bluuebunny/crossref_metadata_2025 huggingface dataset:
Existence of H2−, a relatively long-lived doubly charged negative atomic hydrogen ion
Authors: Schnitzer Rafael , Anbar Michael
Published: February 2003

Doubly charged negative hydrogen ions H2− and D2− have been indirectly observed in a tandem mass spectrometer using a hollow cathode duoplasmatron ion source. The half-life of these doubly charged ions, determined in a number of drift time experiments, is 23±4 nsec. The existence of these ions is concluded from a velocity, momentum, and energy analysis of hydride ions produced in the autodetachment process: X2−→X−+e (X=H,D).
Exo-electron emission from abraded metal surfaces in the dark
Authors: Wei P. S. P. , Lytle F. W.
Published: February 2003

We report some new results on the exo-electron (EE) emission from abraded metal surfaces as studied by a photographic method. A good correlation between the intensity of EE and the standard heat of formation of the stable oxide is established for the first time.
Theory of self-consistent electron pairs. An iterative method for correlated many-electron wavefunctions
Authors: Meyer Wilfried
Published: February 2003

A rapidly converging iterative method is presented to solve the many-electron Schrödinger equation within a Hilbert space confined to functions with at most two electrons outside an internal space defined by the orbitals of a reference function. The wavefunction is given in terms of external two-electron clusters represented by coefficients and density matrices referring directly to the basis functions. All matrix elements are obtained from generalized Coulomb and exchange operators. Only one operator per correlated electron pair is required for each iteration cycle.
The two lowest energy 2A′ states of NO2
Authors: Jackels Charles F. , Davidson Ernest R.
Published: February 2003

The 2A1 ground state of NO2 and the so-called 2B2 excited state are shown to form a strongly coupled pair of 2A′ states when asymmetric distortion is considered. This coupling is of the Jahn–Teller type although the degeneracy between the states is accidental and the normal coordinates are not equivalent.
Theoretical evaluation of vibrational transition rates and relaxation in CO–He
Authors: Verter Michael R. , Rabitz Herschel
Published: February 2003

An extensive set of vibrationally inelastic transition rates kn+Δn→n(T) is presented for CO–He. Calculations were performed over temperatures 100⩽T⩽3000 °K, final states 0⩽n⩽45, and transitions Δn=1,2,3. Cross sections σn+Δn→n are computed by solving sets of close coupled breathing sphere equations containing 7 to 11 vibrational states. The CO–He potential surface used was computed by the method of Gordon and Kim. Sample relaxation studies are carried out to determine the roles of the different rate constants in the relaxation process. It is shown that while the conditions of the Landau–Teller theory are not satisfied here, the relaxation of the total vibrational energy is still approximately described by a single relaxation time under usual experimental conditions. However, a detailed examination of the relaxation process shows that a canonically invariant temperature dependence generally does not hold. Anharmonic effects at high quantum numbers are shown to play a dominant effect in the relaxation.
Collisional ionization and elastic scattering in alkali–halogen atom collisions
Authors: Faist M. B. , Levine R. D.
Published: February 2003

Close-coupling (two-state) computations for collisional ionization and elastic scattering in Na+I and Li+I collisions have been made, using realistic model potentials, at a large range of postthreshold energies. At the higher energies a generalized JWKB procedure was employed which yields a very accurate scattering matrix when compared to a fully quantal solution. The computed elastic and inelastic total and differential cross sections are discussed and compared to the predictions of more approximate methods. The rainbow angles in the angular distribution are compared to those predicted by the classical deflection function obtained from the trajectories along adiabatic or diabatic potential curves (based on the assumption of a single transition point). It is found that at all energies considered these rainbow positions agree best for trajectories computed via the adiabatic representation rather than the diabatic representation. The double rainbow pattern noted in the elastic ionic channel can be interpreted only within the adiabatic representation. Landau–Zener–Stueckelberg (LZS) computations for the energy dependence of the collisional ionization cross section have also been made. The LZS curve jumping mechanism is in better agreement with the quantal results when the transition is taken to occur at the adiabatic coupling point (Rp) rather than the diabatic crossing point (Rx). Moreover, depending on the nature of the potentials, there may also be an additional transition region at small internuclear distances, which is not present in the LZX scheme but may be the source of oscillations found in the post maximum energy dependence of the collisional ionization cross section. The systematics of the collisional ionization cross sections for all alkali metal–halogen atom (M+X) collisions are discussed and the energy dependence of the cross section is computed using realistic potentials. In terms of the reduced velocity (at the crossing point) v/Kp, it is noted that only Li+I and Na+I are in the strong coupling region. For all other systems the cross section is found to decrease with increasing collision energy.
Computational study of elastic and electronically inelastic scattering of Br by ground state I atoms: Role of potential curve crossing
Authors: Faist M. B. , Bernstein R. B.
Published: February 2003

Two-state, close-coupled quantal computations of the elastic and inelastic scattering of ground-state I atoms by ground-state Br and spin–orbit excited Br* atoms have been carried out over a range of total energies E from 0.01 to 0.94 eV. The possibility of translational–electronic energy transfer arises from the 3Π0+ potential curve crossing at E=0.25 eV, responsible for the well-known IBr predissociation. The Y, B, and V12(R) diabatic potentials have been obtained by judicious extension (and manipulation) of the spectroscopically derived ? and ?′ adiabats. At energies below the threshold for Br* formation (Eth=0.457 eV) collisions of I+Br are necessarily elastic, exhibiting both shape and compound-state resonances. These produce interesting interference patterns in the differential cross sections, but no significant inverse-predissociation resonance (which might have been anticipated for E?0.25 eV). The main features of the elastic scattering can be fairly well approximated considering only the lower adiabat, even at postthreshold energies where the inelastic process becomes important. An oscillatory pattern found in the total elastic cross section has been identified with a barrier effect associated with the maximum in this ? adiabat. Total inelastic cross sections are well reproduced by the closed-form Landau–Zener–Stueckelberg (LZS) approximation. The Boltzmann-averaged LZS rate constant for the collisional deactivation I+Br*→I+Br is calculated to be 1.8×10−11 cm3 sec−1 at 300 K (with only a slight temperature dependence over the range 300–1000 K). This large rate is due to the inverse predissociation mechanism, common
The characterization of some vanadium (100) surface using LEED and AES
Authors: Szalkowski F. J. , Somorjai G. A.
Published: February 2003

Low energy electron diffraction (LEED) supplemented by Auger electron spectroscopy (AES) was used to investigate the properties of the (100) crystallographic face of vanadium metal. A (1×1) surface structure is characteristics of the clean metal surface. Sulfur segregates to the surface upon heating, forming first a C(2×2) structure and then a (√2×2√5) R27° structure. A kinematical analysis of the diffracted beam intensity vs voltage curves proved appropriate for the V(100)−(1×1) structure and yields a value of −9.9 V for the inner potential and a value of 3.02 Å for the surface lattice constant. Room temperature gas adsorption studies on the V(100)−(1×1) surface indicate that oxygen diffuses into the lattice and produces a disordered surface, while carbon monoxide either does not absorb or is desorbed under the influence of the electron beam.
A decoupled l-dominant approximation for ion–molecule and atom–molecule collisions
Authors: DePristo Andrew E. , Alexander Millard H.
Published: February 2003

In the space-fixed frame we develop a novel angular decoupling approximation, which involves the degree of alignment between the molecular and total rotational angular momentum vectors. When grafted onto the recent l-dominant method, this decoupling results in a set of block-diagonalized coupled equations of dimensionality equivalent to that of the body-fixed coupled-states (CS) equations. In direct contrast to the CS method, we expect the ’’decoupled l-dominant’’ (DLD) method to be best suited for the study of collisions in which strong long-range forces play a major role. We have applied the DLD method to the determination of partial and total cross sections for rotationally inelastic Ar–N2 and Li+–H2 collisions. The agreement with exact close-coupling values is very satisfactory, except in those cases where collisions at large impact parameter do not contribute significantly to a particular transition.
The theoretical calculation of polar tensors and dipole moment derivatives: BF3 and Bcl3
Authors: Bruns Roy E. , Bassi A. B. M. S.
Published: February 2003

The experimental and CNDO polar tensors for BF3 and Bcl3 are reported. To satisfy charge and symmetry restrictions the importance of the calculation of limiting values for theoretical polar tensor elements is demonstrated. A convenient method to transform polar tensor elements to ∂p/∂Qi values is presented. The atomic effective charges, mean dipole moment derivatives, and anisotropies of BF3 and Bcl3 are reported and discussed.
A study of the reactions of C+ and O+ with carbon dioxide
Authors: Rutherford J. A. , Vroom D. A.
Published: February 2003

Crossed ion neutral beam techniques have been employed to investigate the reactions C++CO2→CO++CO, O++CO2→O+CO+2, and O++CO2→ O+2+CO. Cross sections for the first reaction have been obtained in the ion energy range from 0.5–25 eV, while for the second process the range of measurement was 0.5–500 eV. Signals attributed to the third reaction could only be detected at ion energies less than 0.5 eV. Both ion–molecule reactions exhibit an energy dependence which falls off with increasing interaction energy faster than the 1/v dependence often found for such processes. The charge transfer process, which is endothermic by approximately 0.15 eV, is observed to have a finite cross section at the lowest ion energy employed.
Primary radiation products in cysteine hydrochloride
Authors: Kou William W. H. , Box Harold C.
Published: February 2003

ESR and ENDOR spectroscopy were used to identify three primary radicals in single crystals of cysteine hydrochloride x irradiated at 4.2 °K. The reduction product is an anion formed by addition of an electron to the carboxyl group. One oxidation product results from the loss of the sulfhydryl hydrogen atom. Another oxidation product is an electron vacancy localized mainly on atomic chlorine.
Internal coordinate formulation for the vibration–rotation energies of polyatomic molecules
Authors: Quade C. Richard
Published: February 2003

The vibration–rotation energies of polyatomic molecules are obtained under the assumption of small oscillations using curvilinear internal coordinates for the vibrational degrees of freedom. The use of curvilinear coordinates introduces anharmonic terms into the vibrational kinetic energy and gives second derivative terms for the moments of inertia that include the motion of an atom along its natural curved path during vibration. Coordinate transformation of the intramolecular potential energy is unnecessary. Perturbation expansion is used to obtain all of the vibration–rotation coefficients, αik, χkk, χkk′, χlsls, χlsls, for linear, symmetric top, and asymmetric top molecules. Vibration–rotation constants are given for the linear X2Y and XYZ and nonlinear X2Y molecules in terms of the molecular structure. Analysis is made of the vibration–rotation and anharmonic vibrational coefficients using previously determined anharmonic force constants for CO2 and H2O. Partial analysis of vibration–rotation coefficients is made for additional molecules.
Dynamic corrections for neutron scattering from molecular fluids
Authors: Blum L. , Narten A. H.
Published: February 2003

The Placzek corrections for neutron scattering from atomic systems have been generalized to include molecular fluids. The correction terms include spherical Bessel functions of the zeroth and second order. As a consequence, the dynamic corrections for molecular systems show sizeable oscillations with a period determined by the internuclear distances within a molecule. These results are compared with neutron diffraction data for liquid carbon tetrachloride.
Studies of molecular motions of CH2Br2 in the liquid state by depolarized Rayleigh and Raman scattering
Authors: Wang C. H. , Jones Dane R. , Christensen Daniel H.
Published: February 2003

We have obtained from the concentration dependent depolarized Rayleigh scattering experiment (using a Fabry–Perot interferometer) the single particle reorientation relaxation time τs of CH2Br2 to be 3.4±0.2 psec at 20 °C. This time is a factor of 2.0 greater than the Raman correlation time τm obtained for the ν2(A1) Raman band. Moreover, the Raman correlation times τm obtained for the other A1 bands do not have the same values as that obtained for the ν2 band. This difference is due to the fact that the nonreorientational correlation functions Ciso(t) and Cβ(t) do not have the same time dependence, hence making the usual Raman line shape analysis difficult to apply to CH2Br2. From depolarized Rayleigh scattering, we have found in neat CH2Br2 that the classical Stokes–Einstein relation for rotational diffusion does not apply, despite the fact that τRay was found to vary linearly with η/T. However, when using the slip boundary condition and modeling CH2Br2 as an ellipsoid, we have found reasonable agreement between theory and experiment. This work is the first temperature dependent study of molecular motion in simple liquids involving the joint use of the Rayleigh and Raman scattering techiques. We have shown that by combining results from temperature and concentration dependent Raman and depolarized scattering experiments, significantly more information concerning molecular motions can be obtained than is possible from either technique alone.
A collinear analytic model for atom–diatom chemical reactions
Authors: Halavee U. , Shapiro M.
Published: February 2003

A collinear analytic model for atom–diatom chemical reactions is developed in a systematic fashion from the exact quantum mechanical formulas. All the approximations involved were outlined and their range of validity indicated. The products’ vibrational distributions of 12 chemical reactions as well as isotope effects were calculated and shown to agree well with numerical collinear calculations, experimental results, and classical trajectory studies (when available). The factors influencing a reaction’s outcome were analyzed and the mass and isotope effects extensively explored; two types of behavior corresponding to the light atom and heavy atom regions were quantitatively mapped. It is hoped that similar results can be obtained from this model via spectroscopic data of the separate diatomics without ever having to precalculate a triatomic potential surface.
Atomic recombination dynamics on a solid surface: H2+W(001)
Authors: McCreery Jane Hylton , Wolken George
Published: February 2003

A model potential to describe the interaction of a diatomic molecule with a rigid solid surface has been described previously. Using this interaction potential, a series of classical trajectory calculations have been carried out, designed to simulate the recombination dynamics of two hydrogen atoms initially adsorbed on a tungsten (001) solid surface to form a gas-phase hydrogen molecule. The vibrational and rotational state distributions of the desorbed hydrogen molecules are discussed in terms of a simple statistical model. The angular and speed distributions for the desorbed atoms and molecules are presented. The angular distributions are found to be substantially noncosine in form and peaked towards the surface normal, in qualitative agreement with experiment.
13C kinetic isotope effects in the pyrolysis of dimethyl ether
Authors: Zielinski Mieczyslaw , Kidd Richard W. , Yankwich Peter E.
Published: February 2003

Intermolecular and intramolecular 13C kinetic isotope effects in the pyrolysis of dimethyl ether were measured at temperatures between 451 and 550 °C; initial pressures were between 14 and 1000 torr. A few experiments on the NO-inhibited reaction were carried out at 501 °C. Both isotope effects are of the order of 1% and decrease with increasing temperature; the temperature dependence of the intermolecular effect is the greater. No significant pressure effects were observed. The isotope effects arise in the destruction of the symmetry of the reactant in hydrogen transfer reactions of the form R+C2H3OC1H3→RH+⋅C2H2OC1H3, where R=CH3, H, or NO. The method of three-element reaction coordinates was used to model the hydrogen transfer, the elements being displacements in the stretching coordinates R⋅⋅⋅H, H⋅⋅⋅C2, and C2-O. With adjustments of the transition state diagonal force field which are physically and chemically reasonable, the best simultaneous fit to the experimental results is obtained for reaction coordinates in which displacements in R⋅⋅⋅H and H⋅⋅⋅C2 are large but that in C2-O is small. The fit is not critically sensitive to the degree of transfer of H from C2 to R in the range 25%–75%, and in that range is essentially the same for R=H, CH3, or NO. Predictions are made of kH/kD for comparative pyrolyses of CH3OCH3 and CD3OCD3.
Molecular structure of 1,1-dichloro-2,2-difluoroethylene
Authors: Lowrey A. H. , D’Antonio P. , George C.
Published: February 2003

The molecular structure of 1,1-dichloro-2,2-difluoroethylene (CCl2=CF2) has been determined by an electron diffraction study in the vapor phase. A Euclidean structural model was obtained from the refined unconstrained least-squares distances with a procedure that varies the coordinates of a Euclidean model until the best least-square fit to the non-Euclidean distances is obtained. This procedure provides a means for estimating shrinkage effects due to molecular vibrations. The bonded distances obtained are rg(C–F) =1.315±0.015 Å rg(C=C) = 1.345±0.025 Å, rg(C–Cl) =1.706±0.008 Å; the associated vibrational amplitudes are l (C–F) =0.049±0.018 Å, l (C=C) = 0.044±0.028 Å, l (C–Cl) =0.055±0.009 Å. The &FCF angle is 112.1°±2.5° and the &ClCCl angle is 119.0°±0.9°. The errors are limits of error at the 95% confidence level. The results indicate a negligible cis Cl⋅⋅⋅F shrinkage (Δra=0.001 Å) and an appreciable trans Cl⋅⋅⋅F shrinkage (Δra=0.01 Å). The unsymmetrical halogen substitution has caused the C=C distance to be 0.008 Å larger than that in ethylene.
Isotope effects in superionic conductors
Authors: Kimball J. C. , Eswaran Mukesh
Published: February 2003

A quantum mechanical calculation of the conductivity of a simple model of a superionic conductor which considers short range interactions betweetn the mobile ions is p’resented. This model yields results which depend strongly on the nuclear spin on the mobile ion. In principle, this isotope dependent conductivity could lead to isotope separation. In practice, ignored interactions between the mobile ions and the nonmobile ions in the lattice could mask this effect.
Influence of molecular nonrigidity on the infrared absorption and Raman scattering line shape in dense media
Authors: Robert D. , Galatry L.
Published: February 2003

Starting from the well-known infrared absorption and Raman scattering line shape expressions, the two situations where a complete frequency degeneracy (unresolved isotropic Raman band) is or is not present (absorption band, anisotropic Raman band, and resolved isotropic Raman band) are examined. Thus the influence of the nonrigidity of the interacting molecules on the line shape is studied. The physical mechanisms responsible for the dependence of the profiles upon the internal molecular vibrations are carefully analyzed. An application to the spectra of a diatomic molecule trapped in a rare gas crystal is made and the vibrational broadening of HCl and CO embedded in Ar, Kr, and Xe crystal is calculated.
An electron paramagnetic resonance study of methylene
Authors: Bernheim R. A. , Adl T. , Bernard H. W. , Songco A. , Wang P. S. , Wang R. , Wood L. S. , Skell P. S.
Published: February 2003

The present paper is a detailed account of the study of the methylene molecule trapped in matrices at 4.2 °K by electron paramagnetic resonance spectroscopy. The methylene is generated by photolysis of its diazirine precursor dissolved in the matrix material. Besides verifying that the methylene ground electronic state was a spin triplet, the first observation of the EPR spectrum revealed that the molecule was nonlinear. Through assumptions concerning the amounts of spectral averaging owing to molecular motion and the relationship of the zero-field splitting parameters D and E with each other through an s–p hybrid model of the electronic structure, other workers showed the bond angle to be substantially bent. This was later confirmed by ourselves and others with a measurement of the isotropic carbon-13 hyperfine interaction. Using 90.5% carbon-13 enrichment in the present work, a more precise determination of the isotropic carbon-13 hyperfine interaction was found to be 239 MHz. The increased resolution also permits a measurement of the anisotropic carbon-13 hyperfine interaction from which a bond angle of 134° can be derived. All three methods of bond angle determination are in substantial agreement. Chemical trapping of the products of the photolysis with a 100% mass balance accounting for all substances supplement the EPR of the isotopically substituted species to show conclusively that the trapped paramagnetic species is methylene. Experiments in a nonnuclear magnetic matrix show that the linewidths are not due to hyperfine interactions with matrix nuclei. In addition to xenon matrices, experiments in krypton and xenon–krypton mixtures are reported where the observed zero-field splitting parameters are modified by molecular motion and interaction with the trapping matrix. The present work also verifies that the heat of formation of methylene is 91.9±1.0 kcal/mole.
Absolute elastic differential electron scattering cross sections in the intermediate energy region. III. SF6 and UF6
Authors: Srivastava Santosh K. , Trajmar S. , Chutjian A. , Williams W.
Published: February 2003

A recently developed technique has been used to measure the ratios of elastic differential electron scattering cross sections (DCS) for SF6 and UF6 to those of He at electron impact energies of 5, 10, 15, 20, 30, 40, 50, 60, and 75 eV and at scattering angles of 20° to 135°. In order to obtain the absolute values of DCS from these ratios, He DCS of McConkey and Preston have been employed in the 20° to 90° range. At angles in the 90° to 135° range the recently determined cross sections of Srivastava and Trajmar have been utilized. From these DCS, elastic integral and momentum transfer cross sections have been obtained.
Electronic structure of some inorganic azides from x-ray electron spectroscopy
Authors: Colton Richard J. , Rabalais J. Wayne
Published: February 2003

The electronic structure of some inorganic azides has been investigated by x-ray electron spectroscopy for the purpose of contrasting the relative ionicities or degree of covalency of the alkali and heavy metal azides. Core and valence band electron spectra are presented along with deconvolutions of some of the overlapping bands. The splitting between the terminal N 1s band and the central N 1s band ΔEI for the inorganic azides has been found to be sensitive to the degree of ionicity of the respective compounds. This data when compared to some physical properties such as crystal structure, ionic radius, standard enthalpy of formation, lattice energy, and solubility provides evidence for a trend in ionicities. The observed trends indicate that the transition metal azides are considerably more covalent than the alkali or alkaline metal azides.
Behavior of the intramolecular scattering function in its representation by Nagai’s series
Authors: Mattice Wayne L. , Carpenter Dewey K.
Published: February 2003

The intramolecular scattering function for a polymer molecule is known to be related by Nagai’s series to the even moments of the distance separating two scattering centers. Numerical values of the coefficients in this series have been computed for a rigid molecule. Application of these coefficients to the computation of the scattering function for several rigid molecules and a flexible molecule establishes that the series converges rapidly only at small values of the scattering angle. Oscillation is obtained at higher scattering angles. The origin of this behavior is readily apparent from an examination of the coefficients. The oscillation will be obtained with any molecule, whether flexible or not, at sufficiently high scattering angle. A more practical approach to the computation of the scattering function for a flexible polymer molecule may be to evaluate 〈 (sinμrij)/μrij〉 directly from a suitable Monte Carlo sample.
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o calculations of the 3Σg+ and 3Σu+ states of singly excited Ar2
Authors: Saxon Roberta P. , Liu Bowen
Published: February 2003

CI calculations were performed for the 3Σg+ and 3Σu+ excimer states of Ar2 using molecular orbitals determined by MCSCF calculations. The 3Σg+ state has a small barrier and shallow well, as expected from analogy with other rare gas dimers. The deeply bound 3Σu+ potential curve has a well depth of 0.68 eV at an internuclear separation of 4.59 ao. Elastic scattering on these curves was calculated and compared with experimental data. The primary rainbow from the 3Σu+ state is found to be in excellent agreement with observation.
A kinetic study of the very fast reaction: O(3P)+CS→CO‡ +S(3P)
Authors: Bida Gerald T. , Breckenridge W. H. , Kolln Werner S.
Published: February 2003

The fast reaction O(3P)+CS→CO‡+S(3P)[Eq .(1)] was studied using a novel flow tube technique in which decay of CS was monitored in the presence of much smaller O(3P) concentrations maintained by rapid O(3P) regeneration via S(3P)+O2→SO+O(3P) [Eq. (2)]. Several measurements of the rate constant for Eq. (1) yield k1= (1.35±0.22) ×1010 liter/mole-sec at 300 °K. The chain reaction consisting of Eqs. (1)+(2), known to be important in many CO chemical laser systems, was shown to have a chain length of ∼50 in the low-pressure (1–2 torr) flow tube. It is concluded that the major chain termination step is not likely to be a homogeneous bimolecular or termolecular reaction of S(3P) or O(3P). Possible heterogeneous chain-breaking reactions are discussed. The implications of our results with respect to stoichiometry assumptions made by other workers in the determination of rate parameters for the important reaction of O(3P) with CS2 are also discussed.
Optical model for molecular rate processes: The inversion of total reaction cross sections
Authors: Roberts R. E.
Published: February 2003

An optical model is derived under the assumption that only a relatively small region of the effective complex potential dictates whether or not a specified reaction will take place. This lowest order localized approximation to the optical potential is uniquely parametrized to fit the total reaction cross section data as a function of kinetic energy for the reactions involving K+CH3I, F+H2, and H+H2 (three-dimensional trajectory simulations on assumed potential surfaces). The optical potential thus obtained is then used to predict the reaction probability (opacity) as a function of impact parameter for each of these cases at several kinetic energies. The agreement with experiment is excellent.
ESR of methylene in neon, argon, and krypton matrices
Authors: Bicknell B. R. , Graham W. R. M. , Weltner W.
Published: February 2003

Triplet CH2 and CD2 have been isolated as partially oriented radicals in neon, argon, and krypton matrices at 4 °K. In neon, the ESR spectra exhibit relatively narrow lines (∼40 G) and indicate that bent CH2 occupies two sites in which it has almost free rotation about the H–H direction and a high barrier to rotation (∼195 cm−1) about the other two axes. The ZFS parameters in neon (assuming g=ge) are DCH2=0.601, ECH2=0.0069, DCD2=0.647, and ECD2=0.0100 cm−1. A minimum value of Dgas=0.761(5) cm−1 is then deduced by accounting for motional averaging. Addition of an estimated 0.03 cm−1 as a correction for matrix electronic effects yields Dgas=0.79 cm−1. This is compared with recent ab initio calculated values. Orientation, motion, and matrix effects in these solid rare gases, xenon, and other matrices are discussed.
Resonant absorption in stationary and nonstationary fields
Authors: Sethu Raman V. , Small G. J.
Published: February 2003

Response of a two-level atom, relaxing owing to strong thermalizing collisions, in a weak near-resonant pulsed radiation field is examined using the second order perturbation solution of the optical Bloch equations. Dependence of the rate of absorption on the collisional relaxation time of the atom, the correlation time, pulse width, and the degree of detuning of the incident field are discussed.
Absorption oscillator strengths for the a′ 1Σ−u–X 1Σ+g and w 1Δu–X 1Σ+g transitions of molecular nitrogen
Authors: Tilford S. G. , Benesch W. M.
Published: February 2003

Measurements have been made of the intensities of absorption bands of molecular nitrogen in the vacuum ultraviolet (vuv) in order to determine the lifetimes of the metastable singlet species a′ 1Σ−u and w 1Δu. The method consists of comparing line and band areas among neighboring features. Interest in these states has recently been heightened by recognition of the energy storing capability of their lowest levels and, in particular, of their role as possible precursors in the formation of NO produced by auroral activity. It is found that the v=0 level of the nitrogen a′ 1Σ−u has a lifetime of 0.013 sec. This is governed by transitions to the ground state. The w 1Δu state, on the other hand, should essentially be radiatively disconnected from the ground state at auroral altitudes due to the lack of collisions. There is no evidence for the w–X transitions at low pressures.
Collision induced dissociation of Ar2. Details of the collision dynamics
Authors: DelleDonne Michael , Howard Robert , Roberts R. E.
Published: February 2003

The reaction Ar2+Ar→3Ar is a prototypical problem in the study of collision induced dissociation since (a) the Ar3 system is expected to behave classically for energies near and beyond the dissociation threshold and (b) the Ar2 pair potential has been determined very accurately from gas kinetic properties and molecular beam experiments. The exact three dimensional cross sections for dissociation and exchange are known from previous work. Here we consider two dimensional trajectories in an effort to assess the effectiveness of various collision geometries. All results refer to Ar2 initially in the ground state. We examine two specific types of initial configuration which we label orthogonal (⊥) and parallel (∥). Detailed opacity functions were calculated at several energies for each configuration. In addition we have carried out zero impact parameter calculations for the dissociation probability as a function of energy. One can conclude from these studies that long range forces significantly affect the collision dynamics and that the orthogonal configuration is more important and representative of the three dimensional results than the collinear model frequently used in dissociation studies.
A unified approach to the interpretation of displacive and order–disorder systems. II. Displacive systems
Authors: Aubry S.
Published: February 2003

In the displacive limit of a one-dimensional coupled double-well model, three sets of propagating solutions of the corresponding nonlinear field equation are exhibited. It is shown that knowledge of their amplitude and their energy suffices to interpret the variation with the temperature and the wave vector of the numerically calculated three-peaks structure of the dynamical correlation function. In addition, Lorentz invariance of the nonlinear field equation shows that localized solutions (propagating walls) have the properties of a relativistic particle, the limit velocity of which is a sound velocity. These propagating walls exhibit nontrivial interactions and, therefore, do not have the usual soliton properties. The behavior of the central peak and specifically the nonzero frequency of the latter, for a nonzero wave vector, is explained by assuming a perfect relativistic gas of such moving walls.
Laser photoluminescence of Bi2
Authors: Gerber G. , Sakurai K. , Broida H. P.
Published: February 2003

Bismuth diatomic molecules are produced in an inert gas flow system and in a heat-pipe oven at pressures from 10−4 to 10 torr. Photoluminescence of Bi2 has been studied with various laser sources and with white light. Strong photoluminescence is due to different vibrational–rotational transitions of the A–X system. Observed weaker photoluminescence series are due to vibrational–rotational transitions of four previously unknown electronic systems of Bi2 with ωe= 154.29±0.45 cm−1; ωe=141.23±0.35 cm−1; ωe=127.05±0.1 cm−1 and ωe=105.68±0.25 cm−1. Long v″ progressions extending over the whole Franck–Condon distribution (including both maxima) are measured for several electronic transitions. Molecular constants for the lower electronic states involved are obtained from spectroscopic analysis. The laser-excited photoluminescence experiments suggest that A is not the first excited state in Bi2 and that X is not the ground state of Bi2 but that the X ′ state is probably the lowest energy state. Photoluminescence with white light excitation consists of emission from Bi2 and atomic Bi. In addition to discrete line photoluminescence, continuum spectra arising from an unbound upper state are observed with 514.5 nm laser excitation and with white light excitation.
Resonance fluorescence study of the gas phase reaction rate of nitric oxide with molecular fluorine
Authors: Kolb C. E.
Published: February 2003

Resonant fluorescent scattering of uv light absorbed by the 0,0 transition of the nitric oxide γ band at 226 nm was used to detect the decrease in NO concentration due to reaction with F2 in a standard flow tube geometry. Rate measurements were performed under pseudo-first-order conditions with F2 in excess at temperatures between 168 and 359 °K. These measurements yielded a temperature dependent bimolecular rate constant for the reaction of NO with F2 of 7.0×10−13 exp(−1150/T) with an estimated accuracy of ±30%.
Heat-pipe-oven reactor (HPOR): I. A new device for flame studies; photon yields in the reaction of Na with CCl4 and N2O
Authors: Luria M. , Eckstrom D. J. , Benson S. W.
Published: February 2003

A new device based on the heat-pipe oven has been demonstrated to have a unique capability for studying reactions of metal atoms with oxidizers. This device allows spherical diffusion flame studies under known, uniform, and easily adjustable, metal atom concentration. low gas pumping rate, clean window operation, and minimized problems of reactive solid disposal. The reactor was used for the study of chemiluminescence of the reactions of sodium vapors with CCl4, with N2O, and with both CCl4 and N2O. The emission from these reactions in the range 2000–9000 Å was identified to be from various excited atomic levels of sodium and from excited C2, mostly in the Swan bands. Even though the emission was very intense, especially in the case of Na+N2O+CCl4 (photon yields of about 10%), it was found that the Na excited-state populations had a Boltzmann distribution corresponding to an electron temperature of 2260 °K. This suggests that the emission of sodium radiation from both the Na+CCl4 and Na+CCl4+N2O flames results from V→E transfer from vibrational reservoir states. In the former case, the excitation mechanism must involve energy pooling of C2‡ with either C(1D) or Na(3p) atoms, while in the latter case, CO‡ contributes as well. Sodium resonance radiation from the Na+N2O flame is believed to result from secondary reactions of oxygen atoms.
The multipole Hamiltonian and magnetic circular dichroism
Authors: Healy W. P.
Published: February 2003

The Power–Zienau–Woolley transformation in molecular quantum electrodynamics is carried out for a molecule in an external static magnetic field. From the gauge-independent multipole Hamiltonian thus obtained the Einstein B coefficients for the absorption of left and right circularly polarized photons by an oriented molecule in a uniform magnetic field are evaluated. The B coefficients are related to the macroscopic absorption coefficients by treating the line shapes phenomenologically and by introducing a Boltzmann factor. A familiar expression for magnetic circular dichroism in a dilute medium is thereby recovered; the calculation is extended beyond the electric dipole approximation so as to include the leading contribution of natural circular dichroism. A simple use of the appropriate Kramers–Kronig dispersion relation leads to the deduction of the optical rotatory dispersion, both natural and magnetic, in spectral regions far removed from absorption bands.
On the relation between collinear and three dimensional collision rates with applications to vibrational energy transfer
Authors: Wassam W. A. , Levine R. D.
Published: February 2003

A simplistic procedure for the generation of three dimensional rate constants is introduced and applied to vibrational energy transfer. The method is based on correcting for the different volumes in phase space available to the products of a collinear and a 3-D collision. Explicit expressions are derived for the 3-D distribution of final vibrational states and the effect of reagent vibrational excitation on the rate for collisions where the initial translational (and rotational) energy is thermally distributed. The procedure is applied to an exactly soluble collinear model of vibrational excitation. The resulting 3-D detailed rate constants are shown to correspond to an exponential decay of the bulk mean vibrational energy. The temperature dependence of the rates is compared to experimental results for the He–CO, Ar–HCl, He–HCl, and He–H2 systems. A surprisal analysis of the rates conforms to an ’’exponential gap’’ representation.
Mass-spectral study of CF3I and C3F7I photolysis
Authors: Tal’roze V. L. , Larichev M. N. , Leipunskii I. O. , Morozov I. I.
Published: February 2003

A mass-spectrometric method for study of the photodissociation of alkyl iodides is developed. It permits the observation of iodine atom concentrations both in the ground and in electronically-excited states. The rate constant of iodine quenching in molecular oxygen is obtained. The relative yields of 5 2P1/2 iodine in the photolysis of CF3I and n-C3F7I are compared.
Oscillator strengths for transitions involving excited states not lowest of their symmetry: NitrogenI and nitrogenII transitions
Authors: Luken William L. , Sinano?lu Oktay
Published: February 2003

Theoretical f values are reported and compared to experiment for the 1134 and 1200 Å transitions of NI and the 660, 746, 747, and 858 Å transitions of NII. These involve states which are not the lowest of their symmetry. These results are obtained with the nonclosed shell many electron theory of Sinano?lu and the proper application of the Hylleraas–Undheim–MacDonald theorem. The results of this work are in much better agreement with experiment than Hartree–Fock calculations and other results found in previous literature.
Oxygen wagging and ring puckering in ? 1A2 cyclobutanone
Authors: Moule D. C.
Published: February 2003

Analysis of the near ultraviolet spectrum of cyclobutanone shows that in its excited state the oxygen atom is bent out of the C2C1C4 plane, producing an inversion doubling of the carbonyl out-of-plane mode in the excited state. The out of plane angle is 39° at the potential minimum, from a fit to the eigenvalues for a quadratic Lorentzian-type potential function. The barrier to inversion of the varbonyl group in the excited state is 1550 cm−1. The interval of 98 cm−1 which is observed throughout the spectrum is assigned as a v′=1, v″=1 sequence transition in the ring puckering mode Q20, from which we have ν′20=134 cm−1, compared to the ground state value of ν″20=36 cm−1. The quartic + quadratic potential for the upper state, which was evaluated from this data, was found to contain only a single minimum, which indicates that the C1C2C3C4 carbon atoms are coplanar in this state. CNDO‖2 potential energy curves for the C1C3 and C2C4 ring puckering coordinates in the two electronic states were found to predict correctly the trends in the spectroscopically derived potentials. The eigenvector coefficients from the CNDO‖2 calculation were subjected to a Mulliken population density analysis. The bond density differences obtained in this calculation attribute the large increase in ring strain energy which occurs on electronic excitation to transannular interaction in the C2C4 direction.
Calculation of one- and two-electron molecular properties by the method of diatomics-in-molecules
Authors: Tully John C.
Published: February 2003

A procedure is proposed for extending the diatomics-in-molecules approach to the calculation of one- and two-electron properties of polyatomic molecules. The method requires an input knowledge of appropriate one- and two-electron properties of the isolated atomic and diatomic fragments. Selected one-electron properties of CH2(3B1) are computed as an illustration. Results are in good agreement with those of an ab initio study reported by Langhoff and Davidson.
Ultraviolet emission in O+–H2 reactive scattering
Authors: Harris H. H. , Leventhal J. J.
Published: February 2003

Luminescence resulting from O+–H2 collisions at relative energies between 1.5 and 35 eV has been studied in order to identify internal energy states of product species; A→X transitions of OH and OH+ are observed. Metastable states of reactant O+ are shown to be responsible for the production of these excited species at low relative energies; ground state O+ ions may contribute to the OH+(A 3Π) yield when sufficient kinetic energy is available to surmount the energy barrier that probably exists in addition to the thermochemical endoergicity of the process. Ground state O+ does not appear to be a major factor in the formation of OH(A 2Σ+). These conclusions are consistent with the nature of the potential energy surfaces of H2O+ inferred from an electronic state correlation diagram.
Coherent energy migration in solids: Determination of the average coherence length in one-dimensional systems using tunable dye lasers
Authors: Shelby R. M. , Zewail A. H. , Harris C. B.
Published: February 2003

The coherent nature of energy propagation in solids at low temperatures was established from the time resolved response of the crystal to short optical pulses obtained from a dye laser (pumped by a nitrogen gas laser). The trapping and detrapping of the energy by shallow defects (x traps) was evident in the spectra and enabled us to extract the coherence length: l≳700 ?=186 molecules for the one-dimensional triplet excitons of 1,2,4,5-tetrachlorobenzene crystals at T<4.2 °K. This length which clearly exceeds the stochastic random walk limit is related to the thermalization mechanisms in this coupled exciton–trap system, and its magnitude supports the notion that exciton–phonon coupling is responsible for the loss of coherence on very long molecular chains (trap concentration is 1/256 000).
Continuum and bound molecular electronic wavefunctions for generalized multiple-scattering potentials
Authors: Siegel Jon , Dill Dan , Dehmer J. L.
Published: February 2003

Existing multiple-scattering treatments of bound and continuum electronic states of molecules are restricted to monopole potentials in the various spherical regions. We extend the treatment within these regions to a general potential. The corresponding multiple-scattering equations should facilitate accurate treatment of, e.g., effects of the buildup of charge between atomic centers due to bonding, of the dipole character of polar molecules, and of external fields.
Complex scaling in two-center wavefunctions
Authors: Hunter G.
Published: February 2003

In the solution of the three-body problem by the Born–Oppenheimer separation it is necessary to scale the electronic wavefunctions in order to produce asymptotically correct internuclear potentials. In general, two different scalings are necessary for any particular three-particle system, corresponding to the two different atoms into which the system may dissociate. Such dual scaling is mandatory for applications to reactive scattering involving both of the different atoms. In the case of equally charged nuclei of unequal mass, the scale parameters are complex numbers.
Potential function for axial–equatorial fluorine atom exchange in PF5, AsF5, and VF5
Authors: Bernstein Lawrence S. , Abramowitz Stanley , Levin Ira W.
Published: February 2003

Gas-phase Raman spectra of the ν7 fundamentals of AsF5 and VF5 were recorded for spectral resolutions approaching 1.5 cm−1. The vibrational transitions associated with ν7 for these systems, as well as for PF5, were interpreted in terms of a two-dimensional anharmonic potential function constrained to a double minimum form for the motions leading to axial–equatorial fluorine atom exchange. The intramolecular exchange barrier heights, determined by the double minimum potentials, lie between 1139 and 995 cm−1 (3.26 to 2.84 kcal/mol where 1 kcal/mol=4.184 kJ/mol) for PF5, 864 and 755 cm−1 (2.47 and 2.16 kcal/mol) for AsF5, and 593 and 428 cm−1 (1.54 and 1.22 kcal/mol) for VF5. A discussion of the dynamics of the fluorine atom interchange pathways suggests that these trigonal bipyramidal (D3h) molecules form C4v intermediates by initially displacing the equatorial fluorine atoms and then by mixing in the axial fluorine distortions as the intramolecular exchange proceeds.
A nearly exact MCSCF+CI calculation of the dissociation energy of OH
Authors: Arnold J. O. , Whiting E. E. , Sharbaugh L. F.
Published: February 2003

The dissociation energy and dipole moment of the ground state of OH have been obtained with a newly developed multiconfiguration, self-consistent field plus configuration interaction CDC 7600 computer program. The computed value of the dissociation energy De is 4.62 eV, which is within the uncertainty limits for the experimental value of 4.63±0.01 eV. The computed value of the dipole moment is 1.645 D, which is very close to the experimental result of 1.66±0.01 D. The present results are also compared to the data obtained from similar calculations with the BISON-MC computer program developed by Das and Wahl.