bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of collision-induced phase shifts on the linewidths and line shifts of rotational spectral lines
Authors: Mehrotra Suresh C. , Boggs James E.
Published: February 2003

A more general theory of the width and shift of microwave absorption lines is developed under the impact approximation with the effect of elastic as well as inelastic collisions being considered in a more rigorous way. Previous formulations (Anderson and Murphy–Boggs) are shown to be limiting cases of this treatment. The theory is applied to pure rotational transitions of the OCS–OCS, OCS–He, and OCS–Ar systems and the results compared with experiment. The agreement is very satisfactory for the quantum number dependence and temperature dependence of the linewidths and for the approximate magnitude of the pressure-induced line shift.
Predissociation linewidths and level shifts for double-curve crossing models
Authors: Sink Michael L. , Bandrauk André D.
Published: February 2003

An analytic scattering model is applied to the problem of double-curve crossings in predissociation. Analytic expressions are obtained for the energy shift ΔE and linewidth Γ by searching for poles in the scattering matrix. The validity of perturbation expressions for these parameters is examined by comparing numerical calculations of these expressions to our results. It is found that contributions from individual crossings are not always additive owing to interference effects.
31P NMR studies of the linear chain magnets: Poly(metal phosphinates)
Authors: Smith L. S. , Newman P. R. , Heeger A. J. , Garito A. F. , Gillman H. D. , Nannelli Piero
Published: February 2003

The paramagnetic shift and linewidth of the 31P NMR have been measured for three isoelectronic magnetic polymers Co[OP (C4H9)2O]2, Cr[OP (CH3)2(C6H5)2O]OH, and Cr[OP (CH3)(C6H5)O] [OP (C8H17)2O] for 77⩽T⩽300 K. Analysis of the paramagnetic shift as a function of the static magnetic susceptibility yields a value for the hyperfine coupling constant A≈4×10−4 cm−1. Previously measured and calculated hyperfine constants for an electron in the 3s and 3p orbitals are used to show the paramagnetic shift is due mainly to transferred magnetic spin density (≈0.1%) in the phosphorus 3s orbital. The linewidths of these materials are approximately 10 Oe at room temperature and broaden slightly with decreasing temperature. The temperature dependences of the linewidths are used to deduce an upper limit (⩽1%) for the amount of transferred spin density in the phosphorus 3p orbital.
Temperature dependence of the laser enhanced reaction NO+O3(001) →NO2(2B1,2)+O2
Authors: Moy Jerry , Bar-Ziv Ezra , Gordon Robert J.
Published: February 2003

The ratio of the rate coefficient of the vibrationally enhanced reaction NO+O3→NO2*+O2 to the thermal rate coefficient was determined over the temperature range 155–303 K. This was accomplished by using a pulsed CO2 laser to excite the ozone molecules in a flow system. The enhancement of the rate coefficient was determined from the ratio of the laser-induced chemiluminescent signal to the thermal dc signal and from the fraction of vibrationally excited O3 molecules in the gas mixture. The latter quantity was determined from measurements of laser beam attenuation in static gas mixtures, using the same cell and laser configuration. It was found that the activation energy was reduced by 1.29+0.25−0.22 kcal/mole or by 42±8% of the O3(001) vibrational energy, while the pre-exponential factor was barely affected. The information theory of Levine and Bernstein predicts a much smaller reduction in the activation energy. On the other hand, the excitation function of Menzinger et al. indicates that an equivalent amount of translational (or rotational) energy would be 100% effective in reducing the observed activation energy.
Chain folding and polymer crystallization: A statistical–mechanical approach
Authors: Allegra G.
Published: February 2003

The essential factor controlling the formation of chain folded polymer crystals is attributed to the statistical distribution of many subcritical nuclei pre-existing within the chains. Each nucleus should consist of at least three parallel segments belonging to the same chain; its statistical probability is higher, the shorter are the chain loops connecting the segments. It is shown that the average fold-to-fold thickness both of the critical nuclei and of the actual crystals may reasonably be expected as equal to the average countour length of the loops within the subcritical nuclei. Under proper simplifying assumptions, statistical–mechanical calculations are carried out for polyethylene crystals precipitated from dilute solutions, showing a reasonable agreement with experimental observations. The present theory readily explains the tendency towards lamellar crystallization even in bulk polymer samples. It also suggests that at sufficiently low temperatures amorphous polymers may contain crystallinelike associations where segments relatively contiguous along the chain sequence are present with a higher probability. The present theory is somehow related with the classical kinetic viewpoint insofar as crystal size is not dictated by its own free energy requirements, and its growth is accompanied by a free energy decrease.
The anisotropic interaction between nitrogen molecules from solid state data
Authors: Raich J. C. , Gillis N. S.
Published: February 2003

The development of an angle-dependent interaction potential for slightly nonspherical molecules is outlined, concentrating on molecular nitrogen. The suggested N2–N2 pair potential combines the radial dependence of the Barker type potential used for the rare gases with a modification of the treatment of anisotropy by the Berne–Pechukas Gaussian overlap model. This potential includes quadrupolar and long range dispersion contributions as determined from experiment or semiempirical studies. The valence potential parameters are determined by a fit of beam scattering and solid state measurements. This potential provides satisfactory agreement with Raman and neutron scattering measurements only when the anharmonic contributions to the lattice potential energy are included. A comparison of the suggested potential with a number of other presently available potentials, determined from a variety of measurements, as well as recommendations as to the limits of possible use are made. Possible steps which would lead to future improvements of the N2–N2 pair potential are outlined.
Chemical perturbation and lattice instability in molecular crystals
Authors: Prasad Paras N. , Stevens Edwin D.
Published: February 2003

A method of successive perturbation is used to investigate the nature of lattice instability associated with a chemical substitution. The p-dihalobenzene series is studied by this method using the techniques of x-ray diffraction and laser Raman spectroscopy. The Raman phonon spectra in the series p-C6H4XY (X=Cl, Br and Y=Cl, Br, I) correlate well but a lattice instability exhibited by the breakdown in phonon correlation is encountered by substituting just one more iodine for X in p-C6H4XI. A temperature dependence study is conducted to investigate the role of dynamic (motional) instability. The lattice structures obtained both by thermal perturbations (three different phases of p-C6H4Cl2) and by chemical perturbations (various p-dihalobenzenes) show five characteristic close contacts. A novel application is made of the principal component analysis using these close contacts and corresponding angles as variables to understand the nature of structural perturbations. The result shows that the principal component scores for thermal perturbations and chemical perturbations define two nearly orthogonal axes; this suggests that at least for this series the nature of these two types of perturbations is widely different.
Structure and ordering in dilute dispersions of spherical particles
Authors: van Megen William , Snook Ian
Published: February 2003

The Monte Carlo method is used to determine the structure of a very dilute colloid electrostatically stablized in a dilute aqueous electrolyte. The colloidal system is treated as a monodisperse collection of structureless spherical particles embedded in a continuous background. The calculated radial distribution functions are converted into structure factors which are subsequently compared with the corresponding quantity determined from laser light scattering experiments.
Angular correlation ranges in flexible polymer chains
Authors: Baram Asher , Gelbart William M.
Published: February 2003

We consider the equilibrium structure of flexible polymer chains within the rotational isomeric state model. This model is solved by an irreducible tensor method which is somewhat different from, and simpler than, the approach of Flory and others. We evaluate in particular the quantities 〈DJmm′(Ωn) 〉, where DJmm′ is the mm′th component of the Jth rank Wigner rotation matrix. Ωn is the orientation of the nth monomer with respect to the first, and the brackets denote a canonical ensemble average over all polymer conformations. We find—for physically reasonable values of the parameters defining the three-state rotational isomeric state model—that 〈DJ=1,2oo(Ωn) 〉 decays to zero as (0.632) (±0.841)n−1 for large n. 〈DJ=3,4oo(Ωn) 〉, however, is anomalously long ranged. When the complement of the valence bond angle is tetrahedral (cosϑ=1/3), in fact, 〈DJ=3oo(Ωn) 〉∼ (0.555) (−1)n−1 for large n. This conspicuously unphysical behavior is shown to be an artifact of the three-state model for rotational isomers. Finally, we use our results for averages of the form 〈D2oo(Ωn) 〉, 〈r2nD2oo (Ωn) 〉 and 〈r2nD2o-m(Ωn) D2-mo(φ?nϑ?n0) 〉 to compute the light scattering intensities (I) from dilute solutions of flexible polymer chains. (Here rn is the vector between the centers of mass for the first and nth monomers.) We find negligible angle dependence for I, and (IVh/IHh)90°=1 to within one part in 105.
General anisotropic hyperfine splitting tensors for double zeta functions with application to neutral radical proton interactions
Authors: Wells James W.
Published: February 2003

General electron-proton anisotropic hyperfine splitting (hfs) trensor elements are determined for all n=2 wavefunctions using ’’double-zeta’’ radial functions. These elements are employed in calculating the resultant hfs tensors in neutral radial models for α-proton and β-proton interactions that include interatomic spin density considerations. The resulting tensors are found to be in good agreement with the corresponding experimental ENDOR results from carbond and nitrogen radicals and indicate the utility of such comparisons in model definition. Two classical point dipole models, used to represent these interactions, are also discussed in relations to tensor calculations.
Brownian coagulation of aerosols in rarefied gases
Authors: Wagner Paul E. , Kerker Milton
Published: February 2003

The kinetics of coagulation of aerosols is well understood when the particle radius is much larger or much smaller than the mean free path of the surrounding gas (very small or very large Knudsen numbers). However, no adequate theory is available in the so-called transition regime between these limiting cases. This is an experimental investigation of Brownian coagulation in the transition regime. Monodisperse aerosol [di(2-ethylhexyl)sebacate, particle radius 0.2 μm] was produced in a falling-film condensation aerosol generator. The evolution of the size distribution, and hence the rate of coagulation, was monitored as the aerosol flowed through a series of light scattering cells. Measurements were performed at five different pressures, ranging from atmospheric pressure to 67 Torr, thereby scaling the Knudsen number from 1 to 12. The influence of diffusional deposition, sedimentation, and gradient coagulation in vaporization was kept to a minimum, and the uniqueness of the inversion technique was demonstrated. The experimental results are in good agreement with Fuchs’ semiempirical interpolation equation, assuming a coalescence efficiency of unity.
Measurement of energy distributions of secondary electrons ejected from argon by fast electrons
Authors: Vroom D. A. , Palmer R. L. , McGowan J. Wm.
Published: February 2003

Energy distributions of secondary electrons emitted in the ionization of argon by fast electrons have been measured. Their spectra and single and double differential cross sections were obtained for electron impact energies ranging from 1 to 10 keV, and electrons ejected with energies up to 300 eV. Where possible, the results are compared with existing experimental data and with theoretical calculations.
Quantum mechanical and crossed beam study of vibrational excitation of N2 by electron impact at 30–75 eV
Authors: Truhlar Donald G. , Brandt Maynard A. , Srivastava Santosh K. , Trajmar S. , Chutjian A.
Published: February 2003

The ratios of differential cross sections for excitation of the first excited vibrational state and for elastic scattering for elec- tron impact on N2 have been measured at scattering angles ranging from 20° to 135° at 30, 35, 40, 45, and 75 eV impact energies and from 25° to 90° scattering angle at 20 eV impact energy. The results at 20 eV are in good agreement with two previous sets of measurements. Using previously measured and normalized elastic differential cross sections for N2, the ratios have been converted to inelastic cross sections. Calculations using a four-state vibrational–rotational basis set and an effective interaction potential developed previously are reported at the five energies in the 30–75 eV region. It is shown that the potential scattering model can account for the magnitude and the qualitative behavior of the cross sections at 35–75 eV but there are some significant quantitative differences between theory and experiment. The most striking of these is the way the theoretical model overestimates the scattering at scattering angles less than about 50°. Core-excited resonances apparently make an appreciable contribution to the vibrationally inelastic scattering at 30 eV.
Prediction of low energy molecular Rydberg states from Feshbach resonance spectra
Authors: Spence David
Published: February 2003

An electron transmission spectrometer is used to study Feshbach resonances in CH3X(X=Cl, Br, I). Combination of data obtained an HX and CH3X indicates a linear relationship between resonance energies Enll′(m) and the corresponding ionization potential I (m) given by Enll′(m) =Anll′I (m)+Bnll′ where Anll′ and Bnll′ are constants independent of molecular species m, and l and l′ are angular momentum quantum numbers of the two excited electrons. In addition to predicting the positions of Feshbach resonances, this relation may be used together with a screening factor σ to calculate the energies of energetically low lying Rydberg states in structurally related molecules. This technique is at least as accurate as a previous technique which calculates Rydberg energies from correlations with Rydberg energies determined in other molecules.
Model of thermalization of quasifree electrons in high-mobility liquids and its relationship with electron mobility
Authors: Sano H. , Mozumder A.
Published: February 2003

The present paper aims at constructing a theoretical model in terms of which the relationship between the free-ion yield and electron mobility may be understood for high-mobility liquids. The framework chosen for that purpose is the Fokker–Planck equation with the Coulomb field of the electron and the sibling cation built into it. This equation describes, in general, the distribution function for the electrons in both the configurational and velocity spaces. Starting from a given initial position and velocity, one can calculate the distribution of thermalization distances through the use of a relaxation time. The same relaxation time appears in the mobility equation, implying the similarity of energy-loss processes of low-energy electrons following an ionization event and of electrons slightly heated up by the external field. The model therefore provides, in principle, a connection between the electron mobility and the free-ion yield. In practice the agreement between theory and experiment is reasonably good for molecular hydrocarbons but is poor for liquefied rare gases.
Mercury photosensitized reaction of H2 in the presence of NO. Rate constants of the H+NO+M→HNO+M and HgH+NO→HNO+Hg reactions
Authors: Oka K. , Singleton D. L. , Cvetanović R. J.
Published: February 2003

Mixtures of H2, NO, and Hg vapor were illuminated in a flow system by intensity modulated 253.7 nm light, and the luminescence from HNO(1A″) was measured with a photomultiplier coupled to a lock-in amplifier. The modulation amplitudes and the phase angles of the luminescence were found to be functions of the wavelength, the concentration of NO, and the modulation frequency, and indicated involvement of two distinct HNO(1A″) precursors, H atoms and HgH radicals. At the pressures used, the lifetime of the luminescence from the H+NO+M reaction is 2–3 orders of magnitude longer than that from the HgH+NO reaction. The following values of the rate constants were determined at 25°C:kH+NO+H2 =1.56±0.06×1010 12 mole−2⋅sec−1 (after a 13% correction for the wall recombination of H atoms) and kHgH+NO=2.71±0.12×1010 1 mole−1⋅sec−1, where the indicated uncertainties are the least squares standard deviations of the individual measurements.The overall uncertainity in these values is estimated at about ± 20%.
ESR and ENDOR of sulfur substituted nucleosides, 6-methylmercaptopurine riboside at 300 K
Authors: Pugh Henry L. , Alexander Chester
Published: February 2003

Single crystals of the sulfur containing DNA nucleoside analog 6-methylmercaptopurine riboside have been prepared and irradiated by x rays. Electron spin resonance and electron–nuclear double resonance techniques have been used to study the radiation induced radicals at room temperatures. Three radical species (I, II, III) were observed to be stable at room temperature. Radical species I and II showed little or no hyperfine structure, but had large g-value variations about average values of I, 2.0275; and II, 2.0112. These two radical species are attributed to π-electron monosulfide radicals formed by rupture of the S–CH3 bond. Radical species III is formed by H addition at C2 of the purine ring. This radical has g value and hyperfine principal values: 2.0052, 2.0048, 2.0028; H8, −8.9, −6.3, −3.3 G; N3, 20.5, 1.1, 1.1 G; N1, 7.0, 0.5, 0.5 G; CH2, 40.0 G.
Fluorescence emissions from mixtures of Hg with the noble gases
Authors: Woodworth J. R.
Published: February 2003

Fluorescence emissions from mixtures of Hg with high pressure Xe, Kr, and Ar (∼1 torr Hg, 103–104 torr noble gas) have been studied using a short-pulse relativistic electron beam as an excitation source. Hg–noble gas molecular bands were observed on the red sides of the Hg lines (1849 and 2537 Å) as well as on the red sides of the Hg visible lines (7 3S1→6 3P0,1,2). Temporal histories and production efficiencies of the molecular emissions were determined and a model was formulated for the time histories of the HgXe 2600 Å bands. Possible applications to high power laser systems are discussed.
Monte Carlo trajectory study of Ar+H2 collisions. II. Vibrational and rotational enhancement of cross sections for dissociation
Authors: Blais Normand C. , Truhlar Donald G.
Published: February 2003

Integral cross sections and properties of the dissociated trajectories were calculated for the reactions Ar+H2→Ar+H+H and Ar+H2(qb) →Ar+H+H, where H2(qb) is a quasibound H2. Integral cross sections were also calculated for Ar+H2?Ar+H2(qb). Twenty-four different (21 bound, 3 quasibound) initial vibrational–rotational states of H2 are considered. The potential surface (which we presented previously) is based on modified statistical calculations at small distances and dissociates to an accurate potential in the van der Waals region and to accurate diatomic curves. At a total energy 1 eV above the energetic threshold for dissociation, we find reagent vibrational energy is very effective in promoting dissociation (the vibrational bias parameter of Kiefer, Joosten, and Breshears is about 11 to 13), rotational energy is second most effective, and relative translational energy is least effective. Except for the topmost vibrational level we find dissociation cross sections are much smaller than predicted by the hard-sphere available-energy theory.
Kerr constant and related quantities in terms of atom–atom correlation functions
Authors: Ho/ye J. S. , Stell G.
Published: February 2003

We find an expression for ℱPl(ŝ1ŝ2) h (12) dΩ1dΩ2dr in terms of atom–atom correlation functions, where h (12) is the full molecular pair correlation function, ŝi is a unit vector that describes the orientation of the ith molecule, and Pl is the Legendre polynomial. For l=1 the result yields our earlier expression for the dielectric constant of a polar (nonpolarizable) fluid. For l=2, the result yields the Kerr constant for a nonpolar fluid of linear molecules, and is also relevant to the theory of dipolarized light scattering and quadratic field effects in NMR theory.
H2 and D2 photon impact predissociation
Authors: Borrell Peter , Guyon Paul Marie , Glass-Maujean Michèle
Published: February 2003

Atomic Lyman α and Balmer α, β, and γ emission have been observed by irradiating H2 and D2 in the 680–860 Å region with synchrotron radiation from the ACO storage ring at Orsay. Comparison of the observed excitation spectra with the absorption spectrum shows clear evidence for the predissociation of molecular Rydberg states above the dissociation thresholds which are observed for the dissociation reactions: H2+hν→H(1S) +H* (n=2, 3, 4, 5). Undispersed visible fluorescence is also observed on excitation in the 750–860 Å region which is attributed to molecular emission from excited Rydberg states. The results of these photon impact experiments are compared with those from electron impact processes.
Photoelectron spectra of polynuclear aromatics. V. Correlations with ultraviolet absorption spectra in the catacondensed series
Authors: Schmidt W.
Published: February 2003

IP’s derived from the high-resolution PE spectra of 61 catacondensed hydrocarbons are presented, most of them for the first time, and correlated with the energies of the four main singlet–singlet transitions. For the p and β′ bands (1La and 1Ba) a linear correlation with IP1 and IP2, respectively, is established. The α and β band energies (1Lb and 1Bb) correlate linearly with the mean of IP1 and IP2. The regressions encompass planar as well as nonplanar hydrocarbons of the helicene type; standard deviations range from 0.070 to 0.126 eV. Comparison with Hückel and PPP theory shows the empirical correlations to be superior; their potential in disentangling diffuse uv spectra is demonstrated. Many β′ bands are assigned for the first time, and estimates are given for the position of the α band in anthracene, tetracene, and pentacene. From the slopes and intercepts of the regressions, important conclusions regarding the wavefunctions of the excited states and the role of electron repulsion and CI can be drawn. In some hydrocarbons the energies of the four uv transitions are shown to correlate with IP1 and IP3 rather than with IP1 and IP2. An analysis of this finding in terms of perimeter orbitals is given, and a novel classification of PE bands based on vibrational fine structure and correlation with perimeter states is presented. Correlations between the IP’s and chemical reactivity are pointed out.
Low-temperature magnetic characteristics of tetrahedral CoCl42−. III. Magnetic exchange in paramagnetic Cs2CoCl4
Authors: McElearney J. N. , Merchant S. , Shankle G. E. , Carlin R. L.
Published: February 2003

The results of low-temperature studies of the magnetic characteristics of Cs2CoCl4 are reported. Single-crystal principal-axes magnetic susceptibility measurements between 1.5 and 20 K have been fit to an exchange-modified spin-3/2 single-ion model. Heat capacity measurements between 1 and 30 K on Cs2CoCl4 and between 4 and 25 K on Cs2ZnCl4 have been used with a corresponding states procedure to obtain the magnetic heat capacity of Cs2CoCl4. All the results generally indicate a large zero-field splitting of 14 to 16 K with the possibility of a rhombic distortion of the CoCl42− ion. Both the susceptibility and heat capacity measurements indicate the presence of significant magnetic exchange. At the lower temperatures the heat capacity results appear to be describable by an XY linear chain model, in agreement with structural considerations in conjunction with the theoretically expected behavior of tetrahedral cobalt(II).
Force–velocity cross correlations and the Langevin equation
Authors: London Robert E.
Published: February 2003

Cross correlations of the velocity with the total force, the mean force, and the fluctuating force are evaluated for the Langevin equation and for a generalized version which has been suggested for a Brownian particle in an incompressible fluid of nonzero density. In the latter case all of the cross correlations follow a power law decay with the first ∼t−5/2 and the second two ∼t−3/2 for large t so that the t−3/2 decays cancel. It is shown that the failure to consider the effects of these cross correlations leads to inconsistencies, in particular, the failure to satisfy the stationarity condition. The cross correlation of the fluctuating force with the velocity measured at the same time gives the rate at which energy is put into the system by this force and is shown to be equal to the rate at which energy is lost due to the dissipative effect of the mean force. The inertial response of the fluid to the motion of the Brownian particle is also evaluated. Comparisons are made between the original general treatment by Kubo and more recent developments in which a specific frequency dependent hydrodynamic drag is used. A relationship between the velocity autocorrelation function, the fluctuating force autocorrelation function, and the velocity–fluctuating force cross correlation is proven.
Mass spectrometric observations of some polyatomic gaseous rare earth oxides and their atomization energies
Authors: Kordis J. , Gingerich K. A.
Published: February 2003

From Knudsen effusion mass spectrometric examination of reactions of the type M(g)+MO(g) =M2O(g), 2MO(g) =M2O2(g), and 2MO(g) =MO2(g)+M(g), the atomization energies ΔH°0 (kcal/mole) of the following new species have been estimated: Eu2O(g), 174±12; Gd2O(g), 236±10; Tb2O(g), 243±12; Ho2O(g), 216±14; Lu2O(g), 266±14; Eu2O2(g), 324±17; Gd2O2(g), 427±17; Tb2O2(g), 432±21; Ho2O2(g), 407±26; GdO2(g), 314±17; and HoO2(g), 307±25. Atomization energies ΔH°0 (kcal/mole) revised from literature are presented for the following: Sc2O(g), 236±16; Y2O(g), 249±13; La2O(g), 265±13; Y2O2(g), 438±28; La2O2(g), 459±28; Ce2O2(g), 472±15; CeO2(g), 344±5; and NdO2(g), 318±20. The variation of the atomization energies of the M2O(g), M2O2(g), and MO2(g) species along the lanthanide series follows a similar pattern observed for the atomization energies of the MO(g) species and the heat of sublimation of the corresponding metals. Predictions of the atomization energies of the yet unobserved rare earth oxide species of the types above have been made. The standard heats of formation at 0°K of the gaseous rare earth oxides are also presented.
Group theoretical classification of the tunneling–rotational energy levels of water dimer
Authors: Dyke Thomas R.
Published: February 2003

A permutation-inversion group theoretical classification scheme for the tunneling–rotational levels of the water dimer molecule is given. Electric dipole selection rules and nuclear spin statistics are discussed. Application of these results to the microwave spectrum of water dimer, observed by molecular beam techniques, is also presented.
The structure of water dimer from molecular beam electric resonance spectroscopy
Authors: Dyke Thomas R. , Mack Kenneth M. , Muenter J. S.
Published: February 2003

Molecular beams of hydrogen bonded water dimer, generated in a supersonic nozzle, have been studied using electric resonance spectroscopy. Radiofrequency and microwave transitions have been observed in (H2 16O)2, (D2 16O)2, and (H2 18O)2. Transitions arising from both pure rotation and rotation–tunneling occur. The pure rotational transitions have been fit to a rigid rotor model to obtain structural information. Information on the relative orientation of the two monomer units is also contained in the electric dipole moment component along the A inertial axis μa, which is obtained from Stark effect measurements. The resultant structure is that of a ’’trans-linear’’ complex with an oxygen–oxygen distance ROO of 2.98(1) Å, the proton accepting water axis is 58(6) ° with respect to ROO, and the proton donating water axis at −51(6) ° with respect to ROO. This structure is consistent with a linear hydrogen bond and the proton acceptor tetrahedrally oriented to the hydrogen bond. The limits of uncertainty are wholly model dependent and are believed to cover variations from the zero-point vibrational structure observed to the equilibrium structure. μa shows strong dependence on J and K and is about 2.6 D. Centrifugal distortion constants have been interpreted in terms of the monomer–monomer stretching frequency and give ω=150 cm−1.
Angular entropy: The information content of molecular scattering angular distributions
Authors: Faist M. B. , Levine R. D. , Bernstein R. B.
Published: February 2003

The concept of the angular entropy arises from consideration of the information content of a scattering pattern, i.e., an angular distribution of collision products. It is shown that information theory (I.T.) provides the framework for evaluation and interpretation of the entropy (and entropy deficiency) of an angular distribution of reactive, inelastic, or elastic scattering. The differential cross section σ (ϑ) is converted to a normalized probability density function (pdf), P (u) [u= (1/2)(1−cosϑ)], from which the angular surprisal is obtained as −lnP (u). The average over u of the surprisal yields the angular entropy deficiency. (A histogrammic approximation to the continuous pdf can provide a simple estimate of ΔS). Examples are presented of reactive and inelastic molecular scattering patterns and of various prototype angular distributions giving insight into the angular entropy. The I.T. method is also applied to elastic scattering of atoms and molecules. It inherently demands the elimination of the well-known ’’classical divergencies’’ (the forward infinity and rainbow spike). These problems disappear when quantal (or semiclassical) differential cross sections are used. Nevertheless, the forward cone makes the dominant contribution to the angular entropy deficiency for elastic scattering at moderate energies. The rainbow structure introduces some entropy deficiency, but the quantal interferences in σ (ϑ) contain little information (in the strict I.T. sense). However, nuclear symmetry effects are found to be significant.
Divanadium
Authors: Ford T. A. , Huber H. , Klotzbücher W. , Kündig E. P. , Moskovits M. , Ozin G. A.
Published: February 2003

The metal concentration and matrix conditions which favor the dimerization of vanadium atoms to divanadium molecules are quantitatively assessed using optical spectroscopy. A simple kinetic theory is presented which enables small metal clusters to be identified in the presence of atomic species. This approach makes use of the fact that a metal atom being deposited is capable of diffusing either on the matrix surface or within a narrow region (the reaction zone) near the matrix surface before its kinetic energy is dissipated sufficiently to immobilize it. The surface diffusion pathway is found to predominate over the statistical generation of dimers. The kinetic result, which suggests that V2 is formed on the matrix surface rather than in the gas phase, is also borne out by the intriguing observation that for a given metal deposition rate the dimer-to-monomer ratio decreases as one increases the atomic weight of the noble gas used to isolate them, with Ar giving the most V2 and Xe the least. Careful concentration experiments in Ar, Kr, and Xe matrices permit the uv–visible transitions of V2 to be identified and the extinction coefficient ratio εV/εV2 to be determined. A qualitative molecular orbital description of V2 is presented in the light of iterative extended Hückel calculations. These computations suggest that high spin divanadium has a strong metal–metal bond which is mainly 4s in character with only small contributions from the degenerate dxz,yz π-bonding set. Visible absorptions observed in the 600–450 nm region are tentatively assigned to electronic transitions localized mainly between the V–V σ-bond and the d-orbital manifold.
Coupled states cross sections for rotational excitation of H2CO by He impact at interstellar temperatures
Authors: Garrison Barbara J. , Lester William A.
Published: February 2003

Cross sections for rotational excitation of ortho formaldehyde due to collision with helium are computed following the coupled states (CS) formalism and compared with recent coupled channel (CC) results obtained employing the same ab initio configuration interaction intermolecular potential. The CS equations are integrated at 9 scattering energies between 25 and 95°K using a basis set of 16 ortho H2CO states (1⩽j⩽5). The CS procedure with the orbital angular momentum quantum number l set equal to the total angular momentum J yields the correct order of magnitude for scattering cross sections. Qualitative differences are found, however, in the energy dependence of some inelastic transitions.
The phosphorescence blue shift of anthracene in a charge transfer configuration: A consequence of the donor acceptor overlap in anthracene–PMDA crystals
Authors: Haarer D. , Keijzers C. P. , Silbey R.
Published: February 2003

A 500 cm−1 blue shift of the phosphorescence origin of anthracene in anthracene–PMDA single crystals has been measured. Calculations based on a perturbation theory approach show that the symmetry of the highest occupied and lowest unoccupied orbitals of the anthracene and PMDA molecule, together with the specific geometry of a charge transfer complex, are responsible for the observed shift. The simple model yields the surprising result that the energy of the excited triplet state is less affected by the interaction with a charge transfer state than the energy of the ground state.
A
b i
n
i
t
i
o study of the electronic structure of Li2 −
Authors: Dixon D. A. , Gole J. L. , Jordan K. D.
Published: February 2003

We have performed SCF–CI calculations on the ground states of Li2 and its anion. We predict a value aof 0.45 eV for the adiabatic electron affinity of Li2 and a value of 0.86 eV for the dissociation energy of Li2 −. Thus, the bond strength of the anion is nearly the same as that of the neutral parent. This indicates that the simple MO picture, describing the anion as being formed by the addition of an electron to the antibonding 2σu orbital, is not adequate. The present results are compared to the recent equation-of-motion calculation of the Li2 electron affinity by Andersen and Simons.
The surface structures of phthalocyanine monolayers and vapor-grown films: A low-energy electron diffraction study
Authors: Buchholz J. C. , Somorjai G. A.
Published: February 2003

Monolayer structures and epitaxial growth of vapor-deposited crystalline phthalocyanine films on single-crystal copper substrates were studied using low-energy electron diffraction. At monolayer coverage, ordered layers of copper-, iron-, and metal-free phthalocyanines were observed on both Cu(100) and Cu(111). With increasing film thickness, ordered multilayer structures of these phthalocyanines were also seen on both substrates at 300 K surface temperature. All three phthalocyanines had identical monolayer and multilayer surface structures on Cu(100). On the Cu(111) substrate, the three phthalocyanines exhibited slightly different surface structures, indicating the importance of the central metal atom in the interaction with the substrate that influences the monolayer surface structure and the ordering during crystal growth. The multilayer crystalline films did not have surface structures characteristic of the bulk crystal structure but rather had a surface unit mesh compatible with a single molecule per unit mesh, oriented parallel to the surface. Electron beam damage and space charge effects were negligible for film thicknesses up to 500–1000 Å.
Solutions of the Yvon–Born–Green and Kirkwood equations for hard spheres at very high densities
Authors: Co King U. , Kozak John J. , Luks Kraemer D.
Published: February 2003

The Yvon–Born–Green and Kirkwood equations with superposition approximation for g(2)(x) for a system of hard spheres are found to have multiple solutions over broad ranges of density. These solutions are related to those of an earlier study of high density solutions to the Yvon–Born–Green equation for hard spheres. The extension of these findings to systems of square-well molecules is discussed. The results of the study are interpreted in the framework of a bifurcation theoretical analysis of the problem.
Chemiionization of the hydrogen molecule by He(2 3S) and He(2 1S) atoms: potential surfaces, autoionization widths, and cross sections
Authors: Cohen James S. , Lane Neal F.
Published: February 2003

Potential energy surfaces have been calculated for He(2 3S) and He(2 1S) atoms on H2 using the stabilization method. Energies have been obtained for several He*–H2 separations X and relative orientations ϑ of the H–H and He–H2 axes; the H–H distance was fixed at 1.4 a0. Aside from shallow van der Waals minima at large separations, cuts of both surfaces at fixed ϑ are repulsive. The adiabatic He*–H2 interactions are resonances lying in the HeH+2+e continuum at all points; hence chemiionization can occur in collisions. The autoionization width Γ as a function of nuclear coordinates has been calculated in two ways: (i) by Fermi’s ’’golden rule’’ utilizing expansions in square-integrable functions to avoid having to directly evaluate integrals over continuum orbitals, and (ii) by a new, less rigorous technique based on observations about the convergence characteristics of the stabilization procedure. The widths are found to fall off exponentially with He*–H2 separation and to be only weakly dependent on the angular orientation. The scattering problem has been formulated in terms of the local complex potential V (X,ϑ)−1/2iΓ (X,ϑ) and total chemiionization cross sections have been calculated in the spherical potential and sudden approximations at energies below 10 eV. The two approximations yield very similar results. Ionization rate constants and the singlet-to-triplet cross section ratios are also presented. The cross sections and ratios exhibit a strong dependence on collision energy (or temperature), and it is shown that the well known disagreement between beam and afterglow experiments about these quantities may largely be a result of their different velocity distributions.
Light scattering studies of orientational pair correlations in liquids composed of anisometric molecules
Authors: Bertucci Sidney J. , Burnham A. K. , Alms G. R. , Flygare W. H.
Published: February 2003

The nature of the intermolecular forces which are important in determining the magnitude of orientational pair correlations (OPC) in liquids is studied by combining high pressure and depolarized Rayleigh light scattering techniques. For molecules of similar shape (toluene, bromobenzene, and nitrobenzene) we find that the magnitude of the OPC as measured by depolarized light scattering is not simply a function of the packing of the molecules. For 16 monosubstituted derivatives of benzene, we find that the magnitude of the OPC correlates well with the square of the molecular dipole moment. This correlation of OPC with dipole moment is understood by relating both the molecular dipole moment and the magnitude of the OPC to the detailed distribution of molecular electronic charge. The light scattering reorientational relaxation times for toluene, bromobenzene, and nitrobenzene as a function of pressure have also been measured and found to be linear functions of viscosity.
A systematic study of an isomorphous series of organic solid state conductors based on tetrathiafulvalene
Authors: Wudl F. , Schafer D. E. , Walsh W. M. , Rupp L. W. , DiSalvo F. J. , Waszczak J. V. , Kaplan M. L. , Thomas G. A.
Published: February 2003

The solid state physical properties of an isomorphous series of TTF salts (TTF11 (SCN)6, TTF11 (SeCN)6, and TTF7I5) were examined. While there was no noticeable trend in the conductivity (as a function of temperature and anion), the effective Fermi energy (?F) and the magnetic susceptibility transition temperature exhibited a definite trend as a function of anion. There is a correlation between the above properties and subtle variations in solid state structure of each of the above salts.
Statistical–mechanical theory of gaseous ion–molecule reactions in an electrostatic field
Authors: Viehland Larry A. , Mason E. A.
Published: February 2003

A theory is developed for gas-phase swarm measurements of ion–molecule reactions in electrostatic fields of arbitrary strength. The theory allows measurements of reaction rate coefficients made at low temperatures and strong electric fields to be converted directly to equivalent thermal rate coefficients at elevated temperatures inaccessible by direct methods. It is not necessary to calculate the ion velocity distribution function explicitly, or to unfold the reaction cross section from the rate data. In first approximation the measured rate coefficient is equal to the thermal rate coefficient at an effective temperature calculated directly from the measured ion drift velocity. Higher approximations are obtained from more detailed analysis of the dependence of the rate coefficient and drift velocity on electric field strength. Comparison is made with experimental data reported in an accompanying paper by Albritton et al. In another accompanying paper, Lin and Bardsley compare the present theory with their detailed Monte Carlo calculations.
Pressure dependence of NMR proton spin–lattice relaxation times and shear viscosity in liquid water in the temperature range −15–10 °C
Authors: DeFries T. , Jonas J.
Published: February 2003

The NMR proton spin–lattice relaxation times T1 and shear viscosities have been measured as functions of pressure in the temperature interval −15–10 °C. At low temperatures the low pressure boundary of the experiments is ice I, whereas ice V represents the high pressure extreme of our measurements. The initial compression at all temperatures covered in our study results in higher motional freedom of water molecules so that the pressure dependence exhibits a minimum in viscosity and a maximum in T1. This is a consequence of significant distortion of the hydrogen bond network due to compression which also seems to weaken the hydrogen bonds. Further compression leads to restricted motional freedom due to increased packing of the molecules. This anomalous behavior of spin–lattice relaxation and shear viscosity with compression is more pronounced at lower temperatures since the hydrogen bond network is better developed at lower temperatures. In agreement with our earlier data covering the 10–90 °C temperature range, we find that compression under isothermal conditions distorts the random hydrogen bond network, leading to diminished coupling between the rotational and translational motions of water molecules. The data indicate that the Debye equation describes the relationship between the reorientational correlation time and shear viscosity at constant volume but is not applicable to describe the density effects on water reorientation. In general, pressure and temperature have parallel effects on many dynamic properties at temperatures below 40 °C and pressures below 2 kbar, whereas at higher temperatures and pressures their effects are just the opposite. Hard core repulsive interactions become more important than the directional interactions of hydrogen bonding at high compression.
Analytic representation of oscillations, excitability, and traveling waves in a realistic model of the Belousov–Zhabotinskii reaction
Authors: Tyson John J.
Published: February 2003

On the basis of their thorough investigation of the mechanism of the malonic acid–bromate–cerium reaction, Field and Noyes have proposed a simple model for the sustained oscillations observed in this system. In this paper, I present a scheme of analysis of their differential equations which yields simple analytic formulae characterizing: (a) the domain in parameter space of local asymptotic stability of the steady state solution, (b) the amplitude, period, and waveform of limit cycle oscillations, (c) the direction of bifurcation of small amplitude periodic solutions, (d) the existence of large amplitude stable periodic solutions simultaneously with a locally stable constant solution, (e) the threshold of excitation and transient response to perturbations from a globally, asymptotically stable solution, and (f) the spatial and temporal development of the concentrations of key intermediates in periodic traveling waves (plane, axisymmetric, and rotating-spiral waves). These formulae facilitate the choice of parameter values to give reasonable agreement between model calculations and observed oscillations.
Fluorescence detected circular dichroism and circular polarization of luminescence in rigid media: Direction dependent optical activity obtained by photoselection
Authors: Tinoco Ignacio , Ehrenberg B. , Steinberg I. Z.
Published: February 2003

The fluorescence detected circular dichroism (FDCD) and circular polarization of luminescence (CPL) of rigid systems is related to molecular parameters. It is found that FDCD can yield the CD for light incident along the direction of the fluorescence transition moment. The average CD can also be measured as in a fluid medium. From CPL measurements one can evaluate the optical activity for light incident along the direction of the absorption transition moment. Thus, photoselection provides a method for measuring the direction dependent CD or optical activity of a molecule without actually orienting the molecule.
Self-diffusion of liquid hydrogen and deuterium
Authors: O’Reilly D. E. , Peterson E. M.
Published: February 2003

Coefficients of self-diffusion are reported for liquid H2, D2, and H2–D2 mixtures from the melting points to within about 10 °K of the gas–liquid critical points. The data are interpreted with the quasilattice theory for self-diffusion with corrections for quantum mechanical tunneling. Satisfactory agreement is obtained between the theory and experiment for hydrogen as well as liquid argon.
Reactions between NO+ and metal atoms using magnetically confined afterglows
Authors: Lo H. H. , Clendenning L. M. , Fite W. L.
Published: February 2003

A new method of studying thermal energy ion–neutral collision processes involving nongaseous neutral atoms is described. A long magnetic field produced by a solenoid in a vacuum chamber confines a thermal-energy plasma generated by photoionization of gas at very low pressure. As the plasma moves toward the end of the field, it is crossed by a metal atom beam. Ionic products of ion–atom reactions are trapped by the field and both the reactant and product ions move to the end of the magnetic field where they are detected by a quadrupole mass filter. The cross sections for charge transfer between NO+ and Na, Mg, Ca, and Sr and that for rearrangement between NO+ and Ca have been obtained. The charge-transfer reaction is found strongly dominant over the arrangement reaction that forms metallic oxide ions.
Partial summations and ESR line shapes in the slow tumbling region
Authors: Monroe James L. , Friedman Harold L.
Published: February 2003

Beginning with the results of Hwang, Anderson, and Friedman we derive an expression for the line shape of a spin-one system having an axially symmetrical isotropically tumbling g-tensor and a zero-field splitting term. The expression is in the form of a continued fraction and is obtained by a series of partial summations of subsets of an infinite set of graphs appropriate to the problem. We are able to vary the relative strengths of the g and D tensors, thereby obtaining any mixture of the two. The expression allows us to go to arbitrarily slow tumbling rates with a minimum of computation.