bluuebunny/crossref_metadata_2025 huggingface dataset:
Track-core radius of charged particles at relativistic speed in condensed media
Authors: Mozumder A.
Published: February 2004

A practical procedure is described to determine the core-size of tracks of charged particles of relativistic speed penetrating a condensed medium. It is based on equating the energy flow outwards at the maximum impact parameter obtained from Fermi's theory to the neglected part of stopping-power arising from Bohr's application of the principle of adiabatic invariance. The core size so obtained increases with the speed of the impinging particle approaching a finite limit at the speed of light (about 132 Å for liquid water). The direct energy trnsfer from the charged particles at distances greater than the core-radius arising from Črenkov radiation and molecular excitation has been estimated and shown to be negligible in most cases.
Field dependence of nuclear magnetic relaxation of 119Sn in SnCl4, SnBr4, and SnI4
Authors: Sharp Robert R.
Published: February 2004

Longitudinal and transverse relaxation times of 119Sn have been measured as a function of temperature at several field strengths in SnCl4, SnBr4, and SnI4. T2 in all three liquids is field independent and is governed by scalar coupling to the halogen. T1 is strongly field dependent in SnBr4 and SnI4 and exhibits a minimum with increasing temperature due to competing scalar and spin-rotation interactions. Coupling constants and correlation times previously computed for SnCl4 and SnI4 have been confirmed by measurements at 3.3 kg. Analysis of the data for SnBr4 yield J(119Sn–81Br)=920 Hz, T2(81Br)=0.748×10−6 sec at 294°K, and τθ(2)=3.1×10−12 sec at 294°K. Molecular reorientation in SnBr4 is highly unusual in that the reduced angular correlation time τθ* reaches a minimum value owing to the onset of dynamically coherent reorientation at a temperature (80°C) that is not far above the melting point (30°C), and τθ* remains in the inertial rotation region over much of the liquid range. Various other indications of significant dissimilarity of microdynamical behavior in SnCl4, SnBr4, and SnI4 are pointed out. Assuming the validity of the J-diffusion model, we complete the spin-rotation and magnetic shielding constants for SnBr4, but these results are not consistent with the chemical shifts and shielding constants previously inferred for SnCl4. The inconsistency is believed to be associated with the extremely small reduced frictional constant of SnBr4 and with the possibly inappropriate use of extended diffusion theory to describe reorientation in this liquid.
Exciton transport in substitutionally impure molecular crystals
Authors: Greer William L.
Published: February 2004

The exciton transmission characteristics of a linear molecular crystal containing a nondilute concentration of substitutional impurities are studied theoretically. This model also simulates energy transport in certain long polymers. In the event of exciton scattering by the impurities, we determine exactly the energy flux resulting from the ultimate evolution of an initial nonstationary electronic state. The result is exact for an arbitrary number of impurities located at arbitrary lattice positions within the crystals. We focus attention on two cases, the randomly and the periodically substituted crystals. In the randomly alloyed crystal, we show that the exciton transport is not diffusive; the steady state exciton flux decreases exponentially as the length of the disordered region increases and does not satisfy the ordinary diffusion equation. For the special case of periodic substitution, we are able to present very simple expressions for the exciton transmission probability and flux and to show that the transport is coherent. Similarities and differences between this problem and the one of phonon transport in an isotopically substituted harmonic lattice are noted.
Effects of curvilinear motion in large-amplitude bending of C3
Authors: Hansen C. Frederick , Henderson B. J. , Pearson Walter E.
Published: February 2004

Large-amplitude bending of linear, homonuclear, triatomic molecules is analyzed, with curvilinear departure of the atoms from normal coordinate bending included. The curvilinear motion is deduced from the observed variation of rotational constant with bending quantum number. The effect of this motion is to decrease the energy eigenvalues; energy states with lowest angular momentum are decreased most. These effects counteract the effects of a steeper than harmonic bending potential. A harmonic oscillator model of C3 is fitted to the observed rotational constant within 0.6%, and the bond distance between atoms in the linear configuration of C3 is found to be 1.287 Å; noticeably larger than the average internuclear distance of the vibrational ground state, 1.277 Å. First order perturbation results, including the effect of a quartic perturbation potential, approximately duplicate the pattern of energy levels observed. Square well bending potential wavefunctions are presumed to represent a limiting approximation for high vibrational levels of C3 since these provide the divergence of upper levels required to fit the measured vapor pressure of graphite. In this case, the effects of curvilinear motion are similar to those obtained for the harmonic oscillator model, although somewhat larger decreases in energy are obtained. In either case, the average energy decrease is relatively constant—the order of 10%—over a wide range of vibrational quantum number. This means that the usual normal mode analysis can be adjusted to approximate the effects of curvilinear motion and explains why normal mode approximations can often be extended beyond the limits of small-amplitude bending and yet give reasonably good results.
Exact solutions of a certain set of coupled differential equations
Authors: Knirk Dwayne L.
Published: February 2004

The Schrödinger equation for N-electron atoms has been shown to be equivalent to a denumerably infinite set of coupled, second-order differential equations. These equations differ significantly from those usually encountered by having a strict Coulombic coupling between all functions over the entire domain of the independent variable. We consider the solution of a finite number of these. The coordinate origin is a regular singular point of the equations, and the linearly independent sets of particular solutions are expanded around that point. The solutions which are obtained are explicit for a Hamiltonian which contains only coulombic potentials, but the method is applicable to any set of equations for which the coupling persists at the origin.
Formation of excited states and ions in the radiolysis of benzyl alcohol
Authors: Kira A. , Thomas J. K.
Published: February 2004

Transient species in benzyl alcohol and solutions of benzyl alcohol were studied by means of the nanosecond pulse radiolysis technique. Neat benzyl alcohol gives a benzyl radical with G = 2.1 which is produced by dissociative electron attachment. In solutions of lithium iodide, I2− with a G = 2.1 is observed; however, the yield of the solvent cation measured by using t-stilbene is only 0.2. It is suggested that a radical BzCH2O resulting from an ion-molecule reaction of the primary solvent cations and solvent molecules is a probable precursor of I2−; therefore G = 2.1 refers to the cation yield. The yield for the excited singlet and triplet states, 0.70 ± 0.10 and 1.10 ± 0.10 were determined using solutes such as 1,1′-binaphthyl, 1,2-benzanthracene, pyrene, naphthalene, and benzophenone. The interplay of ionization and direct excitation in the formation of excited states are discussed.
Evaluation of linear chain partition functions by consideration of sequence conditional probabilities
Authors: Poland Douglas
Published: February 2004

A simple, systematic treatment for the class of linear chain models whereby each unit can exist in one of several states, giving rise to a partition function that is a sum over all lengths and orders of sequences of states, is given for increasingly long range interactions between the sequences. The technique consists of constructing relations for sequence conditional probabilities which give rise to a system of linear equations for the a priori sequence probabilities and the unit partition function; matrix inversion eliminates the a priori probabilities leaving a relation for the unit partition function. When the unit partition function is calculated, one can immediately evaluate the a priori sequence probabilities from which all other average quantities can be calculated (thus making it unnecessary to calculate average quantities by taking derivatives of the partition function). A short derivation is given of the generating function method of treating independent sequences and it is shown that the partition function for finite chains can be expressed solely in terms of generating functions (circumventing the need to construct eigenvectors). The technique is then used to treat the case where there is an arbitrary interaction between adjacent sequences with application to titration and chain folding models. The extension to higher order interactions is shown to follow in a similar fashion.
Diffusion-controlled intrachain reactions of polymers. I Theory
Authors: Wilemski Gerald , Fixman Marshall
Published: February 2004

A theory of diffusion-controlled intramolecularly catalyzed reactions of polymer chains in dilute solution is formulated. A closure approximation yields a soluble integral equation for (essentially) the transient rate of the reaction. The time dependent solution consists of a sum of simple exponential terms whose coefficients and decay constants can be readily calculated. The theory is also applied to luminescence quenching and ring-chain interconversion.
Diffusion-controlled intrachain reactions of polymers. II Results for a pair of terminal reactive groups
Authors: Wilemski Gerald , Fixman Marshall
Published: February 2004

The formulas and results of the preceding paper are used to calculate the rate of reaction between reactive groups attached to the ends of a polymer chain. Extensive numerical calculations have been made for both transient rate processes and steady state luminescence quenching. The results are restricted to chains in dilute solution, in either the free draining or nondraining limits and with no excluded volume forces present. A discussion of the qualitative effects of excluded volume forces on the results is included. The results suggest that experimental studies of these processes may be feasible.
Examination of approximations to hindered internal rotation in the calculation of energy level sums
Authors: Stein Stephen E. , Rabinovitch B. S.
Published: February 2004

An examination has been made of the errors involved in various commonly used physical approximations for symmetric hindered rotors that are employed in the calculation of molecular energy level densities and sums. Some illustrative energy level sums obtained with use of these approximations are compared with true hindered rotor sums for a number of molecular systems. Several practical illustrations are also given of experimental systems reported in the literature. A facile higher energy approximation is offered for the accurate calculation of energy level densities or sums of states for molecules containing true hindered rotors wherein an energy-dependent correction is made to the total active internal energy of the corresponding molecule having free internal rotors. At lower energies where a librator approximation works well, a simple rule is offered for selection of an optimum equivalent librator frequency.
Photoionization cross sections in the valence electron approximation. III. Nonlinear molecules (theory)
Authors: Schweig A. , Thiel W.
Published: February 2004

A theoretical method involving the plane-wave approximation and the use of semiempirical LCAO-SCF-MOs of the CNDO and MINDO type is developed for the calculation of photoionization cross sections of arbitrary molecules consisting of atoms from the first three rows of the Periodic Table. Three qualitative concepts for the interpretation of the formalism are discussed, and the dependence of the cross sections on the photon energy, on the electron density distribution in the initial orbital, and on the different behaviour of s, p, and d electrons is treated explicitly. Examples for the application to HeI and HeII photoelectron spectroscopy are given, too.
Interpretation of the magnetic properties of manganese (II) phthalocyanine
Authors: Barraclough C. G. , Gregson A. K. , Mitra S.
Published: February 2004

Detailed crystal field calculations are presented to determine the ground and excited states of manganese(II) phthalocyanine which belongs to the rare spin S = 3/2 state. It is deduced that the ground state of Mn(II) ion is predominantly 4A2(± 1/2) while 4A2(± 3/2) state lies above 40 cm−1 above it. It is shown that 6A1(± 1/2) lies fairly close to the ground state (600 cm−1) and appreciably mixes with it. Two magnetic exchange models are investigated, and their effects are superimposed over the crystal field to quantitatively explain the low temperature experimental data.
Effects of intermacromolecular interactions on diffusion. II. Three-component solutions
Authors: Phillies George D. J.
Published: February 2004

The effect on the diffusion coefficient of nondissipative interactions between diffusing macromolecules is considered for three-component solutions containing solvent and two species of macromolecules. The spectrum of light scattered quasielastically from a system containing two species of interacting Brownian molecules is calculated. Treatment of a dilute minimum-salt protein solution as a three-component system indicates a possible way of performing light scattering measurements on charged species which do not scatter light. Extension of the techniques of paper I of this series to include the effect of interspecies interactions clarifies the relation between the tracer and mutual diffusion coefficients as these have been defined by different authors. This extension also provides a criterion for phase separations in three-component solutions.
Primary processes in the photolysis of water vapor at 174 nm
Authors: Chou C. C. , Lo J. G. , Rowland F. S.
Published: February 2004

The quantum yield for the photolysis of HTO at 174 nm by Reaction (4) is estimated to be ≤0.003 relative to Reaction (5): HTO + hv → HT + O,HTO + hv → T + OH or H + OT.These values were obtained by scavenging T atoms with C2D4, with measurement of the yields of HT, DT, and of tritiated hydrocarbons.
Formation of HO2+ by reaction of metastable O2+ ions with H2
Authors: Ajello J. M. , Huntress W. T. , Lane A. L. , LeBreton P. R. , Williamson A. D.
Published: February 2004

The photoionization efficiency curves of H2+, O2+, and HO2+ have been studied in a mixture of hydrogen and oxygen over the wavelength range 650–810 Å. The HO2+ ion appears at 804 Å, the threshold for ionization of H2, by the reaction H2+ + O2 → HO2+ + H. The relative photoionization efficiency curves of H2+ and HO2+ are the same from 804 to 764 Å. Below 764 Å production of the a 4Πμ metastable electronic state of O2+ leads to the formation of HO2+ by the reaction O2+ (a 4Πu) + H2 → HO2+ + H.
Detailed balance in chemical kinetics as a consequence of microscopic reversibility
Authors: Boyd R. K.
Published: February 2004

The idea that the chemical principle of detailed balance is a consequence of microscopic reversibility is examined in terms of the stochastic theory of chemical reaction rates, particularly as developed by Snider. It is emphasized that detailed balance is a purely macroscopic requirement for all cases except those in which the chemical reaction mechanism contains a closed loop, or where forward and reverse rate constants for an elementary reaction step are measured in different experimental situations, each designed to isolate one unidirectional elementary reaction from its counterpart. Snider's theory is extended to a simpe triangular (closed-loop) isomerization mechanism, and it is shown that detailed balance is not obtained as a result; this is in contrast to local-equilibrium theories, for which the relationship between microscopic reversibility and detailed balance is well established. An attempt to overcome this difficulty by reformulating Snider's theory, in terms of the theory of lumping error, was only partly successful. The limitations of the present approach are discussed, and consequences for universally valid interpretations of phenomenological chemical kinetics are suggested.
Studies in molecular dynamics. XIII. Singlet and pair distribution functions for hard-disk and hard-sphere solids
Authors: Young David A. , Alder Berni J.
Published: February 2004

Molecular dynamics calculations have been carried out on the singlet distribution function (sdf) in hard-disk and hard-sphere solids. The dependence of the distribution function upon particle number and density and the deviations of this function from spherical symmetry are examined. The possible connection between the sdf and the entropy and the behavior of the sdf near the melting point are discussed. The surprisingly close resemblance between the extreme anharmonic hard particles solids and harmonic oscillator solids is emphasized. Calculations of the angular dependence of the pair distribution function at pair contact for hard-disk and hard-sphere solids are also presented.
Coexistence curve properties of Mermin's decorated lattice gas
Authors: Mulholland G. W. , Rehr J. J.
Published: February 2004

Mermin's decorated lattice gas, noteworthy for its singular coexistence curve diameter and previously studied in connection with the breakdown of the law of rectilinear diameters, is shown to display in addition a rich variety of coexistence curve shapes and kinds of critical behavior as the interaction parameters of the model are varied. For an attractive decoration interaction, the coexistence curve of the model resembles, and can closely approximate, the liquid-vapor coexistence curve of real fluids. For a sufficiently repulsive decoration interaction, however, the model is shown to possess (at fixed temperature) three transitions to increasingly dense phases. These coexistence curves may feature peculiar shapes, such as necks and cusps, and they can appear inverted near the critical point; these curves terminate at either a critical point or at a maxithermal point (an analog of an azeotropic point). For discrete values of the interaction parameters, the model possesses a critical double point (the coalescence of two critical points) or a cuspoidal critical point (critical azeotropy), in which cases the critical exponents become renormalized. Qualitatively these results are found to be independent of lattice structure and spatial dimensionality d ≥ 2, and representative coexistence curves are plotted for the simple cubic lattice. Possible applications of these results are mentioned.
Vibrational potential function for diimide, HNNH
Authors: Nibler Joseph W. , Bondybey Vladimir E.
Published: February 2004

The general valence force field for the in-plane motions of diimide has been determined from a total of 24 vibrational frequencies of HNNH, HNND, DNND, and 15N substituted counterparts. The unusual upward shifts observed for the NN stretching frequency on deuteration can be obtained when these shifts are directly constrained, but an exact fit was not possible due to convergence problems. Most of the force constants are well determined and a comparison is offered with recent results for CH3NNH, CH3NNCH3, FNNF, and C2H4. The latter force fields suggest that the unobserved torsional frequency of HNNH should lie above 1300 cm−1 and hence this spectral region has been carefully reexamined.
Molecular collisions. XXI. Semiclassical approximation to atom-symmetric top rotational excitation
Authors: Russell Dennis , Curtiss C. F.
Published: February 2004

In Paper XIX of this series a distorted wave approximation to the T matrix for atom-symmetric top scattering was developed which is correct to first order in the part of the interaction potential responsible for transitions in the component of rotational angular momentum along the symmetry axis of the top. A semiclassical expression for this T matrix is derived by assuming large values of orbital and rotational angular momentum quantum numbers.
Dispersion of the nonlinear optical susceptibilities of organic liquids and solutions
Authors: Levenson M. D. , Bloembergen N.
Published: February 2004

The dispersion of the third order optical nonlinear polarizabilities of benzene, chlorobenzene, and nitrobenzene is measured in the vicinity of Raman active vibrations. Observation of interference between resonant and nonresonant contributions determines the magnitude and phase of the electronic hyperpolarizability. Shifts in the frequency and linewidth of the interference minima occur when additional components such as CCl4, CS2, or 2,5-diphenyloxazole are added to the solution. Analysis of the resulting line shape yields the hyperpolarizability of the solute species.
Low temperature magnetic susceptibility of UCl4
Authors: Gruber John B. , Hecht Harry G.
Published: February 2004

The magnetic susceptibility of powdered UCl4(s) has been measured in the temperature range 4–300 °K using the Faraday method. The data are qualitatively consistent with our previous analysis of the optical spectrum, but suggest that the first excited Γ5 level should lie at approximately 110 cm−1 above the ground state.
Microwave spectrum and rotational isomerism of ethyl nitrate
Authors: Scroggin Don G. , Riveros Jose M. , Wilson E. Bright
Published: February 2004

The existence of two rotational isomers of ethyl nitrate (CH3CH2ONO2) has been confirmed by gas phase microwave spectroscopy. The isomers are designated trans and gauche to indicate the relative positions of the CH3- and -NO2 groups. In the trans form all the heavy atoms lie in a plane. The gauche form differs from the trans by a rotation about the C–O bond of 95° and by an opening of the CON angle by ≈ 3°; both changes appear to indicate considerable nonbonded interaction of the CH3- and -NO2 groups. Stark effect measurements determined the dipole moments to be: μtrans = 3.39 D and μgauche = 3.23 D. The gauche form is less stable by 143 ± 70 cm−1. Several low-lying excited states of the torsional vibration have been assigned for each form. A four-term potential function for internal rotation about the C–O bond was determined. The uncertainties and error limits of a four-term potential function are discussed.
First ionization potentials of some refractory oxide vapors
Authors: Rauh E. G. , Ackermann R. J.
Published: February 2004

The ionization potentials of the gaseous atoms, monoxides, and dioxides over the refractory oxides of Ti, Zr, Hf, Th, U, Y, and La have been obtained from ionization efficiency curves and appearance potentials by electron impact. A method of simultaneous and intercomparative measurements with known standards was used. The measured values for the gaseous metal atoms are in agreement with the spectroscopic values except for Zr and Hf which are about 0.4 eV lower. These results and some of previous studies show that the values for the monoxides are somewhat less than those for the metals, and the dioxides of those metals in their highest oxidation state are generally larger by 3–4 eV.
Reflection of light by a semi-infinite dielectric
Authors: Philpott Michael R.
Published: February 2004

A microscopic theory of the reflection of S-polarized light by a perfect semi-infinite molecular crystal is developed. The interactions between molecules are assumed to be of the dipole-dipole type. When excited state van der Waals interactions are neglected, so that molecular polarizability tensors are independent of distance from the surface, the reflection power factorizes into separate normal mode contributions. Each mode is characterized by a complex amplitude and refractive index. For normal incidence, the theory of Mahan and Obermair is recovered. If interactions between crystal planes are negligible there is only one polariton mode and the reflection formula of classical optics is obtained. If the molecules in the surface plane have excitation energies different from those in other planes, then the reflectivity formula contains an additional frequency dependent factor. When the coupling between planes is weak this surface factor causes the appearance of either a deep narrow minimum within the stopping band or a sharp spike outside the band. Model calculations exploring this effect are described.
Computer study of a classical quadrupole solid
Authors: Mandell Myron J.
Published: February 2004

The Monte Carlo method is applied to a system of 32 classical free rotors on a face-centered cubic lattice with toroidal boundary conditions. The particles interact via nearest-neighbor electric quadrupole interactions, characterized by an energy Γ=6Q2/25R5. A first-order phase transition is found at about kBT=11Γ. Results are given for energy, specific heat, correlations of the form < Y2m (k)* Y2n (k) >, and various other quantities. Suggestions are made for further experimental and theoretical studies.
Molecular beam study of CO oxidation on a (111) platinum surface
Authors: Palmer Robert L. , Smith Joe N.
Published: February 2004

The heterogeneous oxidation of CO on epitaxially grown Pt(111) surfaces has been investigated using modulated molecular beam techniques. Within the range of pressures from 10−7 to 10−5 torr and temperatures from 450 to 950°K, the overall reaction is apparently first order. The observance of higher order kinetics in these experiments is argued to be an artifact of the beam technique used here and suggests a high surface diffusion rate for adsorbed CO. The measured angular dependence of the CO2 desorption rate from the Pt(111) surfaces does not follow the Knudsen law of random (cosine) desorption, but is much more sharply peaked about the surface normal. As with the desorption of hydrogen, the desorption law for CO2 is given by cosdθr where in this case d ≤ 6, depending on the condition of the surface. The adsorption of CO is also shown to have an angular dependence and is not activated on Pt(111). The results of this study strongly suggest that a Langmuir-Hinshelwood mechanism dominates the reaction, with CO being the principal diffusing species.
Graph theoretic approach to the stability analysis of steady state chemical reaction networks
Authors: Clarke Bruce L.
Published: February 2004

A form of graph theory is developed which makes it possible to write the Routh-Hurwitz stability conditions for any network of chemical reactions as sums of graphs. These sums, which must all be positive for stability, can contain negative terms only through two mechanisms: first, by having an odd number of certain types of cycles in formally positive graphs, or second, by having an even number of these cycles and at least one cycle in formally negative graphs. The set of graphs in each stability inequality may be represented as a set of points which defines a convex polytope in a higher dimensional space. For large parameter values only the vertices of this polytope affect the stability of the network. For each vertex corresponding to a graph which is a negative term in a stability inequality there is a convex coneshaped contribution to the network's unstable region. For large parameter values, this region is the union of the interiors of these convex cones in parameter space whose boundaries are hyperplanes, and the equation of each hyperplane is determined by the graphs of a pair of adjacent vertices of the polytope. The full region of instability is the projection from near the origin of certain faces of a dual polytope: faces which correspond to negative vertices of the original polytope. It is proven that, if the reaction network has a sufficiently rich set of graphs, the vertices of the polytope are all maximum overlap graphs from the diagonal term of the Hurwitz determinant. In other cases, the polytope is truncated and vertices in the truncated region can make the network unstable only if they are negative through the second mechanism mentioned above.
Analysis of conflicting theories of dielectric relaxation
Authors: Titulaer U. M. , Deutch J. M.
Published: February 2004

Two different forms have been proposed for the relation between the frequency-dependent dielectric constant of a polar substance and the autocorrelation function for the net dipole moment of a small spherical region inside the dielectric. We find that one of them is correct, while the other one describes a spherical sample embedded in a medium with a frequency-independent dielectric constant equal to the static dielectric constant of the sample. A number of recent papers dealing with this question are analyzed. We discuss in particular the role played by fluctuations in the polarization of the surrounding medium near the surface of the embedded sphere.
Equation of state of the square-well fluid
Authors: Tago Yoshio
Published: February 2004

The Percus-Yevick equation is solved for the square-well (SW) potential with λ = 1.5 and λ = 1.85, where λ is the ratio of the diameter of the attractive well to the diameter of the hard core. The results for the equation of state and the Gibbs free energy are calculated using a method proposed by Chen, Henderson, and Barker. The results for the equation of state are in good agreement with the simulation results for the SW fluid with λ = 1.5 over a wide range of temperatures and densities. The result for the radial distribution function is in satisfactory agreement with the simulation result for the SW fluid with λ = 1.85 in the liquid state. A first order gas-liquid phase transition occurs in the form of a typical van der Waals type in the region where the Percus-Yevick equation has solutions. The effect of the width of the SW potential on the radial distribution function and the thermodynamic properties is examined. It is shown that, for the SW fluid, the same approach to obtain the equation of state may be used with the Yvon-Born-Green theory.
Rotational isomerism. XI. Raman spectra of n-butane, 2-methylbutane, and 2, 3-dimethylbutane
Authors: Verma A. L. , Murphy W. F. , Bernstein H. J.
Published: February 2004

Raman spectra have been obtained for n-butane, 2-methylbutane, and 2, 3-dimethylbutane in the vapor and solid (−196°C) phases. The low temperature spectra of the solids undergo a marked simplification due to the disappearance of all but one of the rotational isomers. This enabled band pairs in the vapor spectrum to be identified with the appropriate rotational isomers. From the temperature dependence of selected band pairs the enthalpy difference between the rotational isomers was obtained for each compound in the vapor phase.
EPR investigations of Dy3+ and Yb3+ in alkaline-earth oxide single crystals
Authors: Reynolds R. W. , Boatner L. A. , Chen Y. , Abraham M. M.
Published: February 2004

The cubic-site EPR spectra of Dy3+ and Yb3+ have been investigated in single crystals of MgO, CaO, SrO, and BaO. In each host crystal the cubic-site Dy3+ and Yb3+ spectra were isotropic, and the observed g values indicated that the electronic ground states for both ions were Γ6 doublets. The Dy3+ and Yb3+ EPR spectra observed for the MgO single crystal host were characterized by narrow linewidths (≈2G for H‖ 〈 111 〉) and correspondingly by excellent resolution of the hyperfine structure. The spectra for both ions exhibited linewidth variations with applied magnetic field orientation, and the narrowest linewidths were obtained with H‖ 〈 111 〉. Appropriate spin-Hamiltonian parameters were obtained for sample temperatures of 4.2°K or below. A tetragonal EPR spectrum was also observed for Yb3+ in CaO, and the wavefunctions determined from the observed g values indicate an appreciable departure from cubic symmetry.
Thermal diffusion of Br2 and Cl2 in the noble gases
Authors: Maya J. , Davidovits P.
Published: February 2004

We have measured the thermal diffusion factors of Br2 and Cl2 in He, Ne, Ar, and Kr. The halogen-noble gas mixture is contained in a closed two bulb apparatus, the ends of which are maintained at different temperatures. The halogen density at the cold end is determined by the equilibrium vapor pressure above the temperature controlled halogen reservoir. The halogen density at the hot end is determined by measuring the total reaction rate; γ of sodium atoms with halogen molecules. Since the cross sections for the reactions of Na atoms with Br2 and Cl2 are known, measurement of γ yields directly the halogen density. The thermal diffusion factors computed from the transport equations are compared with the first Kihara approximation to the Chapman-Enskog theory. The agreement is within experimental error.
CO2 laser initiated decomposition of ethyl chloride
Authors: Bailey R. T. , Cruickshank F. R. , Farrell J. , Horne D. S. , North A. M. , Wilmot P. B. , Win Tin
Published: February 2004

The decomposition of ethyl chloride (EtCl) has been studied at pressures in the range 0.2–100 torr in a CO2 laser beam (105–160 W cw) in order to evaluate the processes occurring in such a system. Selective excitation of the EtCl would lead to the observed CH4 production, but it is shown that ``proof'' of such a process is not readily attainable despite the well-characterized thermal decomposition of EtCl in contrast with systems previously supposed to decompose selectively. The important effects of slight laser power irreproducibilities, induction periods and the high ``temperatures'' produced are discussed.
Theoretical foundations of purely semiempirical quantum chemistry
Authors: Freed Karl F.
Published: February 2004

All the purely semiempirical quantum chemical theories of molecular electronic structure, such as the Pariser-Parr-Pople theory and its all valence electron generalizations like MINDO, assume the existence of an abstract true effective Hamiltonian which acts only within the space spanned by a minimum basis set of valence shell orbitals. Instead of following the customary procedure of attempting to determine the properties of this true effective Hamiltonian by fitting its matrix elements to experiment, this effective Hamiltonian is derived from the full N-electron molecular Schrödinger equation under the condition that a complete configuration interaction calculation within the valence shell using this true effective Hamiltonian reproduces the exact ground and valence state energies. It is shown that this true effective Hamiltonian also exactly reproduces the projection of the exact ground and valence state wavefunctions on the space spanned by the minimum valence shell basis in the fixed core approximation. Explicit expressions are presented enabling the ab initio evaluation of the matrix elements of this true effective Hamiltonian in the valence shell basis, the true parameters. A discussion is presented of how these true parameters may be evaluated by slight modifications of standard methods such as degenerate Rayleigh-Schrödinger perturbation theory (linked cluster form), Nesbet's hierarchy method, and related cluster function approaches. For the first time, this enables the ab initio evaluation of the true parameters which are empiricized in the semiempirical theories, thereby providing systematic methods for the testing of all of the assumptions of these semiempirical theories and for the development of new and improved parametrization schemes. A detailed discussion is also given concerning the nonclassical aspects of the true parameters.
Absorption spectra of Mo atoms isolated in rare-gas matrices
Authors: Green David W. , Gruen Dieter M.
Published: February 2004

The ultraviolet spectra of Mo isolated in Ar, Kr, and Xe matrices at 14°K have been correlated with the gas phase spectra. Contrary to the interpretations of the spectra of the P ← S transitions of a number of matrix isolated atoms, no crystal field effects leading to a breaking of the orbital degeneracy are required in order to obtain a satisfactory explanation of the spectra. The different matrices affect the spin-orbit coupling and configuration interaction of the Mo atom leading to changes in the triplet splittings of the y7P ← a7S transition. The diffusion of Mo atoms in an Ar matrix lead to the formation of polymers. No definitive assignment of bands to the dimer could be made.
Coupled channel operators and rearrangement scattering. I. Comparative study of one-state s-wave e-H scattering
Authors: Kouri Donald J. , Craigie Michael , Secrest Don
Published: February 2004

Equations are derived for the amplitude density functions for the channel T matrix for rearrangement scattering. These equations are applied to electron-hydrogen atom exchange scattering. A calculation is made in the one-state approximation in which only the 1S state of hydrogen is considered. This calculation is compared with an earlier calculation using an alternate definition of the channel T matrix and with a number of other more complete s-wave calculations. The new definition of the channel T matrix is shown to be superior to the earlier definition for numerical computation. The one-state result is surprisingly accurate when compared to essentially exact variational results. This exceptional accuracy with such a limited basis is attributed to the fact that the formalism explicitly includes both the direct and the rearrangement channel descriptions.
Study of molecular reorientation: Pressure and temperature dependence of deuterium relaxation in liquid CDCl3
Authors: VanderHart D. L.
Published: February 2004

Deuterium NMR spin-lattice relaxation measurements have been performed on the neat liquid CDCl3 over the range 28°C<T<165°C and at pressures up to 5 kbar (5×108 N/m2). These measurements enable one to determine τθ,2, the correlation time for molecular reorientation about the axes perpendicular to the symmetry axis. The data are presented as a function of all three state variables, P, V, and T. An attempt is made to describe the data in terms of various simple models for reorientation including activation and free volume models. Each of these models predicts certain aspects of the data but fails in other areas. In particular, the activation model fails to predict the correct behavior at constant temperature and the free volume theories generally fail to predict the constant volume experiments. Deviations from the Debye and microviscosity theories are also noted. The mean time between collisions τcoll was calculated from τθ,2 assuming both Gordon's J-diffusion model and the equivalence of τcoll and τJ, the angular momentum correlation time. These values of τcoll were then compared with the predictions of the cell model for liquids in which τcoll is simply the mean free path divided by the mean velocity. Generally, the data did not fit the cell model; however, at constant pressure, the cell model did predict the changes in τcoll quite well. Qualitative suggestions are made to account for the discrepancies. Finally, the possibility that H-bonding exists in CHCl3 and affects the generality of these results by changing the quadrupole coupling constant and disturbing the dynamics of the system is discussed. It is concluded that these effects are small if they exist at all.
Manganese diborate: Crystal structure, magnetization, and thermal extinction dependence
Authors: Abrahams S. C. , Bernstein J. L. , Gibart P. , Robbins M. , Sherwood R. C.
Published: February 2004

MnB4O7 is paramagnetic at room temperature, with effective paramagnetic moment of 5.84 μB, and orders antiferromagnetically at about 3°K. The lattice constants of orthorhombic MnB4O7 are a =8.62354±0.00003, b =14.0071±0.0001, c =8.07237±0.00002 Å at 298°K, with space group Pbca and eight formulas in the unit cell. A total of 11 317 reflections were measured with PEXRAD resulting in 1060 symmetry-independent structure factors. The structure was solved by a combination of Patterson and Fourier series and refined by the method of least squares. The final R factor for a model with all atoms undergoing anisotropic motion is 0.046. The structure is shown by half-normal probability plot analysis to be accurately isomorphic to that of CdB4O7. The B4O7 group consists of two BO4 tetrahedra with one oxygen atom in common, each sharing two other oxygen atoms with two triangular BO3 groups. Each triangular group contains an oxygen atom from both tetrahedra. The dihedral angle between planes containing the two tetrahedral group boron and each of the triangular group boron atoms is 106.2°. The average tetrahedral B–O distance is 1.473 Å, and average triangular B–O distance is 1.368 Å. The BO3 groups are exactly planar, within experimental error. The Mn2+ ion occupies a distorted tetrahedron of oxygen atoms at an average distance of 2.109 Å: four others at distances ranging from 2.442 to 3.188 Å lie approximately along the normals to the tetrahedral faces. The secondary extinction coefficient in MnB4O7 on gently heating the cold-worked crystal initially increases significantly, consistent with a temperature-dependent increase in mosaic block dimension.
Configuration of an adsorbed polymer molecule: Solvent effect
Authors: Lax Melvin
Published: February 2004

The configurational behavior of isolated adsorbed polymer molecules subjected to a solvent effect were simulated by generating self-avoiding walks confined to the diamond lattice, excluding first-nearest-neighbor occupancy, in the presence of an interacting solid barrier. The method of exact enumeration was employed. The configurational properties obeyed an asymptotic equation of the form  αNγ, where P represents a generalized configurational property, e.g., the average number of segments on the surface, etc., and γ is a critical index. Some insight into the mechanism of chain adsorption is advanced from an analysis of the energy dependence of the configurational properties.
Nuclear magnetic resonance of 14N in ammonium aluminum alum
Authors: Bailey W. C. , Story H. S.
Published: February 2004

Nuclear quadrupole coupling constants eQVzz/h have been measured for 14N (196 kHz) in NH4Al(SO4)2 · 12H2O and 87Rb (12.9±0.1 MHz) in RbAl(SO4)2 · 12H2O. If we assume regular tetrahedral symmetry for the NH4 ion, such that the intraionic charges produce, aside from the effect of antishielding, zero effective electric field gradient at the N nucleus, then the ratio of the coupling constants implies an antishielding factor of about −4.
Franck-Condon principle as involving torsional oscillations: Height of the potential barrier to internal rotation in an electronically excited state of ethane
Authors: Custer Edward M. , Simpson William T.
Published: February 2004

The effect of restricted internal rotation on electronic spectra can be considered in the light of the Franck-Condon principle. For usual values of the temperature and the molecular constants, internal rotation can have a heavy influence on the width and shape of a vibronic line, conceivably even to the extent of masking the shape due to external rotation. These ideas are pursued in the case of the zero-zero and one other vibronic line of ethane. Spectra at ∼ 1400 Å for ethane and perdeuteroethane from −78−260°C are presented, as are spectra theoretically calculated for various temperatures, and barriers ranging from zero to 27 kcal/mole. A detailed comparison of calculated and actual spectra leads to an educated guess for the excited-state barrier height as 1.7 kcal/mole, close to the calculated value of Salahub and Sandorfy of 1.3 kcal/mole. In a discussion of the assignment of the electronic transition, the Innes and Pearson finding that the transition is perpendicular is accepted, but it is suggested that the transition lies over an intense parallel transition in the same region. Various previous efforts to assign these transitions can, it is felt, be reconciled.
Surface catalytic efficiency of a sputtered molybdenum layer on quartz and Pyrex for the recombination of nitrogen atoms
Authors: Hays G. N. , Tracy C. J. , Oskam H. J.
Published: February 2004

The temporal properties of the number density of nitrogen atoms were studied during the decay period of plasmas produced in nitrogen by measuring the intensity of the first positive bands during this period. From the analysis of these data the value of the surface catalytic efficiency γ of a sputtered molybdenum layer on quartz and Pyrex for the recombination of nitrogen atoms was determined for gas pressures varying from 0.1 to 5 Torr and gas temperatures in the range 85–500 °K. A theoretical model explaining the data was derived.
Infrared spectra and characterization of the products of fluorine atom reactions with matrix isolated NO molecules
Authors: Smardzewski Richard R. , Fox William B.
Published: February 2004

Simultaneous matrix deposition of F2 and NO molecules at high dilution in argon and nitrogen onto an 8°K CsI window produces three new infrared absorptions in addition to those of normal, FN-bonded nitrosyl fluoride, FNO. The infrared features at 1886.6, 734.9, and 492.2 cm−1 (Ar matrix) and their 15N and 18O isotopic counterparts were assigned to the previously unobserved FO-bonded isomer NOF, which appears to be a bent molecule having a molecular geometry similar to that of nitrosyl fluoride. Controlled fluorine atom diffusion at 20°K, in the absence of photolytic radiation, results in increased yields of the NOF molecule, while matrix photolysis experiments at 8°K have established that this species undergoes intramolecular photorearrangement to the more stable FNO form when exposed to radiation comprised of wavelengths below 280 nm.
Predissociation model for formaldehyde
Authors: Yeung Edward S. , Moore C. Bradley
Published: February 2004

The collisionless nonradiative decay of single vibronic levels of the first excited singlet state of formaldehyde (1A2) is explained in terms of coupling with vibronic levels of the ground (1A1) state. Interaction is provided by the breakdown of the Born-Oppenheimer approximation through the nuclear kinetic energy operator. A full, Franck-Condon-factor-weighted summation of states calculation is undertaken for both H2CO and D2CO. Harmonic oscillator wavefunctions derived from published spectroscopic studies are used throughout. Anharmonic mixing of the ground state vibrational levels is artificially accounted for by averaging the rate over a range of final state energies. The bound-to-bound state calculation is consistent with a two-step dissociation process, in which the excited state is less strongly coupled to the ground state than the ground state is coupled to the continuum. Direct coupling to the continuum is neglected. The results are in good agreement with experiment as regards the dependence of rate on total energy and on vibrational modes, the absolute magnitude of the rates, and the effect of isotopic substitution. Rates for levels with CH stretches excited are underestimated. The largest contributions to the rates come from coupling to ground state levels with the CO stretch and the out-of-plane bend highly excited.
Formation of O− in ZnO from the dissociation of adsorbed N2O
Authors: Wong Ning-Bew , Taarit Younes Ben , Lunsford Jack H.
Published: February 2004

Photoinduced O− ions or V-type centers have been detected in ZnO following ultraviolet irradiation at −196°C. The EPR spectrum of the ions is characterized by g⊥=2.021 and g∥ = 2.0026. The concentration of O− was greatly enhanced when N2O had been adsorbed on the surface prior to irradiation. No oxygen-17 hyperfine structure was detected when N2O enriched with 17O was adsorbed. The mechanism, therefore, for the formation of the observed O− ions is not the same on a semiconductor as on an insulator such as MgO, which was previously studied. The paramagnetic center on ZnO was formed only on samples that were degassed at temperatures near 450°C. It is proposed that both anion and cation vacancies exist on the surface and in the bulk following this pretreatment. Ultraviolet irradiation produces electron-hole pairs, and the electrons react with N2O forming O− ions and N2. The O− is then trapped at an anion vacancy where it gives up its hole (traps another electron) and becomes an oxide ion. Holes formed in this manner, as well as those produced directly by the ultraviolet light, migrate through the lattice and are trapped at zinc ion vacancies. These centers are stable at −196°C, but they rapidly decay at 25°C. Reactions of O− with CO and O2 suggest that the ions may be near but not on the surface of ZnO.