bluuebunny/crossref_metadata_2025 huggingface dataset:
Sudden rotation reactive scattering: Theory and application to 3-D H+H2
Authors: Bowman Joel M. , Lee Ki Tung
Published: February 2003

An approximate quantum mechanical theory of reactive scattering is presented and applied to the H+H2 reaction in three dimensions. Centrifugal sudden and rotational sudden approximations are made in each arrangement channel, however, vibrational states are treated in a fully coupled manner. Matching of arrangement channel wave functions is done where the arrangement channel centrifugal potentials are equal. This matching is particularly appropriate for collinearly favored reactions. Integral and differential cross sections are calculated for the H+H2 reaction for H2 in the ground and first excited vibrational states. These calculations employ the Porter–Karplus potential energy surface mainly to allow for comparisons with previous accurate and approximate quantal and quasiclassical calculations.
The temperature dependence of absolute rate constants for the F+H2 and F+D2 reactions
Authors: Wurzberg E. , Houston P. L.
Published: February 2003

Multiphoton dissociation of SF6 has been used to generate a transient concentration of fluorine atoms in mixtures with argon and H2 or D2. Absolute rate constants for the F+H2 and F+D2 reactions have been determined as a function of temperature by monitoring the appearance rate of HF or DF product chemiluminescence. In the temperature range from 190 °K to 373 °K the results are fit by the expressions kH=1.0×10−10 exp(−(860±100)/RT) and kD=9.1×10−11 exp(−(1100±100)/RT), both in cm3 molecule−1 sec−1. These values are in rough agreement with those obtained recently using a similar technique. The value of the isotope effect kH/kD is in good agreement with two previously determined values.
Absolute rate coefficients for F+H2 and F+D2 at T=295–765 K.
Authors: Heidner R. F. , Bott J. F. , Gardner C. E. , Melzer J. E.
Published: February 2003

The rate coefficients of the F+H2 and F+D2 reactions must be accurately known over a wide temperature range if the HF and DF chemical lasers are to be properly modeled. Although the pulsed and cw chemical lasers operate at elevated temperatures (500 to 2000 K), no absolute rate data exist for T≳400 K. Extension of the infrared multiphoton dissociation–infrared fluorescence technique permitted the following Arrhenius equations to be determined between 295 and 765 K: kF+H2=(1.3±0.25)×1014 exp[−(1182±100)/RT]; kF+D2=(6.4±2.2)×1013 exp [−(1200±142)/RT]; kF+H2/kF+D2=(2.1±0.8) exp[(18±250)/RT].
A dimensional model for the calculation of thermodynamic properties of gaseous molecules and ions
Authors: Frurip D. J. , Blander M.
Published: February 2003

A dimensional analysis of the classical partition function for ionic molecules leads to simple universal relations for the calculation of nonelectronic entropies and free energy functions. The model is tested for a large number of species with different stoichiometries and degrees of ionicity (MX, MX2, M2X2, and ions MX±1). The success of these tests demonstrates the validity and usefulness of the model. Since the only quantities required for the calculation are the interatomic distance and atomic masses, this procedure can greatly simplify the calculation of these thermodynamic functions for an unknown species. As an illustration, calculated thermodynamic data are presented for the seven alkali halide dimers for which data are not available.
A computer model of molecular arrangement in a n-paraffinic liquid
Authors: Vacatello Michele , Avitabile Gustavo , Corradini Paolo , Tuzi Angela
Published: February 2003

A computer model of a bulk liquid polymer was built to investigate the problem of local order. The model is made of C30 n-alkane molecules; it is not a lattice model, but it allows for a continuous variability of torsion angles and interchain distances, subject to realistic intra- and intermolecular potentials. Experimental x-ray scattering curves and radial distribution functions are well reproduced. Calculated properties like end-to-end distances, distribution of torsion angles, radial distribution functions, and chain direction correlation parameters, all indicate a random coil conformation and no tendency to form bundles of parallel chains.
Vibrational spectrum of H and D on the (0001) graphite surface from scattering experiments
Authors: Ghio E. , Mattera L. , Salvo C. , Tommasini F. , Valbusa U.
Published: February 2003

We have investigated the vibrational spectrum of hydrogen and deuterium atoms adsorbed on the (0001) surface of a graphite single crystal by means of atomic beam scattering in UHV conditions. Bound state resonances, which appear as minima in the specularly reflected beam have been detected over a wide range of incidence and azimuthal angles. Energy levels at 31.6±0.2 and 15.3±0.3 meV for 1H and at 35.4 ±2; 21.35±0.1; 12.0±0.1, and 5.9±0.3 meV for 2H have been determined. By fitting the experimental energy levels to those calculated with the potential the following values of the parameters are obtained: D=43.3±0.5 meV; λ=1.36±0.03 Å−1; p=4.4±0.6. The hypothesis that the interaction potential is the sum over all crystal layers of an atom–single layer potential V*(z) is used. V*(z) has a well depth D*=39.2±0.5 meV and a long range behavior given by C*4/z4 with C*4=5.2±0.3 eV Å4.
Integrated infrared intensities and transition moments in SF6
Authors: Kim Kwan , McDowell R. S. , King W. T.
Published: February 2003

The integrated infrared band intensities of the fundamental vibrational modes in SF6 have been measured by the Wilson–Wells–Penner–Weber method. The results are 1361±34 km mole−1 for the fundamental at 948 cm−1, and 74±2 km mole−1 for the fundamental at 615 cm−1. The transition moments derived from these measurements are 0.437±0.005 D for the 948 cm−1 fundamental and 0.126±0.002 D for the 615 cm−1 band. The transition moment was also derived from single line strengths in the 948 cm−1 band and found to be 0.42±0.03 D, in good agreement with the value found from band intensities.
Rate constants for the reactions of metastable O+ ions with N2 and O2 at collision energies 0.04 to 0.2 eV and the mobilities of these ions at 300 K
Authors: Rowe B. R. , Fahey D. W. , Fehsenfeld F. C. , Albritton D. L.
Published: February 2003

The reactions of metastable O+* ions with N2 and O2 have been examined in a selected-ion flow-drift tube over the relative kinetic energy range from thermal to 0.2 eV. The rate constants are found to be (8.5±3.4)×10−10 and (8.1±3.2)×10−10 cm3 s−1, respectively, at these energies. These values are essentially equal to the gas-kinetic limiting values. The principal reaction mechanism in both cases is charge transfer. Based on molecular beam studies of the ion-source reaction thought to produce the present O+* ions, it appears likely that they are largely O+(2D°) ions. The present results for these reactions are in good agreement with the results of most of the earlier studies, which were limited to either thermal or elevated collision energies. However, the present data for O+*+N2 appear to conflict with the much smaller rate constant predicted for the reaction of O+(2D°)+N2 from analyses of satellite data. As a necessary part of this study, the mobilities of the O+* ions in helium were determined and are 7% to 11% less than those of the ground-state O+ ion.
Dipole moment functions of the hydrogen halides
Authors: Ogilvie J. F. , Rodwell W. R. , Tipping R. H.
Published: February 2003

Experimental intensity data for vibration–rotational transitions and molecular beam data of the hydrogen halide diatomic molecules (HF, HCl, HBr, and HI) have been analyzed, and the coefficients of a power series expansion of the dipole moment functions valid near the equilibrium internuclear separations have been determined. Ab initio quantum chemical computations of the dipole moments have also been carried out over a wide range of internuclear separations for HF, HCl, and HBr, and the factors affecting the accuracy of these results have been critically examined. The long-range values from these computations have been combined with the results near equilibrium in the form of a Padé approximant that embodies the correct asymptotic behavior.
On hyperspherical coordinates and mapping the internal configurations of a three body system
Authors: Johnson B. R.
Published: February 2003

A procedure for mapping the internal configurations of a three body system to points in a three dimensional ’’configuration space’’ is presented. The mapping is based on a modified version of the Smith–Whitten symmetric hyperspherical coordinate system. Several years ago, Kuppermann developed a mapping procedure based on a different hyperspherical coordinate system. Transformation equations relating these two mappings are derived. A comparison shows that the present method produces maps identical to Kuppermann’s method. The coordinate axes in configuration space, however, are rotated by 90 degrees so that the z axis is coincident with the axis of kinematic rotation.
N2–N2 repulsive interactions obtained through a combining rule
Authors: Lee Sangyoub , Kim Yung Sik
Published: February 2003

N2–N2 repulsive interactions are estimated from the Ar–Ar and Ar–N2 electron gas model repulsive potentials through the arithmetic mean combining rule. The results agree rather well with the the empirical interactions for the cases in which they are available.
On symmetry in the polarization expansion for intermolecular forces
Authors: Chipman Daniel M. , Hirschfelder Joseph O.
Published: February 2003

In the usual polarization expansion for intermolecular forces, exchange effects that determine the separations of energy levels within the manifold of interacting states are ignored. Previous low order calculations on simple physical systems have indicated that these exchange terms can be described reasonably well by an appropriate ad hoc symmetrization of the polarization wave function (the SYM-P method). But theoretical considerations suggest that the SYM-P method should be good for only one of the interacting states and not for the others in the manifold. Here this long standing apparent conflict between theoretical expectations and actual results is explained by consideration of a simple model system in which the relevant equations can be solved exactly. It is concluded that while the SYM-P method is potentially exact for only one of the interacting states, it may provide good approximations to the other states of the manifold in the case of large separations of the interacting subsystems.
NMR study of hydrogen diffusion and phase determination of the Mg2NiHx system
Authors: Goren S. D. , Korn C. , Mintz M. H. , Gavra Z. , Hadari Z.
Published: February 2003

We have measured the temperature dependence of the hydrogen spin lattice relaxation time in the hydride (β, β′, γ, δ) and solid solution phases of the Mg2NiHx system. These gave the temperature ranges of the various phases. The hydrogen diffusion activation energy for the β, β′, γ and δ hydride phases were found to be 10.6±0.4, ?17, 12.0±0.3 and ?20 kcal/mole, respectively, and 8.7±0.7 kcal/mole for the α phase solution.
Higher resolution photoionization study of NO near the threshold
Authors: Ono Y. , Linn S. H. , Prest H. F. , Ng C. Y. , Miescher E.
Published: February 2003

The photoionization efficiency curve for NO+ has been obtained at a resolution of 0.14 Å (FWHM) in the region 1189–1340 Å using the molecular beam method. The observed autoionization peaks can be satisfactorily correlated with the positions of the Rydberg levels (ns, npσ, npπ, ndσ, ndπ, ndδ, and nf) observed or calculated with the quantum defects deduced from previous absorption experiments. The analysis of the autoionization structure suggests that autoionization via Δv=−1 processess are not strongly favored in comparison to Δv<−1 processes in NO. The degree of relaxation of NO (2Π3/2) in a supersonic expansion has also been examined by comparing the relative intensity of the autoionization structure at different nozzle temperatures. A small step at 1341.5 Å which is approximately 124 cm−1 lower in energy that the first onset at at 1339.32 Å is attributed to the threshold for the NO (X 2 Σ+ v=0) ← NO (X 2Π3/2 v=0) transition. This observation is in agreement with the previous finding that the relaxation from 2Π3/2 to 2Π1/2 in a free jet expansion is efficient.
Nonionic surfactant mixtures. I. Phase equilibria in C10E4–H2O and closed-loop coexistence
Authors: Lang John C. , Morgan R. D.
Published: February 2003

A detailed phase diagram for mixtures of the homogeneous nonionic surfactant 3,6,9,12-tetraoxadocosanol (C10E4) with water is presented. Closed-loop coexistence is shown to be a prominent feature in this system as well as in three other aqueous mixtures of nonionic surfactant to which it is compared: 3,6,9,12,15-pentaoxapentacosanol (C10E5), decyldimethylphosphine oxide (C10PO), and dodecyldimethylphosphine oxide (C12PO). The sizes and locations of these curves are discussed in terms of a decorated lattice-gas model of closed-loop coexistence. The shape of one loop (C10E4) in the vicinity of its lower critical point is nonclassical; β is found to be 0.36±0.02. Sources of complexity in the diagram for C10E4–water are discussed.
A molecular beam study of the catalytic oxidation of CO on a Pt(111) surface
Authors: Campbell C. T. , Ertl G. , Kuipers H. , Segner J.
Published: February 2003

The oxidation of carbon monoxide catalyzed by Pt(111) was studied in ultrahigh vacuum using reactive molecular beam–surface scattering. Under all conditions studied, the reaction follows a Langmuir–Hinshelwood mechanism: the combination of a chemisorbed CO molecule and an oxygen adatom. When both reactants are at low coverage, the reaction proceeds with an activation energy E*LH =24.1 kcal/mole and a pre-exponential υ4 =0.11 cm2 particles−1 sec−1. At very high oxygen coverage, E*LH decreases to about 11.7 kcal/mole and υ4 to about 2×10−6 cm2 particles−1 sec−1. This is largely attributed to the corresponding increase in the energy of the adsorbed reactants. When a CO molecule incident from the gas phase strikes the surface presaturated with oxygen, it enters a weakly held precursor state to chemisorption. Desorption from this state causes a decrease in chemisorption probability with temperature. Once chemisorbed, the CO molecule then has almost unit probability of reacting to produce CO2 below 540 K. The CO2 product angular distribution varies from cosγ to cos4γ depending sensitively upon the adsorbed reactant concentrations.
Integrated infrared intensities and atomic polar tensors in fluoroform
Authors: Kim Kwan , King W. T.
Published: February 2003

The integrated intensities of the fundamental modes in HCF3 and DCF3 have been measured by Wilson–Wells–Penner–Weber method, and the atomic polar tensors were derived. The effective atomic charge for fluorine was found to be χF/e=0.598±0.018, in good agreement with that in other fluorocarbons. The effective atomic charge for hydrogen was found to be χH/e=0.049±0.006, a value somewhat lower than that found in other hydrocarbons and their derivatives.
Energetics of fragmentation of CH5, H3O, and NH4 from neutralized ion-beam experiments
Authors: Williams Brian Wesley , Porter Richard F.
Published: February 2003

Fragmentation energies for radicals of the type RH2 (RH=CH4, NH3, and H2O) produced by electron capture interactions of 5 keV RH2+ ion with Na or K atoms are reported. The experimental technique involves measurement of spatial beam profiles resulting from dissociation of neutral radicals following their formation in a near resonant electron transfer process. Cross sections for RH2+–Na capture reactions are typically 1x10−14 cm2. Fragmentation energies from measurements with Na target atoms are −2.65±0.14, −0.22±0.03, and −1.12±0.07 eV for CH5, NH4, and H3O, respectively. From our results with Na and K targets and published values for proton affinities, the vertical electron affinities of CH5+ and H3O+ are calculated to be 5.3±0.2 eV and 5.1±0.3 eV, respectively. Beam profiles for ND4 show this species to be metastable with a lifetime of about 1 μs. From this we estimate a potential barrier to dissociation in NH4(ND4) between 0.36 and 0.48 eV, indicating this species should be stable at low temperatures. Comparison of these experimental results with theoretical calculations indicates areas of disagreement.
The anisotropic interaction potential of D2Ne from state-to-state differential cross sections for rotational excitation
Authors: Andres J. , Buck U. , Huisken F. , Schleusener J. , Torello F.
Published: February 2003

Differential cross sections for the rotational excitation from j=0 to j=2 of D2 scattered by Ne have been measured at an energy of E=84.9 meV. The experiments have been performed in a crossed nozzle beam apparatus with time-of-flight analysis of the scattered particles using the pseudorandom chopper method. A detailed analysis of the experimental data which are peaked in the backward direction showed that they are mainly sensitive to the repulsive part of the pure anisotropic potential. From a combined analysis of the state-to-state differential cross sections of the j=0 to j=0 and the j=0 to j=2 transition of D2+Ne and the j=0 to j=1 transition of HD+Ne previously measured, the complete potential energy surface for the hydrogen–neon system is obtained using the coupled states method. The anisotropic contribution varies from 37% of the isotropic part in the repulsive region (2.4 Å) to 12% in the attractive region (3.5 Å). The results differ from the other potential models derived for this system from calculations, spectroscopic studies, and bulk properties. However, it is in agreement with the results of a recently developed inversion method based on the exponential distorted wave approximation using the same experimental data.
Pressure broadening and shifting in microwave and infrared spectra of molecules of arbitrary symmetry: An irreducible tensor approach
Authors: Leavitt Richard P.
Published: February 2003

Contributions to the interruption function S(b) used in semiclassical perturbation approaches to impact broadening and shifting of microwave and infrared molecular gas spectra are derived by irreducible tensor methods for molecules of arbitrary symmetry. Results are given explicitly for long-range intermolecular interactions: up to R−5 for the electrostatic interactions and to R−7 for the induction and dispersion interactions. A new contribution to S(b) is found that has no counterpart for the case of two interacting linear molecules; all other terms are appropriate generalizations of the linear molecules case. For the three cases of asymmetric tops, symmetric tops, and linear molecules, group-theoretical arguments are given that simplify the computation of certain reduced matrix elements that occur in the theory. Results are given in detail for the case in which both radiating and perturbing molecules are linear, and errors in the published literature concerning the contributions to S(b) in this case are corrected.
Transition moments and dynamic polarizabilities in a second order polarization propagator approach
Authors: Nielsen Egon S. , Jo/rgensen Poul , Oddershede Jens
Published: February 2003

We have formulated a polarization propagator approach which yields excitation energies, transition moments, and dynamic polarizabilities which are consistent through second order in the electronic repulsion. Certain terms are proven to be missing in our previous second order calculations of transition moments and dynamic polarizabilities and in the equation-of-motion calculations of the same quantities. Numerical calculations on carbon monoxide are performed. The calculations show that the major difference between the polarizability (and some transition moments) in the RPA and in the second order polarization propagator approximation is due to these terms. The total effect of all correction terms has been to improve considerably the agreement between theoretical and experimental estimates of the excitation properties for carbon monoxide.
Derivation of aerosol collision rates for singular attractive contact potentials
Authors: Marlow William H.
Published: February 2003

A rigorous derivation is given of the collision rate for spherical aerosol particles in the free-molecular regime interacting via an attractive potential which diverges at particle contact. The Lifshitz–van der Waals and electrical image potentials are two examples of such particle interactions that play important roles in aerosol dynamics. The method employed in the derivation of this extended free-molecular collision rate is then used to derive an expression for transition regime collision rates by means of Fuchs’ method. This is the first time that the influence of a particle interaction potential upon the collision rate has been incorporated into the Fuchs formula. Results of computations employing the extended free-molecular formula derived here are compared with published data and the computations used in their interpretation. By utilizing the same interaction potentials as in those interpretative computations, the corrected free-molecular formula is shown to improve the data fits.
On statistical and thermodynamic approaches to ion polar molecule collisions
Authors: Celli F. , Weddle G. , Ridge D. P.
Published: February 2003

Capture collision rate constants for ion polar molecule systems obtained from an average free energy function are shown to be equivalent to those obtained from a canonical activated complex theory formalism. Results provide a satisfactory least upper bound for observed reaction rate constants and compare well with Chesnavich, Su, and Bowers’ results obtained using microcanonical variational transition state theory. The average free energy approach is extended to the ion–quadrupole interaction, and capture collision rate constants are obtained providing a satisfactory least upper bound on experimental rate constants. A thermodynamic model for ion polar neutral collisions is developed and the difference between capture collision rate constants and momentum transfer collision rate constants rationalized in terms of a repulsive entropic force which prevents capture but produces isotropic scattering. The success of the results of Barker and Ridge in accounting for momentum transfer rates is thus explained and connected to the success of the average free energy result in accounting for reaction rate constants. It is argued that the agreement between theory and experiment indicates that rotational inelasticity plays a significant role in a gas phase transport property, i.e., the momentum transfer collision rate of ions in polar gases.
Nonequilibrium product distributions observed in the multiple collision chemiluminescent reaction of Sc with NO2. Perturbations, rapid energy transfer routes and evidence for a low-lying reservoir state
Authors: Gole J. L. , Pace S. A.
Published: February 2003

Nitrogen dioxide reacts with scandium to yield the B 2Σ+–X 2Σ+ spectrum of ScO. This reaction has been characterized from 10−5 to 1 Torr in order to study relaxation and rapid intramolecular E–E transfer among ScO excited states. At the lowest pressures, a ground state metal atom interacts with a tenuous atmosphere of oxidant gas (beam-gas configuration). These ’’single collision’’ studies are extended in a controlled manner to higher pressure by entraining the metal atoms in argon and subsequently carrying out the oxidation of this mixture. At all pressures, the measured B 2Σ+ vibrational populations follow a markedly non-Boltzmann distribution. At the lowest pressures, the formation of ScO B 2Σ+ results directly from the reaction Sc+NO2→ScO*+NO. At higher pressures, the B 2Σ+ state is also populated via rapid intramolecular energy transfer from long-lived, weakly emitting ’’reservoir’’ states via the sequence Sc+NO2+Ar→ScO(res)+NO+Ar and ScO(res)+Ar→ScO(B 2Σ+)+Ar. Spin orbit and Coriolis interactions in ScO connect rovibronic levels of B 2Σ+ and low-lying 4Πr or 2Πi reservoir states resulting in the observation of substantial perturbations in B 2Σ+. Collisional energy transfer is particularly efficient for the most strongly perturbed levels of the B2Σ+ state. This energy transfer is manifest by the appearance of ’’extra’’ band heads representing normally forbidden (small electronic transition moment or Franck–Condon factor) ’’reservoir state’’– ground state transitions which become allowed because of a small admixture of B 2Σ+ character. The relative intensities of the extra and ’’main’’ B 2Σ+–X 2Σ+ transitions are strongly dependent on argon buffer gas pressure. A quantitative description of this dependence gives an estimate for the amount of mixing between the reservoir state and B 2Σ+ and for the rate of energy transfer between these two states. Collisional transfer to ScO B 2Σ+ v′=3–9 s found to proceed at rates which for certain levels approach 100 times gas kinetic. The effects observed in ScO demonstrate that the excited states of this molecule interact in the presence of a collision partner as if they were large diffuse entities. These effects are not pathological. This behavior may have important implications for the modeling of energy systems as well as the ability to create population inversions requisite for the construction of visible chemical laser systems.
The application of an atomic effective potential to the electronic structure and bonding of Si2
Authors: Moskowitz Jules W. , Topiol Sid , Snyder Lawrence C.
Published: February 2003

A series of computations has been carried out on the ground (3Σ−g) and first excited state (1Σ+g) of Si2(R=4.244 a0) both with and without the use of an atomic effective or pseudopotential. A Gaussian double-zeta basis set was utilized and partial electronic correlation was introduced into the wave function by means of the generalized valence bond (GVB) approximation. In all cases, the all electron and effective potential computations are in good agreement. The computations indicate that the stability of the ground (3Σ−g) state is due to the tendency of silicon to prefer the divalent s2p2 atomic configuration rather than the tetrahedral sp3 atomic configuration. In contrast to the C2 carbon analog, the first excited (1Σ+g) state of Si2 is shown to possess a relatively weak π bond.
Kinetics of the reaction OH+H2O2→HO2+H2O
Authors: Sridharan U. C. , Reimann B. , Kaufman F.
Published: February 2003

An experimental study of the title reaction over the temperature range 250 to 459 K is described that uses the discharge-flow technique, laser-induced–fluorescence detection of OH and simultaneous monitoring of O and H atoms. In a teflon or halocarbon wax-coated flow tube, the reaction is well behaved and free from surface effect interference, but this is not true on clean Pyrex. OH radicals are generated in three different ways and at low initial concentrations (4 to 8×1010 cm−3) in order to eliminate side reactions. The rate constant is found to be (1.69±0.26)×10−12 at 298 K and (2.96±0.50)×10−12 exp[−(164±52)/T] cm3 s−1 over the above temperature range, a factor of two higher at 298 K and factors of 3 to 5 higher at 10 to 30 km altitude in the terrestrial atmosphere than earlier studies have indicated. These studies are re-examined, and for several of them, arguments are presented that reduce the discrepancy. The effects of the higher rate constant on atmospheric processes and on some recent laboratory measurements of other reactions are discussed briefly.
Gradient expansion of the exchange energy density functional: A complementary expansion of the atomic energy functional
Authors: Shih Chia C. , Murphy Danny R. , Wang Wen-Ping
Published: February 2003

Hartree–Fock–Roothaan–Clementi (HFRC) densities are used to calculate the total exchange energies and the total binding energies of neutral atoms according to the truncated gradient expansion. With the inclusion of the fourth order, the total binding energies for Z=2 to 36 and Z=54 agree very well with the HFRC values. Higher-order corrections for the consistency between HFRC solutions and the density gradient expansions are, therefore, unimportant.
Decoupling approximations in the quantum mechanical treatment of P-state atom collisions
Authors: Aquilanti V. , Casavecchia P. , Grossi G. , Laganà A.
Published: February 2003

Several decoupling schemes are considered, for the effective simplifications of the close coupling equations which arise in the treatment of atomic collisions involving fine structure. Calculations were performed for two models of 2P atoms colliding at thermal energy with a 1S species. From the comparison of exact and approximate results, the relative merits and limits of full or partial decoupling schemes were assessed. Possible improvements and extensions are suggested.
A variable interval variable step method for the solution of linear second order coupled differential equations
Authors: Parker Gregory A. , Schmalz Thomas G. , Light John C.
Published: February 2003

A new method for the numerical solution of the linear second order coupled differential equations of quantum scattering theory is presented. A formal framework is set up which makes clear the interrelationships between many currently used integration methods. Our variable interval variable step method is designed to reduce the total number of matrix (N3) operations required for solution at a given accuracy. The method is tested on several problems and is shown to be uniformly and rapidly convergent, stable, and significantly faster than previous methods.
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o calculation of hyperfine splitting constants of molecules
Authors: Ohta K. , Nakatsuji H. , Hirao K. , Yonezawa T.
Published: February 2003

Hyperfine splitting (hfs) constants of molecules, methyl, ethyl, vinyl, allyl, cyclopropyl, formyl, O3−, NH2, NO2, and NF2 radicals have been calculated by the pseudo-orbital (PO) theory, the unrestricted HF (UHF), projected UHF (PUHF) and single excitation (SE) CI theories. The pseudo-orbital (PO) theory is based on the symmetry-adapted-cluster (SAC) expansion proposed previously. Several contractions of the Gaussian basis sets of double-zeta accuracy have been examined. The UHF results were consistently too large to compare with experiments and the PUHF results were too small. For molecules studied here, the PO theory and SECI theory gave relatively close results. They were in fair agreement with experiments. The first-order spin-polarization self-consistency effect, which was shown to be important for atoms, is relatively small for the molecules. The present result also shows an importance of eliminating orbital-transformation dependence from conventional first-order perturbation calculations. The present calculations have explained well several important variations in the experimental hfs constants.
Kinetics of the reaction of HO2 with ozone
Authors: Zahniser Mark S. , Howard Carleton J.
Published: February 2003

Rate constants were measured for the reaction HO2+O3→OH+2O2(k1) using a discharge-flow system with laser magnetic resonance detection of both HO2 and OH. k1 was determined directly from the first order decay of HO2 in excess O3 when C2F3Cl was added to scavenge the OH product and prevent interference from the faster reaction OH+O3→HO2+O2(k2). The ratio k2/k1 was independently determined from the steady-state [HO2]/[OH] ratio obtained without C2F3Cl. Results from the scavenger method are given by k1= (1.4±0.4)×10−14 exp [−(580±100)/T] cm3 s−1 for 245 K<T<365K. Results of the ratio measurements are in good agreement with these data when combined with a directly measured value of k2?1.0)×10−14 cm3 s−1 at 300 K. These measurements are compared with other studies and some implications for stratospheric ozone chemistry are discussed.
Fluorescence lifetime studies of NO2. IV. Temperature dependence of fluorescence spectra and of collisional quenching of fluorescence
Authors: Keil D. G. , Donnelly V. M. , Kaufman F.
Published: February 2003

In the laser-excited, low pressure, low resolution fluorescence spectrum of NO2, resolved banded features are seen to decrease relative to unresolved ’’continuum’’ emission as the temperature is raised from 175 ° to 356 °K. This behavior is explained in terms of a breakdown of the ΔK=0 selection rule which becomes more important in higher rotational levels of the excited NO2 molecule. The effect of interelectronic mixing in this mechanism is discussed. Quenching rate constants, which differ for banded features and the underlying continuum were measured in the range of 235–360 °K. The faster rate constant for quenching of banded features, kB, is found to be temperature independent while the other rate constant, kC, exhibits a slight negative temperature dependence. This observation is shown to be consistent with a model of rapid, collision-induced, internal V–R relaxation followed by a slower vibrational cascade process in the perturbed excited state.
Kinetics of free radicals generated by IR laser photolysis. IV. Intersystem crossings and reactions of C2(X 1Σ+g) and C2(a 3Πu) in the gaseous phase
Authors: Reisler H. , Mangir M. S. , Wittig C.
Published: February 2003

Rate coefficients (300 K) for the removal of C2(X 1Σ+g) and C2(a 3Πu), hereafter referred to as 1C2 and 3C2 respectively, by H2, NO, and a number of hydrocarbons are reported as well as rate coefficients for intersystem crossing between 1C2 and 3C2 induced by collisions with N2, CO2, CF4, Ar, Kr, and Xe. C2 molecules are produced via ir photolysis of C2H3CN or C2HCl3, and their concentrations are monitored by laser induced fluorescence. We find that collisionally induced intersystem crossing is significant only when it is spin allowed or involves heavy collision partners (e.g., Kr, Xe). 1C2 reacts more rapidly with NO than does 3C2, and excited CN molecules in the A and B states are formed predominantly in reactions of 3C2. 1C2 reactions result (mainly) in ground state CN, as expected from adiabatic state correlations. Radiationless transitions between the X and B states of CN, induced by collisions with Ar, are observed. It is suggested that both 1C2 and 3C2 are removed by hydrocarbons mainly via chemical reactions, and 1C2 reacts more rapidly than 3C2 for every case that we have measured. With the 3C2+H2 reaction, molecular orbital correlation diagrams show that there are barriers in the entrance channel, thereby rationalizing the small rate coefficient for this reaction, compared to the allowed 1C2+H2 reaction. Similar correlations may explain the smaller reactivity of 3C2, compared to 1C2, toward hydrocarbons in general.
Theoretical dipole moment functions of the HF, HCl, and HBr molecules
Authors: Werner Hans-Joachim , Rosmus Pavel
Published: February 2003

Potential energy and dipole moment functions of the HF, HCl, and HBr molecules in their electronic ground states have been calculated from highly correlated SCEP/CEPA ab initio wave functions. Purely rotational transition energies are obtained with an accuracy of about 0.1 cm−1, and vibrational transition energies agree within 10–20 cm−1 with the experimental values. The SCEP/CEPA dipole moments in the vibrational ground states are calculated to be (experimental values in parenthesis) 1.807 D (1.826 D) for HF, 1.120 D (1.1085 D) for HCl and 0.829 D (0.828 D) for HBr. For HF various theoretical approaches, i.e., the SCEP/VAR (including variationally all singly and doubly excited configurations), SCEP/CEPA (accounting approximately for unlinked cluster effects), and MC-SCF (with eight optimized valence configurations and with 66 configurations including atomic correlation) methods are compared. The spectroscopic constants and dipole moment functions calculated from SCEP/CEPA and MC-SCF wave functions are of comparable accuracy. The SCEP/CEPA and MC-SCF dipole moment functions of HF are in good agreement with the experimental function over a range of internuclear distances which covers approximately the nine lowest vibrational states. The theoretical potential and dipole moment functions have been used to calculate vibrational dipole matrix elements. The fully ab initio results of HF and HCl up to v=5 agree within about 5% with the values derived from experiments. For HBr strongly differing slopes of the dipole moment function have been reported in the literature. The present theoretical results are in good agreement with the most recent measurements and enable a reliable estimate of the absolute intensity for the 0–1 vibrational transition.
One-electron model potential calculations. II. 1Σ+ and 3Σ+ excited states of HeNe relevant to electronic energy transfer
Authors: Siska P. E.
Published: February 2003

Semiempirical calculations of the array of potential curves likely to be important for thermal-energy electronic excitation transfer from metastable helium to neon are presented, and probable mechanisms for formation of various neon excited states are discussed. Features of the scattering of helium metastables by neon as well as known product channels and total quenching rates are qualitatively interpreted using the calculated potentials.
Soft-sphere model for the crystal–liquid interface: A molecular dynamics calculation of the surface stress
Authors: Cape J. Neil , Woodcock Leslie V.
Published: February 2003

A molecular dynamics study of the crystal (100)–liquid interface in the inverse twelfth power soft-sphere model for a system of 7680 atoms with infinite periodicity in two of the three dimensions is reported. The interfacial stress tensor has been computed and this gives a value for the excess interfacial free energy to within an uncertainty of about 10%. The structure and dynamics of the interface is analyzed in detail and compared with current theoretical models used in the interpretation of crystal growth processes. Comparisons with earlier results for the Lennard-Jones (12–6) model lead to the conclusion that the structure of the crystal–liquid interface is determined predominantly by the repulsive part of the pair potential and remains much the same from the soft-sphere high-temperature limit to the triple point.
Correlations between proton chemical shift tensors, deuterium quadrupole couplings, and bond distances for hydrogen bonds in solids
Authors: Berglund Bo , Vaughan Robert W.
Published: February 2003

Correlations between parameters characterizing the proton chemical shift tensors for O–H⋅⋅⋅O bonded protons in solids and O⋅⋅⋅O hydrogen bond distances and deuterium quadrupole coupling constants (e2qQ/h) are presented. The isotropic value of the chemical shift correlates very well with both RO⋅⋅⋅O and e2qQ/h, whereas the correlations involving the chemical shift anisotropy show much more scatter. A quantitative agreement is found with theoretical calculations performed by Ditchfield.
Refinement of the structure of solid nitromethane
Authors: Trevino S. F. , Prince E. , Hubbard C. R.
Published: February 2003

The complete determination of the crystal structure of solid nitromethane has been obtained from single crystal x-ray diffraction and neutron powder diffraction data. The structure is orthorhombic, space group P212121 with a=5.1832 Å, b=6.2357 Å, c=8.5181 Å at T=4.2 K, and Z=4. Two models were used to describe the thermal motion of the methyl group, one with anisotropic temperature factors for the hydrogen atoms constrained to correspond to a threefold rotation around the C–N bond, and the other treating the group as a rigid body, with a tortional oscillation about the C–N bond axis.
Experimental heat capacities of LaNi5, α-LaNi5H0.36, and β-LaNi5H6.39 from 5 to 300 °K. Thermodynamic properties of the LaNi5–H2 system
Authors: Ohlendorf Dieter , Flotow Howard E.
Published: February 2003

The heat capacities of LaNi5, α-LaNi5H0.36, and β-LaNi5H6.39 were measured from 5 to 300 °K, and in the case of LaNi5 also from 300 to 350 °K. Various special techniques and unusual circumstances involved in the measurements and calculations are described. A slight enhancement of the heat capacity of β-LaNi5H6.39 was observed in the temperature range 120 to 160 °K. The standard entropies of LaNi5, α-LaNi5H0.36, and β-LaNi5H6.39 at 298.15 °K were found to be 209.9±0.5, 214.1 ±0.5, and 271.7±0.7 J °K−1 mole−1, respectively. These results together with other thermochemical data were used to calculate the entropy and enthalpy of formation of β-LaNi5H6.39 at 298.15 °K. The partial molar entropy of hydrogen in β-LaNi5H6.39 was derived from our results and used to estimate the configurational entropy at 298.15 °K.
Atmospheric quenching of vibrationally excited O2(1Δ)
Authors: Klais Odo , Laufer Allan H. , Kurylo Michael J.
Published: February 2003

Flash photolysis of O3 in the Hartley band has been coupled with absorption spectroscopic detection of O2(1Δ, v=0) and resonance fluorescence detection of O(3P) to analyze the fate of O2(1Δ, v?1) in an atmospheric environment. The results indicate a rapid vibrational deactivation under atmospheric conditions. The same analysis permits an estimate of the relative quantum yield of O2(1Δ, v?1) in the photolysis. Similarly, an upper limit for the channel of the O(1D)+O3 reaction yielding O2(1Σ) is derived.
Low and high coverage determinations of the rate of carbon monoxide adsorption and desorption from Pt(110)
Authors: Fair James , Madix Robert J.
Published: February 2003

The adsorption probability of CO on Pt(110) was determined to be 0.7 by modulated beam (MBRS) and thermal desorption over a temperature range of 195 to 625 K. By MBRS the desorption rate constant, kd, was measured to be 6×1014 exp{−35.3 (kcal mol−1)/RT s−1} in the low coverage limit. This high value for the desorption pre-exponential factor was explained utilizing transition state theory. The low coverage value of kd was employed to calculate the thermal desorption spectrum, describing the adsorbed state by a one-dimensional Ising model. The repulsive interaction parameter which yielded the best fit was ω=1.5+2.5ϑ kcal mol−1.
Spin-orbit coupling and Λ-doubling in LiO
Authors: Copper David L. , Richards W. Graham
Published: February 2003

LiO molecule in the 2Πi ground state is considered. Radio-frequency transitions between the Λ-doublet components are considered. The spin-orbit coupling constants and Λ-doubling parameters are calculated from ab initio molecular orbital methods.(AIP)
Vibrational spectra and normal coordinate analysis of neptunium (IV) borohydride and neptunium (IV) borodeuteride
Authors: Banks Rodney H. , Edelstein Norman
Published: February 2003

Solid state, low temperature IR (25–7400 cm−1) and Raman (100–2600 cm−1) spectra were obtained for Np(BH4)4 and Np(BD4)4 from which most of the allowed fundamentals were assigned based on the Td molecular structure. Those assignments were used in a normal coordinate analysis to derive a simple force field using eight primary and five interaction constants. This field is very similar to those found for Zr(BH4)4 and Hf(BH4)4. Isotopic impurity, overtone, and combination bands were identified in the IR spectra with the help of the normal coordinate calculations. Near IR spectra of Zr(BH4)4 and Zr(BD4)4 were taken in the range 7400–4000 cm−1 and the observed absorption bands were assigned as either overtone or combination levels.
Gel deswelling under reverse osmosis
Authors: Hecht A. M. , Geissler E.
Published: February 2003

When a swollen gel is pressed into contact with a semipermeable membrane, the solvent is squeezed out at the interface and the gel deswells. The rate at which the gel far from the interface advances is found to be proportional to [(P0−E)/ft]1/2, where P0 is the applied pressure, E the longitudinal elastic modulus of the gel, f the friction coefficient between the solvent and polymer, and t the elapsed time. Satisfactory agreement is found between the experimental macroscopic values of E obtained in this way for polyacrylamide gels at two concentrations and the microscopic values of E obtained from inelastic light scattering.
Diffusion coefficients for segmentally flexible macromolecules: General formalism and application to rotational behavior of a body with two segments
Authors: Wegener William A. , Dowben Robert M. , Koester Vaughn J.
Published: February 2003

We present a formalism to calculate diffusion coefficients for macromolecules with segmental flexibility. Macromolecules composed of several segments of different size and shape can be treated including assemblies with arbitrary types of flexible attachments and multiple branching. The frictional resistance tensor R and the diffusion tensor D are evaluated in generalized coordinates involving all degrees of freedom, and their general properties are established. A simplified approximate expression for R is obtained when hydrodynamic interactions between segments are omitted. We apply this approximate expression to evaluate the rotational behavior of a body composed of two cylindrically symmetric segments flexibly attached at their endpoints by a frictionless universal joint. The effect of size and shape differences of the two segments upon the principal rotational diffusion coefficients of each segment is established. Coefficients governing correlations between rotations of both segments are used to evaluate diffusion coefficients for bending and twisting motions. If the body is completely flexible, the rotational relaxation behavior of each segment reduces to that of some equivalent rigid-body with cylindrical symmetry. Effects of rigid to flexible transitions and enzymatic cleavage of segments are considered. The rotational diffusion coefficient for once-broken rods is up to 2.34 times greater than predicted by Yu and Stockmayer [J. Chem. Phys. 47, 1369 (1967)] and in good agreement with dielectric dispersion measurements by Matsumoto et al. [Macromolecules 7, 824 (1974)].
Adsite symmetry and vibrational structure of NO and H2 co-adsorbed on the Ru(001) surface
Authors: Thiel P. A. , Weinberg W. H.
Published: February 2003

The co-adsorption of molecular nitric oxide and hydrogen on Ru(001) has been studied using electron energy loss spectroscopy. Previous work has shown that molecuar nitric oxide occupies sites of single and multiple (probably threefold) coordination. Competitive adsorption between hydrogen and the latter form of molecular NO is interpreted in terms of hydrogen preferentially adsorbing into threefold symmetric sites. This is consistent with experimental results and theoretical calculations for other surfaces.
The adsorption of dipoles at a wall in the presence of an electric field: The RLHNC approximation
Authors: Eggebrecht John M. , Isbister Dennis J. , Rasaiah Jayendran C.
Published: February 2003

The adsorption of dipolar hard spheres in the presence of an external electric field has previously been studied within the context of the mean spherical approximation. In order to quantify the significance of the physical trends found above, the problem is solved within the higher order closure rules afforded by the linearized hypernetted chain approximation. Exact expressions for the reduced dipole moment and the electric field strength are derived using only the asymptotic forms of the direct correlation functions. It is found that the favorable orientational correlations between the dipolar hard spheres and the wall are underestimated by the mean spherical approximation. This is emphasized in the enhanced adsorption of the dipolar species (at the wall itself) for dipoles oriented close to the direction of the field. However, the nonphysical features of the mean spherical approximation (manifested in the negativity of the density profile) are not fully rectified by the use of the linearized hypernetted chain approximation.
Comment on ‘‘Differential cross sections for the j=0→1 rotational excitation in HD–Ne collisions and their relevance to the anisotropic interaction’’
Authors: Fitz D. E. , Khare V. , Kouri D. J.
Published: February 2003

Calculations are performed in the compled states approximation for rotational transitions of HD–Ne at E=31.5 meV, using the correct phase (CP) choice3,4 and the potential of Buck, et al1(AIP)
Rates of some reactions of N(2D and 2P) near 300 K
Authors: Iannuzzi M. P. , Kaufman F.
Published: February 2003

A brief study was undertaken primarily to reduce the uncertainties in the measurements of the N(2D,2P) reactions with O2 by use of resonance fluorescence detection of the metastables. (AIP)