bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational tunneling effects in 2H-NMR spectra of polycrystalline (ND4)2SnCl6
Authors: Ingman L. P. , Koivula E. , Lalowicz Z. T. , Punkkinen M. , Ylinen E. E.
Published: July 2002

Rotational tunneling effects are analyzed in 2H-NMR spectra of deuterated ammonium ions in powders. The spectra are generally composed of a strong central line from A symmetry species and weaker T sidebands. At a high magnetic field (∼6.5 T) the central line contains well-resolved doublets. Their separations supply a direct measure of the rotational tunneling frequency νt in the range 0.1<νt<6 MHz. At low magnetic fields the spectra depend both on νt and the Larmor frequency ν0. Level crossing related effects may be detected in a ±0.75 or ±0.35 MHz range around that Larmor frequency which is equal to νt or 1/2 νt, respectively. Generally, only the fitting of the theoretical spectra to those measured at different ν0 allows an estimation of the tunneling frequency. The measurement of νt up to about 20 MHz becomes thus possible. The theoretical predictions are verified by results on a (ND4)2SnCl6 powder sample. High and low field spectra lead to slightly different values for the tunneling frequency at 4.2 K equal to 7.5 and 7.1 MHz, respectively. Tunneling to reorientation transition is followed through changes in the spectra and their second moments.
On the dipole moment functions of ClO and OH
Authors: Yaron David , Peterson Karen , Klemperer William
Published: July 2002

The electric dipole moment of ClO in the ground and first excited vibrational levels of the 2Π3/2, J=3/2 state was measured using molecular beam electric resonance. The results are 1.2974(10) (v=0) and 1.2758(12) D (v=1), with μ(v=1)−μ(v=0) being more accurately determined as −0.0216(7) D. Combination of the dipole moments with relative intensity data lead to a fundamental transition moment of −0.034(5) D. This is in excellent agreement with the results of Herman–Wallis effect measurements −0.0364(33) and −0.044(12) D and the most recent ab initio calculation, −0.0360 D. For OH, values for the v=0 through v=2 dipole moments show that the maximum of the dipole moment function occurs at an internuclear separation 0.20 to 0.25 Å larger than the equilibrium internuclear separation. The curvature about the maximum is also determined by the dipole moment measurements, however, the asymmetry about the maximum is not. Estimation of the asymmetry from ab initio calculations leads to an estimate of 28(18) S−1 for the Einstein transition probability of the 1–0 band of OH.
The Ne–N2 potential energy surface from high-resolution total differential scattering experiments and a close-coupling and infinite-order-sudden analysis
Authors: Beneventi L. , Casavecchia P. , Vecchiocattivi F. , Volpi G. G. , Lemoine D. , Alexander M. H.
Published: July 2002

High-resolution crossed molecular beam measurements of the total differential cross section (DCS) for the scattering of Ne by N2 at a collision energy of E=75.8 meV are reported. Strongly quenched diffraction oscillations superimposed on the falloff of the main rainbow structure are clearly resolved. A reliable Ne–N2 potential energy surface (PES) is derived by simultaneously fitting second virial, diffusion, and viscosity coefficient data taken from the literature. Information on the anisotropy of the interaction is obtained from the quenching of the diffraction oscillations in the total DCS within the framework of the infinite-order-sudden (IOS) approximation. The reliability of the IOS approximation in deriving a fully anisotropic potential energy surface from the measured scattering dynamics is examined and demonstrated by performing exact close-coupling calculations for the present experimental conditions and then comparing both integral and differential total and rotationally inelastic cross sections. The derived PES is compared with recently proposed theoretical model potential surfaces. Although in satisfactory agreement with bulk properties, none of these surfaces predicts correctly the present scattering data, each having significantly different spherical and/or anisotropic components in comparison with the PES derived here.
State-to-state vibrational predissociation dynamics and spectroscopy of HeCl2: Experiment and theory
Authors: Cline Joseph I. , Reid Brian P. , Evard Dwight D. , Sivakumar N. , Halberstadt Nadine , Janda Kenneth C.
Published: July 2002

The structure and vibrational predissociation dynamics of HeCl2 are studied by pump–probe spectroscopy and by three-dimensional quantum mechanical calculations. Parity selected excitation spectroscopy is used to confirm the essential features of the previous analysis of the HeCl2 B←X laser excited fluorescence spectra. Product vibrational and rotational state distributions are measured for the v′=6, 8, 12, 20, and 24 levels of HeCl2 in the B state. For the v′=6 and 8 levels the dependence of the product state distribution on the initially excited rotational state is also measured. Although the dissociation dynamics are dominated by Δv=−1, V→T energy transfer, several interesting effects are revealed by monitoring the product rotational degrees of freedom. Due to the symmetry of the HeCl2 potential, the parity of the initially excited HeCl2 rotational state is conserved during the dissociation dynamics. Even when a single initial rotational state is excited, the observed product rotational state distribution is bimodal. The product rotational distribution is nearly independent of the amount of kinetic energy released to the product degrees of freedom. Three-dimensional quantum mechanical calculations using a simple potential energy surface are remarkably successful at reproducing the details of the experimental measurements. Only five parameters of the potential were adjusted to calculate the excitation spectrum, the vibrational predissociation product state distributions, and the lifetimes of the excited states. Analysis of the dissociation mechanism in terms of simple models, however, is not straightforward. In particular, the impulsive, quasiclassical half-collision model is not compatible with the observed independence of the product rotational state distribution from the amount of kinetic energy which is released. The close agreement between state-to-state experiment and quantum theory on the HeCl2 dynamics shows that the shortcomings of more approximate theories are fundamental and cannot be attributed to lack of knowledge of the true potential energy surface.
Molecular solvent model for an electrical double layer: Reference hypernetted-chain results for ion behavior at infinite dilution
Authors: Torrie G. M. , Kusalik P. G. , Patey G. N.
Published: July 2002

We have extended our previous reference hypernetted-chain theory of the structure of a multipolar hard sphere fluid with water-like parameters in the vicinity of a large multiply charged macroion [J. Chem. Phys. 88, 7826 (1988)] to calculate the response of an isolated counterion to the charged surface of the macroion in this solvent environment. We have considered a range of macroion sizes from 28 to 84 Å and surface charge densities up to 17.5 μC/cm2 (108 charges on a macroion 56 Å in diameter) and a range of counterion sizes from 1.9 to 4.0 Å. The ion–macroion potentials of mean force Φim in the solvent environment of the largest macroion are qualitatively different from those for more highly curved surfaces, in accordance with the previous predictions of the theory for the solvent reponse. Likewise, there is a relatively abrupt onset of oscillatory behavior in Φim at high surface charges, just as we found previously in the solvent structure under these conditions. At moderate surface charges, however, the ion response appears to assume an extraordinarily simple form in which Φim consists of two independent components, a short-range piece that is independent of surface charge and the simple Coulombic term that Φim approaches far from the surface. The mean electrostatic potential, on the other hand, has little resemblance to Φim and appears to be of limited usefulness in understanding the ionic distribution in a molecular solvent.
The photodissociation of methyl iodide at 229.4 nm: A determination of the fragment recoil anisotropy using energy-selective electron impact ionization and time-of-flight mass spectrometry
Authors: Penn S. M. , Hayden C. C. , Carlson Muyskens K. J. , Crim F. F.
Published: July 2002

Energy-selective electron impact ionization of laser-produced photofragments together with time-of-flight mass spectrometry is a general and sensitive means of studying primary photodissociation processes. Low-energy electrons ionize photofragments without the production of background fragment ions from dissociative ionization of the parent molecules, and the time-of-flight mass spectral peak shapes provide direct information on the photofragment recoil anisotropy. In the first application of this combination of techniques, we have studied the photodissociation of methyl iodide at 229.4 nm, the short-wavelength end of the A band, in order to assess the contribution of transitions to the 1Q1 state to the absorption profile. The results presented here show that I(2P1/2) is the primary iodine–atom product and that the transition is largely parallel [β=(1.6±0.1) for I(2P1/2)] at 229.4 nm. These data together with previous photofragmentation results suggest that excitation to the 3Q0 state dominates the entire A band absorption profile.
Wave packet study of gas phase atom–rigid rotor scattering
Authors: Sun Yan , Kouri Donald J.
Published: July 2002

The close coupling wave packet (CCWP) method has recently been extended to treat gas phase atom–diatom collisions. The total angular momentum representation reduces the complexity of the coupled wave packet equations. In this paper, the theory is extended by modifying the form of the initial packet so that, even though the (JjlM) representation is used, a single wave packet propagation provides complete information for scattering out of a particular  j, mj initial rotor state with total angular momentum J. We present results of further testing the method using the Lester–Bernstein model atom–rigid rotor system for various numbers of coupled channels N, including N=25, 64, 144, 256, 969. The results for 969 channels show clearly the transition from the ‘‘l-dominant’’ regime at lower energies, where the scattering is dominated by the long-range attraction, to a more sudden regime at higher energies, where the scattering is dominated by the short-range repulsive interaction. The dependence on the final orbital angular momentum at higher energy is interpreted in terms of orbital angular momentum rainbow scattering. The results are very encouraging indicating that the wave packet method can treat gas phase collisions involving very large numbers of quantum states.
On electrostatic rigidity of polyelectrolytes
Authors: Qian Caibao , Kholodenko Arkady L.
Published: July 2002

A microscopic nature of electrostatic rigidity of polyelectrolyte chain is investigated with use of Feynman variational method analogous to that used in the theory of large polaron. The obtained dependence of the rigidity on the ionic strength is in satisfactory agreement with the available experimental data for both high and low ionic strength of polyelectrolyte solution containing counterions.
Energy transfer from highly vibrationally excited azulene and azulene-d8 to carbon dioxide
Authors: Jalenak Wayne , Weston Ralph E. , Sears Trevor J. , Flynn George W.
Published: July 2002

Time-resolved diode laser absorption spectroscopy has been used to study vibrational-to-vibrational energy transfer from highly excited azulene and azulene-d8 to carbon dioxide. The energetic azulene molecules were prepared by internal conversion (S2→S0) after excitation into the S2 state by a pulsed laser source of 337 nm photons. Relative populations of CO2 vibrational modes were determined and compared with predictions of a statistical model. The total vibrational energy transferred to CO2 was determined to be about 25% of the initial azulene energy. Similar experiments were carried out using 248 nm photons to excite azulene in the region of the S4 state.
Fourier transform spectroscopy of the ν3 band of the N3 radical
Authors: Brazier C. R. , Bernath P. F. , Burkholder James B. , Howard Carleton J.
Published: July 2002

We have analyzed the ν3 antisymmetric stretching fundamental of the N3 radical. Azide radicals N3 were generated in a multipass cell by the reaction of Cl atoms and HN3 and detected in absorption using a Fourier transform spectrometer. Improved constants for the 000 vibrational level of X̃ 2Πg are reported along with observations of the 001 level. The frequency of the ν3 fundamental at 1645 cm−1 was found to be somewhat lower than expected.
Short-time diffusion coefficients and high frequency viscosity of dilute suspensions of spherical Brownian particles
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

We study the short-time collective and self-diffusion coefficients, the rotational self-diffusion coefficient, and the high frequency effective viscosity of a suspension of spherical Brownian particles. At low volume fraction these transport coefficients are given by integrals of hydrodynamic pair interactions. Using exact series expansions in powers of the inverse distance between centers of a pair of particles, we obtain accurate values for the low density transport coefficients. We consider hard spheres with mixed slip–stick boundary conditions, as well as spherical liquid droplets with an internal viscosity different from the bulk. We show that the transport coefficients depend strongly on the particle model.
Ionic dynamics in computer simulated molten LiNO3. II. Tumbling and spinning motions of nitrate ions
Authors: Katō Toshiko , Machida Katsunosuke , Oobatake Motohisa , Hayashi Soichi
Published: July 2002

The microdynamics of orientational motion (the tumbling of the symmetry axis and the spinning about the axis) of nitrate ions has been investigated in a molecular dynamics simulation of molten LiNO3. Simple Coulomb potential with Born type repulsion has been adopted and all degrees of freedom of vibrating nitrate ions have been included in the simulation. The relaxation of the orientational correlation functions is studied in the cumulant expansion scheme, and a correlation function which reflects only the spinning motion is separated by the method of Lynden-Bell et al. It is found that the angular momentum correlation function for the perpendicular component C⊥J(t) has a pronounced negative portion, which is clear evidence of librations in a temporary solvent cage. C∥J(t) has a shallow negative lobe. The spinning motion is less hindered and the reorientation is faster. Both the tumbling and the spinning motions are found to be predominantly Markovian. The difference between the tumbling and the spinning motions is examined from the magnitude of the torque impulse as compared to the average angular momentum. The coupling between the environmental structure and the rotational behavior of a nitrate ion has been examined through selectively sampled correlation functions. It is found that the rotational diffusion caused by the tumbling motion is slow and that by the spinning motion is fast if the coordination number remains low.
Tunable far-infrared laser spectroscopy of hydrogen bonds: The K
a =0(u)→1(g) rotation–tunneling spectrum of the HCl dimer
Authors: Blake Geoffrey A. , Busarow Kerry L. , Cohen R. C. , Laughlin K. B. , Lee Y. T. , Saykally R. J.
Published: July 2002

The ground state Ka =0(u)→1(g) b-type subband of the rotation–tunneling spectrum of the symmetric 35Cl–35Cl, 37Cl–37Cl, and the mixed 35Cl–37Cl hydrogen chloride dimers have been recorded near 26.3 cm−1 with sub-Doppler resolution in a continuous two-dimensional supersonic jet with a tunable far-infrared laser spectrometer. Quadrupole hyperfine structure from the chlorine nuclei has been resolved. From the fitted rotational constants a (H35Cl)2 center-of-mass separation of 3.81 Å is derived for the Ka =1(g) levels, while the nuclear quadrupole coupling constants yield a vibrationally averaged angular structure for both tunneling states of approximately 20–25 deg for the hydrogen bonded proton and at least 70–75 deg for the external proton. This nearly orthogonal structure agrees well with that predicted by ab initio theoretical calculations, but the observed splittings and intensity alterations of the lines indicate that the chlorine nuclei are made equivalent by a large amplitude tunneling motion of the HCl monomers. A similar geared internal rotation tunneling motion has been found for the HF dimer, but here the effect is much greater. The ground state tunneling splittings are estimated to lie between 15–18 cm−1, and the selection rules observed indicate that the trans tunneling path dominates the large amplitude motion, as expected, provided the dimer remains planar. From the observed hyperfine constants, we judge the dimer and its associated tunneling motion to be planar to within 10°.
Time-independent adiabatic switching: Nonintegrable classical systems
Authors: Jaffé Charles , Watanabe Masakatsu
Published: July 2002

We present a modification of the time-independent adiabatic switching algorithm which is applicable to nonintegrable Hamilton systems. This algorithm transforms the Hamiltonian into a generalization of the Birkoff–Gustavson normal form. We discuss in detail the difficulties which occur when singular motions are encountered and explore the dynamical nature of the adiabatically switched tori using traditional methods of classical mechanics. A quantization scheme based upon this algorithm is proposed and numerical results, i.e., the semiclassical energies, are reported for a simple model of water.
Reaction paths for the dissociation ã 3A″ CH2CO→X̃ 3B1 CH2 + X̃ 1Σ+ CO
Authors: Allen Wesley D. , Schaefer Henry F.
Published: July 2002

An ab initio investigation of the (CIIs) in-plane bent 3A″ CH2CO→X̃ 3B1 CH2+X̃ 1∑+CO and the (CIs) out-of-plane bent 3A′ CH2CO→X̃ 3B1 CH2+X̃ 1∑+CO dissociation paths has been performed. Geometrical structures, vibrational frequencies, and quadratic force constants have been determined at the DZP SCF and DZP CISD levels of theory for the X̃ 1A1, 3A″, and 3A′ states of ketene and for the 3A″ and 3A′ transition states for dissociation. The DZP CISD structure for Ã 1A″ ketene is also reported. Final energetic predictions for triplet ketene dissociation have been obtained from large-basis (QZ2P and QZ2P+f) UMP4(SDTQ) calculations at the DZP CISD geometries. The CIIs stationary point for 3A″ ketene dissociation is a true transition state with r(C–C)=2.071 Å at the DZP CISD level of theory. The corresponding CIs stationary point for 3A′ ketene is actually a super transition state for the interconversion of two equivalent 3A″CIIs transition states for dissociation. Final theoretical predictions of Te=19 400 cm−1 and T0=19 150 cm−1 are made for the adiabatic excitation energy of the ã 3A″ state of ketene, and a value of 22.3 kcal/mol is proposed for the 3A″ dissociation energy.
Direct measurements of vibrational predissociation of p-difluorobenzene–argon
Authors: Jacobson Benjamin A. , Humphrey Stephen , Rice Stuart A.
Published: July 2002

We have measured rates of vibrational predissociation from 12 vibrational levels of S1  p-difluorobenzene–argon (pDFB–Ar) by time-resolved two-color multiphoton ionization. Three features of our results are particularly surprising. First, the vibrational predissociation rate does not necessarily increase with additional quanta in a given ring vibrational mode. Second, direct excitation of the presumed reaction coordinate, the van der Waals stretch, may decrease the vibrational predissociation rate. Third, pDFB mode 8 (mode 16a in the Wilson notation) shows a large vibrational frequency shift upon complexation, as in other aromatic-argon complexes, but in pDFB it is at most weakly active in vibrational predissociation, in striking contrast to the other systems. These results are inconsistent with available simple theories of vibrational predissociation. We also find that the vibrational predissociation lifetime exceeds 2.7 ns even at 1634 cm−1 excess vibrational energy, where rapid vibrational energy redistribution has been predicted. Where direct comparison is possible, the rates we measure are about 40% lower than those obtained in time-integrated experiments by Parmenter and co-workers.
Theoretical study of the interaction of Fe, Fe+, and FeCO with Ar
Authors: Braga M. , Almeida A. L. , Taft C. A. , Hammond B. L. , Lester W. A.
Published: July 2002

A b initio Hartree–Fock calculations were performed on FeAr and FeAr+ in order to determine the interaction of both neutral and singly-ionized Fe atoms trapped in Ar, and on ArFeCO and FeCOAr in order to ascertain the effect of an Ar matrix on the FeCO molecule. Quadrupole splittings and isomer shifts are computed using ab initio orbital populations and charge densities scaled by a relativistic factor. Good agreement between calculated and experimental hyperfine parameters is found in each case. The interaction energy for Fe+–Ar has been computed by ab initio Hartree–Fock and effective-core potential (ECP) methods. Both approaches yield a minimum between Fe+ and Ar at approximately 7.6 bohr.
Studies of chain conformational kinetics in poly(di-n-alkylsilanes) by spectroscopic methods. III. Fourier transform Raman studies of unsymmetric side chain substitution
Authors: Hallmark V. M. , Sooriyakumaran R. , Miller R. D. , Rabolt J. F.
Published: July 2002

Fourier transform Raman (FT-Raman) spectroscopy has been used to acquire spectra of polysilane derivatives in the absence of resonance effects arising from electronic absorptions of these materials between 320 and 370 nm. The nonresonant FT-Raman spectra allow detailed analysis of the conformation of and lateral interactions between alkyl side groups. Comparison of spectra for the series of unsymmetrically substituted compounds poly(n-pentyl,n-hexylsilane), poly(n-heptyl,n-hexylsilane), and poly(n-octyl,n-hexylsilane) with the symmetrically substituted analog poly(di-n-hexylsilane) reveals various degrees of intra- and intermolecular disorder due to mismatch of side chain length. The induced disorder is well correlated with transition temperatures and UV-vis absorption maxima.
Phase stability of dense charged hard sphere fluid mixtures
Authors: Caccamo C. , Malescio G.
Published: July 2002

The mean spherical approximation (MSA), the hypernetted-chain (HNC) approximation, and Monte Carlo (MC) calculations are used in order to investigate the phase stability of dense charged hard sphere fluid mixtures. According to the MSA, spinodal decomposition occurs in these systems in a certain range of the physical parameters. The HNC results indicate the existence of a spinodal line in qualitative agreement with the MSA prediction. Local compositions obtained by MC calculations show the expected behavior in a mixture approaching segregation. The concentrations of the coexisting phases are also calculated within the MSA, and the characteristics of the phase diagrams of different types of mixture are discussed. A detailed analysis is then made of the thermodynamic quantities which define the Gibbs free energy of mixing; this allows one to understand in more detail the segregation process. Finally, the applicability of the present approach for studying the phase stability conditions of molten salt mixtures and ionic micellar solutions is discussed.
Vibration–rotation interaction in the rigid bender: A quantum mechanical phase space view
Authors: Martens Craig C.
Published: July 2002

In this paper, we examine classical–quantum correspondence in a system with strong vibration–rotation interaction. We study the quantum mechanics of a two degree of freedom rigid bender Hamiltonian, previously considered in the context of classical mechanics by Ezra [Chem. Phys. Lett. 127, 492 (1986)] and by Frederick and McClelland [J. Chem. Phys. 84, 4347 (1986)]. Eigenfunctions and eigenvalues of this system are calculated by matrix diagonalization in a harmonic oscillator–rigid rotor basis. The quantum mechanics are compared with the classical mechanics by visualizing the quantum eigenstates on a surface of section, defined in terms of the coherent states of the rotor and oscillator. We find clear connections between eigenstate structure on the quantum surface of section and features of the classical phase space, such as nonlinear resonance, period doubling, and chaos.
Solvent dynamical effects on nonadiabatic electron transfer reactions at low temperature
Authors: Morillo M. , Yang D. Y. , Cukier R. I.
Published: July 2002

The role of solvent dynamics on low temperature nonadiabatic electron transfer reactions is explored. The solvent degrees of freedom orthogonal to the nuclear motion reaction coordinate are represented by a frictional term. Thus, motion along the reaction coordinate is described by a damped quantum oscillator equation of motion. This equation of motion is used to construct the nonadiabatic electron transfer rate constant which describes long range electron transfer phenomena such as occurs in biological oxidation-reduction reactions. The frictional dynamics are correctly described even for temperatures lower than the characteristic frequencies of the reaction coordinate and the friction. We exhibit the effects that friction can have on the electron transfer rate from room temperature down to 4 K, and qualitatively compare with typical biological electron transfer data, as interpreted using the conventional zero friction theory.
A classical trajectory study of the photodissociation spectrum of H+3
Authors: Gomez Llorente Jose M. , Pollak Eli
Published: July 2002

The photodissociation spectrum of H+3 is studied using classical mechanical methods. Tunneling rates and product translational energies are computed for a large range of total angular momentum and energy. We predict that the experimentally measured spectrum of Carrington and Kennedy is dominated by low total angular momentum and low energy (relative to dissociation). There is an almost one to one correspondence between the measured product translational energy and the total angular momentum. The classical dipole spectrum of chaotic trajectories is found to be relatively structureless, changes slowly with total J, and does not show any correspondence or indication of the experimentally measured regular structure found in the coarse grained spectrum. We conclude that the regularity found in the coarse grained spectrum should be associated with a stable manifold of trajectories. We find that the horseshoe periodic orbit previously found to be stable at J=0 exists also for nonzero J and is stable with respect to small perturbations in 3D. The rotational constant of the rotating horseshoe is 30 cm−1 in interesting agreement with the experiment. The properties of the rotating horseshoe are studied in detail, a novel adiabatic switching method is used to study the stability of the orbit. A quantum formalism of Taylor and Zakrzewski that shows how periodic orbits may cause structure in quantal spectra is used to indicate why the features of the rotating horseshoe orbit may appear in the coarse grained spectrum. The experimental coarse grained features are interpreted as an R branch of the ν3 mode of the rotating horseshoe.
A photoionization study of PH: PH2 revisited
Authors: Berkowitz J. , Cho H.
Published: July 2002

The photoion yield curve of PH+ (PH) is presented, from threshold to 1040 Å. The adiabatic ionization potential of PH is 10.149±0.008 eV. Extensive autoionization structure is observed and analyzed. It is attributed to several Rydberg series, whose mutual convergence limit is 11.852±0.002 eV, and corresponds to the onset of PH+ (a 4Σ−). The photoion yield curve of AsH+ (AsH) is juxtaposed, and shown to have a similar pattern. A new photoion yield curve of PH+2 (PH2) is shown, where the source of PH2 is the H+PH3 reaction. The new results corroborate the earlier data (based on the pyrolysis of benzylphosphine) regarding the adiabatic ionization potential of PH2 to form X̃ 1A1, and the presence of broad autoionizing structure. They also display less scatter, and enable one to estimate the onset for ã 3B1 to be about 0.70 eV above X̃ 1A1. From the analogous behavior of the AsH+2 (AsH2) curve, the a 3B1–X 1A1 splitting in AsH+2 is estimated to be 0.58–0.68 eV.
Integral equations for some correlated percolation models
Authors: Given James A.
Published: July 2002

The properties of the continuum Potts model are used to derive integral equations for the properties of correlated percolation. Specifically, extended Born–Greeen–Yvon (BGY) equations are derived for the two-point connectedness function in continuum percolation systems. Two different types of correlations among the percolating elements are considered: those due to a two-body potential and those due to impenetrable inclusions occupying a fraction of the system volume. In both cases, the superposition approximation is derived and solved numerically. The variation of the percolation threshold with increasing correlation is calculated and discussed. Finally, higher order corrections to the superposition approximation are obtained by analyzing the relevant Mayer series.
Nondipole light scattering by partially oriented ensembles. I. Numerical calculations and symmetries
Authors: Tian Duan , McClain W. M.
Published: July 2002

We consider the elastic scattering of light by an ensemble of scatterers with radius of gyration greater than about one-tenth of a wave; e.g., visible light scattered by an aqueous suspension of viruses or bacteria. We model the scattering molecule as an asymmetric rigid array of interacting point polarizabilities, and we include, as the source of anisotropy in the scattering ensemble, a permanent dipole moment on the molecule and a uniform electric field E° in the scattering cell. We calculate the entire Müller matrix M(θ, E○x, E○y, E○z), for scattering angles θ from 0° to 360° and for E○x, E○y, E○z nonzero one at a time, where z is the incidence axis, y is perpendicular to the scattering plane, and x is perpendicular to y and z. Our first major finding is that the nondipole elements of M are enormously more sensitive to partial orientation than are the dipole elements. The second major finding is a set of new symmetries governing the scattering matrix for the case of axially anisotropic scattering clouds. The Perrin reciprocity symmetries for isotropic clouds may be symbolized by M(θ)=PM(θ), where P stands for matrix transpose plus negation of third row and column (with double negation of element 3,3). Using this operator our new axial symmetries may be expressed as PM(+θ, 0, E○y, 0)=M(+θ, 0, −E○y,0)=M(−θ, 0, E○y, 0). The second symmetry may be generalized to fields in any direction as M(+θ,−E○x, −E○y, E○z)= M(−θ, E○x, E○y, E○z). We also show in Appendix A how the nonlocal polarizabilities used in the theory may be calculated by the inversion only of symmetric matrices, with a significant saving in calculation time when the number of subunits is large.
Rydberg–Klein–Rees inversion of high resolution van der Waals infrared spectra: An intermolecular potential energy surface for Ar+HF (v=1)
Authors: Nesbitt David J. , Child Mark S. , Clary David C.
Published: July 2002

A method is described for extraction of two-dimensional (angular and radial) potential energy surfaces for triatomic rare gas–hydrogen halide van der Waals complexes. The approach relies on extensive J rotational term values obtained by high resolution infrared laser jet spectroscopy for a family of bending vibrational states to deduce the radial and angular dependence of the intermolecular potential. First, effective 1D radial potentials for a series of bending states are obtained by rotational RKR analysis of experimentally observed rotational progressions. These 1D potentials, which represent vibrational averages over different bending wave functions, are then inverted to determine the radially dependent coefficients of a Legendre expansion to the full surface, i.e., ∑lVl(R)Pl (cos θ). This relies on adiabatic angular motion with respect to radial degrees of freedom, the validity of which is discussed. This approach is tested with experimental data from the (100 0) (fundamental), (120 0) (HF parallel bend), and (111 0) (HF perpendicular bend) of Ar+HF (v=1), excited up to J≤25. The accuracy of the resulting surface is verified by exact quantum bound state calculations which quantitatively reproduce the rovibrational input data, as well as predict the spectroscopic properties of five other vibrational states observed in the Ar+HF (v=1) system but not used in the fitting procedure.
Carbon monoxide–oxygen interaction on the Pt(111) surface: An electron stimulated desorption ion angular distribution (ESDIAD) study
Authors: Szabó A. , Kiskinova M. , Yates J. T.
Published: July 2002

CO adsorption on the p(2×2)O–Pt(111) surface was studied by the digital ESDIAD (electron stimulated desorption ion angular distribution) method in combination with TPD, LEED and work function measurements. Three ESD products were detected: CO+, O+ and metastable CO. The ESDIAD patterns of each of these species were measured. The most significant difference in the ESD behavior of chemisorbed CO on the oxygen-covered surface from that of CO adsorbed on clean platinum surface was found at low CO coverages. This indicates that there is no preferential adsorption on the surface sites nonaffected by oxygen. A small tilting of CO was found.
Overtone spectroscopy of propyne and propyne-d1
Authors: Baylor Lewis C. , Weitz Eric , Hofmann P.
Published: July 2002

The vibrational overtone spectra of the acetylenic and methyl C–H stretches of propyne were obtained for the v=1 to v=6 and v=1 to v=7 levels, respectively. Propyne-dl was also studied and the methyl C–H stretching overtones were measured from v=1 to the v=7 level. The C–D stretch was observed only in the fundamental and first overtone regions. Lower level overtones were obtained by standard infrared techniques, while higher absorptions (>12 000 cm−1) were obtained by intracavity dye laser photoacoustic spectroscopy. The C–H stretches in both molecules were analyzed in terms of the local-mode model, and harmonic frequencies (ωi) and anharmonicities (Xii) were calculated. In propyne these values were (acetylenic C–H stretch) ω1=3384±5 cm−1 and X11=−50±1 cm−1 and (methyl C–H stretch) ωm =3037±5 cm−1 and Xmm =−65±2 cm−1. In propyne-dl the methyl C–H stretch parameters were ωm =3034±5 cm−1 and Xmm =−64±2 cm−1. For propyne, a hot band (ν9→ν9+vν1) accompanying the acetylenic C–H stretch was observed for v=1–6 and the anharmonic interaction constant (X19=−23±7 cm−1) was calculated. A crossover from normal- to local-mode behavior has been observed for the methyl C–H stretches in propyne and propyne-dl at the v=3 and 4 levels. Below v=3 the symmetric and antisymmetric methyl C–H stretches are designated by the usual normal-mode notation (ν2 and ν6 in propyne; ν1 and ν6 in propyne-dl), while for v≥3 the single observed band is designated as a ‘‘methyl’’ C–H stretch, vνm. Peak absorption cross sections have been measured for Δv1=1–5, Δvm =3–5, Δv2=1 and 2, and the parallel component of 2ν6 in propyne, and for Δv2=1 and 2, Δv1=1 and 2, Δvm =3 and 4, and the parallel component of 2ν6 in propyne-dl. During the course of this work the spectral constants of 3,3,3-trifluoropropyne were redetermined. The harmonic frequency ω1 is 3376±6 cm−1, the anharmonicity

X 1 1 is −49±1 cm−1, and the anharmonic interaction constant X17 is −17±6 cm−1.
Analytical approach to molecular liquids. IV. Solvation dynamics and electron-transfer reactions
Authors: Zhou Yaoqi , Friedman Harold L. , Stell George
Published: July 2002

The solvation dynamics of a pair of ions rigidly held a fixed distance from each other in either a dipolar hard-sphere solvent or a dipolar dumbbell solvent is investigated under the mean spherical approximation (MSA) applied to an interaction-site model in which the pair of ions is treated as a rigid extended dipolar dumbbell. An analytical expression for the reorganization free energy for a charge transfer reaction is obtained and compared with numerical results obtained via a thermodynamic cycle. Solvent effects on the rate of a charge transfer reaction are also investigated using the dielectric Green-function theory developed by Newton and Friedman.
Dynamical Bragg diffraction from crystalline colloidal arrays
Authors: Rundquist Paul A. , Photinos Panos , Jagannathan Seshadri , Asher Sanford A.
Published: July 2002

Polystyrene spheres with attached functional groups that ionize in solution repel one another; at sufficiently high sphere concentrations the spheres self-assemble into a crystalline lattice with lattice constants large enough to diffract visible light. We have experimentally and theoretically examined diffraction phenomena from colloidal crystals of polystyrene spheres of diameters between 69 and 127 nm in water. We relate the diffraction bandwidths to the sphere scattering powers in the context of the dynamical diffraction theory and demonstrate the importance of the dynamical theory for predicting the observed diffraction angles, intensities, and bandwidths. We also discuss the mechanism contributing to the diffuse scattering and show the significance of the coherent scattering by lattice phonons.
Dissociation dynamics of the d 1Πg and C 3Πg v=0–2 vibrational states in O2
Authors: van der Zande Wim J. , Koot Wouter , Los J.
Published: July 2002

Using translational spectroscopy we have studied the d 1Πg and C 3 Πg Rydberg states of O2 . The dissociation of the vibrational levels v=0–2 to all energetically accessible dissociation limits has been followed. The dissociation pathways directly reflect the predissociation mechanisms involved. For the d 1 Πg Rydberg state competition between predissociation by a 3 Πg valence state, due to singlet–triplet mixing, and by a 1 Πg valence state has been observed. Using the Fermi golden rule the observed vibrationally dependent competition is reproduced, corroborating the positions of the lower 1 Πg and 3 Πg valence state curves and yielding various coupling strengths.
Matrix isolation nuclear magnetic resonance studies of methyl group spin–rotation coupling
Authors: Murphy Michael , White David
Published: July 2002

In dilute rare gas matrices containing acetonitrile or methyl acetylene, the methyl group rotates almost freely about a fixed symmetry axis in a large fraction of the matrix trapping sites. The secular spin–rotation interaction of the methyl protons does not vanish in this case, and we show this leads to resolved, temperature dependent splitting of the proton NMR lines. In addition, ordering of the methyl spins with respect to both spin–rotation and dipolar local magnetic fields occurs after a sudden change in the matrix temperature. This is shown to be consistent with the mechanism of the Haupt effect, a spin conversion process which restores rotational equilibrium after the temperature jump. Since the spin–rotation splitting provides a direct measure of the methyl group angular momentum, dynamical information pertaining to the matrix isolated molecule is obtained from the proton spectrum, providing a considerable simplification over spin relaxation measurements. Using a 0.5% CH3CN/Kr matrix as a prototypal system, the experimental observations are found to be consistent with the proposed form of the spin–rotation interaction. Several conclusions are reached regarding the hindering of methyl rotation by the solid rare gas lattice, dynamics in the trapping site, and the distribution of these sites in the matrix.
Jet-resolved vibronic structure in the higher excited states of N2O: Ultraviolet three-photon absorption spectroscopy from 80 000 to 90 000 cm−1
Authors: Patsilinakou E. , Wiedmann R. T. , Fotakis C. , Grant E. R.
Published: July 2002

Ionization-detected ultraviolet multiphoton absorption spectroscopy reveals Rydberg structure in the excited states of N2O within 20 000 cm−1 of the first ionization threshold. This structure persists, with atomic-like quantum defects and vibrational structure well matched with that of the ion, despite evidence for coupling of vibrationally excited Rydberg states with the underlying valence continuum. In the most completely resolved spectrum, which is assigned to the 3pσ 1Π state, hot-band, fundamental and overtone transitions involving the bending mode ν2, indicate Renner–Teller and Herzberg–Teller coupling of electronic and vibrational angular momentum. Vibronic intensities and positions suggest that these couplings can be regarded as properties of the N2O+ ion core, mirroring behavior manifested in the electronic emission spectrum of the ion. Window resonances are observed in the ionization-detected absorption spectrum above the four-photon ionization threshold, which are assigned to vibrationally excited Rydberg states that couple to competing continuua in which predissociation dominates Δv=−1 vibrational autoionization.
Evaluation of the rate constant for the reaction OH+H2CO: Application of modeling and sensitivity analysis techniques for determination of the product branching ratio
Authors: Yetter Richard A. , Rabitz Herschel , Dryer Frederick L. , Maki Robert G. , Klemm R. Bruce
Published: July 2002

Novel modeling and sensitivity analysis techniques are used with experimental data obtained from discharge flow–resonance fluorescence experiments to evaluate the product branching ratio of OH+H2CO. Two channels are considered: the H-atom abstraction reaction (R2) to form HCO and H2O; and the addition reaction (R17) followed by rearrangement and decomposition to form HCOOH and H. The rate constant values obtained at 298 K are kR2 =(7.75±1.24)×10−12 cm3/molecule s and kR17=(0.2+0.8−0.2) ×10−12 cm3/molecule s. The results demonstrate that the reaction proceeds almost exclusively via the H-atom abstraction pathway.
Theoretical characterization of the lowest three potential surfaces of HNO. I. The potential for H atom addition to NO
Authors: Walch Stephen P. , Rohlfing Celeste McMichael
Published: July 2002

The results of complete active space SCF/multireference contracted CI(CASSCF/CCI) calculations with large atomic natural orbital (ANO) basis sets are presented for the H + NO region of the lowest three potential surfaces of HNO (1A′, 3A″, and 1A″). The calculations focus on the minimum energy path for H atom addition to the N end of NO and on the equilibrium geometry region of HNO and HON.
Analysis of complex molecular dynamics in an organic liquid by polarization selective subpicosecond transient grating experiments
Authors: Deeg F. W. , Fayer M. D.
Published: July 2002

We present a subpicosecond time resolved polarization selective transient grating (TG) investigation of pentylcyanobiphenyl (5CB) in its isotropic liquid phase. This system shows complex molecular dynamics with various contributions to the TG signal. With an excitation wavelength λexc=665 nm one induces an anisotropy in the sample from the electronic and nuclear Kerr effects. Exciting with λexc=575 nm generates an excited state grating because of two photon absorption. Solvent relaxation around the excited molecule on a few ps time scale is observed because it spectrally shifts the excited state–excited state transition which is monitored by the TG signal. In addition, radiationless relaxation leads to local heating of the sample in a spatial pattern which mimics the optical interference pattern which established the TG. The heating generates an acoustic standing wave. It is demonstrated that with a single TG setup, polarization selection allows separation and identification of all of the above-mentioned physical processes as they are characterized by different symmetries of the nonlinear susceptibility tensor χ(3).
Decoupling of crystalline and conformational degrees of freedom in lipid monolayers
Authors: Ipsen John Hjort , Mouritsen Ole G. , Zuckermann Martin J.
Published: July 2002

A theoretical study is performed on a microscopic interaction model which describes the transitions between liquid and solid phases of lipid monolayers spread on air/water interfaces. The model accounts for condensation in terms of acyl-chain conformational degrees of freedom as well as in terms of variables which describe the orientations of crystalline domains in the solid. The phase behavior of the model as a function of temperature and lateral pressure is explored using mean-field theory and computer-simulation techniques. Attention is paid to the particular interplay between the two types of condensation processes and effects on the phase behavior due to decoupling of crystalline and conformational order parameters. In the case of decoupling, the model predicts that the high-pressure solid-conformationally ordered phase is separated from the low-pressure liquid-conformationally disordered phase by a liquid-conformationally ordered phase. This prediction is consistent with synchrotron x-ray experiments which show that the chain-ordering transition and the crystallization process need not take place at the same lateral pressure. A characterization is provided of the nonequilibrium effects and pattern-formation processes observed along the isotherms in the phase diagram spanned by lateral pressure and area. A description is given of the kinetics of the nonequilibrium phase transitions and the concomitant heterogeneous microstructure of the monolayer. This leads to an explanation of the peculiarities of the experimentally observed isotherms of lipid monolayer phase behavior. It is pointed out that cholesterol, which promotes lipid-chain conformational order, has a unique capacity of acting as a ‘crystal breaker’ in the solid monolayer phases and therefore provides a molecular mechanism for decoupling crystalline and conformational order in lipid monolayers containing cholesterol. The phase diagram of mixed cholesterol–lipid monolayers is derived and discussed in relation to monolayer experiments.
Approach to the tricritical point in bimodal polymer solutions
Authors: Broseta Daniel
Published: July 2002

The approach to the tricritical point in bimodal polymer solutions is analyzed in the (mean-field) Flory–Huggins approximation. The parameter governing the thermodynamic properties of the system is the ratio of the molecular weights of the two polymer homologues. When this ratio exceeds a certain value rt, a separation in three phases is possible. The different dimensions of the three phase region vary with r as simple characteristic powers of the distance to this tricritical ratio (r−rt). Comparison with experiments reveals some shortcomings of the Flory–Huggins approximation.
Equations of state and gas–gas separation in soft-sphere mixtures
Authors: Kerley G. I.
Published: July 2002

The Monte Carlo method is used to compute equations of state for mixtures having intermolecular pair potentials of the form u(r)=ε(σ/r)n, where n=12 and n=6. Calculations are given for binary equimolar mixtures with additive diameters having the ratio σ1/σ2=3. In contrast to hard sphere mixtures, it is found that soft-sphere mixtures have a positive volume of mixing at high pressures. For the mixture with n=6, this effect leads to the separation of two fluid phases at a pressure that is below the freezing point of the larger component. These results show that, under certain conditions, large differences in molecular size are sufficient to cause this type of phase separation. The ideal mixing and van der Waals one-fluid (vdW1) theories are also discussed, and the predictions of these two models are compared with the Monte Carlo data. The vdW1 approximation gives satisfactory results at low pressures but is poorer than ideal mixing at high pressures.