bluuebunny/crossref_metadata_2025 huggingface dataset:
Anisotropic bead models for molecular hydrodynamics
Authors: Pastor Richard W. , Zwanzig Robert
Published: July 2002

Some qualitative effects of slip hydrodynamic boundary conditions can be incorporated into bead models by replacing the scalar friction constant of a bead with a friction tensor. Translational and rotational diffusion coefficients are calculated analytically and exactly for polygons, analytically but approximately for spherical shells, and numerically by the method of Bloomfield and de la Torre. The calculated diffusion constants of benzene agree with experiment; such agreement is not possible using scalar friction constants. Some comments are made about the transition from overall slip to stick hydrodynamic behavior when tensor friction constants are used.
Predissociation spectroscopy of cadmium dimethyl
Authors: Amirav Aviv , Penner Abraham , Bersohn Richard
Published: July 2002

The absorption and fluorescence excitation spectra of jet cooled Cd(CH3)2 and Cd(CD3)2 were measured in the structured region between 230 and 200 nm. The symmetric Cd–methyl stretch and methyl umbrella modes in the second excited electronic state of the latter molecule were found to the 304 and 790 cm−1, respectively. Widths of the 0–0 lines (FWHM) were 101 and 25 cm−1, respectively from which lifetimes of 53 and 265 fs were deduced. A weak fluorescence due to CdCH3 is excited by the absorption of Cd(CH3)2 and an even weaker fluorescence is excited in Cd(CD3)2. The quantum yield of this fluorescence is below 10−4 at the electronic origin of the structured band and increases to about 10−3 above 2000 cm−1 vibrational energy.
A reevaluation of the viscosity exponent for binary mixtures near the consolute point
Authors: Nieuwoudt J. C. , Sengers J. V.
Published: July 2002

A reevaluation of the critical exponent of the viscosity anomaly for binary mixtures close to the consolute point is presented. Using published data we make a systematic, consistent study in which corrections due to shear gradients and frequency effects are included and the effect of the functional form for the background viscosity is critically examined. The result is that the value of the exponent is generally found to be larger than reported and larger than the most recent theoretical predictions.
A fast Fourier transform method for quasiclassical selection of initial coordinates and momenta for rotating diatoms
Authors: Eaker Charles W.
Published: July 2002

This paper describes a new fast Fourier transform technique to determine the transformations from action-angle variables to Cartesian coordinates for a rotating diatomic molecule. This allows one to obtain the initial values of the coordinates and momenta for a given v and J from properly weighted radial and angular distributions as required in quasiclassical trajectory calculations. This method offers a number of advantages over previous methods for obtaining initial values. It is very accurate, even for cases of high J. This method works equally well for all types of diatomic potentials. Results are reported for Morse potentials and extended Rydberg potentials.
Analytic energy third derivatives for paired-excited multiconfiguration self-consistent-field wave functions
Authors: Duran Miquel , Yamaguchi Yukio , Remington Richard B. , Osamura Yoshihiro , Schaefer Henry F.
Published: July 2002

A theoretical formalism and its implementation are described for the analytic evaluation of energy third derivatives for the paired-excited multiconfiguration self-consistent-field wave function. Cubic force constants and vibration–rotation interaction constants for H2O, CH2(3B1), CH2(1A1), HCN, and NH3 are reported for five different basis sets—from double zeta through triple zeta plus double polarization. These results are compared with those from single configuration SCF wave functions and with the available experimental data.
Theoretical study of the far-infrared A 3Σ−g–X 3Πu transition in Al2
Authors: Bauschlicher Charles W. , Langhoff Stephen R.
Published: July 2002

Theoretical Einstein coefficients for the A 3Σ−g→X 3Πu transition in Al2 are computed from ab initio potentials and electronic transition moments to aid spectroscopic identification of this transition in the far infrared.
Deuterium nuclear fusion at room temperature: A pertinent inequality on barrier penetration
Authors: Rosen Gerald
Published: July 2002

For a D2 molecule with linear vibrational energy E, it is shown that λ(E) in the WKB d–d fusion rate expression Λ(E)≂(5.7×108 s−1)[exp−λ(E)] satisfies a useful scaling inequality. It follows from this inequality that Λ(E)≳2.8×10−20 s−1 for a lattice-caged D2 with vibrational energy E=9.0 eV.
Infrared laser spectroscopy of the 21 and 221 bands of H2O+(X̃ 2B1)
Authors: Brown Philip R. , Davies Paul B. , Stickland Ross J.
Published: July 2002

Infrared laser absorption spectra of H2O+ have been recorded in an ac discharge using velocity modulation detection. Eighty-five lines of the 210 fundamental and 20 lines of the 221 hot band were used to derive rotation, spin-rotation, and distortion parameters for the ground state and the first two vibrationally excited states of the bending mode. The results are compared with those from analysis of the Ã–X̃ band system and from rotational and vibration–rotation laser spectra, as well as with recent ab initio results obtained using MRCI wave functions.
Vibrational spectra and potential energy surface for the ring bending and ring twisting of 5,6-dihydro-2H-thiopyran
Authors: Tecklenburg Mary M. J. , Villarreal John R. , Laane Jaan
Published: July 2002

5,6-Dihydro-2H-thiopyran, CH2CH2CH=CHCH2S, has been synthesized and its far-infrared and Raman spectra recorded. Two series of sharp bands were observed originating from 139 and 235 cm−1 in the infrared spectrum for the out-of-plane ring-bending and the ring-twisting vibrations, respectively. A detailed energy level diagram including numerous excited states was determined for the two coupled vibrations. The two-dimensional potential energy surface, which satisfactorily fits more than two dozen observed transitions, was calculated to be V=2.431×104 x41 −0.383×104x21 +2.258×104x42 −1.966×104 x22 +1.026×105x21 x22 , where x1 is the ring-bending coordinate and x2 is the ring-twisting coordinate. The minimum energy on the potential surface corresponds to a twisting angle of 37.8° (the half-chair conformation). The lowest energy bent (boat) conformation corresponds to a saddle point 4130 cm−1 above the twisted conformation on the potential energy surface. The results are compared to analogous molecules and to molecular mechanics calculations.
Photodissociation of Cr(CO)6 in a molecular beam at 248 nm: Observation of primary and secondary photoproducts by mass spectrometry
Authors: Tyndall George W. , Jackson Robert L.
Published: July 2002

Photodissociation of Cr(CO)6 induced by a KrF* excimer laser (248 nm) was examined under molecular beam conditions, using a quadrupole mass spectrometer with electron impact ionization (11–19 eV) to detect the products as their respective ions. The ions of all Cr(CO)x products (x=0–5) were produced upon photodissociation of Cr(CO)6 at laser fluences from 1 to 10 mJ cm−2. The quantitative behavior of the Cr(CO)+x product ion yields vs laser fluence as well as statistical rate calculations indicate that Cr(CO)4 is the major product formed upon single-photon dissociation of Cr(CO)6 under collisionless conditions at 248 nm. The additional Cr(CO)+x (x<4) product ions observed at low fluences are formed primarily by electron-impact fragmentation of Cr(CO)4 in the mass spectrometer ionizer. At laser fluences ≳5 mJ cm−2, secondary photodissociation of Cr(CO)4 becomes important. Cr(CO)2 is assigned as the major photoproduct formed via secondary photodissociation of Cr(CO)4.
Observation of fine-structure transitions in argon charge transfer at low energies using a crossed-beam technique
Authors: Howard Stephen L. , Rockwood Alan L. , Anderson Stephen G. , Futrell Jean H.
Published: July 2002

The crossed-beam method was used to investigate the charge–transfer reaction of Ar+(2P3/2) with Ar(1S0) as a function of collision energy over the range of 2–100 eV (center of mass). Both the exactly resonant channel and the endoergic fine-structure transition with ΔJ=1 were detected and the cross section ratio σ3/2→1/2/(σ3/2→1/2+σ3/2→3/2) was found first to increase with increasing collision energy to a maximum value of about 0.5 at 30 eV and then decrease at higher energy. The relative abundance of the fine-structure transition channel at low energy is much higher than that predicted by the accepted general theory for these processes. It is suggested that a curve-crossing mechanism not previously considered and the perturbation of trajectories at low energies resulting from the strongly bound Ar+2 intermediate may be responsible for the unexpected observations.
Aerodynamic focusing of heavy molecules in seeded supersonic jets
Authors: Fernandez de la Mora J. , Rosell-Llompart J.
Published: July 2002

The phenomenon of aerodynamic enrichment of heavy molecules seeded in supersonic free jets has been known since 1955. But its systematic exploitation in the generation of intensely focused molecular beams has been prevented by the lack of a quantitative and realistic explanation of the observed facts. Here, the aerodynamic focusing of CBr4, W(CO)6, and C2Cl6 molecules seeded in jets of He or H2 is studied experimentally, and found to be most singular under conditions similar to those known to produce sharply focused beams of microscopic spheres suspended in air jets. The gas mixture expands through thin-plate orifice into a vacuum chamber, forming a supersonic free jet. The spacial distribution of the heavy molecules in the jet is measured at varying distances L to the nozzle by scanning a thermocouple probe across a jet diameter. The probe is sufficiently small to interfere negligibly with the flow. The increment DV in the thermocouple voltage resulting from seeding the heavy gas on a given flow of He or H2 is seen to be a sensitive indicator of the local concentration of seed molecules in the jet. The following behavior is observed in terms of the same Stokes number or inertia parameter S that governs the simpler and better understood phenomenon of aerosol focusing. Below S=0.4 for H2 and S=0.2 for He, heavy molecule and aerosol beam widths are practically identical, and the boundary of the jet of heavy molecules is rather sharp. At higher values of S, aerosol beams show further reductions

in cross section, down to less than 10% of a nozzle throat diameter dn. In contrast, the measured heavy species minimal beam widths or waists at a distance L∼dn from the throat are around 0.5

d n and 0.35 dn for jets of He and H2, respectively. In units of dn, these widths are several times larger than expected from elementary considerations on the defocusing effects due to Brownian motion (of the order of the square root of the molecular mass ratio between light and heavy molecules). Nonetheless, the thin-plate orifice nozzle yields considerably more concentrated jets of heavy gases than previously seen, with far-field enrichment factors for the seed species close to 50 in the

case of H2 jets. This technique, thus, appears to provide a greatly

improved source for intense molecular beams. Aerodynamically focused beams have a sharp distribution of kinetic energies, being ideally suited for cross beam and beam surface studies. But they are not quite so optimal for spectroscopic studies because they require moderate source Reynolds numbers (of order 100), at which the heavy gas undergoes very little translational, rotational, or vibrational cooling.
Laser studies of the reactivity of SiH with the surface of a depositing film
Authors: Ho Pauline , Breiland William G. , Buss Richard J.
Published: July 2002

A new method for studying the interaction of radicals with the surface of a depositing film is presented. This method combines spatially resolved laser spectroscopy with molecular beam techniques and is demonstrated by measuring the state-resolved reactivity of SiH molecules with the surface of a depositing amorphous hydrogenated silicon film. SiH molecules from a silane glow discharge react at the surface with greater than 0.94 probability. The spatial distribution of the desorbing SiH is consistent with a cosine angular distribution. No dependence of reactivity on rotational state of the SiH was observed.
Triplet (T1) state and radical cation resonance Raman investigation of biphenyl derivatives
Authors: Buntinx G. , Poizat O.
Published: July 2002

The time-resolved Raman spectra of the first triplet state—in resonance with the Tn←T1 absorption at 370 nm—and of the radical cation transient—in resonance with the R+.° ←R+.* absorptions at 380 and 690 nm—are reported for biphenyl, fully deuterated biphenyl, p,p′-bitolyl (4,4′-dimethyl biphenyl), and m,m′-bitolyl (3,3′-dimethyl biphenyl). Complete, reliable vibrational assignments are proposed and a detailed discussion of the resonance Raman activity and enhancement is given based on simple molecular orbital considerations. This investigation confirms the planar, quinoidal character of the triplet and radical cation structures. The structural information available from the analysis of the resonance effects is consistent with the assignment of the 370 nm Tn ←T1 absorption and 690 nm R+.* ←R+. absorption to a φ6←φ3 transition (Dewar’s notation), which causes a decrease of the quinoidal distortion and, particularly, a reduction of the interring CC bond strength. The R+.* ←R+. absorption at 380 nm is ascribable to a φ7←φ6 transition, which leads to a reinforcement of the quinoidal distortion. These results are compared to those available for the biphenyl radical anion and for the radical cation and triplet state of the 4,4′-diamino derivative of biphenyl, N,N,N′,N′-tetramethylbenzidine (TMB).
Calculation of the gas–solid energy distribution from the surface potential energy of adsorption
Authors: Cortés Joaquin
Published: July 2002

The calculation of the energy distribution of a solid’s surface for a cleaved perfect crystal from the surface potential energy of adsorption is proposed. The calculation is made for argon adsorbed on the 100 face of potassium chloride, and for the physisorption of a monoatomic molecule on the 100 face of a simple cubic crystal. The same method is also used to simulate some simple cases of other forms of surface heterogeneity such as defects and impurities.
The inelastic scattering of 2Π [case (b)] molecules and an understanding of the differing Λ doublet propensities for molecules of π vs π3 orbital occupancy
Authors: Dagdigian Paul J. , Alexander Millard H. , Liu Kopin
Published: July 2002

The quantum formalism for the scattering of a diatomic molecule in a 2Π electronic state which is well described by Hund’s case (b) limit is investigated here. For a particular JFi→J′F′1 transition, quantum interference effects will lead to preferential population of one of the final state Λ doublet levels. The nonstatistical population of final state Λ doublet levels arises from an interference between terms in the expansion of the two electrostatic potential energy surfaces, of A′ and A″ reflection symmetry, which describe the interaction between a molecule in a Π electronic state and a closed-shell partner. The particular Λ doublet level preferred is opposite for molecules of π1 vs π3 electron occupancy. The physical origin of this reversal in the Λ doublet propensity is a direct reflection of the fact that for the former the A′ potential surface is more repulsive since the sole π electron lies in the triatomic plane in this case, whereas for molecules of π3 electron occupancy the A′ surface is less repulsive than the A″ surface since for the A′ surface only one of the three π electrons lies in the triatomic plane. The magnitude of these Λ doublet propensities is illustrated by calculated cross sections for the CH(X 2Π)–He system using the ab initio potential energy surfaces calculated by the Argonne theoretical group, and these cross sections are compared to those of the crossed molecular study of Liu and Macdonald [J. Chem. Phys. 91, xxxx (1989)]. A similar analysis is carried out for collisions of a molecule of π3 electron occupancy and is illustrated by inelastic collisions of OH(X2Π).
Comment on: ‘‘Mean field theory of nematic order for grafted rods’’
Authors: Moore Brian G.
Published: July 2002

The calculation of Halperin et al. [J. Chem. Phys. 86, 6550 (1987)] has been repeated, but with the orientational distribution function expressed as a sum of Legendre polynomials. The result is that the inflections in the curves of nematic order parameter and surface pressure vs surface concentration are no longer present.
Equilibrium distributions of alkanes at a fixed partial pressure of molecular hydrogen
Authors: Alberty Robert A.
Published: July 2002

In calculating chemical equilibrium distributions in gaseous systems containing hydrocarbons and molecular hydrogen, the degrees of freedom are usually taken to be T, P, and H/C, but there may be reasons for taking T, P, PC2H4 or T, P, PH2. The standard Gibbs energies of formation in homologous series are closely represented by A+Bn, where n is the carbon number and A and B depend only on the temperature. At a specified ethylene partial pressure, the equilibrium distribution within a homologous series depends only on B, and this note shows that at a specified hydrogen partial pressure, the equilibrium distribution depends only on A.
Hyperfine interactions and relaxivities in divalent and trivalent aquoion systems
Authors: Sahoo N. , Das T. P.
Published: July 2002

The electronic structures of aquoions involving the divalent paramagnetic ion Mn+2 and the two trivalent ions Fe+3 and Cr+3 have been studied using the unrestricted Hartree–Fock procedure, with six H2O molecules surrounding the metal ion. From the results of our investigations, good agreement is found between the metal–oxygen distances obtained from the present calculation and those from hydrated ionic crystals and aqueous solutions. From the analysis of the charges on the various atoms of the cluster, the covlaent bonding between the metal ion and water molecules is found to be negligible in the Mn+2 system and significant for Fe+3 and Cr+3. The spin populations however indicate strong localization on the metal ions in all three cases. Very good agreement is found between the calculated contact proton hyperfine constants for all three systems and measured values and for the 17O hyperfine constant in the Mn+2 aquoion system, the only one of those systems for which it is available. The contributions of the dipolar hyperfine fields at the proton sites to the proton relaxivities are studied and it is shown that in all three cases, the use of the point dipole model in earlier literature to explain the data on these relaxivities is well justified. Physical reasons for the various features of our results are discussed.
Ladder approximation for three- and four-particle correlation functions
Authors: Blawzdziewicz J. , Cichocki B. , Szamel G.
Published: July 2002

An example of an application of a recently developed generalized Ornstein–Zernike formalism to the numerical evaluation of equilibrium three- and four-particle correlation functions is given. Using a simple closure approximation leading to the ladder approximation we have numerically evaluated dipole–dipole-interaction correlation functions for a polarizable nonpolar hard-sphere fluid. These functions depend on the three- and four-particle correlation functions and describe a correction to the Clausius–Mossoti formula for the dielectric constant and an integrated intensity measured in depolarized light scattering experiments. Qualitative agreement with computer simulation data was found for a wide range of densities up to the fluid–solid phase transition. For high densities the ladder approximation yeilds much better results than the Kirkwood superposition approximation, which becomes useless in this context at liquid-state densities.
Rotational energy transfer in HF: A computational study
Authors: Vohralik Peter F. , Watts R. O. , Alexander Millard H.
Published: July 2002

Close-coupled scattering calculations have been carried out for rotationally inelastic collisions between HF molecules. Integral cross sections were determined for vibrationally elastic transitions j1 j2→j ′1 j 2(v1v2), which refer to collisions HFa (v1, j1)+HFb (v2, j2)+ΔE→HFa (v1, j ′1 )+HFb (v2, j 2), where ΔE is the energy gap. Total energies between 241.1 and 3900 cm−1 (0.03 and 0.48 eV) were considered. When v1=v2, the rotational exchange transitions j1 j2→j2 j1(v1v2) are energetically resonant (i.e., ΔE=0), and the corresponding cross sections increase as the collision energy decreases. Particularly large cross sections are obtained for the j j+1→j+1 j(vv) transitions, which are both resonant and coupled to first order by the dipole–dipole interaction. When v1≠v2, the j j+1→j+1 j(v1v2) transitions are no longer resonant, and the corresponding cross sections are greatly reduced at small to moderate collision energies. The dependence of the j1 j2→j ′1 j 2(v1v2) cross sections upon the vibrational states of the molecules, the collision energy, and the interaction potential used is discussed.
An adapted form of the collocation method for calculating energy levels of rotating atom–diatom complexes
Authors: Peet Andrew C. , Yang Weitao
Published: July 2002

A method is presented for calculating energy levels of atom–rigid-diatom systems for various values of the total angular momentum (J) of the complex. The technique is based upon the collocation method for the vibrational motions of the system and the Galerkin approach for the total rotation. Unlike the Rayleigh–Ritz variational principle, the method does not require the evaluation of integrals over the Hamiltonian and so is very simple to implement. An important feature of the method is that the wave function is obtained in an analytic form and so it is a simple matter to calculate many quantities of spectroscopic interest such as rotational constants and spectral intensities. It is also shown that contracted basis sets can be used in conjunction with the collocation method to enhance the efficiency of the calculation. The method is demonstrated by calculating rovibrational levels of the van der Waals complex Ar–HCl for J up to 10.
Theoretical study of energy transfer from N2(A 3Σ+u) to molecular hydrogen
Authors: Sperlein Robert F. , Golde Michael F.
Published: July 2002

Quasiclassical trajectory calculations have been performed for motion on an ab initio potential energy surface to investigate energy transfer from N2(A 3∑+u, v=1, 3 and 6) to H2 and D2. Because of the unusual features of the surface, both vibrational relaxation and electronic quenching of N2(A) are observed, the latter process resulting in dissociation of the hydrogen molecule. It is deduced that coupling of the vibrational motions of the N2 and H2 molecules initiates the energy transfer process. The results are compared with experimental information on the quenching of N2(A) by σ-bonded molecules.
Raman excitation profiles and second-derivative absorption spectra of β-carotene
Authors: Mantini Anna R. , Marzocchi Mario P. , Smulevich Giulietta
Published: July 2002

The absorption and second-derivative spectra of β-carotene in n.hexane, isopentane, and CS2, as well as the Raman excitation profiles for six modes in n.hexane and isopentane have been measured. The combined analysis of the absorption and second-derivative spectra in terms of model summing over the vibronic excited states furnished reliable values of the excited state displacement parameters. The input parameters for the calculation of the full spectra were taken from the Raman band relative intensities, while the line shape functions and their widths were determined by the second-derivative absorption spectra. The Raman excitation profiles were then calculated in terms of the transform theory, both the observed and calculated absorption spectra being transformed. Very good fits with the experimental profiles were found by including a small contribution of inhomogeneous broadening to the total width. It was concluded that the homogeneous broadening mainly derives from dephasing of nondegenerate low-frequency vibrations in the resonant electronic excited state. The low temperature profiles were easily calculated by decreasing this homogeneous contribution. The agreement between calculated and experimental results was improved by assuming that the vibrational frequencies of the carbon–carbon stretching modes increase at the excited state. The resulting nuclear diplacements and frequency shifts at the excited state as well as the Gaussian widths correlate very well with the absorption spectral properties of linear polyenes.
Core effects in atomic resonances. A comparison between 1,3P  (3l 3l′) states of He-like and Be-like systems
Authors: Macías A. , Martín F. , Riera A. , Yáñez M.
Published: July 2002

We report a study of the effect of a 1s2 core on the properties of doubly excited states, choosing as benchmark the two lowest 1,3P0 (3l 3l′) resonances of He- and Be-like systems, and using the pseudopotential Feshbach method recently proposed by Martín et al.
Charge carrier generation in p-terphenyl crystal. Autoionization of higher excited states
Authors: Morikawa Eizi , Kotani Masahiro
Published: July 2002

Photoionization of singlet excitons in p-terphenyl crystal is studied with nanosecond laser excitation. The photoionization spectrum and the photoabsorption spectrum of the lowest excited state are measured. The ratio of the two spectra gives the efficiency with which higher excited states, generated by the two-step excitation, ionize. The observed efficiency is constant below 7.3 eV and is remarkably high (∼10−2 ). The results do not support the view that the excess energy is converted directly to the kinetic energy of the generated electrons in the crystal and indicate that the initially prepared higher excited states relax rapidly and the succeeding autoionization yields electrons with a fixed kinetic energy.
Generalizing the Chirikov paradigm: A geometrical view
Authors: Sahm David K. , McWhorter Shane W. , Uzer T.
Published: July 2002

The Chirikov pendulum is the standard Hamiltonian of nonlinear dynamics. While many resonant problems of this rich field can be mapped onto this paradigm, the accuracy of its predictions depends on the coupling strengths in the problem. In this article, we generalize the Chirikov pendulum by introducing an alternative reduction, namely to an asymmetric rotor. We also present a geometrical interpretation of both mappings, which is found to be helpful in displaying the dynamics, in particular local and normal mode formation in nonlinear oscillator systems. Our mapping proceeds by finding the Lie algebra for the particular resonance involved, and interpreting its generators as angular momentum components that generate a ‘‘constant energy surface’’ in three dimensions.
Multipolar propagators near a small metallic sphere: A self-consistent calculation
Authors: Girard C. , Maghezzi S. , Hache F.
Published: July 2002

The response field of a small metallic sphere to fluctuating dipole and quadrupole moments is built from the spherical tensor theory. This field is then used to define various dipolar and quadrupolar propagators between two points outside the sphere. The nonlocal character of the electron’s response inside the sphere is included through a self consistent method and the metallic ions are schematized by a jellium model. We show that the spherical tensors formalism provides a good basis for a simple formulation of the successive field gradient susceptibility (α)S(α′) near a spherical surface. The dipolar contribution of the van der Waals energy between a rare gas atom and a metallic sphere is given as an application of the method. Consistency with other approaches is also examined.
Deconvolution of experimental differential cross sections
Authors: Gislason Eric A. , Mickley Loretta J.
Published: July 2002

A method to deconvolute experimental differential cross sections is presented. The procedure removes the effect of the angular divergence of the primary beam, the finite size of the detector, and multiple scattering. The resulting cross section is the optimal experimental result. The method also determines ε=nlQ0, the average number of times each beam particle is scattered. Here n is the number density of scattering gas, l is the length of the scattering region, and Q0 is the true absolute cross section. If the product nl is known, Q0 can be determined. Since the effect of the finite angular resolution of the apparatus has been removed, this method should yield accurate total cross sections. This, in turn, allows the differential cross section to be put on an absolute scale.
Fragmentation of N2 in the inner-shell to Rydberg-orbital excited states (K)−1(n
l)1
Authors: Suzuki Isao H. , Saito Norio
Published: July 2002

Fragment ions from N2 in the states of (K)−1(nl)1 have been measured at 54.7° with respect to the polarization direction of the incident photon by using a time-of-flight (TOF) mass spectrometer. Simulation fitting calculation to the measured TOF spectra gave distributions of the kinetic energy for these ions. Ion yield spectra observed showed that no singly charged molecular ion was produced from the states of (K)−1(nl)1 in contrast to that of (K)−1(1πg)1. These results are discussed in connection to the dissociation pathway of N2 in these states.
Interaction induced Raman spectroscopy of the dipole forbidden πu mode in neat CO2 and solutions with N2 as supercritical fluids
Authors: Hacura Andrzej , Yoon Jong-Ho , Baglin Frank G.
Published: July 2002

The interaction induced Raman spectra (IIRS) of 1:0, 1:1, and 1:3 CO2 :N2 solutions have been studied at 50 °C and 75 °C at densities up to 2.5 times the triple point density (90–3010 bars). M0, M2 , and M4 evaluations have been made upon the πu ν2 Raman dipole forbidden signal. In addition, the absolute Raman scattering cross sections have been evaluated for the 1:0 and 1:1 solutions. A broad low intensity high frequency shoulder was seen in the 1:0 data of ν2 and is identified with a CO2 oscillating on a large cluster of CO2 ; this perturbs the isolated ν2 frequency; the shoulder is not found in the 1:1 or 1:3 solutions. Significant many body cancellation has been observed in the 1:0 data at 50 and 75 °C at near solid densities of the fluid. The nature of the ν2 signal shows that there is a significant difference between the neat and N2 –CO2 solutions. In particular, depolarization measurements upon ν2 in the 1:0 and 1:3 solutions at 50 °C and 1255 bars of pressure showed very different behavior. This would seem to indicate quite a different local field environment, i.e., local structures. All Raman intensity correlation functions show times in the 30–60 femtosecond regime, we believe that such fast times can be related to the behavior of individual CO2 moieties. Last, this IIRS (‘‘ears’’) study is consistent with the earlier findings of Amos, Buckingham, and Williams that angle averaged pair entities are the principal intermolecular scattering species and must be present to give rise to the quadrupole–induced-dipole (QID) allowed ν2 signal.
Product vibrational distributions and collision energy dependence of chemiluminescent reactions of group IVA elements with O2, N2O, and NO2
Authors: Cheong B. S. , Wang J. H. , Fang C. C. , Parson J. M.
Published: July 2002

Computer simulations of the observed spectra in reactions of group IVA elements with oxygen donors yield information on the product vibrational distributions in the electronically excited states of group IVA oxides. The observed vibrational distributions have been compared with the prior predictions. While the vibrational distributions resulting from the Ge2+O2 reaction show statistical behavior, those of products formed in the Si+N2O reaction are found to be much hotter than the prior distributions. Also, time-of-flight studies have been conducted to determine the collision energy dependence of the reactive rate coefficients and the cross sections for these chemiluminescent reactions.
Studies of valence shell photoionization of Cl2
Authors: Braunstein M. , McKoy V.
Published: July 2002

We report photoionization cross sections and photoelectron angular distributions for the 5σg, 2πu and 2πg orbitals of Cl2 from threshold to ∼20 eV photoelectron energy obtained using Hartree–Fock photoelectron orbitals. These studies were motivated by recent (2+1) resonance enhanced multiphoton ionization (REMPI) experiments via the 2 1Πg(2π3g4sσg) state which show strong non-Franck–Condon effects in the ion vibrational distribution suggesting that shape and/or autoionizing resonances may play a role near threshold. Previous single-photon experiments on the valence orbitals of Cl2 do not give a consistent picture of these cross sections at low energies. Our results show that there is a shape resonance in the kπu continuum. However, preliminary studies show that these πu cross sections have almost no dependence on internuclear distance and could not be the cause of non-Franck–Condon effects observed in the REMPI experiments.
Nonspherical atomic ground-state densities and chemical deformation densities from x-ray scattering
Authors: Ruedenberg Klaus , Schwarz W. H. Eugen
Published: July 2002

Presuming that chemical insight can be gained from the difference between the molecular electron density and the superposition of the ground-state densities of the atoms in a molecule, it is pointed out that, for atoms with degenerate ground states, an unpromoted ‘‘atom in a molecule’’ is represented by a specific ensemble of the degenerate atomic ground-state wave functions and that this ensemble is determined by the anisotropic local surroundings. The resulting atomic density contributions are termed oriented ground state densities, and the corresponding density difference is called the chemical deformation density. The constraints implied by this conceptual approach for the atomic density contributions are formulated and a method is developed for determining them from x-ray scattering data. The electron density of the appropriate promolecule and its x-ray scattering are derived, the determination of the parameters of the promolecule is outlined, and the chemical deformation density is formulated.
Hindered rotational energy levels of symmetric tops in trigonal crystalline fields
Authors: Smith David
Published: July 2002

The energy levels for the hindered rotation of symmetric top molecules or ions in trigonal crystalline fields have been computed. The symmetry group of the Hamiltonian is C̄3v×C3 , where C̄3v is the trigonal group of rotations about the body-fixed axes and C3 is the trigonal group of rotations about the space-fixed axes. The tunneling frequencies of CH3D on a Vulcan III powdered sample have been computed and compared to the observed results.
A composition density functional theory for mixtures based upon an infinitely polydisperse reference. II. Freezing in hard sphere mixtures
Authors: Kofke David A. , Glandt Eduardo D.
Published: July 2002

A theory recently proposed by the authors [Kofke and Glandt, J. Chem. Phys. 92, 658 (1990)] is applied to the study of freezing in hard spheres and hard sphere mixtures. The theory, which expresses the free energy of an arbitrary mixture as a functional of the composition density of an infinitely polydisperse (IP) reference, is used to evaluate the properties of mixtures of hard spheres constrained to the Wigner–Seitz cells of an fcc lattice. Semigrand Monte Carlo simulations are used to determine the properties of the IP reference mixture, which is also constrained to an fcc lattice. Freezing is determined by comparing the predicted properties of the Wigner–Seitz crystal with the known properties of the fluid phase. A freezing transition is found for monodisperse hard spheres; the estimated solid-phase density and the transition pressure differ from the accepted values by 2% and 8%, respectively. The treatment is also used to study freezing in polydisperse mixtures with Gaussian distributions of diameters. In accordance with the findings of others, an upper bound is found to the variance of the distribution, beyond which freezing no longer occurs. However, the maximum variance predicted here is approximately one order of magnitude less than that previously found. Discrepancies here and in the pure-fluid results are attributed largely to ergodic difficulties in the simulations of the IP reference. Finally, the possibility of a phase transition in IP mixtures is demonstrated through a calculation of the freezing point of IP hard spheres.
Theoretical investigation of the relaxed geometry in the first singlet B
u excited state of polyacetylene oligomers
Authors: Brédas J. L. , Toussaint J. M.
Published: July 2002

We present the results of calculations carried out at both Pariser–Parr–Pople and ab initio split-valence basis set levels, aimed at finding the nature of the relaxed geometry configuration in S2 which is the first singlet one-photon optically allowed Bu excited state of three polyene molecules: hexatriene, decapentaene, and tetradecaheptaene. We investigate as a function of chain length the relative stabilities of two types of geometry configuration in the excited state: one configuration derives from the use of traditional bond order–bond length relationships; the other configuration comes from the explicit consideration of the electron–lattice coupling and leads to the formation of a soliton–antisoliton pair. The latter configuration is found to correspond to a stable situation in polyenes containing over ten carbon atoms and provides a smooth evolution to the long chain (polyacetylene) properties.
Rotational state distributions following the photodissociation of Cl–CN: Comparison of classical and quantum mechanical calculations
Authors: Schinke Reinhard
Published: July 2002

We report exact quantum mechanical close-coupling calculations for the photodissociation of Cl–CN using the ab initio potential of Waite and Dunlap. We find that the simple classical calculations of Barts and Halpern based on the rotational reflection principle agree very nicely with the exact rotational state distributions. Furthermore, it is shown that the impulsive model fails severely to account for the high degree of rotational excitation in the photodissociation of Cl–CN.
Observation of the 2ν1 band of HN+2 with a 1.57 μm distributed feedback semiconductor laser
Authors: Sasada Hiroyuki , Amano Takayoshi
Published: July 2002

Near-infrared distributed feedback (DFB) semiconductor lasers have been employed for high-resolution spectroscopy of molecular ions. Thirteen lines of the 2ν1 band of HN+2 have been observed from 6380 to 6405 cm−1. A least-squares analysis was carried out to yield two molecular constants of the v1=2 state. The band origin determined agrees very well with an ab initio theoretical prediction.
A simple analysis of ion–ion correlation in polyelectrolyte solutions
Authors: Penfold Robert , Nordholm Sture , Jönsson Bo , Woodward C. E.
Published: July 2002

In the Poisson–Boltzmann theory of macroion screening in electrolyte solutions the chief simplifying assumption is the neglect of correlations within the ionic atmosphere. Using a generalized van der Waals density functional analysis incorporating ion–ion correlation in a local approximation we obtain a simple correlation corrected Poisson–Boltzmann theory. For point charges in the salt free case, this local correlation approximation leads to asymptotic instability (a ‘‘structuring catastrophe’’) though the correction is well behaved to low orders in perturbation theory. Results in zeroth order and in a zeroth order field approximation are compared with an extended series of Monte Carlo simulations within a cell model (and a planar electrode model). An excellent agreement is found over a wide range of coupling strengths.
Association and fragmentation in reverse micelles
Authors: Balazs Anna C. , Gempe Michael , Brady James E.
Published: July 2002

We have developed a computer simulation to model the formation of reverse micelles from the association and fragmentation of diblock polymeric chains. Using this model, we can observe the evolution of the average cluster size as a function of time. Furthermore, we can explicitly relate the aggregation number N to Ps or the breakup constant, which is inversely proportional to τ, the lifetime of the chain in the cluster. We find that Ps∼exp(−N), which differs from the scaling relationship that exists between these variables when the associating species are particles. We find that the value of τ (or Ps) affects not only the average cluster size but also, the actual mechanism of structural rearrangement for these micelles.
Direct measurement of photodissociation, geminate recombination, and vibrational cooling in iodine using picosecond Raman spectroscopy
Authors: Xu Xiaobing , Lingle Robert , Yu Soo-Chang , Chang Y. J. , Hopkins J. B.
Published: July 2002

The photodissociation and geminate recombination of iodine in cyclohexane has been studied by directly monitoring the vibrational coordinates using transient Raman spectroscopy. Energy relaxation as a function of vibrational energy gap has been measured for vibrational spacings of 210 to 130 cm−1. These vibrational levels correspond approximately to v=3 to v=52 with energies 740 to 9300 cm−1 above the zero point level. The results support earlier experiments in that over 100 ps is required to completely relax the vibrational energy.
A theoretical treatment of multiple quantum nuclear spin coherences in electron spin echo studies of polyacetylene
Authors: Hustedt Eric J. , Thomann Hans , Robinson Bruce H.
Published: July 2002

We present a theoretical investigation of the origin of the unusual 3-pulse electron spin echo envelope modulation (ESEEM) pattern obtained from 13C enriched polyacetylene, which exhibits modulation at multiple harmonics of the nuclear Larmor frequency ωn. ESEEM signals have been calculated for different models of the electron–nuclear spin coupling. It is shown that the local hyperfine tensors observed in electron nuclear double resonance (ENDOR) experiments on (13CH)x cannot account for the ESEEM results. These tensors have isotropic components which are too large to give narrow peaks at ωn, and there are too few nuclei to produce the large number of harmonics observed. Simulations for a point electron coupled to nuclei on a spherical shell, the spherical model, give insight into the conditions required to obtain narrow lines at multiple harmonics of ωn: The electron must be weakly coupled to a very large number of nuclei. Analytical results show that for the spherical model in the limit of small coupling, the intensity profile of the Fourier transform of the three-pulse ESEEM will be a Gaussian of width k̄ N, where k̄ is a function of the magnitude of the electron–nuclear coupling and N is the number of nuclei. Simulations assuming that the unpaired electron is delocalized in one dimension along the polyene chain and dipolar coupled to the 13C nuclei on neighboring chains give multiple harmonic patterns, at a reduced interchain distance. We conclude that the observed ESEEM pattern arises from nuclei on neighboring polyacetylene chains and is closely related to the matrix/distant ENDOR line observed for (13CH)x.
Application of the geometric Gibbs equation: Hard spheres at high density
Authors: Schaaf Pierre , Reiss Howard
Published: July 2002

The ‘‘geometric Gibbs equation’’ derived by the authors in an earlier paper is employed in the derivation of the asymptotic form of the hard sphere equation of state near the close-packing density. It leads to the same asymptotic equation as the ‘‘polytope’’ method. The significance of the geometric Gibbs equation is explored and the importance of vacancies in any rigorous theory at high density is both emphasized and demonstrated.
Molecular adsorption of ethane on the Ir(110)-(1×2) surface: Monte Carlo simulations and molecular beam reflectivity measurements
Authors: Kang H. C. , Mullins C. B. , Weinberg W. H.
Published: July 2002

Experimental results, obtained using a molecular beam reflectivity method, for the probability of molecular physical adsorption of ethane on the Ir(110)-(1×2) surface are presented. We analyze these results using Monte Carlo simulations and show that molecular adsorption can occur either ‘‘directly’’ or through a precursor state in which an ethane molecule is trapped in a second layer of molecularly adsorbed ethane with subsequent migration to a vacant site. From the Monte Carlo simulations, we are able to establish that the energy barrier for the desorption of an ethane molecule from the precursor state is approximately 4.5 kcal/mol. We also find that the energy barrier for diffusion of an ethane molecule on top of a monolayer of ethane molecularly adsorbed on the Ir(110)-(1×2) surface is approximately 3.7 kcal/mol.
Pattern formation in a temporally modulated chemical instability
Authors: Careta A. , Sagués F.
Published: July 2002

A chemical Turing instability resulting in steady spatial patterns is examined here when we superimpose a perturbatively small time periodic modulation on its control parameter. Results for both the modified instability threshold and the corresponding growth of the bifurcating solutions are explicitly obtained. In particular, quasiresonant effects originating significant stabilization or destabilization of the system, depending on the model parameters, are predicted for particular values of the forcing frequency.
Molecular dynamics studies on molten alkali hydroxides. I. Static properties of molten LiOH
Authors: Okazaki Susumu , Ohtori Norikazu , Okada Isao
Published: July 2002

Molecular dynamics calculation for molten LiOH has been executed using a set of effective pair potentials obtained by a small modification of ab initio ones. Some experimental data on static and dynamic properties of the melt were successfully reproduced by the calculation. Anisotropic coordination structures of Li+ and OH− ions around an OH− ion have been elucidated from two-dimensional pair correlation functions, three-particle angular distribution functions, and four-particle distance distribution functions. The most probable location of a Li+ ion is on the O atom side of the OH− ion, being somewhat different from the pair potential minimum position. The correlation is strikingly large irrespective of the libration of the OH− ion. The OH− ion was found to coordinate to the neighboring OH− ion in two ways. The first one consists of a direct contact between ions on their H atom sides in spite of the large pair repulsion. The second is an indirect one on the O atom side through intervening Li+ ions which have strong Coulombic attraction to the OH− ion. This feature has a good correspondence to that of the LiOH crystal. These anisotropic structures result in an approximately isotropic coordination of the O atom around the OH− ion and give rise to a gOO(r) with a considerably sharp and large first peak and long range oscillatory correlations. This is the origin of the sharp and high first peak of the neutron structure function characteristic of the LiOH melt. It was also found that, on the average, OH− ions around a Li+ ion form a regular tetahedron, whereas the Li+ ions are located around the apexes of a square which is slightly folded downward along a diagonal just under the O atom of the OH− ion. These short range framework structures of the liquid are nearly the same with those of the crystal except that the four Li+ ions just under the OH− ion form only one plane in the crystal. Space–time correlation functions indicate that the above structures dissipate substantially in about 1 ps.