bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-component lattice-gas model with a liquid–vapor phase transition in both pure components
Authors: Andersen G. R. , Wheeler J. C.
Published: February 2003

A two-component decorated lattice-gas model is presented that possesses a liquid–vapor phase transition in both pure components with a coexistence surface and critical curve which extend smoothly from one pure component to the other. The model is exactly soluble in terms of the simple one-component lattice gas of Yang and Lee (spin-1/2 Ising model) and exhibits nonclassical critical exponents. It can produce azeotropy and critical azeotropic points, and is used to analyze the vicinity of the critical azeotrope. For sufficiently repulsive interactions between primary and secondary cell occupants the model exhibits multiple phase transitions and a phenomenon analogous to saddle-point azeotropy.
Perturbed angular correlation studies of Hf binding to cyanocobalamin (vitamin B12)
Authors: Ball Jane W. , Kaplan Morton
Published: February 2003

Gamma ray perturbed angular correlation (PAC) experiments have been carried out with 181Hf labeled cyanocobalamin. Evidence is presented which strongly indicates that Hf binds to vitamin B12 at the phosphate group linking the sugar residue to a side chain of the corrin ring system. Analysis of the time-differential PAC spectra for the crystalline Hf–B12 complex indicates a static electric quadrupole interaction at the Hf nucleus, corresponding to the electric field gradient generated by the chemical bonding. The magnitudes of the derived interaction parameters are similar to those found in Hf phosphate compounds. In aqueous solution, the Hf–B12 complex exhibits PAC spectra which appear to originate from two sources. Approximately 3/4 of the Hf nuclei experience a static electric quadrupole interaction with the same characteristic interaction frequency as in the solid, but with an increased asymmetry parameter. Approximately 1/4 of the Hf signal strength is attributable to a time-dependent quadrupole interaction with a relaxation constant indicative of an effective molecular entity comparable in size to the B12 molecule. This effect may be related to molecular motion in the solution. These results demonstrate the utility of the PAC experimental method for the study of macromolecular species in both the solid and solution forms, and opens possibilities for obtaining new information concerning the structure, orientation, and behavior of macromolecules.
Molecular model of phonon sidebands in nonpolar molecular crystals
Authors: Kunsch P. L.
Published: February 2003

The use of a molecular model to calculate mechanically and electrically induced phonon shapes in molecular crystals is discussed. The model rests on the defect space assumption also made in the standard use of Green’s functions. It has, however, the merits to be much more simple and transparent in terms of the site group symmetry. Comparison with complete fcc latice calculations (Ar, N2, CO) shows a rapid convergence of weighted and unweighted densities of states with number of shells. Raman and allowed and forbidden IR spectra are presented as further illustrations. One exception to rapid convergence is found for mechanically coupled phonon bands involving very specific breathing motions.
Cross relaxation in NMR studies of crystalline symmetrical trifluorobenzene (C6H3F3)
Authors: Albert Shmuel , Ripmeester John A.
Published: February 2003

Crystalline symmetrical trifluorobenzene (C6H3F3) was studied by 1H and 19F broadline and pulsed NMR techniques between ∼189 °K and its melting point (267.6 °K). The 1H and 19F static frame spin–lattice relaxation is nonexponential in the entire temperature region, and can be described as a sum of two exponential terms. The same pair of time constants were observed for both nuclei at each temperature. This behavior is interpreted in terms of cross relaxation between proton and fluorine nuclei, which originates from fluctuations of the H–F dipolar interactions, which are due to in-plane rotation of the molecule about its triad symmetry axis. The rotating frame spin–lattice relaxation was found to be exponential for both nuclei in the entire temperature range of investigation. A decrease in the 1H and 19F second moments and line widths is obvserved on heating. This is consistent with the onset of the in-plane threefold rotation. In accordance with this line narrowing, a minimum in the T1 vs T−1 curves were obtained for 1H and 19F nuclei along with corresponding minima in the T1ρ vs T−1 plots. Calculations of the T1 and T1ρ temperature dependence using an activation energy (Ea) of 9.0±0.4 kcal/mole and a pre-exponential factor (τ0) of 2.4(±0.2) ×10−15 sec for the motion is in agreement with our experimental results.
The infrared dispersion of hydrogen fluoride
Authors: Wickliffe Morris , Rollefson R.
Published: February 2003

The index of refraction of HF has been measured at 22 wavelengths between 1.150 μm and 8.885 μm. The results have been used to determine the integrated intensity of the vibration–rotation fundamental and the dipole moment of the nonvibrating molecule. These values are S= (377±12) cm−2 atm−1 (at 300 K) and μ0=[(1.84±0.02) ×10−18]esu⋅cm.
The virial theorem and stress calculation in molecular dynamics
Authors: Tsai D. H.
Published: February 2003

In molecular dynamics, the pressure in a homogeneous system in equilibrium may be calculated by two different methods. The first is based on the virial theorem of Clausius and gives the pressure at the boundary of the system. The second is based on the notion of stress, which is the sum of the appropriate components of the interatomic forces intercepted by an area, and of the components of momentum flux across the area, averaged over the area and over time. We show by means of a detailed comparison of the forces involved that the two methods are equivalent in the thermodynamic limit. In a small system with arbitrary boundary conditions, the neglect of a part of the interactions between the system and the wall results in some error in the pressure calculated by the virial method. In the special case of a system with periodic boundaries, there is no external ’’wall,’’ and the internal pressures calculated by the two methods are the same. However, with comparable effort in computation, the stress method makes more efficient use of the data and yields a result of greater precision than does the virial method. In a system not in equilibrium or not homogeneous, the stress method remains valid but the virial method leads to ambiguous results. These considerations indicate that the method of stress calculation is more general than the virial method.
The infrared spectrum of crystalline p-chloroaniline at low temperature
Authors: Tripathi G. N. R. , Katon J. E.
Published: February 2003

High resolution spectra of oriented polycrystalline films of p-chloroaniline were recorded at a nominal temperature of 20 K using polarized infrared radiation and interpreted on the basis of the reported crystal structure. The observed spectra are characteristic of an [hkl] crystal plane. The crystal frequencies are classified into the three infrared active factor group symmetry species B1u, B2u, and B3u of the D2h factor group, having electric dipole moment vectors along the crystal axes a, b, and c, respectively. The symmetry types of the internal molecular modes are determined on the basis of the intensity patterns of their Davydov components and correlation between the molecular and factor groups. Some of the vibrational assignments are discussed.
Populations of BaO states in the Ba+N2O chemiluminescent flame using the BaO C 1Σ+ state as a probe
Authors: Torres-Filho Afranio , Pruett J. Gary
Published: February 2003

The reaction Ba+N2O→BaO+N2 in the presence of argon buffer gas (25 μ) has been studied using laser induced fluorescence to probe the nonchemiluminescent states present. We have observed ground electronic state (X 1Σ+) vibrational levels from v″=7 to v″=32 by exciting them using a N2 laser pumped tunable dye laser to various vibrational levels of a highly excited electronic state (C 1Σ+) and observing the resulting ultraviolet fluorescence to low ground state vibrational levels (v″<12). Low ground state vibrational levels (v″=0–9) have also been observed by exciting the molecules to the A 1Σ+ state and observing vibrationally resolved visible fluorescence. We find the C 1Σ+ state to be highly perturbed, but still useful as a population probe for the X 1Σ+ state. We measure the C 1Σ+ radiative life time to be 25±10 nsec. The X 1Σ+ vibrational population from v″=5 out to v″=32 has a characteristic temperature of about 3600 °K. We find evidence of nonthermal population of v″=13–16. We also observed population in the A′1π and the a 3π states with v″=0–4. The implications of these results on the identity of the nonchemiluminescent precursor states in the Ba+N2O reaction are discussed.
The vibrational spectrum and adsorption site of CO on the Ru(001) surface
Authors: Thomas G. E. , Weinberg W. H.
Published: February 2003

Vibrational spectra and LEED structures of CO on the Ru(001) surface were measured as a function of coverage at 100 K. The LEED patterns indicate that CO occupies a variety of asymmetric sites at high coverage, while the vibrational spectra show a single Ru–C stretching frequency (445 cm−1) and a single C–O stretching frequency (1980–2080 cm−1) at all coverages. This shows that on this surface the CO adsorption site at high coverages is determined by interadsorbate interactions, and that the carbon–oxygen stretching frequency is not site sensitive within the resolution of electron scattering measurements.
Random-phase circular dichroism calculations of the σ/3s←n transition in chiral alcohols
Authors: Texter John , Stevens Eugene S.
Published: February 2003

The circular dichroism of the long wavelength electronic transition in 10 chiral alcohols is calculated in the random-phase approximation using time-dependent Hartree theory. This long wavelength transition (1850–2000 Å) which is associated with the hydroxyl group is assumed to be largely localized on the oxygen atom and to originate in nonbonding density of the ground state, namely in an oxygen 2p orbital. The upper state is mixed, having both intravalence σ and Rydberg 3s character. The σ*/3s←n electric-dipole transition moment (μe) is perpendicularly polarized with respect to the COH plane, while the associated magnetic-dipole transition moment (μm) is polarized in plane. The circular dichroism is calculated as a function of hydroxyl rotation about the CO bond by coupling the zeroth-order σ*/3s←n transition moments to the backbone CC and CH bond density of states via a polarizability approximation. These values are then Boltzmann averaged over an empirical rotational potential, and the resulting circular dichroism is satisfactorilly compared with experiment. The empirical potential functions for hydroxyl rotation were found to be in as good or better agreement with experiment as ab initio potentials in the case of simple alcohols. The 10 alcohols c chiroptically studied belong to three different structural classes, borneols, decalols, and sterols, all of which have well defined, rigid, cyclic carbon backbone structures. The extent of hydroxyl rotational freedom among these alcohols varies from the highly restricted case of a single potential well to the more conventional case of a trimodal staggered potential with barriers less than 1.5 kcal mol−1. This potenital function variability, in addition to the calculated sensitivity of the alcohol classes to the direction of polarization of the σ*/3s←n μm, forms a basis for the following conclusion. The low order σ*/3s←n transition mechanism invoked is sufficient to give qualitative agreement with previously reported circular dichroism values. The sensitivity of the sterols and the insensitivity of the borneols to the in-plane μm polarization suggests that the σ* component of the upper state has greater antibonding character along the CO bond than along the OH bond. The coupling of the σ*/3s←n transition to the backbone density of states is dominated energetically by electric–dipole, electric–dipole interaction terms in the context of Schellman’s μeμm-coupling mechanism of rotatory power and the σ*/3s←n electric–dipole moment provides a basis for explaining the sector rule proposed by Kirk, Mose, and Scopes for saturated chiral alcohols.
Predissociation lifetimes of the rotational and fine structure levels of O2+(b4Σ−g, v=3,4,5)
Authors: Moseley J. T. , Cosby P. C. , Ozenne J.-B. , Durup J.
Published: February 2003

A fast ion beam coaxial with a single mode laser has been used to measure the absorption linewidths of predissociated levels of O+2(b4Σg−, v=3,4,5) by detection of the O+ photofragments. Tuning of the absorption wavelength was accomplished by a combination of intracavity etalons and variation of the ion beam velocity (Doppler tuning). Lifetimes were determined for the b state fine structure levels of rotational levels N=31 and 33 of 16O2+(v=3), N=9–25 of 16O2+(v=4), N=14–27 of 16,18O2+(v=4), and N=5–7 of 16O2+(v=5). The lifetimes vary from 0.06 to 4 nsec. Variation with all relevant quantum numbers and with isotopic composition is discussed.
Scattering of thermal He beams by crossed atomic and molecular beams. IV. Spherically symmetric intermolecular potentials for He+CH4, NH3, H2O, SF6
Authors: Slankas John T. , Keil Mark , Kuppermann Aron
Published: February 2003

Differential scattering cross sections are measured for He+CH4, NH3, H2O, and SF6, using the crossed molecular beams technique. These data, which are sensitive to the van der Waals attractive minima and adjacent regions of the intermolecular potential, are interpreted in terms of central-field models. No evidence is found for quenching of the observed diffraction oscillations. The interactions of the isoelectronic hydrides CH4, NH3, H2O with He are found to have decreasing van der Waals radii in this sequence, and their attractive wells all have similar depths. However, the He+SF6 attractive well is found to be anomalously deep, and provides a counter example to the supposition that only the polarizability of the least polarizable of the interacting partners (atoms or molecules) correlates with the van der Waals well depth. Simple combination rules for predicting unlike-pair potential parameters from the corresponding like-pair ones are tested and found inadequate.
An ESR study of the radical Ph2SeCl trapped in a single crystal matrix
Authors: Geoffroy M.
Published: February 2003

The x-irradiation of a single crystal of Ph3SeCl produces the formation of a radical showing hyperfine interaction with 35Cl and 77Se. The ESR tensors are determinated: g1=1.998, g2 =2.029, g3=2.052, T1(35Cl) =138 MHz, T2(35Cl) =30 MHz, T3(35Cl) =27 MHz and T1(77Se) =987 MHz, T2(77Se) =132 MHz, T3(77Se) =29 MHz. The angles between the g1, T1(35Cl) and T1(77Se) directions are less than 9°. The sign assignments corresponding to a total spin density close to unity lead to the following spin populations: c2p (Cl) =0.26, c2p (Se) =0.80. The radical is identified as being Ph2SėCl and is compared with its homologous Me2ṠBr.
Properties of the hydracids trapped in a nitrogen matrix: Experimental and static field approximation study
Authors: Girardet C. , Maillard D. , Schriver A. , Perchard J. P.
Published: February 2003

The properties of the hydracid monomers and dimers trapped in N2 matrices are studied through an experimental reinvestigation of the near and far infrared spectra. In the nir spectra, two kinds of dimers are identified: a nearest-neighbor (nn) dimer in which the two molecules are nonequivalent and a next-nearest-neighbor (nnn) dimer giving rise to a specific absorption very close to the monomer signal. New data in the fir confirm the previous assignment of the librational modes of the monomer but do not allow the identification of the corresponding dimer bands. These results are interpreted according to intermolecular potential calculations, taking into account the angular distortion of the matrix molecules around the HX impurity. An analytical description of the orientation quantum states for the impurities is developed and conveniently explains the fir monomer data by the lifting of the symmetry properties of the perfect crystal. The theoretical frequencies are in much better agreement with experimental when the N2 crystal dynamics is included. In the same way, calculations performed on the dimers show they lose their entity properties in the N2 crystal. Moreover, the librational response of the (HX)2 species, found in the same frequency range as the monomer, may account for a surprising discrepancy between experimental and calculated fir monomer band intensity ratio. The results obtained in the nir lead to a crucial modification of the electric multipole derivatives with respect to the internuclear coordinate in the hydracid dimers, which is explained in terms of a charge transfer process between the two molecules.
Rate constant for the reaction of atomic chlorine with formaldehyde from 200 to 500 K
Authors: Michael J. V. , Nava D. F. , Payne W. A. , Stief L. J.
Published: February 2003

The absolute rate constant for the reaction of atomic chlorine with formaldehyde has been measured from 200 to 500 K using the flash photolysis–resonance fluoresence technique. The results were independent of substantial variations in [H2CO], total pressure (Ar) and flash intensity (i.e., initial [Cl]). The rate constant was shown to be invariant with temperature, the best representation for this temperature range being k1= (7.48±0.50) ×10−11 cm3 molecule−1 s−1, where the error is one standard deviation. This result is compared with the only previous determination of k1, which was a relative value obtained at 298 K. The rate constant is theoretically discussed and the potential importance of the reaction in stratospheric chemistry is considered.
Electron attachment to perfluorocarbon compounds. I. c-C4F6, 2-C4F6, 1,3-C4F6, c-C4F8 and 2-C4F8
Authors: Christodoulides A. A. , Christophorou L. G. , Pai R. Y. , Tung C. M.
Published: February 2003

Electron attachment rates αw, as a function of the pressure-reduced electric field E/P298 and mean electron energy <ε≳ have been measured for trace amounts (<10−3 Torr) each of c-C4F6 (perfluorocyclobutene), 2-C4F6 (perfluoro-2-butyne), 1,3-C4F6 (perfluoro-1,3-butadiene), c-C4F8 (perfluorocyclobutane), and 2-C4F8 (perfluoro-2-butene) in mixtures with N2 (pressures 500–2000 Torr) and Ar (pressures 500–1500 Torr) at T=298°K. The thermal attachment rates for these molecules were found to be 4.89×109, 1.77×109, 4.26×109, 4.05×108, and 1.56×109 sec−1 Torr−1, respectively. From the αw (<ε≳) data, the attachment cross sections σa(ε) as a function of electron energy ε have been determined using the swarm-unfolding technique and are reported. For all five perfluorocarbon molecules, σa(ε) are very large. They exhibit three distinct negative ion resonances with maxima at: ∼0.0, 0.19, and 0.80 eV for 2-C4F6; ∼0.0, 0.17, and 1.04 eV for 1,3-C4F6; ∼0.0, 0.22, and 0.48 eV for c-C4F8; and ∼0.0, 0.18, and 0.59 for 2-C4F8. For c-C4F6, only a shoulder with a possible maximum at ∼0.14 eV was observed, in addition to the maxima at ∼0.0 and 0.71 eV. Although the positions of the maxima in σa(ε) vary only slightly among these compounds, the magnitude of σa(ε) depends strongly on structure.
Infrared laser photochemistry of silane
Authors: Deutsch Thomas F.
Published: February 2003

The infrared photochemistry of silane has been investigated using CO2 TEA laser excitation. Energy deposition measurements show that only 1.5 photons/molecule are absorbed at a pressure of 1 Torr and a P (20) fluence of 0.8 J/cm2, while at 10 Torr 7.5 photons/molecule are absorbed. The visible luminescence that accompanies the dissociation of silane is due to molecular and atomic hydrogen. The thresholds for both luminescence and dissociation increase sharply as the pressure drops below 5 Torr, indicating that dissociation is a collisional process. Silane d1 and d2 have been produced by irradiating silane–deuterium mixes.
Limiting behavior of 2H spin lattice relaxation in molecules with multiple internal rotations
Authors: Kuo Wen-Sheng , Jacobus O. J. , Savitsky G. B. , Beyerlein A. L.
Published: February 2003

Expressions for the dependence of deuteron relaxation times for CD bond vectors on the number of internal rotations, N + 1, in polymeric chains and side chains of micelles and phospholipid vesicles have been derived for large N. The expressions are valid for molecules in which : (i) the diffusion tensors have ellipsoidal symmetry,(ii) all angles between successive pairs of internal rotation axes are equal, and (iii) the internal rotations are independent and all diffusion coefficients for internal rotation are equal. For molecular entities with spherically symmetric rotational diffusion tensors, the asymptotic relation simplifies to the following relation: where DO is the rotation diffusion coefficient, D the diffusion coefficient for internal rotation, ω0 is the magnetic resonance frequency, and QD is the quadropole coupling constant for the CD bond vector. For hydrocarbon chains the relaxation rate is within 10% of its asymptotic value for relatively small N (about 6) and converges rapidly to the asymptotic relaxation rate as N is increased.
On the calculation of polyatomic Franck–Condon factors: Application to the 1A1g→1B2u absorption band of benzene
Authors: Faulkner Thomas R. , Richardson F. S.
Published: February 2003

Two methods for calculating polyatomic Franck–Condon integrals are reported. The first method uses a coordinate transformation on the normal coordinates of both the ground and excited electronic states. This transformation effectively removes any Duschinsky mixing and allows the multidimensional Franck–Condon integral to be written as a sum of integrals each of which is a product of one-dimensional harmonic oscillator overlap integrals. The second method uses contact transformation perturbation theory to construct a representation of the vibrational wavefunctions. With this representation, the calculation of a polyatomic Franck–Condon integral involves evaluation of matrix elements exclusively within the ground electronic state vibrational manifold. Application of both methods is made to the A0n vibronic series of the 1A1g→1B2u symmetry-forbidden electronic transition of benzene. Relative intensities calculated by either method agree well with observed values. However, the computational efficiencies of the two methods are found to be markedly different, with the perturbation method being the least efficient.
Kinetics of swelling of gels
Authors: Tanaka Toyoichi , Fillmore David J.
Published: February 2003

We present a theory of the kinetics of the swelling of a gel. The characteristic time of swelling is proportional to the square of a linear dimension of the gel and is also proportional to the diffusion coefficient of the gel network, which is defined as D=E/f where E is the longitudinal bulk modulus of the network, and f is the coefficient of friction between the network and the gel fluid. This constitutes an essential difference between the present theory and the previous theory which is based on the assumption that the swelling time is determined by the diffusion coefficient of the fluid molecules. Experimental data are shown for spheres of 5% polyacrylamide gels and are analyzed using the present theory. The value of the diffusion coefficient obtained from the macroscopic swelling experiments shows excellent agreement with that obtained microscopically using laser light scattering spectroscopy.
Infrared spectra of chemisorbed CO on Rh
Authors: Yates J. T. , Duncan T. M. , Worley S. D. , Vaughan R. W.
Published: February 2003

The infrared spectrum of CO chemisorbed on alumina-supported Rh atoms has been investigated. In agreement with previous work, three types of adsorbed species have been clearly distinguished on the basis of their C–O stretching frequencies. Species I, assigned as Rh(CO)2, is formed only with Rh atoms which are isolated from each other. Species II, assigned as Rh–CO, and III, assigned as Rh2CO, are formed on Rh clusters having two or more Rh atoms. CO-species II and III undergo interactions with neighbor CO species causing an increase in wave number as coverage increases. Based on infrared intensity measurements for species I, the OC–Rh–CO angle is ∼90°. Chemisorbed 13CO yields the expected infrared spectrum on Rh, and rapid isotopic exchange between 13CO(ads) and 12CO(g) is observed which cannot be explained by the observed rate of desorption of CO from the supported Rh surface.
Catalytic decomposition of formaldehyde on single rhodium atoms
Authors: Yates J. T. , Worley S. D. , Duncan T. M. , Vaughan R. W.
Published: February 2003

The interaction of H2CO with alumina-supported Rh has been studied by transmission infrared spectroscopy. It has been shown that isolated Rh atoms catalyze the decomposition of H2CO to produce carbonyl hydride species HxRh–CO (x=1,2). Species of the type Rh(HCO) and Rh(HCOH) are not observed to form in detectable amounts. Hydride bonding to rhodium carbonyl species is associated with electron donation to Rh causing a decrease in ?CO due to electron donation into 2π*-CO orbitals. Hydride displacement by CO may occur on exposure to CO(g). The conversion of the species H–Rh(CO)2 to H–Rh(CO) has been followed spectroscopically during desorption.
Laser-stimulated migration of adsorbed atoms on solid surfaces
Authors: Slutsky M. S. , George Thomas F.
Published: February 2003

This paper presents a unified theory of the coherent, multiple-photon excitation and laser-stimulated migration of atoms adsorbed on solid surfaces. We show that under steady-state excitation the migration of the adsorbed atom on the surface is diffusional, with diffusion constant dependent upon field intensity, surface temperature, relaxation constants, dipole moment strength, laser frequency, and the vibrational frequency of the adsorbed atom. In particular, it is shown that the rate of diffusion is proportional to the field intensity. Finally, we discuss the utility of laser-stimulated migration in the enhancement of surface chemical reactions, as well as in the analysis of surface relaxational processes.
Diffusion and relaxation kinetics in stochastic models for crystal growth
Authors: Saito Yukio , Müller-Krumbhaar H.
Published: February 2003

The combined influence of relaxation and diffusion processes on the dynamic behavior of stochastic models for crystal growth is systematically investigated. The models are the kinetic Ising model, the discrete Gaussian model, and the SOS–Kossel model. The relaxation kinetics are introduced by single-site transition probabilities to account for adsorption and evaporation. The diffusion kinetics are introduced by two-site transition probabilities to account for nearest-neighbor exchange. The kinetic equations are studied by Monte Carlo simulation, quasichemical(pair) approximation (QCA), and high temperature expansion. The diffusion is found to generally enhance the growth rate of the crystal. In the range of validity of the QCA, i.e., outside the nucleation regime, the results are in very good quantitative agreement with previous Monte Carlo simulations. In the limit of infinite diffusion speed the growth rate approaches the Wilson–Frenkel rate. The local roughness or surface energy is reduced if surface diffusion occurs. The relative influence of diffusion on the response of the system to a chemical potential difference between the two phases depends significantly upon the particular type of transition probability. For the homogeneous Glauber–Ising chain no influence of the diffusion upon the dynamic behavior could be detected.
Ionic clusters isolated in a metal matrix: The structure and vibrational frequencies of Rb9O2 and Cs11O3 clusters in alkali metal suboxides
Authors: Martin T. P. , Stolz H.-J. , Ebbinghaus G. , Simon A.
Published: February 2003

The stable configurations of Rb9O2 and Cs11O3 clusters are determined by minimizing the total energy assuming purely ionic, two particle interactions. The calculated interatomic distances and angles are found to correspond very closely with those which have been determined from a crystallographic study of alkali metal suboxides. Raman spectra are reported for Rb9O2, Cs11O3, Cs7O, and Rb7Cs11O3 and a comparison made with vibrational frequencies calculated using the same ionic model. The agreement is good. These results offer further evidence that the crystalline alkali metal suboxides may be thought of as composed of independent ionic clusters isolated in a metallic environment.
The orthogonal gradient method. A simple method to solve the closed-shell, open-shell, and multiconfiguration SCF equations
Authors: Golebiewski A. , Hinze Juergen , Yurtsever E.
Published: February 2003

A new, simple and efficient method of solving closed-shell, open-shell, and multiconfiguration SCF orbital equations is presented. It is based on the orthogonal gradient approach introduced originally in conjunction with the maximum overlap principle. Semiempirical test calculations show that the method converges rapidly.
Resonance fluorescence, Raman spectra and relaxation of single vibronic levels in the condensed phase: Azulene in naphthalene
Authors: Hochstrasser Robin M. , Nyi Corinne A.
Published: February 2003

Resonance fluorescence and Raman spectra for excitation of single vibronic levels of the first excited singlet state of azulene in a naphthalene mixed crystal at 2 K are described. The intensity distributions for the resonance emission spectra were used to verify and in some cases reassign previously proposed correlations of the ground and excited state a1 modes. Relaxed fluorescence from levels populated via vibrational relaxation from the resonant vibronic level was observed and measurements of the population ratios for the vibronic levels relative to the zero-point level are given. From these ratios the following two conclusions were made. (1) In addition to the zero-point level, only strongly absorbing nearby vibronic levels are significantly populated through vibrational relaxation from the resonantly pumped level for excitations up to ≈1560 cm−1 above the zero-point level. This suggests that there is little cascading within the S1 vibrational manifold and that vibrational relaxation occurs largely by V–V energy transfer to the naphthalene host modes. (2) The measurements of the population ratios indicate that the vibrational relaxation times of most of the pumped levels are longer than the mean nonradiative decay time for the levels into which their relaxation occurs. When the nonradiative decay time is taken as 2.6 ps, the following values for the total vibrational relaxation times of the lower a1 vibronic levels were found: 8.7 (384), 7.4 (665), 10.8 (856), 1.3 ps (912). Comparison of the relative intensities of the Raman scattering and relaxed fluorescence from the zero-phonon level upon excitation of the phonon sideband of the 0–0 transition led to a value of ≈0.2 ps for the phonon relaxation lifetime. Spectral distinctions between resonance Raman scattering and fluorescence were examined through the temperature dependence of the spectrum of the emission onto the 825 cm−1 ground state vibrational level over the range 4.2–35 K. The total dephasing decay constant T2 was calculated from the ratio of the contributions of the fluorescence and Raman scattering to the emission and from the fluorescence linewidth at various temperatures. The values ranged from ≈5.4 ps at 4.2 K to ≈1.5 ps at 35 K and were similar at a given temperature for both methods of calculation indicating that the same dephasing mechanisms are responsible for both effects. The temperature dependence of the fluorescence linewidth and frequency shift is consistent with mechanisms of dephasing involving phonons of either a single mode or all thermally accessible modes. Finally, vibrational and electronic dephasing are compared.
The temperature dependence of electron attachment to N2O in the gas phase
Authors: Shimamori Hiroshi , Fessenden Richard W.
Published: February 2003

The temperature dependence of electron attachment to N2O has been measured in the gas phase for pure N2O and N2O–C2H6 and N2O–neopentane mixtures. The rate constant found for e−+N2O→N2+O− can be expressed as 2.2×10−7 exp(−5.18×103/T) cm3/molecule⋅sec while that for e− +N2O→N2O−* is 4.2×10−9 exp(−2.65×103/T) cm3/molecule⋅sec. The corresponding activation energies are 0.44 and 0.23 eV. The latter value is discussed by reference to recent calculations on the N2O− energy surface. Formation of N2O−* with such an activation energy seems possible. The temperature dependences of other reactions in the overall mechanism are also determined. Certain rate constants increase with a decrease in temperature. Formation of negative ion complexes is suggested to account for the observation.
Raman spectroscopy of strontium tetraiododiargentate octahydrate, SrAg2I4⋅8H2O, and its aqueous solutions
Authors: Habbal F. , Zvirgzds J. A. , Hurd A. J. , Scott J. F.
Published: February 2003

Raman spectroscopy of SrAg2I4⋅8H2O reveals three low frequency vibrations at 25, 65, and 105 cm−1. The 25 cm−1 line is extremely intense, has a narrow linewidth, and is assigned as a vibration of the Ag–I bond. This mode persists in the melt, verifying the hypothesis of Geller and Dudley that AgI4 entities are present in the saturated solution. Autocorrelation spectroscopy measurements on the saturated solutions do not reveal diffusion of these complexes; however, similar measurements on diluted solutions show diffusion of precipitated AgI.
Transport processes in compressed liquid pyridine
Authors: Fury M. , Munie G. , Jonas J.
Published: February 2003

The self-diffusion coefficients of liquid pyridine have been measured as a function of temperature and pressure over the density range 2.7⩽ρ/ρc⩽3.7 and the temperature range 0.5⩽T/Tc⩽0.7 using the NMR spin–echo technique. The shear viscosities and densities have also been measured under the same experimental conditions. The experimental diffusion and viscosity data are interpreted in terms of the rough hard sphere (RHS) model of liquids and the RHS parameters calculated are compared to those obtained for liquid benzene, a molecule of nearly identical shape and size but lacking the dipole moment of pyridine and its specific intermolecular interactions. A qualitative discussion of the experimental results is presented. The Stokes–Einstein relationship between D and η is found not valid for liquid pyridine.
Excited electronic states of cyclohexene, 1,4-cyclohexadiene, norbornene, and norbornadiene as studied by electron-impact spectroscopy
Authors: Frueholz Robert P. , Flicker Wayne M. , Mosher Oren A. , Kuppermann Aron
Published: February 2003

The excited electronic states of cyclohexene, 1,4-cyclohexadiene, norbornene (bicyclo[2.2.1]-2-heptene), and norbornadiene (bicyclo-[2.2.1]-2,5-heptadiene) have been studied by electron impact at scattering angles from 5° to 80°, and impact energies of 30 and 50 eV. Low-lying features with intensity maxima at 4.24 eV in cyclohexene and 4.10 eV in norbornene are identified as singlet → triplet transitions. Similar features in the spectra of 1,4-cyclohexadiene and norbornadiene extending from 3.4 to 5.4 eV and 2.9 to 4.5 eV, respectively, are believed to result from superposition of two low-lying singlet → triplet transitions in each molecule. In norbornadiene these features have estimated intensity maxima at 3.4 and 3.9 eV, while in 1,4-cyclohexadiene they appear to be more highly overlapped, yielding a single intensity maximum at 4.29 eV. The singlet → singlet excited state spectra of these molecules are discussed from the point of view of a model in which ethylene units interact via through-bond and through-space effects. In each of these four molecules, transitions to several superexcited states are observed.
Virial scaling corrections to Koopmans’ ionization potential in positive ions of He, Ne, and Ar series
Authors: Sen K. D.
Published: February 2003

The ionization potentials calculated for the positive ions isoelectronic with He, Ne, and Ar, respectively, using the virial scaling method are found to be in good agreement with the results obtained from the ΔSCF method.
Temperature dependence of rotational linewidths in HCl perturbed by argon
Authors: Smith E. W. , Giraud M.
Published: February 2003

The temperature dependence of the half-widths of HCl rotational spectra perturbed by Ar atoms is calculated for temperature range 12.5−1200 K.(AIP)
The difference band υ1−υ2 of 12C32S2
Authors: Jolma K. , Kauppinen J. , Anttila R.
Published: February 2003

The infrared spectra of CS2 is reported around 260 cm−1. The spectrum was recorded with a Fourier spectrometer with a resolution of 0.01 cm−1. (AIP)
Radiation-induced 237Np charge states from 241Am–CaF2a)
Authors: Karraker D. G. , Stone J. A.
Published: February 2003

Mössbauer studies were done on 241Am–CaF2 and 241Am–CdF2 sources to detect charge states of 237Np following γ-decay of 241Am in the sources. A Np4+ charge state was found to be radiation-generated in the 241Am–CaF2 source, and 241Am–CdF2 sources showed Np44 and Np5+ charge states that were not formed by radiation. (AIP)
Observation and bond dissociation energy of gaseous AuV by high temperature mass spectrometry
Authors: Gupta S. K. , Pelino M. , Gingerich K. A.
Published: February 2003

The bond dissociation energy of gaseous AuV is reported in a study of Knudson effusion mass spectrometry. The system V-Nb-Mo-Au was studied at 1824–2050°K. (AIP)
Model networks of end-linked polydimethylsiloxane chains. III. Effect of the functionality of the cross-links
Authors: Mark J. E. , Rahalkar R. R. , Sullivan J. L.
Published: February 2003

Fractions of hydroxyl-terminated polydimethylsiloxane (PDMS) of relatively narrow molecular weight distribution were cross-linked by reacting the hydroxyl chain ends, in the undiluted state, with a trifunctional orthosilicate. Stress–strain isotherms were obtained on the resulting networks in elongation at 25° C, and were represented in terms of the equation [f*]=2C1+2C2α−1, where [f*] is the reduced stress or modulus and α is the elongation of the network. The present results on the trifunctional PDMS networks were compared with previously reported results on the corresponding tetrafunctional networks, thereby characterizing the effect of changing the network functionality φ from four to three. The observed changes were found to be in good agreement with predictions based on the most recent molecular theories of rubberlike elasticity. Specifically, experiment and theory were found to be in good agreement with regard to the factor which relates the modulus 2C1 to the structure of the network, and with regard to the ratio 2C2/2C1 which is a measure of the extent to which the elastic deformation changes from affine to nonaffine with increasing stress.
Vibrational force constants for diatomic molecules from rigid pseudoatoms
Authors: Epstein Joel , Stewart Robert F.
Published: February 2003

The quadratic vibrational force constant for a diatomic molecule can be evaluated from generalized x-ray scattering factors if the expansion contains at least dipole scattering factors and if the first derivative of these functions with respect to R are known. Rigid pseudoatoms are defined as the finite multipole set, [L‖K], of generalized x-ray scattering factors determined at Re. For this case, the quadratic force constant estimate depends on the pseudoatom charge density on the neighboring nucleus and on the pseudoatom contribution to the field gradient about the neighboring nucleus as shown by Bader and Bandrauk [J. Chem. Phys. 49, 1666 (1968)]. A previous estimate for H2 is shown to be in error. The estimates of ke for N2, CO, BF, and FH from [2‖2] rigid pseudoatoms are calculated and prove to be too large by more than 50%. The radial relaxation terms reduce these values to the correct ke.
The magnetically ordered state of (CH3)2NH2MnCl3
Authors: Buys J. A. H. M. , van Workum G. J. M. , de Jonge W. J. M.
Published: February 2003

The compound (CH3)2NH2MnCl3 (DMMC) orders antiferromagnetically at TN=3.60 K. Nuclear magnetic resonance experiments have been performed to determine the magnetic ground state. The magnetic moments (4.0 μB) directed in the a* direction, form antiferromagnetic chains in the c direction. Susceptibility measurements in the ordered state are in accordance with the determined structure.
Statistical mechanics of rigid particles: A general zero-separation theorem
Authors: Devore Jerald A. , Mayer Joseph E.
Published: February 2003

Modified distribution functions are defined by F*n(r1,⋅⋅⋅,rn) = Fn(r1,⋅⋅⋅,rn) ⋅ exp[Un(r1,⋅⋅⋅,rn)/kT], where Fn(r1,⋅⋅⋅,rn) is the distribution function and Un(r1,⋅⋅⋅,rn) is the mutual potential energy for a set of n molecules. These modified functions are equivalent to those of empty domains (cavities) and are used to derive several general zero-separation relations for rigid particle fluids.
The stability of molecular motion and intramolecular energy transfer
Authors: Hänsel Klaus Dieter
Published: February 2003

In this article a discussion of the relationship between intramolecular energy transfer and the stability of molecular motion is discussed on the basis of classical mechanics. Assuming that energy randomization is determined by the onset of orbital instability, local and global stability concepts are discussed. It is shown that a parameter characterizing the ergodicity of a molecular system may be given as a functional of the potential only. Applications to molecular problems are sketched. A generalization of the techniques discussed in this paper to quantum mechanical systems seems to be feasible.
Exact hard-disk free volumes
Authors: Hoover William G. , Hoover Nathan E. , Hanson Kenton
Published: February 2003

Properties of exact hard-disk free volumes are determined by a combination of analytical and numerical techniques. Both the high-density fluid phase and the lower-density fluid phase are treated. These one-particle free volumes are used to verify known thermodynamic information for hard disks and to calculate the shear modulus for the hard-disk solid phase. The free volumes are also compared to approximate free-volume estimates made from the known hard-disk entropy. The statistical distributions of free volume and free surface (perimeter of the free volume) are studied. The percolation transition, at which the free-volume changes from extensive to intensive, is found to occur at about one-third of the freezing density.
Stable limit cycles in a two-component bimolecular reaction system
Authors: Kádas Z. , Othmer H. G.
Published: February 2003

It is shown that stable limit cycles can exist in a two-species open system in which only one bimolecular reaction that follows mass action kinetics occurs. The fluxes of the active species between the system and bath are uncoupled and depend linearly on the chemical potential differences. This appears to be the simplest system known that exhibits periodic solutions and multiple steady states.
Dephasing times and linewidths of optical transitions in molecular crystals. Temperature dependence of line shapes, linewidths, and frequencies of Raman active phonons in naphthalene
Authors: Bellowsa) James C. , Prasadb) Paras N.
Published: February 2003

The temperature dependence of linewidths, line shapes, and frequencies is investigated for Raman active librational phonons of naphthalene in order to derive the mechanism of optical dephasing for phonon excitations and to measure their dephasing times. Theoretical background is presented on various manifestations of anharmonic interactions particularly in relation to dephasing and relaxation. For the case of the naphthalene crystal, it is shown that the temperature dependences of the phonon frequencies and of their respective linewidths do not manifest by the same mechanism. The dominant contribution to the temperature dependence of phonon frequencies is from the thermal expansion of the lattice. On the other hand, the line shapes are asymmetric Lorentzian and the temperature dependence of the linewidth is consistent with the dephasing process due to T1 relaxations in which a phonon exchanges energy with two other phonons through cubic anharmonic interaction.
Radial distribution function for hard disks from the BGY2 theorya)
Authors: Ueharab) Yosei , Ree Taikyue , Ree Francis H.
Published: February 2003

The Born–Green–Yvon (BGY) hierarchy of equations is truncated by using the Fisher–Kopeliovich approximation for the quadruplet distribution function. The resulting BGY2 equations are solved for hard disks at five different densities. For comparison purposes, the radial distribution functions g (r) from three first-order theories [the BGY1, the Percus–Yevick (PY1), and the convolution–hypernetted-chain (CHNC1) theories] and the Monte Carlo (MC) g (r) are also obtained. At densities less than 47% of the close-packed density, the BGY2 g (r) gives the best agreement with the MC data. At higher densities the BGY2 g (r) agrees well with the MC g (r) at short distances (r?1.7σ : σ=disk diameter), but the PY1 g (r) appears to be better at larger distances. The results at larger distances are somewhat ambiguous on account of the numerical uncertainties associated with the BGY2 solutions. The pressures predicted from the BGY2 theory are virtually identical to the MC values, demonstrating a rapid convergence of the BGY hierarchy.
Effect of vibrational and translational energies of C2H2+ on the reaction with methane
Authors: Honma K. , Tanaka I.
Published: February 2003

The effect of vibrational energy of the ionic reactant on the rates of the reactions, C2H2++CH4→C3H5++H, C3H4++H2, C2H3++CH3, has been examined using a photoionization mass spectrometer. The translational energy dependence of the cross sections of the above reactions has also been studied by changing repeller voltage in the ionization chamber. The relative cross sections of the reaction channels producing C3H5+ and C3H4+ decreased with increasing vibrational energy in the range of 1.1 to 17.6 kcal/mol relative kinetic energies. On the other hand, the reaction producing C2H3+, which is slightly endothermic, occurred at ν2=1 with 1.1 kcal/mol relative kinetic energy and the reaction cross section increased with an increase of vibrational energy at kinetic energies lower than 12.1 kcal/mol. The C2H3+ ion was also produced by translational excitation above 2 kcal/mol for the reactant ion in the vibrationally ground state. These results were explained by the occurrence of two distinct types complexes, i.e., one has a long lifetime, corresponding to the C3H5+ and C3H4+ formation, and the other has a short lifetime, corresponding to the C2H3+ formation.
Eigenvalues of the Lorentz Fokker–Planck equation
Authors: Shizgal B.
Published: February 2003

A new set of orthogonal polynomials are developed for which the convergence of the eigenvalues of the Lorentz Fokker–Planck equation is particularly rapid. The rate of convergence with the new functions is shown to be very much faster than the rate of convergence with the Laguerre polynomials, traditionally employed in kinetic theory problems.
Infrared measurements of sulfur dioxide thermal decomposition rate in shock waves
Authors: Grillo A. , Reed R. , Slack M. W.
Published: February 2003

The rate of thermal decomposition of sulfur dioxide dilute in argon has been measured in a shock tube, using IR emission from the ν3 band to monitor SO2. These results yielded a rate coefficient k=8(±2) ×1015 exp(−108 000/RT) cm3 mole−1 sec−1 for the reaction SO2+M→SO+O+M in the temperature range 2800 to 3880°K and the total concentration range 8×10−6 to 3.4×10−5 mole/cc. The measured rate coefficient agrees well with the results of Just and Kiefer, determined respectively from O-atom formation rate observations and densitometric measurements, and the combined data yield a common rate coefficient between 2500 and 5200°K.