and quartet states, are reported and the implications for reaction kinetics are discussed. In particular, we predict the reaction H*(2s or 2p)+H2→H+H+H to have a large cross section.
Dielectric relaxation processes of some almost spherical dipolar organic molecules
Authors: Crossley J. , Hossain M. S. , Huque Md. E. , Saleh M. A. , Walker S.
Published: July 2002

Dielectric constant and loss measurements have been made, between 10 and 105 Hz, on seven dipolar substituted methanes from liquid nitrogen temperature up to and above the melting point. All these components have been examined in the pure solid state, four in cis-decalin and one in carbon tetrachloride. Two detailed studies have been made on the influence of concentration on the loss factor maximum value. All of the substances gave broad loss curves which were analysed to give relaxation times and activation enthalpies. Six compounds yielded an α process in the pure solid state. In the case of 1, 1, 1-trichloroethane in the pure solid state, cis-decalin, and carbon tetrachloride the β process may probably be ascribed to molecular relaxation.
Quantum diffusion of hydrogen on metal surfaces
Authors: Whaley K. B. , Nitzan A. , Gerber R. B.
Published: July 2002

A quantum mechanical theory is presented for the low temperature diffusion of atomic hydrogen on metal surfaces, based on a band model for the hydrogen motion. The theory is applicable to the diffusion of many interacting particles obeying quantum statistics. At low coverage the hydrogen band motion is limited by collisions between adsorbates, giving rise to a decrease of the diffusion constant with concentration. Other aspects of the hydrogen–hydrogen interaction are introduced to explain the coverage dependence at higher adsorbate concentrations. Comparison with recent low temperature diffusion measurements for H, D, and T on W(110) show that the above model reproduces satisfactorily the main features of the experimental coverage dependence of diffusion. The usefulness and limitations of band treatment for heavy particle diffusion are discussed in the light of these results.
Numerical tests of a generalized multilevel vibronic coupling formalism
Authors: Won Y. , Lagos R. , Friesner R.
Published: July 2002

An approximate matrix continued fraction method for calculating optical line shapes of molecular systems with multilevel excited state manifolds is compared with converged basis set results. It is shown that the theory gives correct spectral envelopes for all parameter values studied.
Electron–inert gas pseudopotentials for use in path integral simulations
Authors: Li Xiao-Ping , Broughton Jeremy Q. , Allen Philip B.
Published: July 2002

A method is devised to obtain an 1=0 pseudopotential for the electron–argon system using the local density functional approximation. The quality of the potential is checked against low-energy scattering data and is found to be good. The method should generalize quite simply to other inert gas–electron and to closed-shell ion–electron systems.
Forward and inverse functional variations in rotationally inelastic scattering
Authors: Guzman Robert , Rabitz Herschel
Published: July 2002

This paper considers the response of various rotational energy transfer processes to functional variations about an assumed model intermolecular potential. Attention is focused on the scattering of an atom and a linear rigid rotor. The collision dynamics are approximated by employing both the infinite order sudden (IOS) and exponential distorted wave (EDW) methods to describe Ar–N2 and He–H2, respectively. The following cross sections are considered: state-to-state differential and integral, final state summed differential and integral, and effective diffusion and viscosity cross sections. Attention is first given to the forward sensitivity densities δ0/δV(R,r) where 0 denotes any of the aforementioned cross sections, R is the intermolecular distance, and r is the internal coordinates. These forward sensitivity densities (functional derivatives) offer a quantitative measure of the importance of different regions of the potential surface to a chosen cross section. Via knowledge of the forward sensitivities

and a particular variation δV(R,r) the concomitant response δ0 is generated. It was found that locally a variation in the potential can give rise to a large response in the cross sections as measured by these forward densities. In contrast, a unit percent change in the overall potential produced a 1%–10% change in the cross sections studied indicating that the large + and − responses to local variations tend to cancel. In addition, inverse sensitivity densities δV(R,r)/δ0 are obtained. These inverse densities are of interest since they are the exact solution to the infinitesimal inverse scattering problem. Although the inverse sensitivity densities do not in themselves form an inversion algorithm, they do offer a quantitative measure of the importance of performing particular measurements for the ultimate purpose of inversion. Using a set of state-to-state integral cross sections we found that

the resultant responses from the infinitesimal inversion were typically small such that ‖δV(R,r)‖≪‖V(R,r)‖. From the viewpoint of an actual inversion, these results indicate that only through an extensive effort will significant knowledge of the potential be gained from the cross sections. All of these calculations serve to illustrate the methodology, and other observables as well as dynamical schemes could be explored as desired.
Structure and thermodynamics of the dipolar hard sphere fluid from the reference-hypernetted chain equation with minimized free energy
Authors: Lado F. , Lombardero M. , Enciso E. , Lago S. , Abascal J. L. F.
Published: July 2002

The reference-hypernetted chain equation, generalized to molecular fluids, is optimized by choosing the reference system so as to minimize the free energy. This procedure, which assures a significant improvement in the internal thermodynamic consistency of the theory, is here applied to a fluid of dipolar hard spheres, using both the complete dipolar potential and one with a reaction field (RF) truncation. We confirm that a recent reformulation of the relation between the dielectric constant ε and the mean square dipole moment for the RF geometry indeed brings ε for the truncated potential into reasonably good agreement with the infinite-range values, but that the important correlation functions nevertheless differ qualitatively in their long-range behavior. The method of solving the molecular integral equation, developed earlier, can be applied to other multipolar potentials, or alternatively, to molecules with distributed point charges.
A generalization of the pressure equation for fluids to macromolecular systems
Authors: Weiner J. H.
Published: July 2002

The pressure equation for fluids of classical spherically symmetric molecules provides a decomposition of the pressure into a kinetic part and a part due to intermolecular forces. A generalization of this equation to model macromolecular systems is developed which introduces two additional terms involving mean forces and mean energies in the covalent bonds. In an amorphous network of long-chain molecules, the bond force term may be interpreted as a covalent tension which helps balance the kinetic pressure, while the bond energy term acts to reduce the kinetic pressure due to the loss of atomic mobility caused by the covalent bonding.
Correlation and prediction of dispersion coefficients for isoelectronic systems
Authors: Koutselos A. D. , Mason E. A.
Published: July 2002

Combination rules for dispersion coefficients are greatly extended by way of a parameter corresponding to an effective number of equivalent electrons, the same parameter N that appears in the Slater–Kirkwood formula for the dipole–dipole coefficient. The same N can be used for all members of an isoelectronic sequence, and new formulas are given for higher-order two-body dispersion coefficients and three-body nonadditive coefficients, in which the same N can also be used. The only additional input data needed are static multipole polarizabilities. A theoretical justification is given using screening-constant wave functions. Extensive empirical testing suggests that the results are usually accurate within about 5% for dipole coefficients and about 5%–10% for higher coefficients. The results apply only to S-state atoms and ions, but should be capable of extension to other systems.
Electron transport in liquids: Effect of unbalancing the sphere-like methane molecules by deuteration, and comparison with argon, krypton, and xenon
Authors: Floriano M. Antonio , Freeman Gordon R.
Published: July 2002

The mobility of electrons in liquid methane is reduced when D atoms are substituted for H atoms in CH4. The reduction is greatest in the least symmetrical system CH2D2. The mobilities decrease in the order CH4>CD4≈CHD3>CH3D>CH2D2. The fractional decrease is greatest at the density of the mobility maximum nμmax ≈11×1027 molecule/m3: μ(deuterated)/μ(CH4)=0.60 in CH2D2, 0.63 in CH3D, and 0.67 in CHD3 and CD4. In the low density liquid near the critical region (nc =6.1×1027 molecule/m3), the isotope effect is obscured by quasilocalization of the electrons in density fluctuations. The isotope effect is attributed to (1) disruption of the sphere-like symmetry of the methane molecule, which increases the potential fluctuations in the conduction band in the liquid, and (2) enhanced inelastic interaction of the electrons with the hindered rotation of the molecules in the liquid. Inelastic scattering is nearly negligible in the liquids xenon, krypton, and argon at nμmax, but it makes progressively larger contributions in methane and neopentane.
Photodissociation dynamics and potential surfaces of hydrogen peroxide
Authors: Bersohn R. , Shapiro M.
Published: July 2002

The dynamics of photodissociation of H2O2 from low lying excited electronic states was studied by classical trajectory calculations. Potential energy functions were constructed for both the ground and two excited states of H2O2. Parameters in the ground state function were chosen to fit vibrational frequencies, potential barriers, and the equilibrium structure. Parameters in the excited state functions were chosen to fit the observed OH rotational state distribution and to be consistent with the electronic spectrum. The moderate rotational excitation of the OH radicals is mostly explained by the fact that the repulsive O–O force exerts a small torque around the center of mass of the OH radicals. The results at 5.00 and 6.42 eV cannot both be fit with the same potential surface suggesting that at least two if not all three of the low lying excited states predicted by Evleth play a role in the electronic absorption. The importance sampling technique was used to weight the initial conditions with the Wigner distribution. As a consequence only a modest (103) number of trajectories were needed to generate a representative rotational distribution. One useful result which may be general is that the rotational distributions resulting from a given surface were little dependent on the photon energy but that use of a steeper potential gave rise to more rotational excitation.
Rotation-induced vibrational mixing in S1 H2CO near E′vib =2000–2300 cm−1: 2143 and 214161 levels
Authors: Apel Eric C. , Lee Edward K. C.
Published: July 2002

Coriolis-induced vibrational mixing in S1 H2CO near Evib =2000–2300 cm−1 is manifested by the rotational quantum number (J′,K′a) dependent variation of the emission intensity in the rotationally resolved, dispersed fluorescence spectrum. The c-axis Coriolis resonance between 2143 (Ka=5) and 214161 (K′a=4) is observed with ξ(c)st ≂0.2 cm−1. The b-axis Coriolis interaction between 214161 (Ka=5) and 4461 (K′a=4) in S1 as well as the a-axis Coriolis interaction between 42 and 4161 in S0 are observed. A significant degree of vibrational mixing is observed in the energy range, Evib =2000–2300 cm−1.
Wave packet theory of the three body photodissociation of cadmium dimethyl
Authors: Kulander Kenneth C. , Orel Ann E.
Published: July 2002

The molecular photodissociation of cadmium dimethyl has been studied using time dependent, Gaussian wave packet methods. The model for this system dissociates to produce three fragments. The fragment velocity and energy distributions are calculated for a number of exciting photon wavelengths and are compared to distributions calculated using classical trajectory techniques. Absorption line shapes are calculated in various levels of approximation to illustrate the relative accuracies of the calculations.
Excited state-selected transition metal cations from one-color UV multiphoton ionization
Authors: Sanders Lary , Sappey Andrew D. , Weisshaar James C.
Published: July 2002

We have used one-color, resonance enhanced multiphoton ionization in the near-UV to create selectively the first excited terms of the transition metal ions Fe+, Ti+, and V+ in the gas phase. The term and level distributions of the resulting photoions are measured using time-of-flight photoelectron spectroscopy. We generally find better electron configuration and term selectivity for two-photon ionization via 4p resonant Rydberg levels than was found in previous work on three-photon ionization of 5s Rydberg levels. In certain Fe cases, a single ion total angular momentum level dominates the photoelectron spectrum. The simple picture of the 4p Rydberg levels as consisting of an ion core of well defined Lc–Sc and Jc weakly coupled to a Rydberg electron is remarkably useful. Deviations from this simple picture are discussed in terms of configuration interaction of resonant states and autoionizing structure in the continuum. The results will find application in studies of state-selected metal ion chemistry.
Statistical mechanics of rigid particles in an external field
Authors: Vanderlick T. K. , Scriven L. E. , Davis H. T.
Published: July 2002

A new density functional is employed which simplifies and unifies the generalized van der Waals (GVDW) theory and the exact one-dimensional theory of hard particles in the presence of an external potential or solid walls. Numerical solution of density profiles in planar systems is made easy by introduction of the density functional. Numerical results for one-and three-dimensional fluids reveal that the GVDW theory fails at high chemical potentials by admitting negative densities. We have some evidence that the cause of the failure is a negative eigenvalue of the weighting function defining the local mean density or excluded volume in the theory. Density profiles of the exact one-dimensional theory are compared to those of the GVDW theory, those of the former exhibiting similar but stronger oscillations and of course no regions of negative density.
Theoretical study of the ground-state vibrations of the linear polyenes
Authors: Hemley R. J. , Brooks B. R. , Karplus M.
Published: July 2002

Vibrational force fields for the ground states of 1,3-butadiene, 1,3,5-hexatriene, and 1,3,5,7-octatetraene are calculated from extended Pariser–Parr–Pople CI theory. For the ground state of butadiene, the principal π-system force constants are compared to those determined by empirical, other semiempirical, and ab initio methods. The calculated vibrational frequency shifts for a series of deuterium substituted butadienes are found to be in good agreement with experiment. Trends in the principal π-system force constants of the polyenes for the ground states of ethylene through octatetraene are described, including the effects of introducing cis linkages in the polyene chain and of using single- vs single-plus-double-excitation CI. Correlations between the force constants and the calculated vibrational frequencies are discussed, and current assignments of the vibrational spectra of the polyenes are examined in view of the theoretical results.
Photodissociation of energy selected C4H+6 ions: The isomerization barrier between butyne and 1,3 butadiene ion isomers
Authors: Bunn Thomas L. , Baer Tomas
Published: July 2002

Ions are energy selected by photoelectron photoion coincidence (PEPICO) and subsequently photodissociated by a pulsed excimer pumped dye laser. The laser is triggered by the delayed zero energy electron signal so that the laser pulse intercepts the single ion at a fixed point in the ion trajectory. The product ions are separated from the undissociated parent ions by their time of flight. Evidence is presented for both single and multiphoton dissociation of the butadiene ions. The derived kinetic energy release distribution (KERD) of the C3H+3 product ions is consistent with the statistically predicted KERD. Finally, the isomerization barrier between 1- and 2-butyne and butadiene ions is measured by probing the ion structure with the laser as a function of the C4H+6 internal energy. Because this rearrangement requires only H atom shifts, the measured barrier is associated with the ubiquitous H atom scrambling process commonly observed in unsaturated hydrocarbon ions.
Rotational analysis and vibrational predissociation in the ν2 band of HCN dimer
Authors: Wofford B. A. , Bevan J. W. , Olson W. B. , Lafferty W. J.
Published: July 2002

The rovibrational infrared spectrum of the bound C–H stretching vibration, ν2, in the HCN dimer has been analyzed. Observed transition frequencies have been combined with previously recorded microwave data to obtain the following molecular parameters (in cm−1): ν2=3241.5696(8), α2=−0.000 110(1), B″=0.058 233 92(1), B′=0.058 344(1), D″J =0.7013(52)×10−7, DJ =0.6636(18)×10−7. The observed full widths at half-maximum intensity of the observed transitions are consistent with excited state lifetimes of 1.7(4)×10−9 s.
Is N-protonated hydrogen isocyanide, H2NC+, an observable interstellar species?
Authors: DeFrees D. J. , Binkley J. S. , Frisch M. J. , McLean A. D.
Published: July 2002

A b initio molecular orbital theory is used to examine the singlet and triplet potential energy surfaces for the CH2N+ system. The results confirm those of earlier studies which suggested that the singlet H2NC+ isomer could be formed via the corresponding triplet isomer. Also, it is shown that the reaction HCN++H2 might lead to this metastable isomer without invoking the triplet species. The best test of the hypothesis that this molecule can be formed by gas phase, ion molecule reactions and may be an important precursor in the interstellar synthesis of HCN and HNC is to search for it in space. To this end, theoretical predictions are made of its rotational frequencies and its vibrational frequencies and intensities to serve as a guide to laboratory spectroscopists and radioastronomers.
Photodissociation of doubly excited states of H2, HD, and D2
Authors: Glass-Maujean M.
Published: July 2002

The cross section for the formation of H(n=2) or D(n=2) by photodissociation for the three isotopes H2, HD, and D2 has been measured using synchrotron radiation between 500 and 200 Å. The comparison with computed cross section curves shows the importance of the Q11Πu Q21Πu(1) neutral states of the 2pπ2Πu ionization continuum. Contribution from Rydberg states converging to the 2pπ2Πu state of H+2 is suggested. No isotopic effect can be confirmed.
Quantum Monte Carlo approach to electronically excited molecules
Authors: Grimes R. M. , Hammond B. L. , Reynolds P. J. , Lester W. A.
Published: July 2002

Quantum Monte Carlo (QMC) is used to compute the electronic energies of H2(B 1Σ+u) and H2(E 1Σ+g). The E state calculation represents the first application of QMC to a molecular excited state with the same symmetry as a lower state. In this QMC approach a trial function specifies the nodes of the QMC distribution. The role of these nodes in excited state calculations is discussed. QMC energies that contain over 95% of the correlation energy are computed using MCSCF wave functions as trial functions.
Extended Brusselator: Multiple attractors
Authors: Li Ru-sheng
Published: July 2002

A model, which is an extended version of the Brusselator, with additional steps Y ⇄k C, is put forward to model the phenomena of multiple attractors.
Temperature dependence of electron attachment to NO2
Authors: Shimamori Hiroshi , Hotta Hiroshi
Published: July 2002

Thermal electron attachment to NO2 in He, Ar, and N2 as buffer gases has been studied at various temperatures between 257 and 354 K with the pressure range of a few to 900 Torr. At buffer-gas pressures from a few to several tens Torr, the effective two-body attachment rate constant (keff) initially increases and then shows a saturation at all temperatures for three buffer gases. Further increase of the pressure up to near 1 atm gives a gradual increase of keff for Ar and N2 systems, whereas no such an increase is seen for He at three different temperatures. These behaviors are explained by a mechanism of a combination of the two-step three-body attachment and the attachment to van der Waals molecules containing NO2. A simulation analysis supports this explanation. A remarkable finding is that in all systems studied keff’s become larger at lower temperatures. An Arrhenius plot for the initial two-body attachment rate constant gives an activation energy of −0.17 eV. It has been suggested that an excited triplet state of NO−2 ion may be the species formed by the initial electron attachment.
Fluorescence from VUV excitation of formaldehyde
Authors: Suto Masako , Wang Xiuyan , Lee L. C.
Published: July 2002

The photoabsorption and fluorescence cross sections of H2CO were measured in the 105–180 nm region using synchrotron radiation as a light source. Vacuum ultraviolet (VUV) emission from excited photofragments was dispersed and identified to be the CO(A 1Π→X 1Σ+) system. The vibrational population of CO(A) was determined from the fluorescence spectrum and was used to study the photodissociation mechanism. The threshold for the production of the VUV emission is at 140.3 nm. UV emission from HCO* was observed below a threshold of 147.5 nm. The upper limit of the dissociation energy, D0(H–HCO), determined from the HCO emission threshold is 3.61±0.03 eV. The quantum yield for the production of either CO* or HCO* is a smooth function of the excitation wavelength, from which two dissociative states with vertical energies at 8.69 and 10.7 eV are derived. The VUV and UV fluorescences have maximum quantum yields at 116 nm of about 1.6% and 0.23%, respectively. The photodissociation process of H2CO in VUV is discussed.
Intra- and intermolecular energy transfer in H2+OH collisions
Authors: Brown N. J. , Rashed O.
Published: July 2002

We have used the method of quasiclassical dynamics to investigate intra- and intermolecular energy transfer in H2+OH collisions. Energy transfer has been investigated as function of translational temperature, rotational energy, and vibrational energy. The energy transfer mechanism is complex with ten types of energy transfer possible, and evidence was found for all types. There is much more exchange between the translational degree of freedom and the H2 vibrational degree of freedom than there is between translation and OH vibration. Translational energy is transferred to the rotational degrees of freedom of each molecule, and this occurs more readily for OH than H2. Both molecules exhibited intramolecular energy transfer from vibration to rotation, and this was a major pathway for vibrational deactivation. Evidence was also found for the intermolecular transfer of energy from vibration to the rotational and vibrational degrees of freedom of the other molecule.
The derivation of the rotational potential function from atom–atom potentials. II. Ammonium–fluorine compounds
Authors: Smith David
Published: July 2002

The rotational potential functions of a number of ammonium–fluorine compounds have been derived from atom–atom potentials of the Buckingham (exp-6) type. The potential functions for the ammonium pervoskites NH4ZnF3, NH4MnF3, NH4MgF3, and NH4CoF3 with Oh site symmetry for the ammonium ion are derived. The hexagonal ammonium hexafluorides (NH4)2SiF6, (NH4)2GeF6, and (NH4)2TiF6 are analyzed including the J=7 term in the potential function. Of all the compounds studied, the potential function converges most slowly for these compounds. The effect of the J=6 and J=7 terms in the potential function are discussed in terms of their effect on the librational and tunneling frequencies. The J=7 and J=8 terms are included in the potential function of cubic (NH4)2SiF6. Three ammonium compounds with strong hydrogen bonds are studied, NH4F, NH4SnF3, and NH4HF2. The computed librational frequencies of NH4HF2 are compared with the librational frequency derived from the heat capacity. It is shown that the librational frequencies of the tetrahedral ions NH+4, BH−4, AlH−4, and GaH−4 derived from heat capacity results are in good agreement with the librational frequencies determined by spectroscopic studies. The potential functions for NH4AlF4 and NH4CrF4 are also derived. For all the compounds studied, the activation energies, librational frequencies, and when applicable, the tunneling frequencies are computed for the ammonium ion.
Tricritical phenomena in quasi-binary mixtures. III. An extended classical treatment
Authors: Scott Robert L.
Published: July 2002

The phenomenological (classical) treatment of tricritical points is extended by adding higher-order terms to the sixth-order free-energy polynomial of the Griffiths asymptotic theory. The free energy is expanded in powers of the experimental (laboratory) densities of a quasi-binary mixture, e.g., the mole fraction x or the molar volume Vm. The higher-order corrections introduce asymmetry into the temperature-composition coexistence curve and yield an approximately parabolic relation between the two densities. This extended classical theory yields, for experimental susceptibilities (i.e., those defined in terms of one of the laboratory densities) of the three coexisting phases, a Griffiths first sum χ1/2α +χ1/2γ −χ1/2β whose limiting value (as the tricritical point is approached) is not zero, as the asymptotic theory predicts, but rather a nonzero constant. The square-gradient one-density approximation is applied to give an extended mean-field treatment of interfacial tensions in the tricritical region.
Nonequilibrium statistics of flexible macromolecules in dilute solutions. II. Macromolecular extension and comparison with nonequilibrium Brownian dynamics
Authors: Saab H. H. , Dotson Paul J.
Published: July 2002

We derive an exact nonequilibrium expression for the probability density of the scalar extension of an arbitrary section of a Rouse chain in dilute solution. We use this new result to assess the accuracy of a new non-equilibrium Brownian dynamics (NEBD) computer simulation method for macromolecular fluids. The NEBD method allows the examination of systems which, because of their complexity, are intractable to analytical methods. Comparison of simulation results for Rouse chains in shear flow with analytical results for the distribution of the end-to-end distance and the distribution of individual chain segment extensions shows that NEBD does indeed produce reliable results.
High sensitivity, high-resolution IR laser spectroscopy in slit supersonic jets: Application to N2HF ν1 and ν5+ν1−ν5
Authors: Lovejoy Christopher M. , Nesbitt David J.
Published: July 2002

A difference frequency IR spectrometer is combined with a slit supersonic expansion for high-resolution (≤50 MHz FWHM) direct absorption investigations of jet-cooled species. The 1.25 cm long nozzle provides a long path length and high densities suitable for synthesis and observation of van der Waals clusters, with a gradual spatial temperature gradient that permits experimental control of low frequency vibrational populations. Due to collisional quenching of velocity distributions, absorption linewidths are reduced and peak absorbance increased five- to sevenfold compared to pinhole expansions. Minimum detectable concentrations of HF containing complexes are 2×109 molecules/cm3/quantum state in a 2.5 cm path length. The combination of high sensitivity, sub-Doppler resolution, long path lengths, and temperature control make direct absorption in slit nozzle expansions a powerful and general technique for high-resolution study of jet-cooled species. The spectometer is used to obtain the near-infrared spectrum of N2HF. The ν1 (HF stretch) fundamental is observed at 3918.2434(2) cm−1, red shifted by 43.1795(2) cm−1 from the HF origin. In the warmer regions of the expansion close to the nozzle the ν5+ν1−ν5 Π←Π hot band is also observed, blue shifted by 2.7160(4) from the ν1 fundamental. Rotational analysis of these spectra reveals changes in vibrationally averaged molecular geometries upon excitation that are consistent with a near linear equilibrium geometry. The ν5 (N2 bend) frequency is estimated at 85±20 cm−1, based on the relative intensities of the two bands and on an analysis of the l doubling. The linewidths of the N2HF transitions show no increase over the HF monomer and are limited by instrumental resolution to 50 MHz FWHM; the lifetime of the upper level is therefore at least ≥3 ns.
(2+1) resonant enhanced multiphoton ionization of H2 via the E,F 1Σ+g state
Authors: Rudolph H. , Lynch D. L. , Dixit S. N. , McKoy V. , Huo Winifred M.
Published: July 2002

In this paper, we report the results of ab initio calculations of photoelectron angular distributions and vibrational branching ratios for the (2+1) REMPI of H2 via the E,F 1Σ+g state, and compare these with the experimental data of Anderson et al. [Chem. Phys. Lett. 105, 22 (1984)]. These results show that the observed non-Franck–Condon behavior is predominantly due to the R dependence of the transition matrix elements, and to a lesser degree to the energy dependence. This work presents the first molecular REMPI study employing a correlated wave function to describe the Rydberg–valence mixing in the resonant intermediate state.
Absorption, resonant light scattering, and distributed relaxation
Authors: Schomacker K. T. , Šrajer V. , Champion P. M.
Published: July 2002

We explore the consequences of distributed nonradiative electronic relaxation rates on absorption and resonant light scattering intensities. Several example cases are explicitly treated and lead to stretched exponential or enhanced power law decay of the correlation function. A case applicable to the nonradiative decay of cytochrome c is developed in detail and it is shown that for relatively mild coordinate distributions, a factor of 3 incease in the total (ensemble averaged) light scattering intensity is easily achievable. This may be a partial explanation of recent observations that find the resonance Rayleigh scattering of cytochrome c to be anomalously large when compared to the predictions of Kramers–Kronig dispersion theory.
Comment on ‘‘Orientation, alignment, and hyperfine effects on dissociation of diatomic molecules to open shell atoms’’
Authors: Havey M. D. , Vahala L. L.
Published: July 2002

A recent paper in this journal [Y. B. Band et al., J. Chem. Phys. 84, 3762 (1986)] reported parameters describing orientation and alignment produced, in an axial recoil limit, by one photon dissociation of diatomic molecules. Reported also were values, applicable to the resonance transitions of the alkali atoms, for orientation and alignment depolarization coefficients. Most of the numerical values reported for the coefficients were incorrect, in some cases by as much as a factor of 2. We report a tabulation of correct depolarization coefficients applicable to the resonance transitions of common alkali isotopes. Further, the coefficients depend on measured hyperfine splittings and radiative lifetimes. Thus, uncertainties in the coefficients reflecting those in measured quantities are also presented. An application of our results to a recent Na2 photodissociation experiment [E. W. Rothe et al., Chem. Phys. Lett. 22, 100 (1980)] is also made.
Electron attachment and ionization processes in CF4, C2F6, C3F8, and n-C4F1
Authors: Hunter S. R. , Carter J. G. , Christophorou L. G.
Published: July 2002

Measurements are reported of the electron attachment (ηT/N) and ionization (αT/N) coefficients for the perfluoroalkanes n-CNF2N+2 (N=1 to 4) over the density-reduced electric field (E/N) range 5×10−17≤E/N≤400×10−17 V cm2 using pulsed Townsend (PT) experimental techniques. The present ηT/N measurements are the first to be obtained for pure C2F6, C3F8, and n-C4F10 at low E/N values. The ηT/N measurements in C3F8 and n-C4F10 are dependent on gas pressure over a wide E/N range in agreement with previous high pressure electron attachment rate constant ka measurements in these gases. The dissociative and nondissociative electron attachment processes for C3F8 and n-C4F10 have been quantified from the pressure dependence of the measured electron attachment coefficients ηT/N as a function of E/N. The thermal electron attachment rate constants (ka)th and the high voltage limiting electric field strengths (E/N)lim obtained from the present measurements are in good agreement with previous literature values.
Coupled states calculations on vibrational relaxation in He+CO2(0110) and He+CO
Authors: Banks A. J. , Clary D. C.
Published: July 2002

Computations of vibrational relaxation rate coefficients for He+CO2 (0110) and He+CO (ν=1) over the temperature range 100–300 K are reported. The calculations used the quantum mechanical coupled-channel method for vibrations and the coupled states approximation (CSA) for rotations. These calculations were used to test the accuracy of the vibrationally close coupled, rotationally infinite order sudden approximation (VCC-IOSA) and a semiclassical method. For He+CO2, the VCC-IOSA results compare very well with those obtained using the CSA, while the agreement is not so good for He+CO. This is because CO has a much larger rotational constant and vibrational frequency than CO2.
Quantal fluctuations in fluorescence lifetimes of individual rovibronic levels
Authors: Engel Y. M. , Levine R. D. , Thoman J. W. , Steinfeld J. I. , McKay R. I.
Published: July 2002

Measured fluorescence lifetimes of individual rovibronic levels in SiH2 (Ã 1B1, 0n0, n=1,2,3), vary irregularly from one level to the next. The distribution of these fluctuations is well fitted by the procedure of maximal entropy. This suggests that the intramolecular evolution at that energy range (∼18 000 cm−1) is chaotic and uniformly samples its available phase space.