bluuebunny/crossref_metadata_2025 huggingface dataset:
Ferroelastic KIO2F2: Crystal structure and ferroelastic transformation
Authors: Abrahams S. C. , Bernstein J. L.
Published: February 2003

Potassium fluoroiodate, KIO2F2, is ferroelastic at room temperature, crystallizing in space group Pca21 with four formulas in the unit cell. The lattice constants at 298.2 °K are a=8.39430±0.00004, b= 5.97918±0.00005, c=8.44680±0.00003 Å, for λCuKα1=1.540562 Å. The integrated intensities of 4169 reflections within a hemisphere segment of reciprocal space with radius (sinϑ)/λ=1.02 Å−1 were measured by diffractometer, and resulted in 772 symmetry-independent Fmeas and σFmeas. The crystal structure was solved by use of a combination of Patterson and Fourier series, and refined by the method of least squares. The final agreement factor R=0.046, and the δR normal probability plot is essentially linear. The atomic coordinates are in agreement with the 1940 values of Helmholz and Rogers. Within the IO2F−2 ion, F–I–F is linear (average I–F=2.000 Å) with O–I–O forming a plane (average I–O=1.768 Å) normal to F–I–F. Compressive stress along [001] ferroelastically transforms a crystal with orientation abc into one with orientation cba. The compressive strength of KIO2F2 is comparable to the coercive stress. Atomic displacements for some atoms in the ferroelastic transformation, as IO2F−2 ions rotate, are as large as 2.4 Å.
The fluorescence excitation spectrum of the HeI2 van der Waals complex
Authors: Smalley R. E. , Levy D. H. , Wharton L.
Published: February 2003

The HeI2 van der Waals complex was prepared from a dilute mixture of iodine in helium at a pressure of 100 atm by supersonic expansion through a nozzle into a vacuum. Laser-induced fluorescence excitation spectra were recorded for the B̃←X̃ transition of HeI2 as well as corresponding spectra for the He2I2 and I2 molecules in the expanding gas. I2 was found to be cooled by the expansion to a rotational temperature of 0.4 K, and a vibrational temperature of 50 K. Similarly, cold internal temperatures were attained by the van der Waals complexes. Evidence was found for vibrational predissociation of the HeI2 complex in both the X̃ and B̃ electronic states. The vibrational predissociation rate was found to depend weakly upon the degree of excitation of the I–I stretching mode, ν1. For ν1=1 in the X̃ state the predissociation rate was found to be greater than 5×106 s−1. In the B̃ state the vibrational predissociation rate is ∼5×1010 s−1 for ν1=27, decreasing to <5×109 s−1 for ν1⩽7. The small (3.4–4.0 cm−1) blue shifts of the vibronic bands of the HeI2 spectrum relative to corresponding bands of I2 indicate (1) the van der Waals complex is slightly more strongly bound in the X̃ state than it is in the B̃ state, and (2) the I–I bonding in both the X̃ and B̃ states of iodine is largely unaffected by the formation of the van der Waals bond with helium.
Fluorine 1s correlation states in the photoionization of hydrogen fluoride: Experiment and theory
Authors: Martin R. L. , Mills B. E. , Shirley D. A.
Published: February 2003

The x-ray photoemission spectrum of the F 1s region of gaseous HF was measured using Al Kα1,2 radiation. Several satellite (’’shakeup’’) peaks were observed at ∼25–35 eV higher binding energy than the main F 1s peak. The spectrum was interpreted in terms of a many-electron theory that includes configuration interaction (CI) in both the hole state and the ground state. A CI calculation yielded eleven physically meaningful excited states based on the HF+ (F 1s hole) state. Six were predicted to have intensities over 0.1% that of the main peak. The four most intense of these were clearly present by visual inspection of the spectrum. Six were readily fitted by the addition of constraints. Derived intensities were in very good agreement with theory if ground-state CI was included. The four most intense peaks can be crudely interpreted as arising from 3σ to 4σ, 1π to 2π, and 1π to 3π excitations, but careful inspection of the orbitals shows that these designations are oversimplified.
Resonant exchange of rotational energy and the thermal conductivity of gases
Authors: Clifford Anthony A. , Dickinson Eric
Published: February 2003

The impact theory of Raman line broadening is used to estimate the influence of resonant exchange of rotational energy on the thermal conductivity of gases composed of molecules possessing an axis of symmetry. By taking account of the relative rotations of the colliding molecules, and modifying the integration of the exchange probability function at short range, we obtain cross sections that are smaller than those previously estimated. Exchange cross sections are calculated for hydrogen and hydrogen fluoride at 300 K. In agreement with recent experimental measurements, it is concluded that rotational resonant exchange in hydrogen makes a negligible contribution to the thermal conductivity of the pure gas and its mixtures.
Low-frequency structure in the depolarized spectrum of argon
Authors: An Sung-Chung , Montrose C. J. , Litovitz T. A.
Published: February 2003

Depolarized Rayleigh scattering spectra have been obtained in argon at five thermodynamic states. For the three spectra at liquid-like densities a relatively narrow component (half-width ≲10 cm−1) was discovered in the spectra. This component exhibits a roughly Lorentzian line shape and with it is associated about half of the total depolarized intensity. Its origin is the so-called ’’dipole-induced dipole’’ effect and it is assumed that it reflects the correlated motions of an atom and its near neighbors.
Reactions of N+3 with NO and O2 and mobility of N+3 in nitrogen
Authors: Lindinger Werner
Published: February 2003

Rate constants for the reactions of N+3 with NO and O2 were measured from thermal energy to approximately 0.3 and 0.4 eV kinetic energy in the ion–reactant center of mass. In both cases there is an initial decrease of the rate constant with increasing energy followed by an increase. The mobility of N+3 in N2 was measured over the range from E/N=30 to 240 Td (1 Townsend unit =10−17 V cm2). In the overlapping region from 30 to 40 Td agreement exists with earlier data.
Stark effect on the lowest excited state of the 2,4,5-trimethylbenzyl radical in a durene host crystal
Authors: Udagawa Y. , Hanson D. M.
Published: February 2003

The magnitude of the vector difference between the dipole moments of the ground state and the first excited electronic state of the 2,4,5-trimethylbenzyl radical in a durene host crystal is determined to be 0.40 D with 10% accuracy from the frequency modulation of the fluorescence in an alternating electric field. From the Stark data and the temperature dependence of the spectra, it is concluded that several different orientations of the duryl radical in the host crystal can be present, depending upon the experimental conditions. These orientations result from the recoil of the duryl radical in the photodissociation process.
O(1S) quenching by O(3P)
Authors: Slanger Tom G. , Black Graham
Published: February 2003

The quenching of O(1S) by O(3P) was measured as a function of temperature, between 200 °K and 365 °K. The resulting Arrhenius expression is k=5.0×10−11 exp(−610/RT) cm3 molecule−1⋅sec−1. The possible effect of O3 on the measurements was considered, and it is concluded that O3 does not contribute to the observed quenching. A sizable discrepancy now exists for this reaction between experiment and the most recent theoretical calculations.
Nonequilibrium velocity distribution and dissociation rate in dissociation of diatomic molecules
Authors: Koura Katsuhisa
Published: February 2003

The nonequilibrium velocity distribution and dissociation rate in the dissociation of diatomic molecules in a heat bath of inert atoms are studied by solving the Boltzmann equation with the Monte Carlo simulation. The explicit time-dependent velocity distribution and dissociation rate from the initial equilibrium to the quasisteady state are obtained for the dissociation model, where the vibrational distribution is taken to be the Boltzmann equilibrium distribution; the effects of the vibrational transition are taken into account by a simplified model, and the cross section is taken to be a hard sphere model with the line-of-centers dissociation cross section. The quasisteady state velocity distributions and dissociation rates of upper vibrational levels, from which the dissociation occurs effectively, indicate significant deviations from equilibrium. The decreases in the dissociation rates of the upper vibrational levels from the equilibrium dissociation rates amount to 75% for H2–Ar and 20% for N2 or O2–Ar, which are comparable with the decrease in the dissociation rate due to the nonequilibrium vibrational distribution. For halogens–Ar, the decreases in the dissociation rates are less than 20%. The decrease in the overall dissociation rate from the equilibrium dissociation rate due to the nonequilibrium velocity distribution is 60–30% for H2–Ar at temperatures 1500–4000 °K, where the nonequilibrium dissociation rate is still larger than the experimental rates. The difference between the nonequilibrium dissociation rate and the experimental rates may be attributed to the nonequilibrium vibrational distribution. It is of interest that the nonequilibrium of the dissociation rate due to the nonequilibrium velocity distribution takes place even at low temperatures, where the nonequilibrium of the vibrational distribution may not occur.
Vibronic mixing and vibronic coupling of nearby electronic states. II: CNDO calculations of vibronic mixing in p-benzoquinone
Authors: Merienne-Lafore M. F. , Trommsdorff H. P.
Published: February 2003

The electronic structure of the lowest excited states of p-benzoquinone is given by calculations in the CNDO approximation. It is shown that for the description of the two nearly degenerate nπ* B1g and Au states the molecule can approximately be considered as a dimer. [A single p-benzoquinone molecule is considered as a dimer, the monomer unit being one-half of the molecule, i.e., a (CH)2C=O fragment.] The vibronic mixing of these two states is evaluated from calculations at displaced nuclear positions, which specify the relative importance of the different coordinates of b1u symmetry. The computed parameters are compared with parameters obtained, within the dimer model, from experimental data. The results of the calculations support the interpretation of Stark measurements of this compound which suggested a pseudo Jahn–Teller distortion of the lowest excited states.
Extended calculations of the anomalous Rayleigh linewidth and shear viscosity
Authors: Garisto Frank , Kapral Raymond
Published: February 2003

In this article the results of a fairly extensive set of mode coupling calculations of both the anomalous Rayleigh linewidth and shear viscosity of xenon are presented. The trilinear kinetic equations for the order parameter fluctuations are presented and solved. Although the effects of explicit trilinear coupling on the Rayleigh linewidth are found to be small in agreement with earlier work, there are some quantitative differences. The major results of this paper are related to an examination of the approximation of neglecting diffusive modes when they occur in combination with viscous modes, and the study of non-Ornstein–Zernike corrections to the anomalous shear viscosity. Both effects are found to be large.
NMR study of adsorbed water. I. Molecular orientation and protonic motions in the two-layer hydrate of a Na vermiculite
Authors: Hougardy J. , Stone W. E. E. , Fripiat J. J.
Published: February 2003

The well characterized two-layer hydrate of Na-Llano vermiculite has been studied by continous wave (CW) wide-line NMR and by pulse NMR of 1H nucleus in a wide temperature range. The absorption spectrum consists of a doublet and of a central line. The doublet splitting is orientation dependent between 0 and 50 °C and orientation independent below −60 °C. Below 60 °C, the symmetry of the hydration shell may be destroyed. The linewidth is compatible with rapidly rotating water molecules. An octahedral distribution of water molecules around the Na+ cations fits the experimental data, the six rotation axes around which the water molecules are spinning rapidly being tilted by about 65° with respect to the C* axis. The cation hydration shell is characterized by a rotational diffusion motion with an activation energy of 8.5 kcal, the diffusion coefficient at room temperature being about 0.5×10−8 cm2 sec−1. The diffusion coefficient of free water or protons is about 0.2×10−6 cm2 sec−1. From +50 to −100 °C, the observed T−11 is contributed successively by the diffusion of the cation hydration shell, the diffusion of free water or proton with respect to the paramagnetic centers, and finally by the paramagnetic contribution. Water molecules within the hydration shell and/or water molecules ’’between’’ the hydration shells exchange protons with a frequency between 10−4 and 10−5 sec−1 at room temperature. This exchange occurs within the hydration shell or it is relayed by the ’’free’’ water molecules between the hydration shells.
Temperature dependence of intrinsic carrier generation in anthracene single crystals
Authors: Chance R. R. , Braun C. L.
Published: February 2003

The pulse technique is used to measure free carrier yields in anthracene single crystals as a function of temperature, photon energy, and applied field. Field dependence plots, measured over the temperature range 200–300 K, exhibit slope-to-intercept ratios described by S/I=3.2×10−5 (298/T)2 cm/V. This result is in quantitative agreement with that predicted by Onsager’s theory of geminate recombination. Activation energy (Ea) results are reported over a wide range of photon energies. We find that Ea is essentially constant (Ea=0.090 eV) over the photon energy range 4.4–5.2 eV and again constant (Ea=0.068 eV) over the range 5.4–6.2 eV. This observation is consistent with autoionization from two spectroscopically observed states: system II (1B2u) in the lower energy range, and system III (1B1u) in the higher energy range. The average thermalization lengths that would be associated with these Ea values are in the range 10–70 Å depending on the form chosen for the distribution of charge pair configurations.
Dimensionality of spin fluctuations in highly anisotropic TCNQ salts
Authors: Butler M. A. , Walker L. R. , Soos Z. G.
Published: February 2003

An anisotropic three-dimensional random walk model is developed and used to describe the frequency dependence of proton spin–lattice relaxation rates in several TCNQ salts. The triplet exciton system (φ3AsCH3) (TCNQ)2 at 145 °K is shown to display two-dimensional diffusive behavior. Measurements of the proton relaxation rate in NMP–TCNQ at room temperature are reported and interpreted as displaying two-dimensional diffusive behavior with a transition to three-dimensional behavior. These results suggest that the nuclear resonance time scale is so long as to preclude observing one-dimensional behavior even in highly anisotropic one-dimensional systems. The random walk model is extended to display the dependence of relaxation rate upon charge transfer and thus explain the unusually rapid relaxation in NMP–TCNQ.
Trajectory calculation of the effectiveness of reagent vibration in the H+2+He→HeH++H or He+H+H+ reactions
Authors: Whitton W. N. , Kuntz P. J.
Published: February 2003

Total cross sections for the reaction of H+2(v′) with He were calculated for v′=0 to 5 over a range of total energies between 1 and 5 eV using classical mechanics. The potential surface used was an appropriate diatomics-in-molecules function adjusted to fit existing ab initio points. In agreement with experiment, reagent vibrational excitation is very effective in promoting formation of HeH+ or dissociation, a fact which we explain in terms of the increased range of the attractive forces when the H+2 bond is extended. This effect is not properly described by comparison collinear calculations.
The kinetic isotope effect in the dissociative chemisorption of methane
Authors: Winters Harold F.
Published: February 2003

Investigations involving the dissociative chemisorption of CH4 and CD4 on clean tungsten have now been extended to include the other partially deuterated methanes. The ratio of the sticking probabilities S (CD3H)/ S (CD4), S (CD2H2)/S (CD4), S (CDH3)/(SCD4), and S (CH4)/ S (CD4) are approximately 1.7, 2.7, 3.5, and 4.5, respectively, for 2250 °K≳T≳1250 °K. These large ratios have an unexpectedly small temperature dependence. The apparent activation energies (i.e., the slope of R lnS vs 1/T) are all 9.3±1 kcal/mole for 1350 °K<T<2359 °K. It is shown that a model involving (1) the tunneling of hydrogen atoms through a potential barrier, (2) vibrational excitation, and (3) the lifetime of the undissociated molecule on the surface can qualitatively explain the experimental results. Model calculations suggest that the dissociative chemisorption reaction, and in particular the kinetic isotope effect, is dominated by tunneling. The apparent activation energies are primarily determined by vibrational excitation while the change in activation energy observed at lower temperatures can be accounted for by considering the lifetime of the undissociated molecule on the surface.
Theory of spatial distributions of restricted random walk models of polymers
Authors: White R. Allen
Published: February 2003

A polymer can be represented by a restricted random walk of order s in which loops in s steps or less are excluded although larger loops are allowed. All restricted walks of order s can be characterized by the configurations of their last s−1 steps; a set of coupled finite difference equations with s−1 step configuration and spatial distribution as variables can be written describing all possible walks. A complete matrix theory is presented which solves the set of coupled distributions through moment analysis to yield a complete spatial distribution. The theory, which makes no assumptions on the initial distribution of configurations, is directly amenable to symmetry reduction by techniques described herein and is highly suitable for obtaining numerical results. The theory demonstrates that the second moment is asymptotically linear in the number of steps taken and techniques for computing the asymptotic slope are demonstrated. A simple example is solved to demonstrate the numerical feasibility of the theory.
Resonance Raman scattering of light from a diatomic molecule
Authors: Rousseau D. L. , Williams P. F.
Published: February 2003

Resonance Raman scattering from a homonuclear diatomic molecule is considered in detail. For convenience, the scattering may be classified into three excitation frequency regions—off-resonance Raman scattering for incident energies well away from resonance with any allowed transitions, discrete resonance Raman scattering for excitation near or in resonance with discrete transitions, and continuum resonance Raman scattering for excitation resonant with continuum transitions, e.g., excitation above a dissociation limit or into a repulsive electronic state. It is shown that the many differences in scattering properties in these three excitation frequency regions may be accounted for by expressions derived from simple perturbation theory. Scattering experiments from molecular iodine are presented which test and verify the general scattering theories. Spectral measurements, time decay measurements, and pressure broadening measurements were made on I2 in the discrete resonance Raman scattering region; and spectral measurements at several excitation frequencies were made in the continuum resonance Raman scattering region. In each case calculations based on general theories correctly describe the experimental data.
Total cross sections for Na(3 2P1/2→3 2P3/2) + rare gas collisions
Authors: Pascale J. , Olson R. E.
Published: February 2003

Quantum-mechanical calculations that use the recent pseudopotentials of Pascale and Vandeplanque were employed to obtain the 3 2P1/2→ 3 2P3/2 fine structure transition cross sections for Na+He, Ne, Ar, Kr, and Xe collisions. The collision energy range investigated was from threshold (0.00213 eV) to 0.5 eV. The cross sections are found to rise rapidly from threshold to a plateau, with the Na+Ar, Kr, Xe systems also exhibiting a large secondary maximum at approximately 0.1 eV. Regular undulatory structure was observed superimposed on all the cross sections except for the Na+Ne system, while resonances caused by quasibound states were found to be most prominent on the Na+Ar system. Thermally averaged cross sections were calculated from 100 to 1000 °K and are found to be in reasonable accord with experiment.
π backbonding in Cr(CN)63− and Cr(SCN)63−: A re-examination of the Cr 3s multiplet splitting
Authors: Lazarus M. S. , Chou T. S.
Published: February 2003

We have observed multiplet splittings in the Cr 3s x-ray photoelectron spectra of K3Cr(CN)6 and K3Cr(SCN)6. These results, which contradict the previously reported work on the same compounds, are in agreement with EPR and optical spectra for the Cr(CN)63− ion. The Cr 3s spectrum appears to confirm the view that the extent of π backbonding in the cyanide complex is small. Likewise, it may be concluded that the extent of π backbonding in Cr(SCN)63− is also small.
Rayleigh–Brillouin scattering of gases with internal relaxation
Authors: Lao Q. H. , Schoen P. E. , Chu B.
Published: February 2003

We have measured the light scattering profiles of CO2, N2, C2F6, and C2H6 at 29 °C in both kinetic and hydrodynamic regimes. The generalized theory by Weinberg, Kapral, and Desai was applied for the first time, we believe, to analyze the power spectra of C2F6 and C2H6, for which the Mountain theory was inadequate. We have also observed the rotational relaxation times of C2H6 and C2F6 and the vibrational relaxation time of the latter.
Utilization of an arc-heated jet for production of supersonic seeded beams of atomic nitrogen
Authors: Bickes R. W. , Newton K. R. , Herrmann J. M. , Bernstein R. B.
Published: February 2003

Intense supersonic beams of atomic nitrogen (≳1017 atoms sr−1 sec−1) have been produced from the dissociation of N2 in an Ar arc (at temperatures in excess of 6000 K) using the arc-heated nozzle beam source of Young, Rodgers, and Knuth. Experiments characterizing the N2 dissociation and the translational energies of the N, N2, and Ar components in the beams are described. Evidence is presented for the formation of atomic C as well as C2 and CH from the pyrolysis of CH4 and C2H4 in the Ar arc.
An NMR study of the alkali metal intercalation phase LixTiS2: Relation to structure, thermodynamics, and ionicity
Authors: Silbernagel B. G. , Whittingham M. S.
Published: February 2003

The Knight shift and electric quadrupole coupling constant have been measured as a function of lithium concentration in the continuous intercalation phase LixTiS2 where 0⩽x⩽1. These NMR data, when compared with observations of c axis dilation and thermodynamics of this phase, suggest the donation of an appreciable fraction of the Li 2s electron to the TiS2 layers during intercalation, with the fraction diminishing as x increases. The relation of the charge donated to each of the observed quantities is discussed.
Mode expansion method in polyelectrolytes. II. Flexible chains
Authors: Iwasa Kunihiko
Published: February 2003

The free energy of polyelectrolytes with flexible chain conformation was evaluated by taking an average with respect to polymer conformations after the configurational average over small ion species had been taken. The result agrees with Manning and Zimm’s result if the Debye–Hückel length 1/κ is smaller than the average distance b′ between neighboring charged groups along the chain, but a remarkable difference is shown if 1/κ is greater than b′. This indicates that some sort of rigidity, except for charge repulsion of the polymer chain, should be considered to derive Manning’s limiting law.
Solvent and temperature effects on natural radiative lifetimes of some substituted polyenes
Authors: Hug Gordon , Becker Ralph S.
Published: February 2003

The extent to which the concept of a natural radiative lifetime (τ0) can be used as an indicator of what is taking place on the molecular level as the solvent or temperature changes is considered. The observation is made that since the universal solvent shift can be explained by perturbation theory, this indicates that the fluorescing state is really a mixture of states of the isolated molecule. With this as a basis, a simplified coupling model is introduced and resolvent techniques are used to solve for the time-dependence of the fluorescing state as a function of the solvent–solute interaction strength. The results indicate that much of the dependence of τ0 on the solvent and temperature for the retinols and diphenylpolyenes may be due to the state mixing introduced by the solvent perturbations. In these molecules as the strength of solvent induced state mixing increases, τ0 decreases. The temperature effect is such that τ0 decreases as T decreases because of an increase in solvent perturbation correlated to density increase.
Cage effects and steric hindrance in van der Waals solids, with application to alkyl iodide photolysis in rare gas hosts
Authors: Brus L. E. , Bondybey V. E.
Published: February 2003

Molecular luminescence in the near ir is produced by 259–290 nm pulsed ’’photodissociation’’ of CH3I, CD3I, and perfluoroalkyl iodide guests in rare gas hosts at 4.2 °K. For CD3I and CH3I in Ne, the 15 member progression observed is assigned as fluorescence into high lying (near v″=37) vibrational states of the methyl–iodide stretch in the ground electronic state. Photoselection studies confirm the transition dipole lies along the methyl–iodine bond. The photodissociation cage effect is near complete, with only a minor quantum yield of permanent dissociation. Detailed analysis of the spectra suggests that the 0+ ’’repulsive’’ excited state is actually slightly (<2000 cm−1) chemically bound, and that the ground state potential is negligibly perturbed from its free molecule shape. A simple semiquantitative theory of the cage effect is proposed. In the impulsive limit, the excited state dynamics reflect a strong cage effect. However, in the adiabatic limit, the absorption and emission spectra reflect only a weak, van der Waals cage effect. The theory is applied to known examples of photodissociation in solids, and to rearrangements of larger molecules. A cage potential experiment of Schnepp and Dressler is reinterpreted.
Infrared absorption spectra of gaseous HD. II. Collision-induced fundamental band of HD in HD–Ne and HD–Ar mixtures at room temperature
Authors: Prasad R. D. G. , Reddy S. Paddi
Published: February 2003

The collision-induced infrared absorption spectra of the fundamental band of HD in binary mixtures of HD with Ne and Ar at room temperature have been studied with an absorption path length of 105.2 cm for different base densities of HD in the range 8–20 amagat and a number of total gas densities up to 175 amagat. The observed features of the profiles of the enhancement of absorption in these mixtures resemble closely those of the corresponding profiles of the fundamental band of H2 in binary mixtures with Ne and Ar. The binary absorption coefficients of the band obtained from the measured integrated intensities are (1.84±0.06) ×10−35 and (4.41±0.06) ×10−35 cm6 s−1 for HD–Ne and HD–Ar, respectively. The characteristic half-width parameters, δd and δc of the overlap transitions and δq (and δq′) of the quadrupolar transitions, are obtained from an analysis of the profiles of the enhancement of absorption in both these mixtures. The quantity δc which is the half-width of the intercollisional interference dip of the Q branch increases with the density of the perturbing gas Ne or Ar, and for HD–Ne it varies in a manner similar to that for HD–He as described in Paper I of this series.
H2+3 orbitals as a model for Walsh’s AB2 rules
Authors: Hicks W. V. , Schaad L. J. , Innes K. K.
Published: February 2003

The molecular orbitals of H2+3, calculated with a basis of s and p Gaussian atomic orbitals, are examined as a possible model for Walsh’s AB2 diagram. The lower orbitals are more satisfactory than Walsh’s for predicting triatomic geometries, but the upper are not.
Self-consistent molecular orbital methods. XVI. Numerically stable direct energy minimization procedures for solution of Hartree–Fock equations
Authors: Seeger Rolf , Pople John A.
Published: February 2003

Direct energy minimization methods are examined for solution of the Hartree–Fock equations in a basis expansion. Given a set of trial spin– orbitals, an initial direction for a univariate search of spin–orbital space is specified using general energy-weighted coordinates. The first energy minimum in such a search then provides an initial point for the next iterative step. Techniques are proposed for the maintenance of spin–orbital continuity during this process. Comparison of various energy-weighted coordinates indicates that most effective convergence is achieved if the search path is chosen to pass through the point corresponding to the classical procedure involving the construction of successive approximations to the Fock matrix. The utility of the new method is illustrated by the completion of a previous systematic study of Hartree–Fock energies for small organic species.
Restrictions on chemical kinetic models
Authors: Wallwork David , Perelson Alan S.
Published: February 2003

When constructing kinetic models of chemical processes, one needs to insure that certain qualitative features are incorporated. Ideally one would like to have a minimal set of postulates such that any system obeying these postulates would necessarily exhibit the behavior observed in chemical systems. One of the most primitive properties of a chemical system is that concentrations cannot be driven to zero. On the basis of topological arguments we show that any system which obeys the laws of thermodynamics will have this property. Furthermore, by imposing constraints, such as structural stability, on the behavior of chemical systems under small perturbations of their vector fields, we show that there are no stable steady states on the boundary of a reaction simplex. The existence and uniqueness of thermodynamic equilibrium states is also discussed.
Studies in the mechanism of radiationless conversion of electronic energy. I. p-xylene in solution
Authors: Gregory T. A. , Lipsky Sanford
Published: February 2003

The fluorescence quantum yields of p-xylene in isooctane excited at 184.9 nm have been determined for the transitions S3→S0 (Φ3), S2→S0 (Φ2), and S1→S0 (Φ1) as functions of the concentration of p-xylene (cx) and of the concentration of added quenchers (CCl4, CHCl3, c−C7F14), and compared with the S1→S0 fluorescence quantum yield (φ1) for excitation at 253.7 nm in the same solutions. Also vapor fluorescence quantum yields have been redetermined for the S3→S0 and S2→S0 transitions. It has been found that the total nonradiative decay rate constants of S3  (2.5×1014 sec−1) and of S2  (1.0×1013 sec−1) are essentially the same for dilute solution and vapor phases. Also it is found that in dilute solution, Φ3 is independent of concentration of quencher, whereas Φ2 is strongly quenched and to about the same extent as is the ratio β=Φ1/φ1. In more concentrated solutions (cx≳3M), Φ3 rapidly declines to unobservable levels, whereas Φ2 and β appear to increase—but only slightly. On the basis of these results the following is concluded for a dilute solution of p-xylene in isooctane excited into S3 (at 184.9 nm): (i) The S3 state either radiates (kr=3.0×109 sec−1) or makes internal conversion to S2  (k=2.5×1014 sec−1). No autoionization occurs nor do any other decay channels such as S3→S1 or S3→S0 have appreciable probability. (ii) The S2 state either radiates (kr=1.3×108 sec−1), makes internal conversion to S1  (k=1.0×1013 sec−1), or autoionizes (k≳1014 sec−1). All other decay channels are of negligible importance. (iii) In the absence of a quencher, the ejected electron returns to the parent positive ion to regenerate S2 and not a lower state. In the presence of quencher the electron is scavenged and no longer can contribute to any emission. Possible mechanisms for the disappearance of Φ3 at high cx are considered.
Chemical implantation, isotopic trapping effects, and induced hygroscopicity resulting from 15 keV ion bombardment of sapphire
Authors: Gruen D. M. , Siskind B. , Wright R. B.
Published: February 2003

Targets of single crystal sapphire (α-Al2O3) have been bombarded with 15 keV H+, D+, He+, Ne+, and Ar+. The H+ and D+ chemically implant, as indicated by the production of ir absorption bands at 3400 and 2450 cm−1, attributed to the hydroxyl and deuteroxyl stretching frequencies, respectively. At ion fluences of 1×1017 cm−2 the chemical trapping efficiency of D+ as calculated from the integrated deuteroxyl band intensity is of the order of unity. The number of D+ chemically trapped as deuteroxyl saturates at about 2×1017 cm−2 of target surface area, and at higher D+ fluences physical trapping of deuterium in gas blisters becomes the dominant trapping mechanism. By contrast, chemical trapping of H+ saturates at about 7×1016 cm−2. This isotopic effect can be rationalized in terms of the greater displacement damage resulting from D+ bombardment. The surfaces of sapphire targets subjected to 15 keV H+, D+, and He+ bombardments at fluences above 2×1017 cm−2 exhibit an induced hygroscopicity characterized by a growing hydroxyl band absorption on exposure to water vapor. Blisters indicative of physical trapping are observed for H+ and He+ as well as for high fluence D+ bombardments. There is no evidence of blistering or induced hygroscopicity due to Ne+ or Ar+ bombardments. The results for H+ and D+ are extrapolated to T+ and the implications for chemical sputtering are considered.
Extension of the Gordon–Kim overlap interaction to open shell systems
Authors: Harrison W. A. , Sokel R.
Published: February 2003

An attempt is made to extend the theory of interatomic interactions for closed shell systems to bonded solids by proceeding in two steps. In the first step, the states are restricted to prevent the formation of bonds and the Gordon–Kim approach is used. In the second step, the change in energy due to bond formation is estimated using the bond orbital model and parameters obtained from the optical reflectivity peak. Calculated bond lengths, cohesive energies, and bulk moduli for diamond, silicon, and germanium show correct trends and semiquantitative agreement with experiment in most values, but considerably less accuracy than that obtained for closed-shell systems. Consideration of the various contributions suggests that the discrepancies arise from the use of free-electron exchange and free-electron kinetic energy for a system (in step one) which is not in the ground state.
Condensation of a supersaturated vapor. III. The homogeneous nucleation of CCl4, CHCl3, CCl3F, and C2H2Cl4
Authors: Katz Joseph L. , Mirabel Philippe , Scoppa II Carman J. , Virkler Terry L.
Published: February 2003

The upward thermal diffusion cloud chamber was used to measure the supersaturations required to cause an observed rate of homogeneous nucleation of 2 to 3 drops cm−3⋅sec−1. Measurements were made on carbon tetrachloride, chloroform, Freon 11, and 1,1,2,2-tetrachloroethane over as much as a 90 K range of temperatures. The results obtained were compared to the predictions of the classical theory of homogeneous nucleation and to the predictions of the Lothe-Pound theory. The classical theory was found to be in excellent agreement with all these measurements even though measurements on chloroform and Freon 11 by another technique have been reported to agree with the Lothe–Pound theory.
A derivation and comparison of two equations (Landau–Ginzburg and Cahn) for the kinetics of phase transitions
Authors: Metiu Horia , Kitahara Kazuo , Ross John
Published: February 2003

We postulate a master equation and derive by path integral methods the Cahn equation for the most probable evolution of a nonuniform system. To do so we need to impose a constraint of local conservation of the variable of interest. The same derivation without this constraint has been shown previously to lead to a Landau–Ginzburg equation. Thus the two equations have a common origin. The Cahn equation is applicable to conserved variables in systems in which spatial inhomogeneities occur over distances much larger than a characteristic distance λ of the system (like mean free path, or nearest neighbor separation). The Landau–Ginzburg equation is applicable to nonconserved variables, and to conserved variables when spatial inhomogeneities occur over distances much smaller than the characteristic distances for those variables. For conserved variables a precursor of the Cahn equation in the derivation reduces to the Landau–Ginzburg equation as λ is increased.
Molecular statistical model of the smectic A and smectic C phases
Authors: Priest R. G.
Published: February 2003

A molecular statistical model of the smectic A and smectic C liquid crystal phases is presented. It is found that the maximum tilt angle in the C phase is 49.1°. Free rotation about the long molecular axis is allowed in the C phase. The model allows smectic C states with chiral constitutive molecules to be ferroelectric. The second rank order parameters needed to describe a smectic C state are discussed.
Collision-induced absorption of gaseous silane in the far infrared
Authors: Rosenberg Arieh , Ozier Irving
Published: February 2003

The collision-induced spectrum of SiH4 has been studied between 32 and 400 cm−1 with a Michelson interferometer. From a band shape analysis and a spectral moment calculation, it is concluded that the octopole moment Ω dominates the induction process, but that current treatments of octopolar induction cannot completely account for the data. Either these treatments are incomplete in some way, or a second induction mechanism makes a significant contribution to the absorption. If the discrepancies are attributed to the hexadecapole moment Φ, then it is found that ‖Ω‖= (2.6±0.3) ×10−34 esu cm3 and ‖Φ‖= (5.9±1.9) ×10−42 esu cm3. An algebraic error in the symmetrized calculation of Ozier and Fox (1970) is corrected and the minor changes in the spectra due to the correction are discussed.
Dielectric constant data and the derived Clausius–Mossotti function for compressed gaseous and liquid ethane
Authors: Weber L. A.
Published: February 2003

Measurements are presented for the dielectric constant of ethane in the saturated liquid and compressed fluid states at temperatures between 95 and 323 K. Pressures ranged up to 390 bar. The data, which have an estimated uncertainty of 0.01%, are combined with accurate density data from several sources to produce the Clausius–Mossotti function over a wide range of temperature and density. An analytical expression of the Clausius–Mossotti function is given, and the consistency of the available density data is discussed.
Quantum mechanical streamlines. IV. Collision of two spheres with square potential wells or barriers
Authors: Hirschfelder J. O. , Tang K. T.
Published: February 2003

Quantum mechanical streamlines and probability density contours help in the understanding of collision dynamics by showing what happens during the collision. This is illustrated by considering the elastic scattering of two particles which interact with a spherically symmetric square potential, V (r) =0 when r≳a and V (r) =C when r<a, where the constant C is negative for a potential well and positive for a barrier. The streamlines and density contours completely determine the wavefunction (which is separable in r and ϑ and independent of φ) since the pre-exponential factor is the square root of the density and the phase is everywhere perpendicular to the streamlines. The streamlines often form quantized vortices surrounding wavefunction nodes. Since the nodal regions are circular rings, the vortices are toroidal (like smoke rings). If one or both of the particles is charged, the vortex ring corresponds to a collisional magnetic moment. The distortion of the streamlines provides a visual explanation of the large collision cross sections. In S-wave resonances 4πr2=σ0, where r is the distance from the origin to the center of the outermost vortex and σ0 is the cross section of the S-wave component. Thus, r is essentially the ’’scattering length.’’ In P-wave resonances (where again r is the radial distance to the outermost vortex), 1.85πr2?σ1 and in D-wave resonances, 1.45πr2?σ2. In the square well resonances, the density inside of the interaction sphere has the shape of the dominant wave and often has maxima hundreds to thousands of times greater than the density of the incident wave. By happenstance, whenever the potential well has the proper depth to form a newly bound D state, there is a Ramsauer–Townsend effect so that (for small energy of the incident wave) the total scattering cross section is very small;outside the interaction zone, the streamlines are only slightly deflected by the potential well;inside, the streamlines are deflected towards the symmetry axis and the probability density is concentrated near this axis
Dielectric constant in terms of atom–atom correlation functions
Authors: Ho/ye Johan S. , Stell G.
Published: February 2003

A general formalism developed by us earlier is used to obtain an expression for the dielectric constant of a diatomic polar fluid in terms of atom–atom correlation functions. These functions are then shown to be of comparatively short range, decaying line r−6 for large r for almost all densities and temperatures.
Hellmann–Feynman and virial theorems and the total energy of molecules
Authors: Gopinathan M. S. , Whitehead M. A.
Published: February 2003

Using the Hellmann–Feynman and virial theorems, a simple method has been developed to correct the potential energy curve of molecules obtained by an approximate calculation. The method is applied to the potential energy curves of N2, O2, F2, and CO calculated by the multiple scattering SCF-Xα theory, as an illustration.
The semiclassical coupled states method
Authors: Billing Gert Due
Published: February 2003

A semiclassical method, in which only the translational degree of freedom is treated classically, is presented. The coupling scheme is reduced by neglecting the intermultiplet transitions. Thus, the method is the semiclassical version of the CS (coupled states) method. Some calculations on the He–H2 system are carried out. These calculations show that vibration to rotation energy conversion is dominating at low collision energies.
Distribution of the radii of gyration of star and comb random-flight chains
Authors: Gupta Santosh K. , Forsman W. C.
Published: February 2003

The distributions of the radius of gyration of some regular star and uniform comb random-flight chains have been evaluated as functions of chain length. It is observed that these distributions approach their asymptotic limit more slowly than do the linear chain molecules. This would lead to a greater dependence of the excluded volume expansion factor on molecular weight than for the linear chains.
Optical model for molecular rate processes. II. The inversion of rotationally inelastic cross sections
Authors: Roberts R. E. , DelleDonne Michael
Published: February 2003

We have used an effective optical potential in order to obtain inelastic differential cross sections as a function of kinetic energy and scattering angle for collision induced rotational excitations of H2 or D2. Our data source is from the close-coupled quantum calculations of Allison and Dalgarno for the j=0→2 transition. In each case the optical predictions are in excellent agreeement with the exact calcuations, thus providing additional validation of the optical methodology to problems of chemical dynamical interest.
The length of an isoelectronic sequence of molecules
Authors: Laurenzi Bernard J.
Published: February 2003

When studying classes of molecules, the isoelectronics are especially attractive since they possess exact interrelationships among their properties. In the work described below the length of an isoelectronic sequence is examined. It is possible to describe an upper and a lower critical charge for a sequence of homonuclear diatomic molecules. Between these critical limits of charge the behavior of the internuclear distance, dissociation energy, and force constant are given as functions of charge.
On the Lippmann–Schwinger equation for atom–diatom collisions: A rotating frame treatment
Authors: Kouri Donald J. , Heil Timothy G. , Shimoni Yair
Published: February 2003

The use of a rotating frame description of molecular collisions is reconsidered within the framework of the Lippmann–Schwinger equation for the transition or T operator. The present approach explicitly displays the proper boundary conditions which apply to descriptions of such collisions in the rotating frame whose Z axis follows the scattering vector. The resulting body frame equations are shown to lead naturally to the introduction of ’’body frame Bessel and Hankel functions,’’ 𝒥Jjλλ′ and ℋJjλλ′ (BFBF), which are solutions of the unperturbed Hamiltonian for the collision transformed to the rotating frame. It is found that the BFBF can be defined in several ways differing by phase factors that affect their asymptotic form. Two particular choices are examined, one of which leads to a simple asymptotic form of the wavefunction, and the other leads to a somewhat more complicated form. Both are shown to yield the jz-conserving coupled states equations of McGuire and Kouri but slightly different approximations are required in the two cases. The implication of these results as to the accuracy of the jzCCS method are discussed.
Anomalous temperature dependence of the frequency and damping constant of phonons near Tλ in ammonium halides
Authors: Matsushita Mitsugu
Published: February 2003

The mechanism which gives rise to the anomalous temperature dependence of nonsoft phonons near the lambda transition point in ammonium halides is discussed. The orientation of an ammonium ion in its unit cell is treated as a pseudo-Ising spin, and the Hamiltonian of a pseudospin–phonon coupled system is derived from simple assumptions. Perturbation theory is used to evaluate pseudospin–phonon scattering terms in the Hamiltonian. The resulting expressions for the frequency and the damping constant of phonons are seen to include a long-range order parameter and a correlation function of the pseudo-Ising spins, which are well known to show characteristic temperature variations near a lambda point. The results qualitatively explain the anomalous temperature variation in the frequency and the damping constant of nonsoft optical phonons near the lambda point, as measured by means of Raman scattering experiments in NH4Cl and NH4Br. The difference between the temperature dependences of the frequency in these two materials is also discussed.
Static and dynamic structure factors for interdimensional systems. II. Layered crystals
Authors: Gavish B. , Imry Y.
Published: February 2003

The displacement–displacement space–time correlation functions and the static and incoherent dynamic structure factors are calculated for a layered harmonic lattice. The continuous transition from 3D to 2D behavior of the various quantities as functions of the interlayer coupling is analyzed. Criteria for probing the 2D behavior are obtained along with suggestions and predictions for relevent scattering experiments.