bluuebunny/crossref_metadata_2025 huggingface dataset:
Polydispersity effects in the small-angle scattering of concentrated solutions of colloidal spheres
Authors: van Beurten P. , Vrij A.
Published: February 2003

Calculations are presented of the intensity of radiation scattered by a polydisperse system of spherical particles at finite concentrations where interparticle interference of the scattered waves becomes important. The results apply to small-angle (x-ray and neutron) scattering and to light scattering of a solution of particles in the colloidal size range. Use is made of a closed expression for the scattering of a multicomponent mixture of hard spheres recently derived by one of us. The derivation is based on Baxter’s solution of the Percus–Yevick approximation for hard sphere fluids.
Intense depolarized Rayleigh scattering in Raman spectra of acids caused by large proton polarizabilities of hydrogen bonds
Authors: Danninger Walter , Zundel Georg
Published: February 2003

The depolarized Rayleigh scattering of liquid H2O and D2O and of aqueous solutions of HCl and DCl was studied as a function of concentration and temperature. This scattering is caused by time dependent changes of the anisotropy of the molecular polarizability. The scattering below 30 cm−1 is determined by a Lorentzian component. As shown in Ref. 1 with pure water, the scattering of this component is caused by the change of the molecular anisotropy of the polarizability due to breaking and formation of hydrogen bonds, whereby the mean lifetime of the hydrogen bonds determines the half-width. This mechanism is confirmed by our results obtained with pure liquid H2O and D2O. With the acid solutions, the intensity of the scattering of this Lorentzian increases strongly with increasing acid concentration. This intensity increase is caused by the large anisotropic proton polarizability of the hydrogen bond in the H5O2+ groupings. Thus, this intense scattering is in addition to the intense continua in the IR spectra, a second access to study proton polarizabilities of hydrogen bonds. The time dependence of these proton polarizabilities is caused by changes of the local electrical fields due to breaking and formation of hydrogen bonds in the neighborhood of the H5O2+ groupings and therefore determined by the mean lifetime of these bonds. The half-width of this Lorentzian becomes smaller with increasing acid concentration, indicating a larger mean lifetime of the hydrogen bonds in the neighborhood of the H5O2+ groupings. From the fact that the scattering intensity in the DCl solutions is less than in the HCl solutions, it is concluded that the easily polarizable hydrogen bonds are already considerably polarized by their polar environments.
Vibrational spectroscopy at very high pressures. Part 28. Raman and far-infrared spectra of some complex chlorides A2MCl6 under hydrostatic pressure
Authors: Adams David M. , Berg Rolf W. , Williams Alan D.
Published: February 2003

Raman and far-IR mode frequency shifts with pressure have been observed under hydrostatic conditions in a gasketed diamond anvil cell (d.a.c.). Using compressibilities calculated from unit cell constants and lattice energies, Grüneisen parameters γi have been obtained for all observed modes. Where previous data exist for comparison, it is shown that use of a d.a.c. without gaskets (i.e., applying a shear stress to the sample) greatly underestimates the true shifts of IR modes in particular. The Grüneisen parameter γ1 for a1g ν̃1 of the complex ions is always greater than γ2. It is shown that this is due to the greater value of anharmonicity of ν̃1, as is the increase in γ1, γ2, and γ3 in the series A2MCl6, where A=Cs, Rb, K. Grüneisen parameters for K2SnCl6 and Rb2TeCl6 are greater than for non-transition metal complexes. Anomalies in the ν̃i versus pressure curves for K2SnCl6 and [(CH3)4N]2MCl6 (M=Sn, Te, Pt) are discussed in relation to their structures. Shifts of ν̃i with temperature for K2ReCl6 and K2PtCl6 are analyzed into explicit and implicit anharmonic contributions.
Experiments on selective excitation of multiple-quantum transitions in NMR spectroscopy
Authors: Warren W. S. , Pines A.
Published: February 2003

Irradiating a spin system with an appropriate sequence of phase shifted pulses excites only certain orders of multiple-quantum transitions. excited. Using such sequences in preparation and detection produces a large signal/noise enhancement for the multiple-quantum spectra over nonselective excitation, as predicted from theory. In a previous paper the theory was presented in detail. In this paper some aspects of the theory are first outlined briefly. Experimental results are then presented demonstrating four-quantum, six-quantum, and A-symmetry selection in oriented benzene and eight-quantum selection in oriented 1-bromobutane. A six-quantum selective sequence applied to benzene with nonequilibrium initial reduced density matrix proportional to Ix produces one- and five-quantum spectra. The n-quantum signal/noise enhancements are obtained and the selectivity is measured as a function of pulse sequence parameters. The behavior is that expected if one assumes that the limit to selectivity is determined by leading nonselective terms in an average Hamiltonian expansion.
A model for reactive nonadiabatic transitions: Comparison between exact numerical and approximate analytical results
Authors: Child M. S. , Baer M.
Published: February 2003

Exact diabatic/adiabatic branching ratios and final state distributions are presented for a reactive model for nonadiabatic transitions, applicable to situations where the coupling term is approximately constant over the region where the interpotential seam crosses the two valleys. Comparison is made with the Bauer–Fischer–Gilmore (BFG) and Franck–Condon (FC) models for a variety of situations. A new index γ=(vRΔGR/vrΔGR), where subscripts R and r denote translational and vibrational variables, respectively, is introduced as a measure of the validity of the two approximations. The FC approximation is shown to become exact for γ≳≳1, while the BFG approximation is preferred for γ<<1.
Perturbation theoretical study of the fluid phase of a binary mixture of hard-core molecules with square wells. Chemical potential surface and phase diagram
Authors: Ichimura Takao , Ueda Akira
Published: February 2003

Leonard–Henderson–Barker perturbation theory is applied to a binary mixture of hard-core molecules with square wells. Molecules are of equal core sizes, equal ranges of forces, and equal well depths, but of a relatively weak unlike-pair depth. The effect of change in the weakness parameter a of an unlike pair is examined in detail on the topological nature of the chemical potential surface of the fluid phase and hence on the phase diagram. How a positive or heterogeneous azeotrope appears and how the critical line changes with a are clearly demonstrated. Three liquid phases can coexist in equilibrium in the same range of a.
14N nuclear quadrupole interaction in salicylaldoxime and in Cu(II) doped bis(salicylaldoxime) Ni(II)
Authors: Murgich Juan
Published: February 2003

14N NQR in pure salicylaldoxime is studied at 77 °K. ENDOR measurements on single crystals of CU(II) doped bis (salicylaldoxime) also provide quadrupole data. The two data are compared.
On the search for O2−
Authors: Hird B. , Ali S. P.
Published: February 2003

The search for negative doubly charged atomic ions of oxygen is conducted in an apparatus used to measure charge transfer cross sections. An RF ion source is used. No evidence for 02− was found in the beam. (AIP)
Comment and addendum on ‘‘Single and double ionization of nitrogen dioxide by electron impact from threshold up to 180 eV’’
Authors: Märk T. D. , Märk E. , Stephan K.
Published: February 2003

In a recent paper by the authors (Ref. 1), it was claimed that NO2 2+ was observed for the first time via electron impact of NO2. The existence and the appearance potential of NO2 2+ has been reported earlier by Cooks, Terlwillinger, and Beynon (Ref. 2). The results of Ref. 2 are compared with the author’s measurements. (AIP)
Photonucleation in supersaturated CCl4 vapor investigated in a diffusion chamber
Authors: Cordier B. , Papon P. , Leblond J.
Published: February 2003

We report an experimental investigation of the nucleation of supersaturated carbon tetrachloride vapor in the presence of an impurity, Cl2, induced by light in the visible part of the spectrum. This photonucleation phenomenon has been studied with a thermal diffusion chamber. The process is shown to be strongly dependent upon the intensity and wavelength of light and upon the supersaturation of the vapor. A tentative interpretation of this photonucleation phenomenon is given, that is, it relies upon the formation of CCl3 free radicals.
Calculation of vibrational preionization in H2 by multichannel quantum defect theory: Total and partial cross sections and photoelectron angular distributions
Authors: Raoult M. , Jungen Ch.
Published: February 2003

Multichannel quantum defect theory has been used to calculate the effect of vibrational preionization on the total and partial oscillator strength distributions and photoelectron angular distribution in H2 for excitation between 790 and 760 Å. The total oscillator-strength distribution obtained agrees well with the high-resolution photoionization data of Dehmer and Chupka. The partial oscillator strength resonance profiles are predicted to have different shapes in different vibrational ionization channels, while their widths change little with channel. The preionization resonances are also predicted to affect the angular distribution asymmetry parameters b over a broader range than they affect the oscillator strength distribution. The gross features of the preionization resonances are discussed in terms of approximate solutions of the MQD equations.
Effect of changing reagent energy on reaction dynamics. XI. Dependence of reaction rate on vibrational excitation in endothermic reactions HX(vreag)+Na→H+NaX(X≡F,Cl)
Authors: Bartoszek F. E. , Blackwell B. A. , Polanyi J. C. , Sloan J. J.
Published: February 2003

The chemiluminescence depletion (CD) method has been applied to two endothermic reactions to obtain approximate relative rates of reaction kendo(vreag) out of specified reagent vibrational levels vreag for a range of vibrational energies Vreag extending to well above the energy barrier Q. The reactions were (1) HCl(vreag = 1–4)+Na→H+NaCl(Q = −4.2 kcal mole−1, Qc = −10 kcal mole−1) and (2) HF(vreag = 1–5)+Na→H+NaF (Q = −12 kcal mole −1, Qc = −18 kcal mole −1). The major finding is that for high Vreag, with over 90% of the reagent energy present as vibration, the collision-efficiency for both reactions is approximately unity. From this we conjecture that the barrier crest is ’’late,’’ with little extension into the entry valley (type IIS). The reactive cross sections exceed by an order of magnitude the (large) cross section of several Å2 obtained in part VI for endothermic reactions HX+Y(e.g., HF+Cl). The salient difference is likely to be the fact that the present reactions involve a switch from covalent to ionic bonding. To within experimental error the same absolute vibrational energy in either HCl or HF gives rise to the same absolute cross section for reaction with Na.
A diatomics-in-molecules case study on the system Be+HF→BeF+H. II. The sensitivity of the potential surfaces to the diatomic input
Authors: Roach A. C. , Kuntz P. J.
Published: February 2003

Potential energy surfaces are presented for a series of diatomics-in-molecules (DIM) models of the reaction Be+HF→BeF+H. This system involves low-lying electronic states of Be and ionic contributions to bonding and is therefore quite complex from the DIM point of view. The various DIM models involve between three and 18 structures and make extensive use of configuration interaction in the diatomic fragments. The potential surface is predicted to have a barrier ∼1.9 eV high for collinear BeFH; at bent configurations this barrier gradually increases. A potential surface having this feature is obtained only if the Be(2s2p) configuration is included in the DIM model. The general problem is raised of how to decide whether a given structure in a DIM model should be retained or not, and of how best to obtain reliable diatomic configuration interaction input data.
MINDO/3-FP atom-in-molecule polarizabilities of TCNQ, TTF, TMPD, and of their radical ions
Authors: Metzger Robert Melville
Published: February 2003

The MINDO/3-FP method was used to obtain molecular polarizabilities a and atom-in-molecule polarizabilities ai for the neutral molecules 7,7,8,8-tetracyanoquinodimethan (TCNQ), tetrathiofulvalene (TTF), and N,N,N′,N′-tetramethylparaphenylenediamine (TMPD), and their radical ions TCNQ−, TTF+, and TMPD+. Except for the direction perpendicular to the molecular plane, the a and the ai describe fairly well the covalent bonding environment. The radical ions are more polarizable than their parent neutral molecules, but not spectacularly so. The ai appear to be covalent bond polarizabilities, and are the largest for the atoms that lie on the longest molecular axis.
A comment on the dynamics of excitation trapping in molecular crystals
Authors: Klafter J. , Silbey R.
Published: February 2003

In this paper we consider the long-time trapping rate, by a deep sink, of an excitation which migrates incoherently in a one-dimensional molecular crystal. The approaches based on the first passage time version of the continuous time random walk and on the master equation formalism are compared and put in a unified framework. Time domains are predicted for the different behaviors of the trapping rates for both one- and two-dimensional systems.
The experimental determination of ground state dipole moments from dielectric constant measurements using ellipsoidal cavity correction factors
Authors: Myers Anne B. , Birge Robert R.
Published: February 2003

An equation is derived from the Onsager–Böttcher formalism to calculate permanent dipole moments of molecules of ellipsoidal shape from dielectric constant data. An expression is also derived which allows dipole moments calculated by spherical cavity formalisms to be corrected without repeating the experiment. This expression is applied to literature values calculated using the Debye equation, and it is shown that the correction brings the published values for nonspherical molecules closer to the vapor phase values. Tables of the ellipsoidal shape factor Aa as a function of the ratios of the axes of the molecular ellipsoid are presented. Interpolation algorithms can be used in conjunction with the above tables to calculate Aa to at least three significant digits.
The mercury–ammonia system at high mercury densities
Authors: Mandl A. , Hyman H. A.
Published: February 2003

The properties of optically excited Hg/NH3 and Hg/ND3 mixtures have been studied as a function of mercury pressure in the range 1–40 Torr. At low mercury pressures ≲1 Torr, the well-known UV emission band due to HgNH3* is dominant; this regime has been investigated extensively over the past ten years. However, at higher pressures the UV band disappears and is replaced by a strong green emission band, centered near 500 nm. From a detailed study of the spectral and temporal characteristics of the green band, it is concluded that it arises from the formation of the Hg2NH3* (or Hg2ND3*) excited molecular complex. The lifetime of this exciplex has been measured and is found to be 3.8 msec (±25%). Because of interest in the system as a potential energy storage laser medium, optical transmission measurements have been carried out at selected wavelengths. Net absorption has been observed across the green band at high mercury density, as well as in the UV band in the low density regime.
Rayleigh–Brillouin scattering studies of liquid and supercooled liquid o-terphenyl
Authors: Higashigaki Y. , Wang C. H.
Published: February 2003

A comprehensive study of the Rayleigh–Brillouin scattering spectra of o-terphenyl in the liquid and supercooled liquid state has been carried out. Anomalies in the Brillouin frequency and linewidth data are observed at the glass transition temperature (Tg) and at a temperature (TK) about 36 K higher than the melting point. The anomaly at Tg is due to the absence of free volume associated with the freezing-in of the density fluctuation. The longitudinal hypersonic frequency kink as well as the linewidth maximum and the dimunition of the transverse sound frequency at TK are interpreted as due to the relaxation of translational order which disappears above TK. The result of the depolarized Rayleigh scattering spectral study shows that cluster formation as previously proposed is unlikely. Hydrodynamic slip boundary conditions for molecular reorientation are found to be appropriate in liquid o-terphenyl. Rotation about the short and long axes lying in the plane of the parent phenyl ring is found to be consistent with the experimental data, whereas rotation about the axis perpendicular to the plane of the parent phenyl ring gives too high a hydrodynamic volume. Also observed is quasidiscontinuity at Tg in the integrated intensity versus temperature plot for the central Rayleigh component of the VV scattering spectral density. The Brillouin intensity is found to follow the temperature dependence of the configuration entropy, suggesting the occurence of the adiabatic configurational changes. The configuration changes are found frozen in below Tg, leading to a large residual entropy.
High pressure study of viscosity effects on the luminescence of tetracyanobenzene EDA complexes
Authors: Thomas Michele Moisio , Drickamer H. G.
Published: February 2003

High pressure fluorescence studies fron 0–10 kbar have been performed on electron donor–acceptor (EDA) complexes of s-tetracyanobenzene (TCNB) with a series of aromatic hydrocarbons. Four solvents were used, 2,2,4,4,6,8,8 heptamethylnonane (HMN), methylcyclohexane (MCH), 2,6,10,14 tetramethylpentadecane (TMPD), and a mixture of MCH and HMN. A viscosity range from 0.006 to 10 000 poise was covered at constant temperature. As pressure (viscosity) increased the fluorescence spectrum shifted from one dominated by emission from the equilibrium (EQ) excited singlet state to one dominated by Franck–Condon (FC) singlet emission. Lifetime measurements for the complexes of o-xylene and p-xylene with TCNB as well as one mesitylene complex yielded the two radiative rates (kEQ and kFC) as well as the rate of internal conversion from FC to the EQ excited state to (kIC). The results are discussed in terms of the rate of relaxation of the solvent compared with the rate kFC. It was found that kIC correlated very well with the solvent viscosity.
Third moment of the satellites in quadrupole-split NMR spectra
Authors: Hughes D. G.
Published: February 2003

A general expression for the dipolar third moment of a nuclear magnetic resonance absorption is presented. This is used to calculate the dipolar third moment of the satellites in the quadrupole-split spectrum of a spin system with I = 3/2. It is shown how comparison of the measured and theoretical values of the third moment of the satellites should enable the sign of the quadrupole coupling constant to be determined.
A study of the rotational state dependence of predissociation of a polyatomic molecule: The case of ClO2
Authors: Michielsen Stephen , Merer Anthony J. , Rice Stuart A. , Novak Frank A. , Freed Karl F. , Hamada Yoshiaki
Published: February 2003

This paper reports extensive measurements of the linewidths of rotational transitions in the Ã 2A2←X̃ 2B1 absorption of ClO2. It is found that: (1) The rate of predissociation is independent of the state of rotation of the initially excited molecule, (2) the rate of predissociation depends on the spin state prepared, being greater for the F1(J = N+1/2) state than for the F2(J = N−1/2) state, (3) for v1<3 the rate of predissociation from the states (v100) is independent of v1, but for v1≳3 the rate increases with v1, and (4) the bending mode is a promoting mode for the predissociation. It is argued that spin–orbit coupling is the dominant interaction between the prepared 2A2 state and the intermediate manifold in this indirect predissociation. Analysis of a model of this process leads to the prediction that, if the prepared and intermediate states have nearly the same geometry, whether the intermediate state is real or nascent, the rate of predissociation should be independent of initial rotational state. Furthermore, it is suggested that the spin–rotation interaction mixes the F1 and F2 states; using a two state analysis, the predicted ratio of rates of dissociation G(F1)/G(F2) = 1.2 is in qualitative agreement with that observed, and the lack of dependence of the ratio on N explained.
17O NMR–NQR study of molecular motion in the liquid, and α and β solid phases of CO
Authors: Li F. , Brookeman J. R. , Rigamonti A. , Scott T. A.
Published: February 2003

A comprehensive NMR−NQR study of carbon monoxide enriched 24.5% in 17O (I = 5/2) is reported, which provides interesting information on the motions of the CO dipoles in the liquid and in the a and b solid phases. In the liquid phase the 17O NMR spin-lattice relaxation due to the tumbling rotational motions of the molecular axis, combined with the experimental evaluation of the rigid electric field gradient (EFG) at the 17O site has been utilized to achieve a direct and precise measurement of the reorientational correlation time of the dipole and its temperature dependence. In the b phase, the quadrupole perturbed NMR spectra and the spin-lattice relaxation indicate the occurrence of a precessional motion of the dipole around the c axis. An effective EFG is consequently present, whose order of magnitude is about 103 smaller than the static one. The precessional correlation time is derived, which is surprisingly very close to the reorientational correlation time of the liquid phase, suggesting a quasifree rotation character for the precession. 17O NQR signals in the a phase have been detected and resonance frequencies and linewidths have been measured. From the temperature behavior of the NQR frequency, information on the local order parameter for the librational motion of the dipoles is obtained. The linewidth exhibits a peculiar temperature dependence, with a maximum around 37 K and a dramatic divergence for T→29 K. While an orientational static disorder could be expected in view of the residual entropy, it is shown how the linewidth, as well as some pure quadrupole relaxation measurements, seem to indicate the occurrence in the a phase of a dynamical disorder, with sudden head-to-tail reorientations of the dipole, at least for T≳29 K. In the framework of such a dynamical model, a theoretical picture for the motional contribution to the NQR allows one to obtain insights on the features of the orientational disordering processes. The hypothesis of a phase transition of antiferroelectric type below T?37 cannot be ruled out. Finally, some evidence of softening of the libronic frequencies on approaching the a–b phase transition is obtained.
A spectroscopic investigation of the OCS discharge system
Authors: Clark William W. , De Lucia Frank C.
Published: February 2003

Millimeter and submillimeter rotational spectroscopy are shown to be a sensitive diagnostic probe of the OCS discharge system. In this system the simple decomposition reaction OCS→CO+S is dominant while more complex reaction pathways result in small concentrations of CS and SO. Over a very wide range of pulse conditions, we find the chemical composition of the discharge to be determined almost entirely by average current. The rotational and translational degrees of freedom were found to be in equilibrium, and the temperature that characterized this equilibrium was measured. In addition, vibrational temperatures for a number of states were measured, and temperatures near the rotational/translational temperature were found. The time evolution of these parameters was also studied and shown to be in agreement with dilute gas transport theory.
On the fragmentation of benzene by multiphotoionization
Authors: Rebentrost Frank , Ben-Shaul Avinoam
Published: February 2003

The MPI fragmentation of benzene is analyzed on the basis of experimental data and the dissociation dynamics of C6H+6 at low excess energies. A multiple fragmentation mechanism with branchings is discussed in which vibrational energy is pumped into the C6H+6 ground state by photon absorption and subsequent radiationless transitions. Calculations are performed for a statistical, products phase space model and show remarkably good agreement with the experimentally observed fragment patterns vs laser intensity. About 60 eV/molecule are required to make C+ the most abundant ion, if all the energy were initially present in the C6H+6. By RRKM estimates of the C6H+6 decomposition rate it is concluded that such an assumption is not realistic and a model with multiple absorption and fragmentation steps applies.
Consideration of the exit channel velocity effect upon inelastic cross sections computed by the infinite-order sudden approximation
Authors: Agrawal P. M. , Raff L. M.
Published: February 2003

A modified form for the state-to-state inelastic cross section in infinite-order sudden calculations that is explicitly dependent upon the exit-channel energy is derived. This modified IOSA formalism has been applied to the rigid-rotor CO2–He system using the electron-gas surface previously obtained by Parker, Snow, and Pack. It is shown that the modified IOSA equations yield zero cross sections for inelastic channels that are closed by energy conservation constraints whereas significant nonzero values are obtained from standard IOSA calculations. In addition, it is shown that the suggested modification brings the infinite-order sudden results into much better accord with quasiclassical trajectory calculations for all exit channels.
Effects of memory on the coherent scattering function of a Rouse chain
Authors: Jhon Myung S. , Fesciyan Sezar , Dahler John S.
Published: February 2003

Truncated continued fractions are used to form estimates of the memory function associated with the coherent scattering function of a dilute polymer solution. Calculations specific to a freely draining, Rouse chain are presented for the three lowest order approximations, namely, the memoryless single-pole ’’mean field’’ approximation and the two- and three-pole approximations. The importance of memory is assessed by comparing these results with one another and with exact numerical values of the scattering function.
Steady state properties of far from equilibrium diffusive systems
Authors: Termonia Yves
Published: February 2003

A stochastic model is proposed for the study of the effect of the interactions on transport through a system placed between two parallel external baths. The exchange of matter of a system at local equilibrium with a single external bath is conveniently treated with the help of a stochastic theory described previously. That theory is extended here to the case of a system placed between two parallel external baths. The extension is realized as follows. The system is viewed as an array, perpendicular to the external gradient, of hypothetical layers at local equilibrium. Each layer exhibits a ’’two-faced exchange,’’ exchanging matter with two baths (viz., layers on its two sides) and provides, in turn, a bath to these two layers. The theory is applied to a study of the steady state properties of far from equilibrium systems composed of two and of three molecular species. It is shown that, in the case of a binary system, our two-faced exchange model leads to an equilibrium in each layer, whatever the value of the external gradient. However, that convenient feature evaporates on passing to systems with more than two molecular species. The effect of the interactions on the stationary flows and concentration profiles is extensively studied. The relevance of our results to the diffusion of hydrogen through palladium is also discussed.
Specular and off-specular high resolution electron energy loss spectroscopy of acetylene and ethylene on tungsten (100)
Authors: Hamilton J. C. , Swanson N. , Waclawski B. J. , Celotta R. J.
Published: February 2003

High resolution electron energy loss spectroscopy (EELS) in both specular and off-specular directions has been used to identify the vibrational modes of acetylene and ethylene on tungsten (100). The off-specular data were essential to this study since, at low coverages, some of the vibrational modes were detectable only for off-specular scattering. In addition, analysis of the relative intensities of the specular and off-specular loss peaks allows us to infer orientations of molecular dipole derivatives and bond directions for the adsorbed species. We find at 135 K that ethylene on tungsten (100) dissociates to acetylene and hydrogen for exposures less than 1 L. At saturation coverage molecular ethylene is also adsorbed. Warming of this adsorbed ethylene to 300 K causes dissociation to acetylene. Analysis of specular and off-specular loss intensities suggests a geometry with the C–H bonds lying parallel to the surface. Previous UPS data for this adsorption system can be reinterpreted giving a C–C bond length of 1.35 Å and a C–C–H bond angle of 180 °: This reinterpretation is consistent with the geometry suggested by our EELS measurements. We also present data for acetylene adsorption at room temperature and for ethylene physisorption at 82 K. Physisorbed ethylene shows vibrational losses identical to those seen in gas phase IR.
Carbon–carbon double bonds can be catalytically hydrogenated on hydrated alumina
Authors: Hall James T. , McBride Duncan E.
Published: February 2003

We use inelastic electron tunneling spectroscopy to study catalytically induced hydrogenation of several unsaturated dicarboxylates chemisorbed on undried, hydrated alumina. Using isotope substitution we estabilish that hydroxyl groups on the alumina are the source of hydrogen. Fumarate and maleate hydrogenate completely at room temperature forming a succinate species. Gutaconate, hydromuconate, and muconate hydrogenate in a way that is exponentially temperature dependent, only slightly pressure dependent, and time independent in the range of a few minutes to an hour. We have investigated the temperature dependence over the range 25–400 °C, and we believe it is related to the thermal dehydration mechanism in alumina. From the variation in temperature dependence among the surface species we develop an index of the relative ease of hydrogention that correlates well with double bond distance from the surface and with bond resonance stabilization. Species whose double bonds are constrained to be closer to the surface hydrogenate more easily, and conjugated double bonds stabilize a species against hydrogenation. The alumina surface can be regenerated with water vapor and used for further hydrogenation.
Temperature dependent mobilities: NO2−, NO3−, CO3−, CO4−, and O2+ in O2
Authors: Perkins M. D. , Eisele F. L. , McDaniel E. W.
Published: February 2003

The zero-field mobilities of NO−2, NO−3, CO−3, CO−4, and O+2 in oxygen gas are measured as a function of temperature over the range 281−573°K.
Comment on the structure of liquid ammonia
Authors: Klein M. L. , McDonald I. R.
Published: February 2003

Recently reported neutron scattering data (Ref. 6) on liquid ND3 is commented upon. The potentials considered are models A and C of Ref. 4. (AIP)
Absolute rate constants for the reactions of CH with O and N atoms
Authors: Messing I. , Filseth S. V. , Sadowski C. M. , Carrington Tucker
Published: February 2003

CH(X 2R) was produced by multiple infrared photon dissociation of CH3OH in the presence of excess atomic oxygen or nitrogen. Time-resolved measurements of relative CH concentrations were made at 298 K with a tunable dye laser. Rate constants deduced from the dependence of CH decay rate on atom concentration are (9.5±1.4)×10−11 cm3 s−1 for CH+O and (2.1±.5)×10−11 cm3 s−1 for CH+N.
Close-coupling and coupled state calculations of argon scattering from normal methane
Authors: Smith Leslie Neil , Secrest Don
Published: February 2003

Close-coupling and coupled states integral, differential, and total cross sections in an argon–methane scattering system are presented for all the irreducible representations of the tetrahedral symmetry. From symmetry considerations the rotational wave functions, which transform according to each of the irreducible representations, are defined. A new interaction potential for the Ar–CH4 system is used. The spherically symmetric part of the potential is due to U. Buck, and was found by fitting a flexible Morse-spline–Morse-spline–van der Waals curve to total differential cross section. In the present paper an anisotropic term was added to the potential. Scattering from normal methane consisting of a mixture of all of the symmetries is discussed. Close-coupling and coupled states cross sections are compared for all the representations. Errors in the coupled states approximation run to 30%. Calculations with a Lennard-Jones interaction potential show that the magnitude of the percentage error in the coupled states cross sections is potential dependent. Coupled states integral, differential, and total cross sections for normal methane are presented over the energy range from 246 to 582 cm−1 (30–72 meV). The effect of compound state resonances on this system is illustrated.
Studies of the O2 3Πg(V) valence states and 3Πg(R) Rydberg state in the Schumann–Runge continuum from ejected and scattered electron spectra
Authors: Spence David
Published: February 2003

Electron-impact energy-loss spectra are obtained in O2 in the energy-loss range of 7 to 10 eV. These spectra contain some discrete features resulting from scattered electrons following excitation of Rydberg states, and other features from ejected electrons following decay of O2−Feshbach resonances into valence electronic states in the continuum via the reaction e+O2 (X 3Sg−)→(O2−)→O2 3Rg(V) +eejected. From the known O2− energies and from measurements of the ejected electron energies, we find the 3Rg(V) state to extend over an energy range of only about 0.20 eV in the Franck–Condon region of the ground state, contrary to some previous experimental and theoretical results. From measurements of scattered electrons, effective excitation functions of several vibrational levels of the 3Rg(R) Rydberg state are obtained. Of the many O2− Feshbach resonances known to exist in this energy regions, only one decays strongly into the 3Rg(R) state.
Mode specificity in unimolecular reaction dynamics: The Henon–Heiles potential energy surface
Authors: Waite Boyd A. , Miller William H.
Published: February 2003

Energies and lifetimes (with respect to tunneling) for metastable states of the Henon–Heiles potential energy surface [V(x,y) = 1/2x2−1/3x3+1/2y2+xy2] have been computed quantum mechanically (via the method of complex scaling). This is a potential surface for which the classical dynamics is known to change from quasiperiodic at low energies to ergodic-like at higher energies. The rate constants (i.e., inverse lifetimes) for unimolecular decay as a function of energy, however, are seen to be well described by standard statistical theory (microcanonical transition state theory, RRKM plus tunneling) over the entire energy region. This is thus another example indicating that mode specificity in unimolecular reaction dynamics is not determined solely by the quasiperiodic/ergodic character of the intramolecular mechanics.
Simple analytical approximation for rotationally inelastic rate constants based on the energy corrected sudden scaling law
Authors: Smith Neil , Pritchard David E.
Published: February 2003

We have recently demonstrated that the energy corrected sudden (ECS) scaling law of De Pristo et al. when conbined with the power law assumption for the basis rates kℓ→0∝[ℓ(ℓ+1)]−g can accurately fit a wide body of rotational energy transfer data. We develop a simple and accurate approximation to this fitting law, and in addition mathematically show the connection between it and our earlier proposed energy based law which also has been successful in describing both theoretical and experimental data on rotationally inelastic collisions.
Empirical force field calculations for bridged annulenes
Authors: Favini Giorgio , Simonetta Massimo , Sottocornola Manuela , Todeschini Roberto
Published: February 2003

A systematic study of a number of bridged annulenes by empirical force field calculations is described. The theoretical results obtained for the isolated molecules confirm the different geometrical situations found for the compounds in the crystalline state. The packing forces do not modify the geometry of the rigid molecules with respect to the gas phase.
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o calculation of higher order corrections to Λ doubling and spin splitting in diatomic molecules
Authors: Cooper David L. , Veseth Leif
Published: February 2003

Higher order corrections to the L doubling in 2R states and spin splitting in 2S states of diatomic molecules have been derived by use of a Van Vleck transformation to third order. The corrections are expressed in terms of the parameters pJ, qJ, and gN, normally introduced through the phenomenological replacements pv→pv+pJJ(J+1), qv→qv+qJJ(J+1), and gv→gv+gNN(N+1). The molecules investigated are OH, OD, and SH, and ab initio values for pJ, qJ, and gN are computed for the X 2R,v = 0 and A 2S+,v = 0 states. The electronic matrix elements were obtained within the Hartree–Fock approximation, and vibrational eigenfunctions were computed from Morse curves for OH and OD and from RKR potentials for SH. Comparison of ab initio values and experimentally derived parameters shows reasonable agreement.
A systematic classification of atomic integrals in a
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o molecular orbital calculations. I. The core valence approximation molecular orbital method
Authors: Campbell J. C. , Hillier I. H. , Saunders V. R.
Published: February 2003

A computational scheme for the systematic approximation of certain two-electron integrals in ab initio molecular orbital calculations is described in which core–valence overlap distributions are expanded in terms of monocentric functions centered at the core. The integral evaluation time now varies as (Nvalence)4 rather than (Ntotal)4, thus resulting in substantial saving of computer time. The method is shown to be invariant to unitary transformations of the basis set and to be capable of yielding wave functions to any required accuracy. For most purposes a level of approximation denoted B1 is found to be adequate, yielding molecular energies accurate to 10−3 a.u. with no appreciable error in other molecular properties.
An approximation to frozen natural orbitals through the use of the Hartree–Fock exchange potential
Authors: Feller David , Davidson Ernest R.
Published: February 2003

A modification of a previously defined virtual orbital transformation based on the Hartree–Fock exchange potential is shown to yield a set of orbitals which effectively approximate the frozen natural orbital CI convergence properties.
Quantum statistical mechanics with flexibly constrained degrees of freedom
Authors: Bruch L. W. , Goebel C. J.
Published: February 2003

The statistical mechanical configurational probability distributions are analyzed for two models in which the imposition of constraints in some degrees of freedom affects the reduced distribution function of other nonconstrained degrees of freedom. The classical, semiclassical, and extreme quantum regimes are exhibited for the models when the constraints are imposed by adding harmonic potential energy terms for the constrained variables to the Hamiltonian. In general, the limit obtained for large force constants in the added energies does not correspond to the distribution of the rigidly constrained system. This is in accord with discussions given by Helfand and by Rallison; the present results illustrate their conclusions.
Field-induced magnetic ordering in uniaxial nickel systems: A second example
Authors: Paduan-Filho Armando , Chirico Robert D. , Joung K. O. , Carlin Richard L.
Published: February 2003

The zero-field susceptibilities parallel and perpendicular to the tetragonal axis of NiCl2⋅4SC(NH2)2 have been measured between 1.2 and 30 K and fitted to the usual spin Hamiltonian. The resulting parameters are D/kB = 7.6±0.4 K, z0J/kB = −4.5±0.3 K, g∥ = 2.26±0.03 and g⊥ = 2.34±0.03. The susceptibility has also been measured parallel to the tetragonal axis at constant temperatures between 0.3 and 2 K as a function of external field. Magnetic ordering is found to occur, and portions of the phase boundary between paramagnetic and antiferromagnetic states have been determined.
Matrix interactions with rare-gas oxide excimers
Authors: Taylor R. V. , Scott W. , Findley P. R. , Wu Zenglie , Walker W. C. , Monahan K. M.
Published: February 2003

Matrix interactions of rare-gas oxide molecules formed in solid matrices were studied by photoluminescence. Excitation energies between 8–12 eV were provided by both synchrotron radiation at the Stanford Synchrotron Radiation Laboratroy (SSRL) and a hydrogen discharge source at UCSB. Photoluminescence emission and excitation spectra of Ar:CO2(1%), Kr:CO2(1%), Kr:N2O(1%), and Kr:O2(1%) mixtures were obtained. Radiative lifetimes of ArO and KrO emissions from CO2 doped Ar and Kr matrices were also measured. The direct photodissociative threshold of CO2 to CO(X 1S+)+O(1S) in an Ar matrix was found to be 10.6 eV, in agreement with that of the gas phase. Further unambiguous evidence for radiationless, dissociative energy transfer, between the matrix free exciton and CO2, probably by a harpooning mechanism, was seen in Kr:CO2 below the direct photodissociation threshold. Temperature, matrix annealing, and character of the photofragment were found to have significant effects on the rare-gas oxide molecular binding and production rate.
Calculation of Raman intensities for the ring-puckering vibrations of trimethylene oxide and cyclobutane. The importance of electrical anharmonicity
Authors: Bocian David F. , Schick G. Alan , Birge Robert R.
Published: February 2003

Raman intensities are calculated for the ring-puckering vibration of trimethylene oxide (TMO) and cyclobutane using an anisotropic atom–point dipole interaction model to calculate the elements of the molecular polarizability tensor. Three different models for the ring-puckering motion are examined: (i) a model in which the methylene groups are held rigid to the molecular frame as the ring puckers, (ii) a dynamical model in which the methylene groups rock (TMO and cyclobutane) and wag (TMO) as the ring puckers, and (iii) a second rigid model in which all of the polarizability of the molecule is localized on the atoms of the ring skeleton. All three models for the ring-puckering motion predict unusually large second-order terms in the expansion of the polarizability tensor elements in the ring puckering coordinate [‖∂2amn/∂Z2)0‖≳0]. These terms result in intense Dv = 2 overtone transitions. The calculated relative intensities of the members of the Dv = 2 overtone progression are in good agreement with those observed for both molecules. The calculations also predict that for TMO the intensities of the Dv = 2 overtones are comparable to the Dv = 1 fundamentals. This result when evaluated in terms of the expected differences in the band shapes of the two types of transitions readily accounts for the predominance of the overtone transitions in the Raman spectrum of TMO.
Effect of the oxygen protonation on the electronic structure of urea in the solid state: A 14N NQR study
Authors: Murgich Juan , Santana R. Magaly
Published: February 2003

The 14N NQR frequencies of urea complexes with H2O2 (1:1), NH4Cl (1:1), oxalic (2:1), phosphoric (1:1), and nitric acid (1:1) at 77 °K are reported. The analysis of the NQR data indicates that the population of the N nonbonding orbital decreases and that the population of the s N–H and N–C bonds increases as the degree of protonation of the O atom of urea increases. These changes are consistent with a larger weight of structures like C = N+H2 as the protonation increases. The NQR results are in agreement with those obtained from a CNDO/2 calculation for the uronioum ion [Yu. A. Panteleev and A. A. Lipovskii, Zhu. Struk. Khim. 17, 2 (1976)].
Ultraviolet absorption spectrum of nitrous oxide as a function of temperature and isotopic substitution
Authors: Selwyn Gary S. , Johnston Harold S.
Published: February 2003

The ultraviolet absorption spectra of nitrous oxide and its 15N isotopes over the wavelength range 197 to 172 nm and between 150 and 500 K show a weak continuous absorption and a pattern of diffuse banding that became pronounced at higher temperatures. The temperature dependence of the absorption spectrum results from the activation of the n″2 bending mode. Deconvolution of the data shows that absorption by molecules in the (010) vibrational mode results in a spectrum of vibrational bands superimposed on a continuum. A weaker and nearly continuous spectrum results from the ultraviolet absorption by molecules in the (000) vibrational mode. Analysis of the structuring indicates n′2 = (490±10) cm−1. No rotational structure can be observed. Measurement of the n′2 isotope shift is used to identify the quantum number of the upper state vibrational levels. Normal coordinate anaylsis of the excited state is used to determine a self-consistent set of molecular parameters: bond angle (115°), the values of n′1 and n′3 (1372 and 1761 cm−1, respectively), and the force constants of the upper state. It is suggested that the transitions observed are 1S−(1A′′)←X~¯ 1S+ and 1D←X̃ 1S+.
Long-range retarded interaction energies. I. Calculation of two-body coefficients
Authors: Peña M. Diaz , Pando C. , Renuncio J. A. R.
Published: February 2003

Expressions for the coefficients of the 1/r expansion of the retarded dipole–dipole interaction energy between two ground-state (nondegenerate) atoms at distance r are obtained in terms of the oscillator-strength sums. Values are calculated for the coefficients c3, c7, c9, and c11 of 66 atomic pairs formed from hydrogen, rare-gas, and alkali atoms. Lower and upper bounds are established for the coefficients c2 and c4 of the same 66 systems.
Long-range retarded interaction energies. II. Combination rules for two-body coefficients
Authors: Peña M. Diaz , Pando C. , Renuncio J. A. R.
Published: February 2003

Combination rules are derived from which the long-range retarded two-body coefficients for interactions between unlike atoms may be estimated in terms of the self-interaction coefficients and the oscillator-strength sums. The rules are tested by comparing their results with the coefficients obtained previously. Fifty-five systems involving H, He, Ne, Ar, Kr, Xe, Li, Na, K, Rb, and Cs are studied.