bluuebunny/crossref_metadata_2025 huggingface dataset:
A CASSCF study of the potential curves for the X 1Σ+, B 1Σ+, and A 1Π states of the BH molecule
Authors: Jaszuński Michal/ , Roos Björn O. , Widmark Per-Olof
Published: February 2003

The complete active space (CASSCF) method was applied to compute the potential curves for three singlet states of the BH molecule. The vibrational and rotational levels were calculated by solving numerically the radial Schrödinger equation and the spectroscopic constants were obtained by a least square fit of the term values to the Dunham expansion. For each of the states X 1Σ+, A 1Π, and B 1Σ+, only four vibrational levels are known from experiment. The results for the ground state are in excellent agreement with experiment: the maximum deviation between calculated and experimental Gv+1/2 (v = 0,2) is 3.5 cm−1. Spectroscopic constants obtained by fitting 160 term values (v = 0,9, J = 0,15) are reported. The existence of a double minimum in the potential curve for the B 1Σ+ state is confirmed. The calculation shows that all four experimentally observed levels correspond to the inner minimum. The differences between calculated, for this minimum, and experimental ΔGv+1/2 (v = 0,2) values for the B 1Σ+ state do not exceed 22 cm−1. A fifth vibrational level and a number of levels corresponding to the outer minimum or lying above the maximum are also obtained in the calculation. For the A 1Π state a dissociation energy of 0.50 eV (exptl. = 0.70 eV) was obtained, and the potential curve was found to have a maximum lying 0.20 eV above the dissociation limit.
Quantum dynamical tunneling in bound states
Authors: Davis Michael J. , Heller Eric J.
Published: February 2003

Tunneling involves an allowed quantum event which fails to take place classically. Dynamical tunneling is the subset of such events which do not involve a classically insurmountable potential barrier. In this paper, we present unambiguous evidence for dynamical tunneling in bound state quantum systems.The tunneling occurs between two distinct regions of classically trapped quasiperiodic motion. Close analogies are shown to exist between this situation and ordinary barrier penetration in a double minimum potential. In the cases we study, tunneling occurs between equivalent or nearly equivalent local mode motions, which have arisen out of a resonance between the symmetric and nonsymmetric stretch.
Complete set of steady states for the general stoichiometric dynamical system
Authors: Clarke Bruce L.
Published: February 2003

The complete set of steady states for rate constants in the range 0?ki<∞ and concentrations in the range 0?Xi<∞ is given explicitly in parametric form for the general chemical reaction system. The only assumptions are that the stoichiometries are real numbers, and the reaction rates are proportional to functions of class Ck, k≳0; the functions are assumed to be positive in the interior of the domain. Hence, these results apply far more generally than just to chemical systems and should be valid for many ecological and economic models as well. The set of steady states in the interior of the domain is in general a simply connected differentiable manifold M of dimension n+r−d, where n = number of species, r = number of reactions, and d = rank ν (ν = stoichiometric matrix). The full set of steady states M* consists of those in the interior (M) plus a frequently very complicated set of steady states lying in the boundary. M* is the union of a set of differentiable manifolds but is not itself a differentiable manifold in general. If the reaction rate laws are all monomials (i.e., products of concentrations raised to some ’’order of kinetics’’), a working APL algorithm is given which provides enough information to construct explicitly the set of manifolds comprising M*. This algorithm is used to treat an early version of the Belousov–Zhabotinski reaction mechanism. The algorithm may be used for more general rate laws with minor modifications.
The two-dimensional Lennard-Jones system: Sublimation, vaporization, and melting
Authors: Phillips James M. , Bruch L. W. , Murphy R. D.
Published: February 2003

The solid–vapor equilibrium line and the liquid–vapor equilibrium line near the melting region for classical two-dimensional systems of particles interacting by Lennard-Jones (12,6) pair potentials are determined from Monte Carlo simulations with 224 particles. Structural and thermodynamic properties are evaluated for homogeneous single phases on both sides of the melting transition. The Monte Carlo results show that a cell model approximation to the solid phase has good accuracy for most of the anharmonic 2D solid conditions of this work. Free energies for solid states are constructed starting from the low temperature quasiharmonic solid; for the 2D liquid states the starting point is the virial expansion for the dilute gas. From these free energies the triple-point melting of the periodically extended 224-particle system is found to be first order and the latent heat of melting is determined. Graphic displays are obtained showing the occurrence of dislocation pairs, voids, particle exchange, and mixed phases. The implementation of the simulation is discussed, including problems of superheating and the achievement of equilibrium.
Potential energy surfaces of the reaction C++H2 → CH++H
Authors: Sakai Shogo , Kato Shigeki , Morokuma Keiji , Kusunoki Isao
Published: February 2003

Potential energy surfaces of six low lying states for the reaction C++H2 have been calculated with an ab initio MCSCF–POLCI method. An emphasis has been placed on the reaction channel leading to the chemiluminescent product CH+(A 1Π)+H(2S). Based on the characteristics of the surfaces, a mechanism for this reaction has been proposed which involves a nonadiabatic transition from 2 2A′ to 3 2A′ in the vicinity of the C2v symmetry. An alternative path involving a 1 2A″ → 2 2A″ transition cannot be denied.
A calorimeter for heat capacity measurements of high precision from 1.4 to 100 K. The heat capacity of hexa-ammine cobalt (III) chloride from 2.1 to 309 K
Authors: Clayton P. R. , Staveley L. A. K. , Weir R. D.
Published: February 2003

An adiabatic calorimeter is described for determining heat capacities with a precision of one or two tenths of one percent between 2 and 100 K. Temperatures are measured with a germanium thermometer. It has been demonstrated that even at the upper end of this range (≳80 K) the accuracy of the measurements is still better than one percent, so that by combining determinations with this calorimeter with those made on another instrument operating from the boiling point of liquid nitrogen upwards, the range 2 to ∼600 K could be accurately covered. Measurements have been made of the heat capacity Cp of hexa-ammine colbalt (III) chloride [Co(NH3)6Cl3] from 2 to 310 K. Cp has a somewhat unorthodox temperature dependence between 165 and 185 K. Also, at the lowest temperatures studied, Cp was found to embody a contribution proportional to T−2, which is regarded as the high-temperature ’’tail’’ of Schottky anomaly arising from the splitting, due to quantum mechanical tunneling, of the lowest energy level of the torsional oscillation of the ammonia molecules about the Co–N axes. The magnitude ΔE of this splitting has been evaluated, and assuming a simple sinusoidal potential energy function for the one-dimensional restricted rotation of the ammonia molecules about the Co–N axis, the value of ΔE has used to assess the height of the barrier V0 separating two potential energy minima. The contribution from this mode of motion to Cp has then been estimated. Calculated values of the total Cp are found to be in excellent agreement with the experimental values from 10 to 310 K, including the range 165 to 185 K. It appears that the cause of the apparent ’’anomaly’’ in this region is simply that it is here that the contribution from the restricted rotational movement of the ammonia molecules reaches its maximum. The Schottky anomaly is estimated to make its maximum contribution to the heat capacity at 18 mK, while the value of the barrier height Vo is 4.7 kJ mo1−1.
Polarization density profiles for dipoles against an electrified wall in the MS and RLHNC approximations
Authors: Rasaiah Jayendran C. , Isbister Dennis J. , Eggebrecht John
Published: February 2003

The polarization density profiles P(z, E) for dipoles in the vicinity of a wall, from which an electric field emerges, have been calculated using the mean spherical (MS) and renormalized linearized hypernetted chain (RLHNC) approximations. The profiles for the two approximations are significantly different. The presence of a component of the polarization density that is perpendicular to the field E, when the field is neither perpendicular nor parallel to the wall, is noted. The anisotropic response of the fluid to the electric field in the vicinity of the wall is discussed in terms of a local dielectric tensor when the relationship between the polarization density and the electric field is linear.
Depolarized light scattering studies of liquids: 1,5- and 2,4-pentanediol
Authors: Fytas G. , Dorfmüller Th.
Published: February 2003

The temperature dependence of the shear viscosity of the isometric pair 1,5-pentanediol and 2,4-pentanediol exhibits some remarkable differences in the liquid phase. The purpose of this study is to rationalize these differences in terms of molecular structure of the two diols. In order to achieve this, we performed low frequency depolarized light scattering measurements and auxiliary viscosity measurements of the bulk liquids and their solutions over a wide temperature (230–350 K) and concentration range (0.2< χ <1). The integrated scattering intensity is shown to be mainly induced by H-bonded intermolecular structures and indicates marked structural differences between the two liquids obviously due to the capacity of 2,4-pentanediol to form bulky ring structures. The relaxation times obtained from the spectral linewidth measurements indicate that the local microviscosity to which molecular segments are subject during rotational motions is markedly smaller than the macroscopic shear viscosity especially at lower temperatures in 2,4-pentanediol, whereas this difference is negligible in 1,5-pentanediol. Even small concentrations of a solvent tend to reduce this effect drastically.
Quasidecay of harmonic oscillator coherent states in nonharmonic potentials
Authors: Brickmann Juergen , Russegger Peter
Published: February 2003

The time evolution of harmonic oscillator coherent states (HOCS’s) (displaced ground state wave functions) in symmetrical and nonsymmetrical nonharmonic potentials is studied numerically. The potentials were modeled with an ansatz V(ε, q) = (1−ε) VHO+ε VNH, where VHO is the harmonic oscillator potential and VNH is chosen as a convenient Morse oscillator (nonsymmetrical) or as a negative Gaussian (symmetrical) so that the potential near the minimum is not distorted. The initially well located HOCS’s decay within a medium time scale of 5–100 classical oscillations to a wave packet which is delocalized in the position area of the potential well between the classical turning points. The time period up to this delocalization is designed as quasidecay time τQ, and it is demonstrated with some examples that in the symmetrical potentials the HOCS is refocused after a period of τR = 8τQ, while, as a rule, in the nonsymmetrical potential the recurrences need much longer time and could not be observed in the time scale under investigation. It is shown that the absolute values of the quasidecay times cannot be related either to the energy moments 〈H〉 and ΔH nor to the contribution of a quasicontinuum to the initial HOCS but that there is a strong correlation of ΔH and τQ for the nonsymmetrical potential considering only initial wave packets of the same energy.
Generalized Langevin theory for many-body problems in chemical dynamics: The quantum equivalent chain
Authors: Adelman S. A.
Published: February 2003

A quantum generalized Langevin representation for condensed phase atomic dynamics, which provides a physically useful formal framework for a quantum condensed phase molecular collision theory, is presented. Basic formal properties of the representation are worked out. A treatment of atomic quantum relaxation is presented.
Solubility of xenon in n-alkanes: n-pentane through n-hexadecane
Authors: Pollack Gerald L.
Published: February 2003

Measurements of the Ostwald solubilities (L) for 133Xe gas in the 12 liquid n-alkanes from n-pentane through n-hexadecane at 20.0 °C are reported. The results range from L(133Xe→n-C5H12) = 5.39 to L(133Xe→n-C16H34) = 3.14. The corresponding values of the partial molar Gibbs energy Δμ20 range, respectively, from 2150 to 1920 cal/mol.
Optical analogs of NMR phase coherent multiple pulse spectroscopy
Authors: Warren W. S. , Zewail A. H.
Published: February 2003

A new technique is presented for producing optical pulse sequences using acousto-optical modulation. The method is applied to gaseous I2 at −l5 °C.(AIP)
Bound level resonances in rotationally inelastic HD/Pt (111) surface scattering
Authors: Cowin James P. , Yu Chien-Fan , Sibener Steven J. , Hurst Jerry E.
Published: February 2003

The rotationally inelastic scattering of HD(J = 0→, 1, 2, 3) from a clean Pt(111) surface is reported. Discrete peaks for each final J state are resolved in the in-plane angular distributions, with the inelastic peaks typically being large compared to the elastic peak. Sharp modulations of these inelastic transition probabilities were found as the incident angle was varied, suggesting bound level resonances. These resonances are inconsistent with G-vector mediated (elastic) selective adsorption, but are consistent with rotationally inelastic selective adsorption into the bound levels of the physisorption potential. Observation of bound level resonances can be used to determine molecule–surface potentials. However, unlike elastic selective adsorption, highly probable rotationally inelastic HD selective adsorption is applicable to metallic surfaces of low corrugation.
Theoretical investigation of the electron affinity of CO2
Authors: Yoshioka Y. , Schaefer H. F. , Jordan K. D.
Published: February 2003

A b initio SCF and configuration interaction calculations are reported for CO2 and CO2. The electron affinity of CO2 is discussed in terms of contribution of electron correlation. (AIP)
The rotational spectrum and molecular structure of the acetylene–HCl dimer
Authors: Legon A. C. , Aldrich P. D. , Flygare W. H.
Published: February 2003

Three isotopic species of a weakly bound T-shaped π complex formed between acetylene and HCl have been observed through the assignment of their rotational spectra by the use of a pulsed, Fourier-transform spectrometer with a high pressure gas being pulsed into an evacuated Fabry–Perot cavity. The spectra clearly show the complex to be a nearly prolate asymmetric top (κ = −0.9898) with the HCl on the a-inertial axis and perpendicular to acetylene and with the hydrogen atom pointing towards the middle of the acetylene triple bond. The chlorine is situated, on average, 3.699 Å from the edge of the acetylene molecule. The following molecular constants have been obtained for the C2H2, H35Cl isotopic species: The rotational constants are A = 36 084(838), B = 2481.065(6), and C = 2308.602(6) MHz; the nuclear quadrupole coupling constants are χaa = −54.342(4), χbb = 26.862(5), and χcc = 27.480(9) MHz, and the rotational centrifugal distortion constants are DJ = 7.9(4) and DJK = 497(8) kHz.
Kinetic mean-field theories
Authors: Karkheck John , Stell George
Published: February 2003

A kinetic mean-field theory for the evolution of the one-particle distribution function is derived from maximizing the entropy. For a potential with a hard-sphere core plus tail, the resulting theory treats the hard-core part as in the revised Enskog theory. The tail, weighted by the hard-sphere pair distribution function, appears linearly in a mean-field term. The kinetic equation is accompanied by an entropy functional for which an H theorem was proven earlier. The revised Enskog theory is obtained by setting the potential tail to zero, the Vlasov equation is obtained by setting the hard-sphere diameter to zero, and an equation of the Enskog–Vlasov type is obtained by effecting the Kac limit on the potential tail. At equilibrium, the theory yields a radial distribution function that is given by the hard-sphere reference system and thus furnishes through the internal energy a thermodynamic description which is exact to first order in inverse temperature. A second natural route to thermodynamics (from the momentum flux which yields an approximate equation of state) gives somewhat different results; both routes coincide and become exact in the Kac limit. Our theory furnishes a conceptual basis for the association in the heuristically based modified Enskog theory (MET) of the contact value of the radial distribution function with the ’’thermal pressure’’ since this association follows from our theory (using either route to thermodynamics) and moreover becomes exact in the Kac limit. Our transport theory is readily extended to the general case of a soft repulsive core, e.g., as exhibited by the Lennard-Jones potential, via by-now-standard statistical–mechanical methods involving an effective hard-core potential, thus providing a self-contained statistical–mechanical basis for application to such potentials that is lacking in the standard versions of the MET. We obtain very good agreement with experiment for the thermal conductivity and shear viscosity of several saturated simple liquids.
Perturbation theory of the electron correlation effects for atomic and molecular properties. Second- and third-order correlation corrections to molecular dipole moments and polarizabilities
Authors: Diercksen G. H. F. , Sadlej A. J.
Published: February 2003

The many-body perturbation theory is applied for the calculation of the second- and third-order correlation corrections to the SCF HF dipole moments and polarizabilities of FH, H2O, NH3, and CH4. All calculations are performed by using the finite-field perturbation approach. The pertinent correlation corrections follow from the numerical differentiation of the second- and third-order field-dependent correlation energies. This computational scheme corresponds to a completely self-consistent treatment of the perturbation effects. The third-order corrected dipole moments are in excellent agreement with the experimental data and the best results of other authors. A comparison of the present perturbation corrections for polarizabilities with the PNO–CI and CEPA results of Werner and Meyer reveals that some cancellation of the third- and fourth-order correlation contributions can be expected. The second-order corrected polarizabilities are as a rule better than the results of the third-order perturbation approach. It is concluded that also for polarizabilities the low-order many-body perturbation scheme is able to account for the major portion of the relevant correlation effects.
The interaction of surface sulfur with carbon on Fe(110)
Authors: Kelemen S. R. , Kaldor A.
Published: February 2003

The interaction of carbon and sulfur was studied on an iron single crystal with a (110) surface orientation. Sulfur was deposited from the decomposition of H2S at 150 °C and first formed a P(2×2) followed by a C(3×1) LEED pattern. The fully developed patterns correspond to 1/4 and 1/3 sulfur atom per surface iron atom. The ease of graphite formation from the dehydrogenation of acetylene as well as the decomposition of surface graphite are inhibited by the presence of a three-dimensional carbide, as compared to the initially clean surface. Once formed, surface graphite facilitates carbide dissolution. The formation and decomposition of surface graphite is inhibited by the presence of surface sulfur. Carbon surface segregation measurements at constant sulfur coverage were evaluated using the Langmuir–McClean equation. As the concentration of surface sulfur increases up to its saturation value of 1/3 monolayer, there is a corresponding decrease in the enthalpy of segregation of carbon. An order transition from a C(18×3) to a C(3×1) LEED pattern, concomitant with a discontinuous change in the concentration of carbon with temperature, is observed near 3/4 surface sulfur saturation, precluding analysis within an ideal solution framework.
Dynamical effects on conformational isomerization of cyclohexane
Authors: Hasha D. L. , Eguchi T. , Jonas J.
Published: February 2003

The pressure dependence is studied for the isomerization rate of cyclohexane in solvents of acetone-d6, carbon disulfide, and methycyclohexane-d14. (AIP)
Conducting complexes of polyphenylene sulfides
Authors: Shacklette L. W. , Elsenbaumer R. L. , Chance R. R. , Eckhardt H. , Frommer J. E. , Baughman R. H.
Published: February 2003

Poly(p-phenylene) sulfide, poly(m-phenylene) sulfide, and the newly synthesized polymer poly(thio-2,8-dibenzothiophenediyl) have been treated with strong electron acceptors (AsF5, SbF5) to form conducting complexes with p-type electronic conductivities up to 3 S/cm. Near IR to UV absorption spectra and temperature-dependent conductivity measurements suggest a localization of charge carriers even at high doping levels. Elemental analysis and IR spectroscopy demonstrate that heavy exposure to AsF5 causes substantial changes in the backbone structure of these polymers. The dopant appears to predominantly induce the formation of carbon–carbon bonds bridging the sulfur linkages to form thiophene rings. This chemical modification enhances the conductivity of the complex and, in the case of poly(m-phenylene), is shown to be an actual prerequisite for achieving high conductivity.
Memory kernels from molecular dynamics
Authors: Berkowitz Max , Morgan John D. , Kouri Donald J. , McCammon J. Andrew
Published: February 2003

A numerical procedure is presented for the direct computation of memory kernels in generalized Langevin representations of time correlation functions. The procedure will facilitate analysis of the microscopic dynamics of a wide variety of systems, ranging from simple solids to globular proteins.
An analysis of crystallization by homogeneous nucleation in a 4000-atom soft-sphere model
Authors: Cape J. Neil , Finney John L. , Woodcock Leslie V.
Published: February 2003

A molecular dynamics simulation of crystallization by the process of spontaneous homogeneous nucleation for a model of 4000 soft spheres with periodic boundary conditions is reported. When the amorphous system in the metastable state, at a point for which classical homogeneous nucleation theory predicts a vanishing of the free energy barrier, is annealed, many crystallites with variable symmetries are formed in the system. The nature and structures of the growing crystallites are analyzed by examination of the primary Voronoi polyhedra. The results support the qualitative correctness of the classical theory. Comparisons are made with previous nucleation studies for the Lennard-Jones model at low temperatures and supercooled liquid metals as studied recently by Raman and co-workers.
On the thermodynamics of elementary chemical reactions in homogeneous systems
Authors: Lengyel S. , Gyarmati I.
Published: February 2003

The authors intend to contribute to the solution of the problems posed by the inconsistency of thermodynamics with the classical theory of chemical kinetics. It is, however, mainly intended only to clarify the basic question of linear theory, i.e., trivial and real satisfaction of the Onsager reciprocity relations (ORR). To this end, the thermodynamic basic equations of homogeneous closed chemically reacting systems are presented and direct transformation of a stoichiometrically equivalent independent system is given. Nonlinear constitutive equations of reaction kinetics equivalent to the Guldberg–Waage equations are presented and a method is given for their derivation. It has been proven that in stoichiometrically independent systems the ORR’s are always trivially satisfied in a linear approximation. A real satisfaction of the ORR’s occurs only in the case of independent fluxes and affinities that equivalently represent dependent systems.
Depolarized Rayleigh spectra of siloxane polymers
Authors: Lin Y-H. , Fytas G. , Chu B.
Published: February 2003

We report measurements of depolarized Rayleigh spectra IVH(ω) of polyphenylmethyl siloxane (polymer A) in bulk and in carbon tetrachloride, of polydimethylphenylmethyl siloxane (copolymer B) in bulk, and of depolarized Rayleigh intensity IVH of polydiphenyl siloxane in carbon tetrachloride. From the temperature dependence and the concentration dependence of IVH, we were able to determine positive intermolecular and intramolecular static pair correlations of polymer A and of copolymer B, and negative intermolecular and intramolecular static pair correlation of polydiphenyl siloxane solutions. The results are explained in terms of the most preferred orientation of phenyl groups attached to the polymer backbone. The linewidths of depolarized Rayleigh spectra of polymer A and of copolymer B can be attributed as due to the crankshaft type of motion of the polymer backbone which orients the polarizability anisotropy of the phenyl groups. The local reorientation time is virtually independent of the macroscopic viscosity. The apparent activation energy (∼2.3 kcal/mol) is much lower than that determined by the dielectric relaxation technique but is of the same order as that calculated from Brillouin spectra.
Selective vibrational excitation by stimulated emission pumping
Authors: Kittrell Carter , Abramson Evan , Kinsey James L. , McDonald Stephen A. , Reisner David E. , Field Robert W. , Katayama Daniel H.
Published: February 2003

Stimulated emission pumping (SEP) is a new method for sub-Doppler spectroscopy and efficient state-selective preparation of highly excited rotational–vibrational levels of small molecules. The sensitivity of SEP has been examined by exciting the I2(B 0+u–X 1Σ+g) 17–0 P(38) line and stimulating the P(38) and R(36) lines of the 17–11, 17–10, and 17–9 bands. All observed effects when the two laser pulses were temporally separated could be accounted for by a master equation model.
Theoretical model for enhanced photochemistry on rough surfaces
Authors: Nitzan Abraham , Brus L. E.
Published: February 2003

A simplified theory of enhanced ultraviolet, visible, and infrared photochemistry near rough dielectric and metallic surfaces is described and numerically investigated. Protrusions on a rough surface are modeled as isolated microscopic spheres. We formulate classical equations of motion for molecules interacting with electromagnetic fields and such material spheres. The model incorporates (a) dipole–dipole coupling between absorbing molecules and the large, induced dipoles created in microscopic spheres irradiated near Mie resonances, and (b) dissipative energy transfer from excited molecules to higher order (l≳1) multipole resonances in the spheres. Calculations show that substantial enhancements in photochemical yields are possible for relatively slow chemical reactions as well as fast reactions. The similarities and differences between enhanced photochemistry and surface enhanced Raman scattering (SERS) are discussed in detail. Dielectric materials for enhanced infrared photochemistry at CO2 laser wavelengths are proposed.
Theory of cusped rainbows in elastic scattering: Uniform semiclassical calculations using Pearcey’s integral
Authors: Connor J. N. L. , Farrelly D.
Published: February 2003

The first numerical application of the uniform Pearcey asymptotic approximation to a problem in semiclassical collision theory has been carried out. The problem investigated is the calculation of the differential cross section for elastic scattering by a potential energy curve that possesses a maximum and a minimum. The corresponding classical deflection function also possesses a maximum and a minimum, which means there are three contributing saddle points in the semiclassical analysis, two of which may be complex. The collision system investigated comes from a model curve crossing study of He++Ne scattering. In order to apply the uniform approximation, it is necessary to calculate Pearcey’s integral and its derivatives for many values of their arguments. Two methods for doing this have been developed. One method uses a series representation while the second method integrates a third order differential equation. It is also necessary to calculate the arguments of Pearcey’s integral from the classical trajectory data as well as the terms that multiply Pearcey’s integral and its derivatives. Iterative and algebraic methods for doing this are described. Both real and complex valued classical trajectories have been included in the semiclassical analysis. The uniform Pearcey approximation gives excellent agreement with results from the partial wave series. The accuracy of the transitional Pearcey, uniform Airy, and transitional Airy approximations is also investigated. The uniform Pearcey approximation can now be regarded as a practical tool for applications in semiclassical collision theory and other short wavelength phenomena.
Steady, one-dimensional Brownian motion with an absorbing boundary
Authors: Harris S.
Published: February 2003

We consider the solution of the Fokker–Planck equation for the case of steady, one-dimensional flow with prescribed flux at the outer boundary and complete absorption at the inner boundary. An approximate solution is obtained using a modified version of the bimodal Maxwellian moment method due to Lees, which has been used previously with success in treating boundary layer problems in the context of the Boltzmann equation. We obtain explicit results for the density in the physical space which is characterized by a boundary layer of order (inverse velocity relaxation time/thermal velocity)−1 and a Milne extrapolation length (distance beyond the boundary at which the extrapolated asymptotic value of the density is zero) of 1.44 in appropriately normalized units. This latter value compares surprisingly well with recent analytical–numerical results which find a value of 1.46 for this quantity.
Interference effects in pair correlation energies: Helium L-limit energies
Authors: Petersson G. A. , Nyden Marc R.
Published: February 2003

The reduction of the net contribution to the correlation energy from angular momentum, L, ΔEL, relative to the second-order contribution EL(2) is quantitatively described by an interference effect between low-lying natural orbitals.
Bulk properties or not: The electronic structure of small metal clusters
Authors: Demuynck Jean , Rohmer Marie-Madeleine , Strich Alain , Veillard Alain
Published: February 2003

The dependence of the properties of small copper clusters on their size, and their relationships to the properties of the bulk metal have been studied through ab initio SCF calculations (with the emphasis on the clusters Cu8 and Cu13). The basis set used is of double-zeta quality for the valence shells 3d and 4s. The analysis focuses on the following properties of the clusters: geometrical structure, binding energy, and distribution of energy levels. For the 13-atom cluster, the icosahedron is found more stable than the cubo-octahedron corresponding to the fcc structure of the bulk metal. The binding energy per atom increases almost linearly with the number of atoms of the cluster. From the orbital energy values, the sets of 3d and 4s levels are well separated for Cu8 and just begin to overlap for Cu13. This situation looks rather different from the one for the bulk metal where the s band totally overlaps the d band. The relationship between the orbital energies from the ab initio SCF calculations and the ionization potentials is discussed. For the 13-atom cluster, the relaxation effect is probably small enough that the above conclusions, based on Koopmans’ theorem and the use of orbital energies, should not be altered by including it. On the whole, the ab initio results contradict the conclusion of the SCF–Xα–SW calculations that the electronic structure of small metal clusters and of the bulk metal are not fundamentally different and that not many atoms are needed in a cluster to have a resemblance to the bulk. This idea was based on the analysis of the orbital energy values from the Xα calculations, which showed that the ’’d band’’ was completely overlapped by the ’’s band.’’ However, this represents probably a spurious effect, since the Xα eigenvalue differs from the corresponding ionization potential by a ’’self-interaction term,’’ with rather different values for the orbitals of the 3d type and of the 4s type.
Scaled atomic number expansion of Hartree–Fock energies for atomic ten-electron systems
Authors: McDowell Keith
Published: February 2003

The coefficients in a scaled-atomic-number expansion of ten-electron atomic Hartree–Fock energies are computed by using a reduced free-particle Green’s function method. The resulting energy (in a.u.) through third order is given by . . . . . . . The expansion appears to be convergent but does not yield results of chemical accuracy (10−3 a.u.). Two additional energy expressions, one an upper bound expression and the other a Padé approximant expression, are also investigated. The Padé expression is found to produce the best agreement with previously reported values.
Measurements of molecular second order optical susceptibilities using dc induced second harmonic generation
Authors: Singer K. D. , Garito A. F.
Published: February 2003

We have developed an infinite dilution extrapolation procedure for determining the molecular second order optical susceptibility β using dc induced second harmonic generation. We show how the effects of solute–solute and solvent–solvent interactions are minimized by this procedure, and how several local field models used in the procedure affect the calculation of β. Using hexane as a solvent for nitrobenzene, we have determined the best experimental value in solution for the intrinsic β of nitrobenzene of (1.11±0.17)×10−30 esu.
Excitation of the O2(a 1Δg) state by low energy electrons
Authors: Tachibana K. , Phelps A. V.
Published: February 2003

Coefficients for the excitation of the a 1Δg metastable state of O2 by low energy electrons in O2–Ar mixtures have been measured using a drift tube technique. The time dependence of the absolute intensity of the 1.27 μm band emission was measured for 1% and 5% O2 in Ar at total gas densities of (1–20)×1024 m−3. The electric field to gas density ratio E/N was varied from (0.3–10)×10−21 V m2, corresponding to calculated mean electron energies between about 0.8 and 4 eV. The measured decay constants yield a normalized diffusion coefficient for O2(a 1Δg) in Ar of (2.8±0.2)×1020 m−1 s−1 and a rate coefficient for quenching by O2 of (1.57±0.1)×10−24 m3 s−1. Our experimental electron excitation coefficients agree satisfactorily with values calculated using previously derived sets of electron collision cross sections for O2 and Ar.
Electron spin echoes of a photoexcited triplet: Pentacene in p-terphenyl crystals
Authors: Sloop David J. , Yu Hsiang-Lin , Lin Tien-Sung , Weissman S. I.
Published: February 2003

Electron spin echo observations of the photoexcited triplet state of 0.1 mol % pentacene-h14 and -d14 in p-terphenyl crystals at room temperature are presented. Theory is presented for calculation of the echo envelope modulations for an S = 1, I = 1/2 spin system including zero field splittings in the high field limit. Echo envelope modulations due to proton and deuteron hyperfine interactions in the pentacene molecule have been observed. The echo decay data are used to calculate triplet state decay parameters.
An investigation of the transition moment of the A 1Σ+–X 1Σ+ bands of CsH
Authors: Yang Sze-Cheng , Hsieh Yin-Kang , Tam Andrew C. , Zemke Warren T. , Verma K. K. , Stwalley William C.
Published: February 2003

Relative emission intensities from both experiment and theory for the 4579 Å Ar+ laser-excited A 1Σ+ (v′ = 19, J′ = 10) level of CsH are compared. The experimental results include both a previous measurement and two independent recent remeasurements and are in good agreement. The theoretical results have been improved by including explicit internuclear distance dependence of the electronic transition moment using the theoretical calculations of Laskowski and Stallcop. The agreement between experiment and theory is very good and much improved compared to previous work. A radiative lifetime of 84.5 nsec is calculated for the A 1Σ+ (v′ = 19, J′ = 10) level.
Rotational rainbows: An IOS study of rotational excitation of hard-shell molecules
Authors: Korsch H. J. , Schinke R.
Published: February 2003

The hard-shell model for rotationally inelastic scattering is studied within the infinite-order-sudden (IOS) approximation. In addition to the rotational rainbow oscillations of the differential cross sections for j = 0→j′ transitions, a pronounced diffraction structure for small j′ (j′≊0, 2, 4, and 6) and small scattering angles is observed. A classical and semiclassical IOS approximation developed recently is compared with the quantal results for hard-shell He–Na2 collisions at 0.1 eV. The semiclassical IOS approximation leads to a simple (almost closed form) formula for the inelastic differential cross sections. Very good agreement with the quantal results is obtained with the exception of the small angle region for low Δj transitions, which is dominated by diffraction structures. The validity of the IOS approximation for hard-shell molecules is discussed. The model results for He–Na2 are compared with IOS calculations using a soft ab initio potential surface. The results differ drastically with the exception of the region of backward scattering, where the rotational rainbow oscillations are in good agreement. The integral cross sections show the same disagreement—again with the exception of high  j′ excitation, which is dominated by wide-angle scattering.
An information theoretic synthesis and analysis of Compton profiles
Authors: Sears Stephen B. , Gadre Shridhar R.
Published: February 2003

The information theoretic technique of entropy maximization is applied to Compton profile (CP) data, employing single and double distribution moments <pn≳ as constraints. Emphasis is placed on the first and second (n = 1,2) moments—the average momentum and energy, respectively. Studies are made both on semi-infinite and finite ranges of the variable q in the Compton profile J(q). The quality of resulting maximum entropy profiles is judged by how well they predict familiar CP quantities—moments, the profiles’ magnitude at the origin J(0), and the width at half-maximum q0.5. Information theoretic quantities—Shannon entropies, information contents, and surprisals—are also presented. Based upon the ’’sum’’ constraint <q+q2≳, a relation is derived which approximately predicts J(0) given q0.5 (or vice versa) for a large class of molecular systems, i.e., J(0) = (2/N)exp [−K/4][K]1/2[(π1/2/2)erfc(K1/2/2)] −1, where K = ln 2/(q20.5+q0.5). The assimilation of more and more constraints (information) results in generally improved Compton profiles. The average momentum constraint contains the most information of all moment expectation values, as judged by its predictive capacity and by the information theory measures.
Synthesis, hyperfine interactions, and lattice dynamics of a crown ether intercalation compound of FeOCl
Authors: Herber R. H. , Cassell R. A.
Published: February 2003

The anisotropic van der Waals layer compound FeOCl has been intercalated with the cyclic polyether 1,7,10,16-tetraoxa-4,13-diazacyclo-octadecane (Kryptofix-22 or Krypt) to give a matrix in which the ’’guest’’ to ’’host’’ ratio is 1:18. This intercalate has been studied by x-ray power diffraction, 57Fe Mössbauer, and Fourier transform infrared spectroscopy. The Mössbauer spectra above the magnetic ordering temperature (but below 150 K) show the existence of two types of iron atoms in the structure. The majority iron atoms display the sigmoid temperature dependence of the quadrupole hyperfine interaction parameter which has been associated with a transition of the lattice from a three-dimensional solid at low temperatures to a quasi-two-dimensional solid at high temperatures. The minority iron atoms are characterized from the isomer shift and quadrupole splitting parameters as ’’Fe(II)’’, and are those iron atoms which interact directly with the lone pair electrons on the nitrogen atoms of the intercalant. The observed b-axis expansion at room temperature is 4.28 Å, which is significantly smaller than the ring diameter reported for the uncomplexed polyether. These data suggest significant interpenetration of the intercalant guest molecule into the van der Waals layer defined by the chlorine atoms. The midrange infrared spectra of the intercalate do not show any evidence for the type of polyether–metal cation interaction which has been reported for lanthanide complexes of this molecule. The lattice dynamical properties of the solid as probed by the 57Fe Mössbauer active atom are discussed in detail.
Comparison of molecular models of the liquid–vapor interface
Authors: McCoy B. F. , Scriven L. E. , Davis H. T.
Published: February 2003

Interfacial properties of a simple fluid are predicted by exact gradient theory and by the gradient approximations of the Yvon–Born–Green equation and of two free energy models (the ADF and MVDW models) of current interest. Predictions are presented for three different local, homogeneous fluid approximations to the pair correlation function of an inhomogeneous fluid. The gradient limit of the ADF model is identical to the exact gradient theory for all correlation function approximations considered. The influence parameter of inhomogeneous fluid is better predicted by the MVDW model than by the YBG equation. The density profile and surface tension are not very sensitive (say within a tolerance of 10%) to variations among the models considered.
Generating functions for the nuclear spin statistics of nonrigid molecules
Authors: Balasubramanian K.
Published: February 2003

Generating functions are developed for the nuclear spin species and nuclear spin statistical weights of nonrigid molecules as the trace of spin projection operators. These generating functions are obtained in terms of the GCCI’s (generalized character cycle indices) of the PI groups of nonrigid molecules which are expressible as generalized wreath products. The GCCI’s of generalized wreath products can be obtained in terms of the GCCI’s of the composing groups. Thus, the method developed here does not require the character table of the P group of nonrigid molecules. From these generating functions the nuclear spin statistical weights of the rovibronic levels and nuclear spin species of nonrigid molecules can be obtained easily. The method is illustrated with several examples of nonrigid molecules containing up to 3 601 989 nuclear spin functions. Application to molecular electric beam deflection studies of weakly bound complexes such as ammonia dimer is also discussed.
The molecular structure of HCO+ by the microwave substitution method
Authors: Woods R. Claude , Saykally Richard J. , Anderson Thomas G. , Dixon Thomas A. , Szanto Peter G.
Published: February 2003

Precise measurements of the frequencies of the J = 0–1 transition of HCO+, H13CO+, HC18O+, DCO+, D13CO+, and DC18O+ have been used to determine a substitution (rs) molecular structure for HCO+. The bond distances obtained are rs(CO) = 1.1071(2) Å and rs(CH) = 1.0930(1) Å. This is the first such microwave rs structure to be determined for any molecular ion. The determined bond distances and the transition frequencies can be compared to the results of previous high quality ab initio calculations and excellent agreement is found. An initial attempt to characterize the Doppler shifts of the HCO+ frequency due to ion drift in a dc discharge and a measurement of the J = 1→2 transition of DCO+, and thus its centrifugal distortion constant, are also reported here.
The emission spectrum from gaseous HCl+ produced by photoionization
Authors: Tsai Bilin P. , Parrella Lynda L.
Published: February 2003

The emission spectrum of photoinoized Hcl+ is reported. Transitions between A2Σ+(v' = 1−3) and X2Πi(v  = 0) and weaker transtions between A2Σ+(v’ = 1−3) and X2’’ = 1) are reported. (AIP)
Comments on inequalities among atomic expectation values
Authors: Thakkar Ajit J.
Published: February 2003

Inequalities for the atomic expectation values <rk≳ and <pk≳ are discussed. It is continded that momentum densities in atomic system are not always monotonically decreasing functions. (AIP)
A study of the E 1Σ+g state of 7Li2 by pulsed optical–optical double resonance spectroscopy
Authors: Bernheim R. A. , Gold L. P. , Kelly P. B. , Tipton T. , Veirs D. K.
Published: February 2003

The results of pulsed optical–optical double resonance spectroscopic studies of the E 1Σ+g state of 7Li2 are presented. Observations were carried out on the v = 0 through v = 12 levels. A set of Dunham molecular constants was derived and an RKR potential was generated. Franck–Condon factors for transitions to the A 1Σ+u state were determined for the range of observed vibrational levels. An apparent deviation from the expected dissociation energy is observed consistent with the suggestion that a double minimum exists for this state.
A computer simulation method for the calculation of equilibrium constants for the formation of physical clusters of molecules: Application to small water clusters
Authors: Swope William C. , Andersen Hans C. , Berens Peter H. , Wilson Kent R.
Published: February 2003

We present a molecular dynamics computer simulation method for calculating equilibrium constants for the formation of physical clusters of molecules. The method is based on Hill’s formal theory of physical clusters. In the method, a molecular dynamics calculation is used to calculate the average potential energy of a cluster of molecules as a function of temperature, and the equilibrium constants are calculated from the integral of the energy with respect to reciprocal temperature. The method is illustrated by calculations of the equilibrium constants for the formation of clusters of two to five water molecules that interact with each other by an intermolecular potential devised by Watts. The method is compared with other procedures for calculating the thermodynamic properties of clusters.
Electronic properties of polymers. Anisotropic light absorption and photoelectron emission of oriented polyethylene films in the vacuum ultraviolet region
Authors: Hashimoto Shimpei , Seki Kazuhiko , Sato Naoki , Inokuchi Hiroo
Published: February 2003

Anisotropic optical absorption of oriented ultrathin films, (down to 50 nm thickness) of low density polyethylene (LDPE) was measured in the vacuum ultraviolet (VUV) region between 6 and 12 eV. Clear anisotropy was found in the observed spectra. The lowest-energy absorption feature at 9 eV was polarized perpendicular to the chain axis, and the second at about 11.5 eV parallel to the chain axis. The edge of the lowest perpendicular absorption was determined to be 8.05±0.1 eV. The usefulness of the anisotropic absorption data in testing two theories was verified; the anisotropy predicted by the independent system model disagrees with the data, but the energy band model gives some promising results. The photoemission threshold of the oriented LDPE was determined to be 8.5±0.1 eV. The spectral dependence of the anisotropy ratio of the photoemission quantum yield was found to coincide with that of the optical absorption in the energy range above 9.5 eV. This coincidence was explained in terms of the three-step model of photoemission.