bluuebunny/crossref_metadata_2025 huggingface dataset:
Cartesian partitioning of the potential energy in the chemical bond
Authors: Ceulemans A.
Published: February 2003

From the variation principle, a simple proof is presented in which the Hamiltonian of a diatomic molecule is partitioned in Cartesian components along parallel and orthogonal bond directions. Kinetic and potential energy operator components are obtained independently, so that no privileged role can be attributed to the kinetic energy in the formation of a chemical bond. As an example, the partioning is effectively computed for the binding state of the hydrogen molecular ion. It is shown that a uniformly scaled LCAO wave function already incorporates approximately the unidimensional virial relationships formulated by Epstein and Hirschfelder [Phys. Rev. 123, 1495 (1961)].
Interaction of molecules with electromagnetic fields. II. The multipole operators and dynamics of molecules with moving nuclei in electromagnetic fields
Authors: Yang Kuo-Ho (Tom) , Hirschfelder Joseph O. , Johnson Bruce R.
Published: February 2003

This paper presents a thorough unified treatment of the electric and magnetic multipole operators and the dynamics of a moving molecular system of electrons and nuclei in the presence of an arbitrary (semiclassical) electromagnetic field. The multipole operators are expressed in terms of rjc, the position of each of the particles j relative to the center of mass rc, the velocities ?jc and ?c, and the spins sj. Two levels of precision of the multipole operators and dynamics are considered: The ’’nonrelativistic’’ approximation including all terms which vary as 1/c (where c is the velocity of light) suffices for most practical applications. The multipole moments are determined by the Lorentz force on the molecule. Also, the multipole operators are related to the electric and magnetic polarization operators Popc and Mopc, respectively, as well as to the effective charge and effective current on the molecule. The Lagrangian is then determined by rearranging the ’’Newtonian’’ equations of motion into the Lagrangian form. In both the Hamiltonian and the Lagrangian, terms involving Popc and Mopc couple the external fields to the molecular dynamics. The Hamiltonian is also derived in the ’’quantum mechanical fashion’’ by making a Power–Zienau–Woolley type transformation of the usual ’’minimal coupling’’ Hamiltonian. The new coordinates are rc and a set of (N−1) linearly independent combinations of the rjc. In the determination of the electric and magnetic properties of molecules, there are significant advantages in considering moving nuclei and center of mass coordinates rather than assuming clamped nuclei. In order to explain a few very sensitive types of experimental properties, it is necessary to use the semirelativistic approximation which is accurate through all of the α4mc2 or 1/c2 terms and includes all of the fine-structural effects with the exception of the Lamb shift. The electric and magnetic multipole moments were derived in terms of the Kracjik and Foldy semirelativistic coordinates and spins. In addition to the usual ’’dipole length’’ term, the semirelativistic electric dipole operator contains spin–orbit and purely relativistic terms. The related semirelativistic Hamiltonian in the Kracjik–Foldy coordinates is stated but its very complicated derivation will be presented in another paper.
An a
b i
n
i
t
i
o study of the conformational energetics of N-benzylideneaniline
Authors: Bernstein J. , Engel Y. M. , Hagler A. T.
Published: February 2003

A b initio calculations employing both minimal (STO–4G) and split valence (4–31G) basis sets have been used to study the conformational energy surface of N-benzylideneaniline (NBA). The former indicate that the minimum energy conformation of NBA corresponds to a rotation about the N–phenyl bond of ≊ 45° and a rotation about the CH–phenyl bond of 0°. These results are in close correspondence with those from spectroscopic and x-ray diffraction studies. This represents a significant improvement over the results obtained from most semiempirical methods which in general have not been able to account for the rotation about the exocyclic bonds. The compounds N-ethylideneaniline and N-methylbenzylidenimine were also studied and found to be good models for determining the energetics about the two exocyclic single bonds using both the minimal and extended (4-31G) basis sets. A partitioning of the results for these two model compounds yields information which permits an analysis of the origin of the barrier to a planar conformation in benzylideneaniline as well as the difference between the two basis sets. The partitioning also provides a criterion for determining the suitability of particular compounds as models for larger systems. The most stable conformation of NBA has been attributed to a compromise between steric interactions and delocalization of the bridge double bond and/or nitrogen lone pair electrons into the conjugated system. Calculations on additional model compounds were carried out in an attempt to isolate the relative contributions of these factors in determining the most stable conformation. Finally, electron deformation plots have been employed in order to gain insight into the features of the electron density distribution underlying these effects.
On the conformational transition of a self-avoiding walk adsorbed at an interface
Authors: Bellemans A. , Orban J.
Published: February 2003

The conformational transition of a self-avoiding walk at an interface, between nonadsorbed and adsorbed states, is investigated by means of extended numerical data in the asymptotic limit, for five-choice and four-choice walks on the simple cubic lattice, up to 14 and 16 steps, respectively. Given x = exp(βε), where ε is the energy per adsorbed segment (β = 1/kT), the location of the transition is found to be x* = 1.485 (five choice) and x* = 1.502 (four choice). The nature of the transition is also investigated and it is shown that the mean fraction of adsorbed segments yn(x), in the limit of infinite n, tends to 0 below x* and to A[ln(x/x*)]α above x*, with α?0.35–0.40. The mean thickness (perpendicular to the surface) and the mean spread (parallel to the surface) of the walks are also discussed.
Molecular motion in binary systems of molecular crystals
Authors: Britcher A. R. , Strange J. H.
Published: February 2003

Nuclear magnetic resonance (NMR) relaxation has been used to study molecular motion in binary systems of the molecular crystal hexamethylethane with several chemically distinct solutes in concentrations up to 10%. The anisotropic reorientation of HME molecules in the brittle phase is generally enhanced, but endospherical reorientation in the plastic phase is only disturbed by one of the solutes. Self-diffusion of HME molecules is reduced by one solute and enhanced by the others, but effects are small and the activation enthalpy is independent of solute concentration for the range studied. Possible explanations of the observed effects are discussed. The effects of the solvents on the phase transitions are also examined.
Microwave and radio frequency spectra of Xe–HF
Authors: Baiocchi F. A. , Dixon T. A. , Joyner C. H. , Klemperer W.
Published: February 2003

The spectroscopic constants for Xe–HF obtained from molecular beam electric resonance spectroscopy are as follows:     The structure and weak bond force constants derived from the spectroscopic constants are consistent with data on other noble gas–hydrogen fluoride complexes. It is found that the electric field gradient at the Xe nucleus for a given HF orientation scales with the electric field gradient arising from HF.
Laser-induced fluorescence of the C2H5O radical
Authors: Inoue G. , Okuda M. , Akimoto H.
Published: February 2003

Laser-excitation and laser-induced fluorescence spectra of the C2H5O radical are reported. The 0–0 band of the ?–? transition lies at 29 204 cm−1 (342.4 nm). The spectra are characterized by a prominent progression involving the C–O stretching mode, as in the case of the CH3O radical. The fundamental C–O stretching frequencies in the ground and excited states are 1067 and 596 cm−1, respectively.
Raman study of diluted solutions of asymmetric top molecules. Theory of band profiles
Authors: Guissani Y. , Leicknam J-Cl.
Published: February 2003

A theory is proposed to analyze the Raman band profiles of asymmetric top molecules in solution. Vibrational relaxation is assumed to arise from a pure dephasing process. Rotational relaxation is treated for the diffusional and free rotation limits and for the extended diffusion. The rotational diffusion results are obtained by applying the Van Kampen cumulant technique; corrections due to the fourth-order cumulant are given. Free rotation is studied by numerical methods. The extended diffusion is examined by applying the well-known J and M models to the asymmetric top problem. Representative band profiles are described according to the point group of the solute molecule.
Thermodynamic and other characteristic properties of benzopyrene trimer exciplex
Authors: Abu-Zeid M. E. , Lopez J. R. , Marafi M. A.
Published: February 2003

A simple model based on a single process of a three body reaction has been used in this work for trimer formation. This model has successfully explained the dependence of monomer and trimer on temperatures. The binding energy, association energy, and dissociation energy have been evaluated. Also, trimer entropy, enthalpy, and ground state destabilization energy have been calculated.
Electron diffraction studies of laser-pumped molecules. I. Characterization of system and analysis of data
Authors: Bartell L. S. , Kacner M. A. , Goates S. R.
Published: February 2003

The feasibility of investigating the vibrational excitation of laser-pumped molecules in a supersonic jet by electron diffraction is investigated. Studies of SF6 confirm that diffraction patterns can be measured, reproducibly, to a part per thousand of the total intensity, or better, and hence can be analyzed to yield useful information about laser-induced changes in intramolecular motions, provided adequate account is taken of certain systematic errors. Procedures for discriminating between bona fide and spurious signals are outlined. Methods are described for characterizing the gas flow and density profile in the microjets issuing from a small tubular nozzle, and for measuring the distribution of radiant energy in the focused infrared beam used to irradiate the gas jet.
A study of long range anisotropic potentials for CsF, CsCl, and KCl interacting with the rare gases based on measured state-to-state rotationally inelastic cross sections. I. Measurements for CsCl and KCl
Authors: Meyer Gerhard , Toennies J. Peter
Published: February 2003

Absolute inelastic cross sections for rotational quantum state-to-state transitions have been measured in small angle grazing collisions of CsCl and KCl with the rare gas atoms. With CsCl the transitions (j,m) = (3,0)?(4,0) and with KCl the transitions (2,0)?(3,0) were studied. The cross sections for CsCl are similar in size to those reported previously for CsF by Borkenhagen, Malthan, and Toennies [J. Chem. Phys. 71, 1722 (1979)], whereas the cross sections for KCl are found to be considerably smaller. The cross sections are corrected for apparatus effects and analyzed in terms of the Born approximation and the long range anisotropic potential. A comparison of absolute center of mass differential cross sections reveals that the cross sections for CsCl and to a greater extent for KCl are considerably greater than expected for a pure induction potential. The relative size of the additional potential term is determined and is found to be somewhat smaller than the higher polarizability dispersion term predicted by Buckingham [J. Chem. Phys. 48, 3827 (1968)] using the Rittner model.
On the direct energy transfer via exchange to moving acceptors
Authors: Allinger K. , Blumen A.
Published: February 2003

In a recent work [K. Allinger and A. Blumen, J. Chem. Phys. 72, 4608 (1980)] we derived expressions for the energy decay of an excited donor due to its interactions with moving acceptors. As we show here, this approach is related to path-integral methods which occur in different fields. We apply the formalism to interactions mediated by exchange. Analytic expressions are found for the decay due to acceptors moving slowly or rapidly on the time scale of the energy transfer. If the motion is frozen we retrieve the decay law for acceptors imbedded randomly in a solid matrix [A. Blumen, J. Chem. Phys. 72, 2632 (1980)]. For slow diffusive motion, as in the three-dimensional dipolar case [M. Yokota and O. Tanimoto, J. Phys. Soc. Jpn. 22, 779 (1967)], the decay may be expressed by means of a power series in the diffusion coefficients. Here we obtain the coefficients of the series from a recurrence formula and present the first ten terms. An approximate, compact formula for the decay law is also given. In the rapid motion case the decay law depends on the distance of nearest approach between donor and acceptors, but not on the details of the motion.
Photochemistry of acetylene at 1470 Å
Authors: Okabe Hideo
Published: February 2003

The photolysis of acetylene at 1470 Å has been studied over the pressure region from 13 to 1330 N m−2 (0.1 to 10 Torr). The quantum yield of diacetylene formation has been measured as a function of C2H2 pressure and the partial pressures of He and H2. The quantum yields of acetylene disappearance and of ethylene and hydrogen formation were also briefly studied. From these results it was concluded that the major primary photochemical process is direct dissociation C2H2 hν→C2H+H with a quantum yield of 0.3 and the process ot H2 production C2H2 hν→C2+H2 is minor (quantum yield?0.1). The remaining process is the formation of a metastable acetylene which reacts either with ground state acetylene to form C2H radicals or is deactivated by collisions with the walls or inert gases. The C2H radicals react with H2 130±11 times slower and with CH4 31±1.7 times slower than with C2H2. Absorption cross sections of diacetylene have been measured in the 1200 and 1800 Å region. The role of acetylene photolysis in the Jovian atmosphere is briefly discussed.
Rate constants for deactivation of N2(A 3Σ+u, v′ = 0,1) by O
Authors: Piper L. G. , Caledonia G. E. , Kennealy J. P.
Published: February 2003

The removal of N2(A 3Σ+u, v′ = 0,1) by O has been studied in a room temperature discharge–flow apparatus by monitoring the temporal decay of the 0,6 and 1,10 bands of the Vegard–Kaplan system. The measured rate constants are (2.8±0.4) and (3.4±0.6)×10−11 cm3  molecule−1 s−1 for v′ = 0 and 1, respectively.
Muonium reactions in micelles
Authors: Jean Y. C. , Ng B. W. , Stadlbauer J. M. , Walker D. C.
Published: February 2003

The chemical rate constants for muonium (Mu) atoms reacting with iodine, phenol, Ni++, Fe(CN)−36, and naphthalene in three aqueous anionic micellar systems (sodium hexyl sulfate, sodium octyl sulfate, and sodium dodecyl sulfate), were measured by MSR (the muonium spin rotation technique). The results show that the Mu reactivities toward I2, phenol, and naphthalene were significantly increased when these solutes were located inside micelles as compared to homogeneous aqueous solutions. A large increase in the rate constant occurred at the critical micelle concentration, showing that MSR can be used to probe such properties of micelles. The observed rate constants did not change much with the size of the micelles, but the rate was less than that found in a pure organic solvent, supporting the view that the core of a micelle is relatively viscous due to its ordered structure. Since Mu is a light isotope of hydrogen, it may be inferred that H atoms would have analogous behavior in micellar systems—including the efficient penetration of the micelle shell and high reactivity with solutes held in the hydrocarbon core. Finally, these data corroborate the contention that a spur model is not the dominant process of muonium formation.
A statistical mechanical theory for line tension
Authors: Tarazona P. , Navascués G.
Published: February 2003

A statistical mechanical theory for line tension is developed. The consistency of the theory is provided by the equivalence of the thermodynamic and mechanical approaches to line tension. A theoretical calculation for the line tension of a simple system is given. The results agree with previous estimations for the line tension.
Study of the corrosion of aluminum by trichloroethylene using inelastic electron tunneling spectroscopy
Authors: White H. W. , Ellialtiog̃lu R. , Bauman J. E.
Published: February 2003

Inelastic electron tunneling spectroscopy (IETS) has been used to determine the molecular species which occur on an aluminum oxide surface during metallic corrosion by uninhibited trichloroethylene. IETS spectra were obtained for trichloroethylene adsorbed on aluminum oxide and the observed vibration modes were assigned by comparison with infrared and Raman frequencies. Modes were observed which could be associated with AlO–CH2–CH3, or a similar molecular species, and with Al–Cl stretch vibrations. A reaction sequence consistent with the results has been developed. According to this model the corrosive attack is by chlorine ions formed by the reaction of trichloroethylene at AlOH (or AlO) surface sites. The chlorine can initiate corrosive attack by reaction with exposed aluminum atoms at oxygen vacancy sites.
Site symmetry of surface adsorbed molecules
Authors: Nichols Henry , Hexter Robert M.
Published: February 2003

A procedure for determining the allowed local site symmetry groups S for surface adsorbed molecules is presented. An analogy can be drawn between the perturbing field experienced by a matrix isolated molecule in a three-dimensional crystal and that experienced by a molecule adsorbed on a surface. In the former case the field has the symmetry of the host lattice site whereas in the latter case the field has the symmetry of the adsite. If the surface is viewed as a homogeneous two-dimensional plane, then the symmetry of the adsite is described by the group G = C∞v. If the surface cannot be viewed as an unstructured homogeneous plane so that the perturbing field experienced by the adsorbed molecule must be considered on a microscopic level, then the adsite symmetry will be described by the group appropriate for the substrate atoms. This can only be one of the groups G = Cn or G = Cnv. The symmetry operations of the local site symmetry group S consist of an operation in the molecular point group M applied to the molecule combined with an operation in G applied to the substrate. A mapping of the operations in M and G to operations in their isomorphous Longuet-Higgins groups allows the operations to be identified as permutation P or permutation–inversion P operations. The operations in S are restricted to be combinations of P operations in M with P operations in G and P operations in M with P operations in G. An adsorbed molecule having M = D3h symmetry is used as an example to demonstrate the procedure for determining S. The local site symmetry group of the adsorbed molecule is, in general, different for the homogeneous surface approximation as opposed to the microscopic surface approximation. An attempt is made to apply predicted spectroscopic selection rules to adsorbed pyridine and ethylene. Several factors which complicate the interpretation of Raman spectra of molecules adsorbed on metal surfaces are discussed, one of these being depolarization effects due to rough surfaces.
Lattice gas model of the ionic diffuse double layer
Authors: Macdonald J. Ross
Published: February 2003

A liquid lattice gas model of the ionic diffuse double layer is considered which involves mean field corrections. By comparison with recent Monte Carlo results for a liquid electrolyte, the value of the mean field pair interaction parameter α is found for various ionic concentrations. It turns out to be nearly independent of concentration, but the lattice gas site concentration N seems to require some reduction at low concentrations. The relatively simple liquid lattice gas model yields very good agreement with the Monte Carlo results over their entire range of charge and potential difference for the available 1, 0.1, and 0.01 M concentrations. (AIP)
Classical time-correlation functions and the Lanczos algorithm
Authors: Moro Giorgio , Freed Jack H.
Published: February 2003

It is shown how the Lanczos algorithm (LA) can be applied to classical systems, leading naturally to a continued fraction representation equivalent to that obtained by Mori for the correlation functions. The relation of the LA to the method of moments suggests its utility for study of the theoretical structure, while the computational efficiency of the LA suggests its use for computer calculation of correlation functions. (AIP)
The MPB and BBGY potential theories for a 2:2 restricted primitive model electrolyte at low concentrations
Authors: Burley D. M. , Hutson V. C. L. , Outhwaite C. W. , Thomlinson M. M.
Published: February 2003

Monte-Carlo calculations for a 2:2 restricted model electrolyte solution1 are compared with MPB and BBGY potential theories for such electrolyte solution at low concentration. (AIP)
On the phase transition in alkali cyanides
Authors: De Raedt B. , Binder K. , Michel K. H.
Published: February 2003

The phase transition in alkali cyanides is studied starting from a Hamiltonian containing a bilinear coupling term between translational and rotational degrees of freedom, thus leading to an effective rotational interaction. It is shown that the problem can be reformulated in terms of elasticity theory and the effective rotational interaction then becomes an interaction between elastic dipoles. It is demonstrated that the phase transition is of first order. The long range nature of the elastic dipole forces which would lead to a shape dependence of the system for monodomain samples is responsible for the breakup of the ordered phase into domains.
An electrohydrodynamic contribution to the Hall effect in electrolyte solutions
Authors: Hubbard J. B. , Wolynes P. G.
Published: February 2003

We calculate the electrohydrodynamic forces on an ion moving in a polarizable liquid in a magnetic field. The result is a hydrodynamic force which is about 30% of the Lorentz force q/c (v×B) and which acts in the opposite direction. The calculated effect is independent of ion size, charge, and solvent viscosity.
Shock compression of liquid carbon monoxide and methane to 90 GPa (900 kbar)
Authors: Nellis W. J. , Ree F. H. , van Thiel M. , Mitchell A. C.
Published: February 2003

Dynamic equation-of-state data for liquid CO and CH4 were measured in the shock pressure range 5–92 GPa (50–920 kbar) using a two-stage light-gas gun. The liquids were shocked from initial states near their saturation curves at 77 and 111 K for CO and CH4, respectively. The experimental technique used to double-shock CH4 is described. The CO data were examined by using three theoretical models: (1) a chemically nonreactive model, (2) a quasi-chemical-equilibrium model that allows CO to dissociate into gaseous species and graphite, and (3) a chemical-equilibrium model that also includes a dense carbon phase which exists at higher pressures and temperatures than graphite. This dense phase is assumed to be diamond. Our analysis shows that at low pressure chemical equilibrium takes much longer than a typical shock passage time. As a consequence, the experimental data initially follow the nonreactive Hugoniot to pressures well beyond the chemical dissociation limit. Both the experimental data and the Hugoniot computed with case (3) agree satisfactorily at high pressure. Further consequences of these observations to high-explosive studies are discussed. The theoretical analysis for the CH4 data was presented in an earlier paper.
X-ray diffraction study and models of liquid ethylene at 106 K
Authors: Narten A. H. , Habenschuss A.
Published: February 2003

X-ray diffraction data for liquid ethylene near the melting point are presented. The data are analyzed using scattering factors for –CH2 groups which permits extraction of the intermolecular carbon–carbon structure and distribution functions. Atom pair potentials derived from studies on crystalline hydrocarbons are used with the reference interaction site model (RISM) to predict structure and distribution functions for a dense ethylene-like fluid, and these are compared to the curves derived from the diffraction study. Site–site interaction potentials give an adequate description of liquid ethylene, provided that the attractive forces are averaged over the –CH2 groups with repulsive ’’bulges’’ at the hydrogen positions in the C2H4 molecule. Although –CH2 groups are ’’seen’’ by x rays as spherically symmetric, the hydrogen atoms have a significant effect on the carbon–carbon correlations in liquid ethylene near the triple point.
Calculation of Raman intensities for the ring-puckering vibrations of 2,5-dihydropyrrole and trimethyleneimine. Electrical versus mechanical anharmonicity in asymmetric potential wells
Authors: Bocian David F. , Schick G. Alan , Birge Robert R.
Published: February 2003

Raman intensities are calculated for the ring-puckering transitions of 2,5-dihydropyrrole (DHP) and trimethyleneimine (TMI) using an anisotropic atom–point dipole interaction model to evaluate the elements of the molecular polarizability tensor. The calculated relative intensities for the members of the Δv = 1 and Δv = 2 ring-puckering progressions for DHP are in good agreement with those observed. The calculations predict that the observed Δv = 2 overtones of DHP occur not because of the first-order allowedness expected for these transitions in the asymmetric double-minimum potential well which governs the ring-puckering motion, but rather because of unusually large second-order terms in the expansions of the polarizability tensor elements in the puckering coordinate [‖(∂2αμν/∂Z2)0‖≫0]. Raman intensities are calculated for the ring-puckering transitions of TMI using the two different potential functions which have been proposed for the puckering motion. It is found that the intensities calculated for the slightly asymmetric double-minimum potential V(Z) = (0.922 05×106)Z4−(0.379 44×105)Z2+(0.159 13×105)Z 3 proposed by Carreira and Lord [J. Chem. Phys. 51, 2735 (1969)] cannot be reconciled with experiment. On the other hand, the intensities calculated for the highly asymmetric single-minimum potential V(Z) = (0.7553×105)Z2−(0.4336×106)Z3+(0.7035×106)Z4 proposed by Robiette et al. [Mol. Phys. 42,1519 (1981)] are in excellent agreement with experiment. Our calculations confirm that the three most intense ring-puckering transitions observed in the Raman (and far-infrared) spectrum of TMI are the lowest members of a Δv = 1 rather than a Δv = 2 progression. The calculations further indicate that high-order electrical terms in the polarizability expansions contribute significantly to the Raman intensities of both the Δv = 1 and Δv = 2 ring-puckering progressions. Neglect of the electrically anharmonic terms in the intensity calculation for the single-minimum well results in a significant disparity between calculation and experiment.
Primary radiation-induced electron-excess defects and the formation of O−3 in KClO4 and KBrO4 investigated by ESR
Authors: Byberg J. R.
Published: February 2003

The ESR spectra of primary electron-excess defects in KClO4 and KBrO4 crystals irradiated with x rays at 26 K indicate that trapping of electrons on ClO−4 leads to immediate expulsion of O−, whereas BrO−4 may accept an excess electron, forming a doubly charged anion BrO2−4. The thermally activated formation of O−3 involves in both cases addition of O2 to the primary electron-excess defects. Hence, BrO2−4 is chemically equivalent to O−.
Determination of bandwidth and frequency changes by Raman difference spectroscopy
Authors: Laane Jaan
Published: February 2003

The mathematical expressions necessary for analyzing the Raman difference spectra resulting from bandwidth and/or frequency differences between two samples have been derived for both Lorentzian and Gaussian band shapes. Intermediate bands may be analyzed using interpolation. The results show that the difference spectra have two maxima separated by a minimum near zero intensity whenever bandwidth changes are dominant. If the frequency shift is zero, these two maxima have the same intensity. Nonzero frequency shifts result in two unequal maxima and their intensity difference may be used to calculate the magnitude of the shift. Three examples of experimental spectra have been analyzed in order to demonstrate the applicability of the technique.
Electron impact investigation of the electronic transitions of 1,4-cyclohexadiene and 1,5-cyclo-octadiene
Authors: McDiarmid Ruth , Doering J. P.
Published: February 2003

In this electron impact investigation of the excited states of 1,4-cyclohexadiene and 1,5-cyclo-octadiene, variable scattering angle and incident energy have been employed to distinguish between different types of electronic transitions. On the basis of the observed forbidden and allowed p–Rydberg transitions and the selection rules for p–Rydberg transitions to the two ion cores of 1,4-cyclohexadiene, the SA π-electron orbital is demonstrated to lie lower in energy than the SS π-electron orbital. On the basis of the observed forbidden and allowed valence transitions and the selection rules for valence transitions of 1,4-cyclohexadiene, the AA vacant π* orbital is demonstrated to lie lower in energy than the AS vacant π* orbital. Only one unambiguous valence transition was observed in 1,5-cyclo-octadiene, in contrast to theoretical predictions. The mean valence transition energies of these nonconjugated dienes were found to be significantly higher than those of the corresponding monoenes (cyclohexene and cyclo-octene). This result is in contrast to what is observed in norbornadiene and is indicative of the importance of through-bond interaction of the π orbitals in these cases as opposed to through-space interaction in norbornadiene.
An exactly soluble model for the quantum defect formula
Authors: Raina V. , Sane K. V. , Saxena R. P. , Srivastava P. K.
Published: February 2003

It is shown that the Schrödinger equation for a model potential consisting of a zero range square well attached to a coulomb tail can be solved exactly. The eigenvalues are found to agree with the quantum defect formula.
Reply to ‘‘Comment on the ‘Energy level analysis of Np3+:LaCl3 and Np3+:LaBr3’ ’’
Authors: Carnall W. T. , Crosswhite H. , Crosswhite H. M. , Hessler Jan P. , Edelstein N. , Conway J. G. , Shalimoff G. V. , Sarup R.
Published: February 2003

The crystal field parameters in Np3+:Lad3 and Np3+:LaBr3 are discussed. The arguments of Poon and Newman, regarding the scaling of the crystal field parameters are aknowdged as being correct.(AIP)
Generalized reorientation cross section for cylindrically symmetric velocity distributions
Authors: Monchick Louis
Published: February 2003

Generalized reorientation cross sections are derived for the case of atom–molecule collisions where the molecules initially have a velocity distribution cylindrically symmetric about an axis in the laboratory reference frame. This spatial ordering of the velocity can come about, for instance, by exciting molecular electronic states with a light source whose linewidth is much narrower than the Doppler-broadened absorption line. A simple kinetic theory can be set up in terms of state multipoles that are not completely irreducible; the resulting reorientation cross sections are only slightly more complex than the cross sections occurring in a spherically symmetric velocity field. Two approximations are investigated: a McGuire–Kouri mj-conserving model and a semiclassical model where the orientation of the rotation plane is conserved. The import of the generalized cross sections for several types of experiment and the applicability of the approximate models are discussed.
The existence and cure of intrinsic divergence in closed shell SCF calculations
Authors: Stanton Richard E.
Published: February 2003

Fixed point iteration theory is used to clarify the conditions for convergence in closed shell SCF calculations. The existence of intrinsic divergence is then demonstrated by elementary graphical techniques in a series of He and H2 calculations. Finally, a new method of overcoming convergence problems is introduced. This makes use of cubic osculatory interpolation between the results of successive SCF cycles. The technique is computationally cheap. It also preserves idempotency of the density matrix.
Intramolecular rate process: Isomerization dynamics and the transition to chaos
Authors: De Leon Nelson , Berne B. J.
Published: February 2003

The Hamiltonian flow of a system of two degrees of freedom, capable of undergoing geometrical isomerization, is studied as a function of the coupling between the reactive and unreactive degrees of freedom. The reaction dynamics are analyzed from the perspective of the KAM theorem and the transition to chaos. Conditions are found for the validity of linear rate laws and RRKM theory in isolated molecules.
Defects and fluorine diffusion in sodium fluoroberyllate glass: A molecular dynamics study
Authors: Brawer S. A.
Published: February 2003

A mechanism for fluorine diffusion in sodium fluoroberyllate fluid and glass is described based on a molecular dynamics study. It is shown that the separation of a F from the neighborhood of a Be occurs only when the Be has a coordination number of five or more. The diffusion mechanism involves a F entering the coordination sphere of a fourfold Be, making it fivefold, after which a different F leaves the neighborhood, making the Be fourfold again. The diffusion mechanism qualitatively explains the concentration dependence of the viscosity in alkali fluoroberyllate fluids. At temperatures below the computer glass transition, where no macroscopic F diffusion occurs during a computer run, some F can still separate from fivefold Be by distances greater than 0.5 Å. (In the glass, about 40% of the Be are fivefold coordinated by F.) These motions have an activation energy of 0.1 eV (compared to 0.7 eV for macroscopic diffusion). It is suggested that such sites give rise to secondary relaxations and anomalous low-temperature properties of glass.
The equation of state of D2O determined from sound speeds
Authors: Chen Chen-Tung A. , Millero Frank J.
Published: February 2003

The equation of state of D2O valid over the ranges 0–60 °C and 0–1000 bar has been determined from the high pressure sound speeds of Chen and Millero. The precision of the equation of state is ±0.01×10−6 bar−1 in the isothermal compressibility, ±10×10−6 cm3 g−1 in the specific volume, and ±2×10−6 deg−1 in the thermal expansibility. The results agree reasonably well with the equations of Fine and Millero and with the refit equation of Emmet and Millero.
Raman spectroscopic investigation of the structure of silicate glasses. III. Raman intensities and structural units in sodium silicate glasses
Authors: Furukawa Toshiharu , Fox Karen E. , White William B.
Published: February 2003

The Raman scattering intensity of the 1100 cm−1 polarized band, which appears on the addition of Na2O to SiO2 glass, reaches a maximum at the disilicate composition. The intensity of the polarized band at 950 cm−1 increases sharply as the Na2O concentration increases above 30 mole %. These data were interpreted by normal mode calculations and by IR and Raman intensity calculations for the silicate anion structural units: SiO4 isolated tetrahedra, Si2O7 dimers, Si2O6 chain links, Si2O5 sheet units, and Si2O4 framework units. According to these simplified models, the polarized high frequency band is due to symmetric stretching of Si–O− nonbridging bonds and the frequency increases with degree of polymerization of the tetrahedra. The previous assignments of the 1100 cm−1 band to the symmetric stretch of tetrahedra containing one nonbridging oxygen and of the 950 cm−1 band to the symmetric stretch of tetrahedra containing two nonbridging oxygens were confirmed. The other main feature of the alkali silicate glasses, an intense polarized band in the range of 400–600 cm−1, was shown to be a mixed stretching bending mode of the Si–O–Si bridging bond. The model also accounts for the loss of intensity of the high frequency band with increasing degree of silica polymerization.
Tensor patterns and polarization ratios for three-photon transitions in fluid media
Authors: Friedrich Donald M.
Published: February 2003

Excitation of polyatomic molecules by the simultaneous absorption of three photons provides a method for observing and unambiguously assigning molecular electronic and vibronic transitions which may be inaccessible to one- or two-photon spectroscopy. Such three-photon allowed states may be forbidden to one-photon processes by reason of symmetry, even though the states may be parity (g→u) allowed. Also, the states may be one-photon allowed but lie at inaccessibly high energies, such as in the vacuum ultraviolet. Calculation of the polarization dependence of the three-photon absorption intensity requires a knowledge of the third rank tensor pattern for the three-photon transition. These patterns are presented here for the symmetry species of several molecular point groups, and their applications are discussed. Tensor patterns for the special case of three-photon excitation by a single laser are also presented. In several symmetry species the polarization ratio (circular/linear) for excitation of fluid media becomes 5/2. The intensities vanish for a few species possessing antisymmetric tensors. This phenomenon of ’’identity forbiddenness’’ is discussed.
Line shapes and transport for excitons with stochastic coupling
Authors: Jackson Bret , Silbey Robert
Published: February 2003

The effect of nonlocal fluctuations on the line shape and dynamics of exciton states is considered, using a stochastic perturbation approach. The cumulant expansion is truncated at second order. The usual Gaussian and motional narrowing limits are treated and the limitations of the model are discussed.
Molecular mechanics for phase transition and melting of n-alkanes: A spectroscopic study of molecular mobility of solid n-nonadecane
Authors: Zerbi Giuseppe , Magni Roberto , Gussoni Mariangela , Moritz Kurt Holland , Bigotto Adriano , Dirlikov Stoil
Published: February 2003

n-Nonadecane shows a phase transition from orthorhombic to so called α phase near 22.8 °C and melts at 32 °C. Temperature dependence studies of infrared and Raman spectra of n-nonadecane and of selectively deuterated nonadecanes (2D2 and 10D2) allow one to obtain a detailed description of the structural changes which take place at the phase transition and melting. Experiments are interpreted on the basis of normal mode calculations. Comparison with the information available in the literature from other experiments or calculations shows that additional information are derived in this work. A model for the mechanism of phase transition is proposed.
High pressure luminescence of indole and substituted indoles in liquid solution
Authors: Politis T. G. , Drickamer H. G.
Published: February 2003

High pressure luminescence measurements have been made on indole, 5-methoxyindole, and tryptophan in a number of solvents. Radiative and nonradiative rates have been calculated as a function of pressure. It has been shown that the nonradiative rates are determined by viscosity in all solvents except water and, to some extent, methanol for which the peak shift as well as possibly viscosity are the controlling factors. The radiative rates and the peak position and shift have been scaled with the dielectric constant in a continuous and consistent way. Some peculiarities have been observed when water and methanol are used as solvents.
Calculation of Raman intensities for the ring-puckering vibrations of cyclopentene and 2,5-dihydrofuran
Authors: Bocian David F. , Schick G. Alan , Birge Robert R.
Published: February 2003

Raman intensities are calculated for the ring-puckering vibrations of cyclopentene and 2,5-dihydrofuran using an anisotropic atom–point dipole interaction model to calculate the elements of the molecular polarizability tensor. Large second-order terms are calculated for both molecules in the expansions of the molecular polarizability tensor elements in the ring-puckering coordinate [‖(∂2αμν/∂Z2)0‖≫0]. These terms result in intense calculated Δv = 2 overtone transitions, in agreement with experimental observations. The calculations predict that the Δv = 2 transitions of cyclopentene are one to two orders of magnitude more intense than the Δv = 1 fundamentals, while the two sets of transitions are comparable in intensity in 2,5-dihydrofuran. Although the shape of the band contours precludes observation of the Δv = 1 transitions for either molecule, the calculations suggest that the fundamental transitions of cyclopentene would not be observable even if the band shape were amenable to observation.
Study of mode Grüneisen parameters in various quasi-one-dimensional crystals
Authors: Carlone C. , Hota N. K. , Stolz H. J. , Elbert M. , Hochheimer H. D.
Published: February 2003

This work reports the pressure dependence of the Raman spectra of TCNQ0, TTF0, K(TCNQ), Cs2(TCNQ)3, perylene–TCNQ, and phenazine–TCNQ in the range from 0 to 70 kbar. By using the average compressibility of ∼0.01 kbar−1 of TTF–TCNQ, approximate mode Grüneisen parameters are derived spanning a range of several orders of magnitude. In the gas phase spectrum of TCNQ, low lying internal modes are observed at 40, 76, and 145 cm−1, lowering the so far assumed demarcation between internal and external modes from about 200 cm−1 down to less than 50 cm−1. From the pressure and gas phase data it is concluded that all of the observed low frequency modes in this class of compounds actually are modes of mixed internal and external character. In TTF0 and TCNQ0 a continuous change in color from yellow to dark red is observed when pressure is applied until the samples become opaque at about 80 kbar. Furthermore, TCNQ shows a phase transition at 12 kbar.
Dynamics of nonadiabatic reactions. I. F (2P3/2, 2P1/2)+HBr (DBr)→HF (DF)+Br (2P3/2, 2P1/2)
Authors: Hepburn J. W. , Liu K. , Macdonald R. G. , Northrup F. J. , Polanyi J. C.
Published: February 2003

A crossed molecular beam study has been performed on the nonadiabatic reactions F(2P3/2) [F(2P1/2)]+HBr(DBr)→HF(DF)+Br(2P3/2) [Br(2P1/2)]. Atomic F came from a seeded supersonic jet, so that the cross sections Sr(Br) and Sr(Br) could be measured as a function of collision energy ET = 1–11 kcal/mol. The reagent ratio [F]/[F] was varied by means of a variable temperature F atom source. Products Br and Br were detected by tunable vacuum ultraviolet laser-induced fluorescence (VUV LIF). The reaction cross sections showed no threshold, but a steep decline with increasing ET; the barrier to reaction on the FHBr surface is <1 kcal/mol. The ratio [Br]/[Br] showed no correlation with [F]/[F] in the reagents, indicating that the source of Br was not the adiabatic process F+HBr→HF+Br, but a nonadiabatic process F+HBr→HF+Br, Br. The results at high collision energy indicate that there is a substantial barrier to the reaction F+HBr→HF+Br. The cross section ratio for the two branches of F+HBr, i.e., Sr(Br)/Sr(Br), is 0.056±0.004 at room temperature. This ratio declines slightly with increasing collision energy, and is sensitive to isotopic substitution: for F+DBr at 300 K, Sr(Br)/Sr(Br) = 0.0101±0.0016. The velocity dependence and isotope effect of the branching ratio are discussed in terms of an ’’energy exchange’’ occurring well along the exit valley of the ground electronic state potential-energy surface, in which V–E transfer between nascent HF and Br results in a hop onto the upper potential-energy surface.
Coexistence of nonregistered monolayer and bilayer solid films
Authors: Wei Mein Sieng , Bruch L. W.
Published: February 2003

A statistical thermodynamic theory is formulated for monolayer and bilayer solid films of inert gases in physical adsorption. At temperatures where the lattice vibrations are anharmonic, and allowing for thermally activated vacancies and dilute overlying gas layers, the Helmholtz free energies of the adsorbed phases are evaluated with a cell model approximation which includes part of the correlation energy. Three physical systems are treated, using realistic models for adatom interactions and substrate holding potentials: Xe/Ag(111), Ar/Gr, and Kr/Ag(111). Phase diagrams and thermodynamic functions of the solid films are calculated. The lattice constant discontinuity at the transition under compression from the monolayer to the bilayer is found to be small, in accordance with experiments. The lattice constants at the transition are distinctly larger than that of the corresponding bulk solid under its own vapor pressure. The calculated difference between the latent heats of adsorption of the monolayer and the bilayer is in good agreement with experimental data for Xe/Ag(111) and Ar/Gr, but it is significantly smaller than the experimental difference for Kr/Ag(111).
Piezomodulation spectroscopy of molecular crystals. IV. The first singlet systems of TCNQ and BDP
Authors: Merski J. , Eckhardt C. J.
Published: February 2003

The polarized modulated piezoreflection spectra of the first singlet system of TCNQ and BDP are reported. Kramers-Kronig transformations are obtained and polariton energy strain shifts are determined. In contrast to systems with negligible to intermediate coupling, the piezomodulation spectrum for the mixed transverse-longitudinal polaritons show strong deviations from the logarithmic frequency derivative of the corresponding reflection spectra. Progressions in the piezomodulation spectra in the energy region of the stop band are related to multiparticle states. Piezomodulation spectroscopy is shown to exhibit greatest deviation from derivative behavior when solid state effects become extensive.
The fluorescence and photofragmentation of liquid saturated hydrocarbons at energies above the photoionization threshold
Authors: Schwarz Frederick P. , Smith Diane , Lias Sharon G. , Ausloos P.
Published: February 2003

Fluroescence quantum yields (φF) are reported for saturated hydrocarbons excited by photon absorption in the liquid phase at energies below and above the photoionization threshold. The quantum yields of fluorescence obtained in the subionization region are in excellent agreement with those reported by Lipsky and colleagues. In the phtotionization region, emission occurs as a result of both charge recombination processes and deactivation of the superexcited molecule to the vibrationally relaxed first excited singlet state. The presence of these two populations of fluorescing species above the ionization threshold is manifested in differences in the slopes of plots of φF as a function of energy above and below the ionization onset. Since, for saturated hydrocarbons, photofragmentation is the only nonradiative channel, the variations in the relative importances of the major modes of fragmentation have been examined as a function of energy for selected molecules. It is observed that the two main dissociative processes, H and H2 elimination, show a continuous variation with photon energy. While at the absorption threshold H2 elimination predominates, at the highest energy used in this study (11.6 eV) H atom detachment from the excited molecules (both those formed by direct photon absorption and those formed by charge recombination) is of comparable importance. Liquid phase radiolysis experiments show a ratio of H atom elimination to H2 formation which is substantially higher than that measured at the highest photon energy. The role of equilibrated excited singlet state (S1)0 in saturated hydrocarbons is examined in the context of the wavelength-dependent fluorescence and photochemical data. The results indicate that, at energies above the ionizatin thresholds for the various molecules, ion pair recombination has a probability below unity of leading to the formation of vibrationally relaxed (S1)0 emitting states. In the radiolysis of liquid C6–C10 alkanes, it is suggested that charge recombination does not always lead to the formation of vibrationally relaxed (S1)0 molecules. This is explained in terms of LET effects and a high mean energy associated with the ionization process.