bluuebunny/crossref_metadata_2025 huggingface dataset:
Proton radiolysis of CHCl3 and CHBr3 at high dilution in argon during condensation at 15 °K. Infrared spectra of the CHX+2, CHX−2, and CHX−3 molecular ions
Authors: Ault Bruce S. , Andrews Lester
Published: February 2003

Matrix samples of chloroform and its deuterium and bromine analogs have been subjected to irradiation by a 2.0 keV proton beam during deposition, and the products trapped in an argon matrix at 15 °K. Several infrared absorptions have been observed and assigned to a number of new molecular species. Extremely photosensitive bands observed at 652, 621, 593, and 570 cm−1 have been assigned to the CHCl−3, CHCl2Br−, CHClBr−2, and CHBr−3 molecular anions, respectively. Absorptions at 998, 907, and 839 cm−1, which were decreased by operating an exposed bulb filament, have been attributed to the isolated cations CHCl+2, CHClBr+, and CHBr+2. Bands stable to the bulb filament at 1044, 988, and 898 cm−1 were assigned to the CHX+2 (X−) species and similar bands at 838, 795, and 735 cm−1 were assigned to the CHX2− anions. In addition, bands attributable to the species CX3, CX2CX3+(X−), CHX2, and HX−2, which have been identified previously, were observed here as well.
Calculated and experimental electronic structure of gaseous MnO3F and MnO3Cl
Authors: Jasinski Jerry P. , Holt Smith L. , Wood J. H. , Moskowitz J. W.
Published: February 2003

The gas phase electronic absorption spectra of MnO3F and MnO3Cl have been measured in the visible and near ultraviolet regions. Interpretation has been made by comparison with a molecular calculation for each molecule by the SCF–Xα–SW method in terms of C3v symmetry. The calculated ground and excited states and observed electronic transitions correlate directly with those for the parent MnO−4 cluster. The lowest excited state, 1Ea, which correlated with the first 1T1 state of MnO−4, retains the characteristics of the parent state. The charge transfer originating from the halide atom is first observed in the 1Ec excited state in each molecule correlating with the second lowest 1T2 state in MnO−4.
Fluorescence of monochlorobenzene in the vapor phase
Authors: Ichimura Teijiro , Hikida Takumi , Mori Yuji
Published: February 2003

The vapor phase fluorescence of monochlorobenzene has been measured in the presence of ethane, propane, or n-butane. The excitation wavelength effect on the fluorescence quantum yield indicates that there should be a fast nonradiative decay channel, an intermediate isomer, at around 240 nm. Pressure effect of a foreign gas on the fluorescence quantum yield is also studied and the nonradiative relaxation processes are discussed.
Direct vibrational excitation via charge exchange in symmetric ion–molecule collisions
Authors: Flannery M. R. , McCann K. J. , Moran T. F.
Published: February 2003

Integral and differential cross sections for vibrational excitation in symmetric H+2(X 2Σ+g, v′0) + H2(X 1Σ+g, v″0) collisions have been determined for incident kinetic energies E (keV) in the range 0.01<E<2.0. A multistate impact parameter description of the collision was used. At low kinetic energies, the inclusion of a relatively small number of states in the wavefunction expansion of the system achieves converged cross sections while at high energies a large number of product states are needed. The inelastic channels in this symmetric system are strongly coupled to each other and to the energy resonant charge exchange channels. Differential cross sections for excitation of individual product channels have been computed as a function of scattering angle within the multistate impact parameter approach. Small angle scattering within several degrees is dominant with forward inelastic scattering becoming more pronounced as the ion kinetic energy is increased. Integral cross sections for individual channels and integral cross sections summed over all product channels are presented as a function of both ion kinetic energy and reactant internal energy.
Study of reactive and inelastic collisions by time resolved gain spectroscopy. I. Chemical vibrational excitation and vibrational relaxation of HCl occurring with the reaction of hydrogen with chlorine
Authors: Menard-Bourcin F. , Menard J. , Henry L.
Published: February 2003

A method, which may be termed ’’time-resolved gain spectroscopy’’ is applied to the study of the reaction of hydrogen with chlorine. This method is a direct way to know at any time (with a precision of 1 μsec) the population of HCl molecules on the ground level or an excited level. At the early beginning of the reaction, the formation rate of molecules in various excited vibrational energy levels can be determined directly. The relative rate constants kv for the reaction to form HCl molecules on vibrational levels v=0–4 have been obtained. When the chemical reaction stops, some relaxation processes go along and may be studied: The rate constants were derived for the de-excitation of HCl from vibrational levels v=1–4 by V–T transfers occurring upon collisions with hydrogen molecules. The concurrent processes of reaction and deactivation, in the intermediate stage, were analyzed and a computer simulation of the complete kinetics was developed.
A new perturbation expansion for fluids of nonspherical molecules
Authors: Mo K. C. , Gubbins K. E.
Published: February 2003

The free energy of a fluid composed of nonspherical molecules is expanded about that for a reference fluid of rigid, nonspherical molecules. The expansion procedure involved separating the intermolecular pair potential into repulsive and attractive regions for each set of relative orientations, and is similar to that used by Weeks, Chandler, and Andersen for monatomic fluids. The properties of the reference system are evaluated using the expansion of Bellemans. Calculated thermodynamic properties are compared with Monte Carlo results for fluids with a potential uLJ + ua, where uLJ is the Lennard-Jones model and ua is either the point dipole, point quadrupole, or an anisotropic overlap potential. For dipoles, agreement is satisfactory for values of the reduced dipole moment μ/(εσ3)1/2 up to about 1, while for quadrupoles good agreement is found for Q/(εσ5)1/2 up to about 0.8. For larger multipole moments the Bellemans expansion fails. For anisotropic overlap potentials the method seemed satisfactory for all values of anisotropic strength tested. A simplified form of the perturbation expansion is also obtained by introducing a generalized van der Waals model. The resulting equation of state is tested against data for CO2 in the dense gas and liquid regions, including the vapor–liquid equilibrium region. The functional form of this equation gives an accurate representation of the data for all temperatures and densities except those in the critical region, and also for the low temperature dilute gas.
A random-walk simulation of the Schrödinger equation: H+3
Authors: Anderson James B.
Published: February 2003

A simple random-walk method for obtaining ab initio solutions of the Schrödinger equation is examined in its application to the case of the molecular ion H+3 in the equilateral triangle configuration with side length R=1.66 bohr. The method, which is based on the similarity of the Schrödinger equation and the diffusion equation, involves the random movement of imaginary particles (psips) in electron configuration space subject to a variable chance of multiplication or disappearance. The computation requirements for high accuracy in determining energies of H+3 are greater than those of existing LCAO–MO–SCF–CI methods. For more complex molecular systems the method may be competitive.
Low energy particle range
Authors: Anderson James B.
Published: February 2003

The penetration of molecules at thermal energies into target gases is examined with Monte Carlo techniques applied to successive collisions of projectile and target molecules. Calculations are reported for hard-sphere, inverse-12th, and inverse-6th repulsive interaction potentials under conditions relevant to low density gas flows in which range effects are important. Effects of molecular weight ratio, entering velocity, and target gas temperature on range distributions are determined systematically. Results are applicable in the study of a number of phenomena occurring in low density gas flows. Application of the techniques developed to prediction of diffusion coefficients for dilute gas mixtures is also demonstrated.
Pressure-broadened linewidths of ozone
Authors: Tejwani G. D. T. , Yeung Edward S.
Published: February 2003

Self-broadened and foreign-gas (N2 and O2) broadened linewidths of O3 at 300 °K for a wide range of quantum numbers J and Ka, for both type A and type B bands, have been calculated using the Anderson–Tsao–Curnutte theory of line broadening. In the case of O3–O3 collisions, dipole–dipole, dipole–quadrupole, quadrupole–dipole, and quadrupole–quadrupole interactions have been included. For O3–N2 and O3–O2 collisions, dipole–quadrupole and quadrupole–quadrupole interactions were taken into account. Good agreement was obtained between the available measured linewidths and the computed values for corresonding transitions on using the molecular quadrupole moments of O3 given by Rothenberg and Schaefer. Air-broadened linewidths of O3 at 200 °K have also been computed, so that the temperature dependence can be established.
Deuteron quadrupole coupling constants in CD3F and 1,3,5-C6D3F3
Authors: Bhattacharyya Pranab K. , Dailey Benjamin P.
Published: February 2003

The deuteron quadrupole coupling constants in the deuterated methyl fluoride and 1,3,5-trifluorobenzene molecules have been determined from nematic phase studies using a pulsed FT NMR spectrometer. The values obtained in the molecular symmetry axis system are as follows: χzz = −47.8±1.5 kHz (in CD3F) and χzz = −94.4±0.7 kHz (in 1,3,5-C6D3F3). If the field gradient tensor is assumed to be diagonal in the C–D bond axis system, the values of the coupling constants referred to the C–D bond are given by χCD = 133±7 kHz (in CD3F, η = 0.03±0.03) and χCD = 180±3 kHz (in 1,3,5-C6D3F3, η = 0.05±0.01).
Energy partitioning and assignment of excited states in the ultraviolet photolysis of HI and DI
Authors: Clear Robert D. , Riley Stephen J. , Wilson Kent R.
Published: February 2003

Photofragment spectroscopy has been applied to the photolysis of hydrogen iodide and deuterium iodide at 37 550 cm−1 (266.2 nm). The extent of excited I(2P1/2) atom production and the symmetry of the excited molecular states leading to dissociation are derived by measuring the translational energy and angular distributions of the recoiling H and D atoms. It is found that for HI, 36%±5% of the I atoms are formed in their excited state, while for DI the fraction is 26%±3%. In both cases, the transition leading to excited atoms is parallel, agreeing with Mulliken’s spectral interpretation. These results, together with measurements reported by others at different wavelengths, are used to analyze the first absorption continuum of HI and DI. This continuum is found to be consistent with overlapping transitions to 3Π1, 3Π0+, and 1Π excited states, as predicted by Mulliken. A nonlinear curve fitting technique is applied to the available data to separate each state’s contribution to the total continuum absorption and to provide estimates of the shape of each excited state potential energy curve.
Theoretical studies in photoelectron spectroscopy: Use of angular distribution data to deduce molecular orobital parameters. IV. Calculation of angular distributions for ejection of low-energy photoelectrons from molecules having 1sσ2g, 2pσ2g, or 2pσ2u configurations
Authors: Ritchie Burke
Published: February 2003

Angular distributions are calculated for the ejection of photoelectrons in the energy range of less than 1 eV to above 50 eV from molecules having 1sσ2g, 2pσ2g, or 2pσ2u configurations. The continuum wavefunction for the photoelectron is calculated in the ’’static-exchange’’ approximation of electron–molecular ion scattering theory by numerical integration of the one-electron integrodifferential equations appropriate for this approximation, accounting accurately for screening and exchange effects in the photoelectron–molecular ionic system. These equations are coupled in the radial partial waves relative to the center of mass of the nuclei, and they are solved both for the case of complete uncoupling and for the cases of coupled s and d waves, for ejection from a 2pσu molecular orbital, and of coupled p and f waves, for ejection from a 1sσg or 2pσg molecular orbital. Formulas are presented for the asymmetry parameter β for each of these cases. Provided enough data are available it may be possible to use these formulas to interpret experimental results in the absence of calculations. We study partial wave coupling in the photoelectron–molecular ionic motion. It is found that a molecular ionic core in the 1sσg state provides a field in which the coupling between partial waves is weak, even at fairly large internuclear distances. A core in the 2pσg state provides a field in which the coupling is weak to moderate, and a core in the 2pσu state provides a field in which the coupling is moderate to strong as a function of energy and internuclear distance. The possibility of the interpretation of the over-all qualitative features of angular distributions for many-electron molecules in terms of a paradigm of results for photoionization of a molecule with a filled nlσi shell is discussed with reference to experimental results for N2, and it is proposed that the effective charge parameter of the molecular orbitals belonging to the neutral and ionic systems and the bare nuclear charge could be treated as adjustable parameters to provide a crude but qualitatively correct description of the screening in the many-electron molecule.
Precision neutron diffraction structure determination of protein and nucleic acid components. XVII. Molecular and crystal structure of the amino acid glycine hydrochloride
Authors: Al-Karaghouli A. R. , Cole F. E. , Lehmann M. S. , Miskell C. R. , Verbist J. J. , Koetzle T. F.
Published: February 2003

The amino acid salt glycine hydrochloride, H3N+⋅CH2⋅COOH⋅Cl−, crystallizes in the space group P21/c, a=7.117(2) Å, b=5.234(2) Å, c=13.745(3) Å, β=97.25(1) °. The crystal structure has been refined from three-dimensional neutron diffraction data to a final R value based on F2 of 0.058. Estimated standard deviations in bond distances and angles are approximately 0.002 Å and 0.2°, respectively. There is a network of hydrogen bonds with the chloride ions linking three neighboring glycines to form layers parallel to the ab plane. One ammonium group hydrogen is involved in two weak electrostatic interactions; these interactions join the single layers to form parallel double layers but are probably too weak to be termed hydrogen bonds.
Short range electrostatic interactions in dielectric media
Authors: Buff Frank P. , Goel Narendra S. , Clay John R.
Published: February 2003

The electrostatic theory for a dielectric sphere with arbitrary charge distribution within it is presented. The sphere is located in a dielectric medium which either contains another similar sphere or is separated from another dielectric medium by a planar interface. It is shown that the case of two dielectric spheres is simply related to that of a sphere near a planar interface. The cases of a point charge or a point dipole at the center of the sphere are analyzed in detail. For the point charge case, the numerical results for both the electrostatic potential energy and forces for a wide range of values of the ratio of dielectric constant of sphere to that of medium are given. In addition, approximate analytical expressions are given and constrasted with the exact numerical results. The asymptotic expression occasionally used is shown to provide a good approximation for equal and oppositely charged cavities embedded in a medium of very high dielectric constant. An analytical expression, similar in structure to the exact one for metallic spheres, is derived and shown to provide an excellent approximation over the whole range of dielectric constants and arbitrary charge combinations. Approximate expressions using Padé approximants and simple variational approaches are also given. For the point dipole case, extensive numerical results are presented. It is shown that, for the case of a dielectric cavity, embedded in a medium of high dielectric constant and in contact with another similar cavity with the dipoles oriented along the lines of centers, the potential energy can be expressed in terms of tabulated analytical functions. This also applies for a dielectric cavity in contact with a planar dielectric interface with dipoles oriented perpendicular to the interface.
The scattering of noble gas atoms (He, Ne, Ar) from (100) copper. (Space and velocity distribution)
Authors: Lapujoulade J. , Lejay Y.
Published: February 2003

The scattering of He, Ne, and Ar from (100) copper has been studied experimentally. Both space and velocity distributions were measured. Special attention was paid to the cleanliness of the surface and to the effect of possible surface contamination. The influence of the beam incidence, the crystal azimuth, and the temperature are discussed. The behavior of the reflection lobe is in agreement with the phenomenological description of Merrill. The velocity distribution suggests that there is always superposition of elastic and inelastic scattering. Many-phonon processes predominate in the scattering of Ne and Ar, while the helium case may be described by an elastic, single-phonon process. A comparison of these experimental data with existing theoretical calculations (many-phonon by G. Armand, single-phonon by F. O. Goodman) is given.
Negative ion formation, vibrational excitation, and transmission spectroscopy in hydrogen halides
Authors: Ziesel J. P. , Nenner I. , Schulz G. J.
Published: February 2003

Three independent experiments are performed in order to elucidate low-energy electron collisions on hydrogen halides. The formation of Cl− and Br− shows a steep onset near threshold and structure at higher energies. This structure occurs at the energies of vibrational levels of the X 1Σ+ ground state of the hydrogen halide. The transmission spectrum of HBr shows structure at the position of the v=1 level of the X 1Σ+ state. In addition, transmission spectra show that the vibrational cross section to v=1 in all the hydrogen halides rises steeply near threshold and levels off about 60 meV above threshold. A trapped-electron experiment in HCl substantiates this observation and in addition, gives the order of magnitude of the vibrational cross section near threshold (2×10−16–2×10−15 cm2).
Solvation process of the trapped electron in polar liquids
Authors: Tachiya M. , Mozumder A.
Published: February 2003

The solvation process of the trapped electron in polar liquids is investigated within the framework of the Tachiya–Mozumder model of pre-existing traps. The orientation of the molecular dipoles and the energy level of the electron are calculated, in a quasistationary approximation, as a function of time after the electron is localized in a pre-existing trap. An estimate of the solvation time is provided.
A quantum kinetic theory of moderately dense gases. III. The effect of bound states on the transport coefficients
Authors: Olmsted R. D. , Curtiss C. F.
Published: February 2003

An approximate solution to the set of coupled integro-differential equations discussed previously is considered. The classical limit of the transport coefficients is obtained and a comparison of the results with previous theory is made.
Sensitized fluorescence in crossed atomic beams: Hg(6 3P0,2)+Tl(6 2P1/2)
Authors: Loh Lambert C.-H. , Sholeen Charlotte M. , Herm Ronald R. , Parrish David D.
Published: February 2003

A velocity selected Tl beam has been crossed by an electron-bombarded Hg beam which contains a small percentage of the Hg(6 3P) metastable levels (an approximate 5:1 ratio of 3P2:3P0). By means of an interference filter–photomultiplier combination beneath the collision zone, the cross section for excitation of the 5350 Å (7 2S1/2 → 6 2P3/2)Tl fluorescence has been determined to increase monotonically with decreasing relative collision speed, g, approximately as g−s for s=2.0±0.5. Implications of these results for the sensitized fluorescence phenomenon in vapor mixtures of metallic elements are briefly discussed.
Tetroxo anions of hexavalent technetium and rhenium
Authors: Astheimer L. , Hauck J. , Schenk H. J. , Schwochau K.
Published: February 2003

[(CH3)4N]2MnO4, [(CH3)4N]2TcO4, and [(CH3)4N]2ReO4 are isotypic and crystallize in an antifluoritelike cubic face centered lattice with a=11.09 Å (Mn), 11.20 Å (Tc), and 11.35 Å (Re). The magnetic susceptibilities obey the Curie–Weiss law over the temperature range 77–298 °K with μeff=1.63 μB (Mn), 1.60 μB (Tc), and 1.47 μB (Re). The magnetic moments of [(CH3)4N]2TcO4 and [(CH3)4N]2ReO4 as well as the splitting and vibrational structure of the 2E→2T2 intrashell transition indicate strong Jahn–Teller distortion of TcO42− and ReO42− in the first excited 2T2 state. Tetrahedral crystal field parameters of Δ=−11 kK, Δ=−17 kK, and Δ=−19 kK are derived for MnO42−, TcO42−, and ReO42−. The electron transfer spectra reflect the lower degree of covalency of the Re–O bond in comparison to the Tc–O and Mn–O bond.
Validity of central field approximations in molecular scattering: Low energy CO–He collisions
Authors: Monchick L. , Green Sheldon
Published: February 2003

Close coupled calculations have been carried out on collisions of helium and carbon monoxide interacting via a theoretical interaction potential which is believed to accurately reproduce the true interaction of this system. These are compared with an equivalent set of calculations for the spherical average of this potential, V0. We conclude that the latter approximation holds reasonably well for transport property calculations but not for differential and total scattering cross sections. As a consequence, conservation of scattering cross section theorems that are based on V0 do not hold well.
Spectra and structure of small ring compounds. XXXII. Ring inversion potential function for 7-oxabicyclo(4.1.0)hept-3-ene and 7-oxabicyclo(4.1.0)hept-2-ene
Authors: Durig J. R. , Lopata A. D. , Carreira L. A.
Published: February 2003

The infrared and Raman spectra of gaseous 7-oxabicyclo(4.1.0)hept-3-ene and 7-oxabicyclo(4.1.0)hept-2-ene have been recorded between 50 and 3300 cm−1. The low frequency ring puckering mode of these molecules has been treated as the ring inversion of a pseudo-four-membered ring and the one dimensional potential function describing this motion is found to have considerable asymmetry due to the presence of the epoxide ring. A series of Q branches beginning at 122 cm−1 in the 7-oxabicyclo(4.1.0)hept-3-ene spectra has been fit to a potential function of the form V (x) =3.238×104x4−0.710×104x2+0.958×104x3. A series of Q branches beginning at 187 cm−1 in the 7-oxabicyclo(4.1.0)hept-2-ene spectra has been fitted to a potential function of the form V (x) =3.948×105x4−0.391×105x2+0.172×105x3.
ψ2,0 in the Fock expansion for He 1S state wavefunctions
Authors: David Carl W.
Published: February 2003

The coefficients of ψ2,0 in the Fock expansion for 1S states of two electron atoms and ions are obtained.
Molecular dynamics study of the structure and thermodynamic properties of argon microclusters
Authors: Briant C. L. , Burton J. J.
Published: February 2003

Microclusters of 2–100 argon atoms were studied using molecular dynamics. The microclusters were ordered solids at low temperatures and energies and disordered liquids at high temperatures and energies. The melting transition occurred considerably below the bulk melting temperature. Radial density functions, interference functions, diffusion coefficients, and surface energies were calculated for both the solid and liquid phases. The surface energies of the microclusters could be expressed as a function of the form Ai2/3+Bi1/3 where i is the number of atoms in the cluster. For 0 °K solid clusters A and B have the values 0.26×10−12 and −0.12×10−12 erg, respectively; for liquid clusters at 40 °K, A and B have the values 0.17×10−12 and −0.045×10−12 erg, respectively. Clusters containing 7 or more atoms melted with the occurence of a first-order-like transition. This transition was studied further through Monte Carlo calculations. A possible model for the first-order-like transition is proposed.
Solute and solvent effects on chemical equilibria
Authors: Ben-Naim A.
Published: February 2003

Using a simplified version of a result due to Kirkwood and Buff, a general formalism is developed for the solute and solvent effect on chemical equilibrium. A specific example—an association reaction—is worked out in some detail to detect the nature of the various factors that combine to induce a shift of the equilibrium in one direction or another.
The delayed absorption of microwaves due to electron thermalization in nanosecond pulse irradiated N2, He, and Ar at atmospheric pressure
Authors: Warman John M. , de Haas Matthijs P.
Published: February 2003

The absorption of microwaves in pulse irradiated C2H6 (ethane), N2, He, and Ar at atmospheric pressure has been investigated on a nanosecond timescale. In the case of C2H6, a time of less than 1 nsec following the pulse is required for the absorption signal to reach a time independent plateau value. For N2, He, and Ar, a considerable delay in the development of the absorption signal is observed. This increase in absorption coefficient over 20, 50, and 300 nsec in the case of N2, He, and Ar, respectively, is attributed to a time dependence of the electron collision frequency resulting from post-pulse thermalization of the electron energy. Using momentum transfer collision frequency data from swarm studies, the time dependence of the electron energy has been derived and is discussed in terms of the energy exchange rate coefficient Ku, defined by −d?/dt=Ku [?−(3/2) kT]N. For N2, Ku is found to be almost independent of electron energy over the range kTe=0.03–0.7 eV and has a mean value of 1.55×10−11 cm3 sec−1. For He, Ku increases from 0.25×10−11 cm3 sec−1 at kTe?0.03 eV to 1.3×10−11 cm3 sec−1 at kTe?1.5 eV. The results are compared with energy exchange parameters from other sources. A further growth of absorption signal over several hundred nanoseconds in He is attributed to Penning ionization of impurities.
Molecular photoionization. I. Development of a computational method
Authors: Chapman Frank M.
Published: February 2003

A computational method for calculating photoionization cross sections and photoelectron angular distributions is presented. The method employs a partial wave expansion for the continuum wavefunction to generate a set of coupled radial equations which are solved exactly using a noniterative integral equation technique. In addition, single center expansions are used for the initial and final electronic wavefunctions. The convergence properties of the partial wave expansions are studied for hydrogen molecule–ion where results are presented for a number of different bond lengths. The results reported here are in good agreement with exact calculations for this system. As a result of these preliminary calculations, it appears that the method is capable of handling photoionization calculations for moderately complex molecular systems.
Numerical comparison between electronically adiabatic and diabatic representations for collinear atom–diatom collisions
Authors: Zimmerman I. Harold , George Thomas F.
Published: February 2003

Within the framework of quantum coupled-channel calculations on nonreactive atom–diatom collision systems involving two electronic potential surfaces, we test the electronically adiabatic representation obtained within the diatomics-in-molecules approximation against a similarly derived diabatic representation. The testing is numerical, with a view toward establishing computational advantages of one representation over the other with regard to (a) convergence of computed probability as the diatomic molecular basis is expanded; (b) convergence with respect to the starting position of the numerical integration; (c) convergence with respect to the distance between stabilizations; and (d) convergence with respect to the integration steplength. The adiabatic representation shows definite advantages in tests (a) and (b), but they are not sufficient to offset the much more rapid execution time possible with the diabatic basis. The significance of these results is discussed.
Approximations for the rotational excitation of molecules by atoms
Authors: Chu Shih-I , Dalgarno A.
Published: February 2003

The applicability of the effective close-coupling approximation of Rabitz and the centrifugal decoupling approximation of McGuire and Kouri is examined for a system which models the rotational excitation of molecular nitrogen in collisions with helium atoms. For small values of the rotational quantum number both methods are more accurate for total elastic scattering cross sections than for inelastic and neither is capable of providing reliable inelastic cross sections close to the rotational threshold. Agreement with the close-coupling cross sections improves with increasing impact energy. The accuracy of the centrifugal decoupling approximation appears to be stable with respect to the magnitude of the repulsive anisotropy whereas the effective close-coupling method is sensitive to it and the approximation becomes inaccurate when the anisotropy is large. The detailed-balancing relationship is not satisfied by the effective close-coupling method and modifications that depend upon energy and upon the magnitude of the anisotropy are needed.
Vibronic coupling in polyenes: High resolution optical spectroscopy of 2,10-dimethylundecapentaene
Authors: Christensen Ronald L. , Kohler Bryan E.
Published: February 2003

High resolution fluorescence and absorption spectra of 2,10-dimethylundecapentaene in 4.2 °K n-nonane matrices are presented. Analysis of these spectra indicates that the transitions between the ground and first excited state are induced by low frequency, nontotally symmetric vibrations. This is in agreement with Herzberg-Teller expectations for forbidden transitions and supports the 1Ag assignment for the lowest lying excited singlet state. Vibrational analysis of the absorption indicates increases in the carbon–carbon double bond stretching frequencies in the 1Ag state. This is unexpected given the decreases in carbon–carbon double bond orders predicted by molecular orbital calculations. Rationalization of this apparent discrepancy may be found in substantial mixings of excited state vibrations. The photochemical implications of these findings also are discussed.
Theoretical determination of the barriers to internal rotation in hydrogen peroxide
Authors: Dunning Thom H. , Winter Nicholas W.
Published: February 2003

The cis and trans barriers to internal rotation in hydrogen peroxide have been accurately determined within the Hartree–Fock approximation using a [4s3pld/2slp] basis set. The calculated trans barrier (384 cm−1) differs from experiment (386 cm−1) by only 2 cm−1. The cis barrier is not well determined by experiment, and the calculated value (2921 cm−1) may well be the most accurate value reported to date. The effects of geometry optimization and polarization functions were also studied. The calculations clearly illustrate the importance of determining the Hartree–Fock geometry for a suitable molecular configuration. With the exception of the OOH bond angle, optimization of the geometrical parameters as the molecule rotates was found to be of little importance; the increase of 6° in &OOH upon rotation from the cis to the trans configuration lowers the cis barrier by 653 cm−1. It was found that the inclusion of oxygen 3d and hydrogen 2p functions was essential for describing the trans barrier, without which the barrier height was just 54 cm−1. In addition, polarization functions lowered the cis barrier by 675 cm−1.
EPR and ENDOR studies of CO2− centers in x- and uv-irradiated single crystals of sodium formate
Authors: Dalal N. S. , McDowell C. A. , Park J. M.
Published: February 2003

Using ENDOR we have characterized the 23Na and 1H hyperfine structure of the CO2− radical ion in sodium formate single crystals. In contrast to previous studies we have detected CO2− in uv irradiated samples. ENDOR studies of the 23Na hfs together with CNDO calculations indicate that the CO2− forms a tight ion pair with the nearer Na+ cation, thus explaining the four line EPR hfs observed. Studies of the temperature and angular dependences of the magnetic resonance parameters suggest that in NaHCO2 the CO2− radical anion is held rigidly in the lattice. Hf interactions have been resolved for four pairs of nearest neighbor protons and show considerable covalency in the NaHCO2 structure, in agreement with recent SCF ab initio calculations on CO2−, which show that the unpaired electron is in a diffuse orbital. Extra so-called ’’forbidden’’ lines have been observed in the 23Na ENDOR and a model of ENDOR enhancements involving cross relaxation with other paramagnetic species is discussed.
Number fluctuation correlation function for light scattering in crossed laser beams
Authors: Duncan G. C. , Olschewsky M. R.
Published: February 2003

The number fluctuation correlation function for light scattered by particles in crossed, equal intensity laser beams is derived. The beams are assumed to possess intensity profiles that are respectively Gaussian transverse to the directions of propagation. A further Gaussian dependence introduces the effect of the detection slit. The general structure of the correlation function is derived and two specific special cases are considered: (1) uniform translation at a known velocity, and (2) Gaussian distribution of velocities. It is concluded that the crossed-beam technique can yield a determination of the x component of the velocity in both cases. A major utility of the crossed-beam number fluctuation technique is that it would allow one to probe the dynamics of particles in the regime intermediate between the volume-determined lengths and the interference fluctuations-determined length.
Band structure, crystal conformation, and hydrogen bond potentials for solid HF
Authors: Zunger Alex
Published: February 2003

The electronic structure of hydrogen fluoride chains is calculated using the small-periodic-cluster approach by representing some high-symmetry points in the Brillouin zone of the infinite solid by the one-electron energies of a finite periodic structure. The LCAO representation is used for the crystal orbitals, and a self-consistent calculation is performed. Problems regarding the convergence of the band structure as a function of the number of K-grid points used to construct the Hartree–Fock elements, the number of interacting neighbors, and the approach to self-consistency in the iteration cycle are examined. Band energies, ionization potential, cohesive energy, charge, and electrostatic potential distribution are computed. The adequacy of other methods currently used to investigate electronic and structural problems in hydrogen-bonded solids is discussed in view of the results obtained. The crystal structure is optimized, and the stability of the crystal against unit cell deformations and atom displacements is examined. Potentials for either collective or single proton movements are computed and discussed in view of the experimental vibrational force constants.
Infrared spectroscopic evidence for the coexistence of two molecular configurations in crystalline fatty acids
Authors: Hayashi Soichi , Umemura Junzo
Published: February 2003

Infrared spectra of 14 normal fatty acids have been obtained in the range of temperatures from room temperature to liquid-helium temperature. Great temperature dependencies are observed in their spectra, especially in the regions of the characteristic frequencies of the carboxyl group, and the band progression of the CH2 wagging modes. Many bands decrease in intensity with decreasing temperature and disappear near liquid-helium temperature. Close to the positions of these lost bands, alternative bands, which increase in intensity with decreasing temperature, are found. The results support the postulate proposed in our earlier work that the two distinct configurations, cis and trans forms for Cβ–Cα–C=O group, coexist in the crystalline state, and indicate that (1) the cis configuration is the lower energy (enthalpy) form for the odd acids (C3–C11) and for the acids containing four or six carbon atoms; and (2) the trans configuration is the lower energy form for the even-numbered (C8–C20) carbon series. Differences of enthalpy and entropy between cis and trans forms of CH3(CH2)12COOH are estimated to be about 200 cal/mole and 3.2 cal/(mole deg), respectively.
Ne–H–H potential energy surface including electron correlation
Authors: Birks John W. , Johnston Harold S. , Schaefer Henry F.
Published: February 2003

An ab initio calculation of the Ne–H–H potential energy hypersurface for a wide range of H2 internuclear separation (0.8⩽R⩽5.0 bohr) and Ne–H2 separation (2.5⩽X⩽6.0 bohr) is reported. Calculations were carried out for both the collinear and perpendicular bisector geometries. For the perpendicular bisector geometry, the potential surface has the property previously found for He–H2 of exerting a contractive force on the H2 molecule as the noble gas atom approaches. The surface demonstrates a crossover point near 3.0 bohr where the contractive force changes to a stretching force. Analytic fits to the potential energy surface are presented and discussed in detail. In particular, the ’’dumbbell’’ model was not found to provide a good description of the potential surface. Because of the wide range in values of X and R for which points were calculated, the surface reported here should be useful for trajectory studies of diatomic dissociation and atom recombination.
The electronic structure of dialkylbenzenes
Authors: Lipari N. O. , Duke C. B.
Published: February 2003

A new CNDO model parametrization, differing from earlier ones primarily by its embodiment of large orbital exponents for conjugated carbon atoms, is proposed for the description of the electronic spectra of aromatic hydrocarbons. In contrast to its predecessors, the new model is shown to describe both the ultraviolet absorption and the valence electron eigenvalue spectra of methyl substituted benzenes. The predictions of this model are compared with those of earlier molecular-orbital calculations (Hartree–Fock, CNDO/2, and SPINDO) and with the ultraviolet-photoemission and ultraviolet-absorption spectra of benzene, p-xylene, m-xylene, and 1,4-bis(trifluoromethyl) benzene. The model also correctly predicts the symmetry of the radical anion ESR spectra of those compounds for which they have been measured.
The electronic structure of polyacenes: Naphthalene through pentacene
Authors: Lipari N. O. , Duke C. B.
Published: February 2003

The new ’’spectroscopic’’ CNDO/S2 model parametrization, constructed to describe the electronic excitation spectra of conjugated hydrocarbons, is applied to determine the electronic structure of naphthalene, anthracene, naphthacene (tetracene), and pentacene. The occupied and virtual orbital eigenvalue spectra are evaluated. Comparison of the former with observed ultraviolet photoemisiion (UPS) spectra reveals that the predicted energies of the highest-energy two or three filled orbitals are in quantitative (ΔE≲0.1 eV) correspondence with these spectra. The ordering of the π-electron levels agrees with earlier assignments, and the deeper levels (both π and σ) are in qualitative (ΔE≲0.5 eV) correspondence with the UPS spectra. The energies and nature of both singlet and triplet transition states are evaluated using configuration-interaction analyses of appropriate dimensionality. All three singlet levels for each molecule agree with observed optical spectra to within ΔE?0.1 eV. The ordering and splittings of the triplet levels correspond well with the available experimental assignments.
Inelastic collisions between an atom and a diatomic molecule. I. Theoretical and numerical considerations for the close coupling approximation
Authors: Choi B. H. , Tang K. T.
Published: February 2003

The close coupled differential equations for rotational excitation in collisions between an atom and a diatomic molecule are reformulated. Although it is equivalent to other formulations, it is computationally more convenient and gives a simpler expression for differential cross sections. Questions concerning real boundary conditions and the unitarity of the S matrix are discussed. Stormer’s algorithm for solving coupled differential equations is introduced for molecular scatterings. This numerical procedure, which is known to be very useful in nuclear scattering problems, has to be modified for molecular systems. It is capable of treating the case where all channels are open as well as the case where some of the channels are closed. This algorithm is compared with other typical procedures of solving coupled differential equations.
Flow through charged membranes
Authors: Mehta G. D. , Morse T. F.
Published: February 2003

A theoretical analysis of transport through charged membranes is presented using a cell model. In an attempt to improve the capillary tube model, we have modeled the membrane by an array of charged spheres. Three sets of partial differential equations describe this system: the generalized Nernst–Planck flux equations, the Navier–Stokes equation, and the Poisson–Boltzmann equation. These equations are averaged over a cell volume to yield a set of ordinary differential equations on the gross scale, that is, a length scale of the order of the membrane thickness. It is shown for hyperfiltration that the cell model will reject salt more efficiently than the tube model, and in the limit of Stokes’ flow the present analysis reproduces the rejection coefficient previously obtained for the capillary tube model. Results for the electrodialysis mode of membrane operation are also presented. Comparison is made with the capillary tube model for the same total charge and equal volume to surface ratio. The effect of a spatially varying wall charge density on membrane performance is discussed.
The polarizabilities of CH and CC bonds
Authors: Amos A. T. , Crispin R. J.
Published: February 2003

A variant of uncoupled Hartree–Fock perturbation theory is described which uses as zero-order function a Hartree–Fock wavefunction built up from localized orbitals. The second-order energy has the form of a sum of contributions from the individual orbitals which, since the orbitals are typically localized along bonds, can be interpreted as the sum of bond contributions. A variational procedure for computing these bond contributions is derived and applied to finding the components of the electric polarizability tensor for the CH and CC bonds in the molecules methane, ethane, ethylene, and acetylene.
Transient effects in time-resolved infrared-microwave double resonance of ammonia
Authors: Dobbs G. M. , Micheels R. H. , Steinfeld J. I. , Wang John H.-S. , Levy Joel M.
Published: February 2003

A number of time-dependent absorption phenomena in laser-pumped infrared-microwave double resonance in ammonia have been studied in detail. A Stark-modulated, infrared-laser enhanced transient nutation experiment permits simultaneous measurement of both T1 and T2 for the (8,7) microwave inversion doublet. The result is 1/2πT1 = [(25.4±2.0) p+0.028] MHz and 1/2πT2=[(24.5±2.0) p + 0.087] MHz, where p (NH3) is in torr. Four-level double-resonance experiments allow a separate measurement of the inversion relaxation rate in the (9,7) absorption doublet, which is kβ= (12±3) ×106 sec−1⋅torr−1. The effect of partial saturation within the infrared Doppler velocity profile has also been investigated.
The Raman spectrum and phase transition in s
y
m-triazine
Authors: Elliott G. R. , Iqbal Zafar
Published: February 2003

The complete Raman spectrum of polycrystalline sym-triazine has been observed at different low temperatures. The Raman active molecular vibrations have been assigned on the basis of the published normal coordinate analysis of the free molecule, while the external Raman active vibrations at 293 °K have been assigned in terms of molecular librations and translations, based on a model lattice calculation of frequencies and Raman intensities. Accurate temperature dependent Raman measurements of the splitting of the internal mode at 1550 cm−1 coupled with differential thermal calorimetric observations confirm the existence of a phase transition of second order character at T0=196±1 °K. The transition proceeds continuously below T0 as shown by sizable shifts of certain phonon frequencies with temperature. The temperature dependences are consistent with the transition displacements proposed by Coppens and Sabine, but do not show the existence of a soft Cochran type phonon.
Comment on ’’Electronic structure of UF−6’’ by M. Boring, J. H. Wood, and J. W. Moskowitz [J. Chem. Phys. 61, 3800 (1974)]
Authors: Edelstein N. , Karraker D.
Published: February 2003

It is pointed out that some of the data used for comparison with the calculation on the UF−6 ion in a recent paper by Boring, Wood, and Moskowitz is generally considered incorrect. This has no effect on the calculations or the main conclusions of the original paper. (AIP)
Rate constants for the quenching of N2(A 3Σ+u; v
A=1–6,8) by rare gases
Authors: Roy C. R. , Dreyer J. W. , Perner D.
Published: February 2003

The techniques of pulsed electron beam excitation and optical absorption spectroscopy have been used to produce and detect metastable nitrogen (A 3Σ+u) in the vibrational levels v=1–6,8. Second order quenching rate constants have been determined in the presence of added rare gases. The rate constants, in units of cm3/sec and for increasing values of v, are as follows: He (6.3±0.6) ×10−16; (3.3±0.4) (−15); (4.8±0.6) (−15); (7.2±0.7) (−15); (9.0±1.4) (−15); (1.2±0.3) (−14); (2.1±0.4) (−14). Ne (4.1±0.6) (−16); (4.3±0.6) (−16); (8.1±0.8) (−16); (2.3±0.2) (−15); (5.3±0.8) (−15); (6.1±1.0) (−15); (2.5±0.6) (−14). Ar (2.1±0.8) (−16); (3.0±0.4) (−16); (1.0±0.2) (−15); (3.9±0.3) (−15); (7.2±0.9) (−15); (4.5±0.9) (−15); (2.1±0.2) (−14). Kr no value; (4.5±0.5) (−15); (5.7±0.8) (−15); (5.4±0.8) (−15); (9.5±1.2) (−15); (2.3±0.3) (−14); (7.5±2.5) (−14). Xe no value; (4.2±0.4) (−15); (1.4±0.4) (−14); (9.9±0.7) (−14); (1.5±0.2) (−13); (1.1±0.3) (−13); (4.4±0.5) (−13). An attempt has been made to explain the results in terms of the standard energy transfer theories.
Anharmonic force field of NCO− isolated in KI
Authors: Smith D. Foss
Published: February 2003

The anharmonic force field of NCO− in a KI lattice has been recalculated using weighted data (instead of assuming unit weights). The new set of force constants was in much better agreement with force constants for NCO− in other lattices. It is concluded that high correlations between the quadratic force constants can cause errors in lattice calculations and a procedure is given to avoid this problem. (AIP)
Calculation of vibronic intensity of the formaldehyde 1A2←1A1 transition
Authors: Johnson W. Curtis
Published: February 2003

The absorption intensity of the electronically forbidden but vibronically allowed nπ* (1A2←1A1) transition of formaldehyde is calculated to illustrate the utility of the formalism involving floating basis orbitals detailed in the preceding paper. The general equations are further simplified by applying the independent systems approximation. The bulk of the intensity is predicted to be polarized in the plane and perpendicular to the CO bond as observed, and the calculated dipole strength of 2.1×10−3 a.u. compares well with the measured 2.3×10−3. This intensity is due to the electron distributions following the nuclear vibrations and is partly cancelled by the intensity which arises from the vibrations of the nuclei themselves. If fixed basis functions are used, intensity arises only from the vibrations of the nuclei. Most of the intensity due to the vibrating electron clouds is found to come from interactions with the overlap distributions of the CH bonds. When electron distributions are considered to simply screen nuclei, such interactions are lost.