bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron–vibration interactions in benzene and deuterobenzene
Authors: Lipari N. O. , Duke C. B. , Pietronero L.
Published: February 2003

An exactly solvable model of the linear interaction of molecular vibrations with the electronic states of molecules is utilized to describe the intensity distributions in band systems observed in the ultraviolet photoelectron and ultraviolet absorption spectra of C6H6 and C6D6. From this analysis, the linear coupling constants describing the vibration-induced energy shifts of the 1e1g(π), 3e2g(σ), and 3a1g(σ) one-electron orbitals and of the 1B2u excited state are determined. These coupling constants also are calculated using the spectroscopic CNDO/S2 molecular-orbital model of Lipari and Duke and the vibrational normal modes of Whiffen. The sensitivity of the calculated coupling constants to the precise form of the molecular orbital model is shown to be substantial. The CNDO/S2 model provides a semiquantitative (±50%) description of the measured coupling constants, whereas prior CNDO models (e.g., CNDO/2) predict values for these quantities deviating by as much as an order of magnitude from the measured ones. Although only linear electron–vibration coupling constants can be extracted from observed ultraviolet absorption and photoelectron spectra, the associated quadratic coupling constants can be evaluated using the molecular-orbital model. This analysis indicates that the linear terms dominate the nuclear-relaxation-induced energy shifts for one-electron orbitals but that the quadratic terms can predominate for excited electronic states. The calculated linear and quadratic coupling constants for the 1e2u(π*), 1e1g(π), 3e2g(σ), 1a2u(π), 3a1g(σ) orbitals and the 1B2u, 1B1u, and 1E2u electronic states are given for all 30 independent molecular vibrations of C6H6 and C6D6.
Bound and resonant molecular states formed from and dissociating to atoms and ions which do not exist
Authors: Taylor Howard S. , Bobrowicz Frank W. , Hay P. Jeffrey , Dunning Jr. Thom H.
Published: February 2003

The importance of considering the existence of bound and resonant molecular states arising from and dissociating to atoms and ions which are thermodynamically unstable is discussed. Physical mechanisms can exist at finite internuclear separations which do not exist at infinite separations that can stabilize such states. As an example, it is shown that such states explain the unexpected minima occuring in the potential energy curves for the 1Πu and upper 1Σg+ states of Zn2. Such states may also be of fundamental importance in understanding experimental phenomena such as the dissociative attachment and vibrational excitation patterns observed in electron impact experiments on HCl.
Laser generation of transient photocurrents in liquids without the application of an electric field
Authors: Bergman A. , Dickson C. R. , Lidofsky S. D. , Zare R. N.
Published: February 2003

The first known observation of the Dember effect in liquids is reported using a pulsed nitrogen laser (337.1 nm) which generates photocarriers in a solution of anthracene in tetrahydrofuran. From the sign of the Dember signal (diffusion current), the majority photocarriers are determined to be positively charged anthracene ions. The decay of the Dember signal is exponential. This differs from the second-order decay kinetics reported previously in photoconductivity studies of aromatic molecules in solution. The exponential decay is interpreted as a bimolecular recombination process which includes trapping. The relaxation of the photocarriers is significantly altered when the solution is in contact with the electrodes. The Dember signal has a quadratic dependence on the excitation intensity. The photoionization mechanism is shown to result from at least two competing biphotonic absorption mechanisms, the consecutive absorption of two photons and triplet–triplet annihilation.
A Monte Carlo investigation of C++ electronic structure
Authors: Gibbs R. L. , Dunn Tucson
Published: February 2003

A Monte Carlo method is applied to C++ to illustrate the capability of such an approach. A correlated wavefunction which, due to integration difficulties, cannot be used in traditional approaches, is used for this illustration. The Hamiltonian is partitioned so the approach might be compared to other approximations. One- and two-body distributions, three-body terms, and several atomic parameters are calculated. The Hartree–Fock approximation is used to compute the same properties for comparison.
A nuclear magnetic resonance calibration of a
b i
n
i
t
i
o and semiempirical molecular orbital calculations of the conformational energies of 2-fluorobenzyl alcohol
Authors: Schaefer Ted , Parr William J. E.
Published: February 2003

Of the 24 spectral parameters describing the high resolution proton and fluorine nuclear magnetic resonance spectra of 2-fluorobenzyl alcohol, the conformational dependence of four is sufficiently well known for a comparison with conformational energy predictions from ab initio (STO−3G) and semiempirical (INDO and MINDO/3) molecular orbital calculations. Such calculations are performed for 18 conformations and the prediction by STO-3G of the four spectral parameters is in good agreement with experiment. The ab initio calculations are much more expensive than the semiempirical calculations but are also much more reliable.
Diatom potential curves and transition moment functions from continuum absorption coefficients: Br2
Authors: Le Roy Robert J. , Macdonald R. Glen , Burns George
Published: February 2003

A program for calculating diatomic molecule absorption coefficients using ’’exact’’ numerically computed radial wavefunctions was developed and used to examine critically a number of approximations which are frequently used in absorption coefficient calculations. These tests showed that both use of the delta function approximation, and fixing the rotational quantum number in the radial overlap integrals at J=0, introduce errors of ?4%–6% of the absorption coefficient maximum εmax. Similarly, errors of ?1%–2% of εmax are introduced by either fixing the initial state J at the average value for the given temperature, or by using Gislason’s Airy-function approximation for the unbound state wavefunction. A simple procedure for shrinking the sum over initial state J’s without significant loss of accuracy was therefore devised. These techniques were then applied to the analysis of the visible absorption continuum of Br2, and a nonlinear least-squares fitting procedure used to determine optimized final state potential curves and transition moment functions for the two overlapping electronic transitions. In the region to which the data are sensitive, 2.10≲R≲2.55 Å, the transition moments so obtained (in debyes, lengths in angstroms) are M0(R) =0.3905+0.265(R−2.3) for the B (3Π+Ou) ←X (1Σ+g) transitions and M1(R) =0.5060–0.154(R−2.3) for the 1Π1u←X (1Σ+g) spectrum. The concomitant repulsive potential curves for the B (3Π+Ou) and 1Π1u states are, respectively (in cm−1, expressed relative to the ground state dissociation limit; lengths in angstroms): V0(R) =−1174+5494 exp[−5.807(R−2.3)] and V1(R) =7654 exp[−4.637(R−2.3)−0.879(R−2.3)2], where the former expression applies only to the region inside the B-state inner turning point for v=8, R1(8) =2.4485 Å. Comparisons with experimental relative intensities in the discrete portion of the B–X spectrum, and with molecular beam photofragment spectroscopy measurements of the relative strengths of the two electronic transitions, are also reported.
B atoms, B2 and H2BO molecules: ESR and optical spectra at 4 °K
Authors: Graham W. R. M. , Weltner W.
Published: February 2003

Elemental boron has been vaporized and trapped in solid argon at 4–10 °K. The Douglas–Herzberg transition of B2 has been observed in absorption at 3300 Å at 10 °K, indicating that the lower Σ state is the ground electronic state. However, B2 was not observed via ESR, which is interpreted as support for a 3Σ−g ground state with a zero-field splitting greater than about 4 cm−1, rather than 5Σ−u favored by ab initio calculations. The ESP spectrum of B atoms in solid argon is also detected. It exhibits axial symmetry and almost complete quenching of the orbital angular momentum of the 2P1/2 free atom, with g∥=2.0014(5), g⊥=1.9645(30). The simple crystal field model has been applied in the interpretation of the g and hyperfine tensors and comparison made with Al and Ga in argon, as studied by Ammeter and Schlosnagle. The ESR spectrum of H2BO, appearing as an impurity, was observed for the first time, and its magnetic parameters determined.
Optical activity of crystals
Authors: Jerphagnon J. , Chemla D. S.
Published: February 2003

The analysis of the crystalline optical activity from the rotational invariance point of view allows one to quantitatively distinguish between molecular enantiomorphism, a polar longitudinal effect, and structural rotatory power. These three effects are discussed in connection with crystallographic symmetry. An alternative formulation of optical activity is presented. The so-called specific rotativity is empirically shown to be dispersionless and almost insensitive to local field effects. In addition it is of the same order of magnitude for all known materials. These features are discussed in terms of a coupled oscillator model.
Temperature-dependent relaxation of CO(v=1) by HD, D2, and He and of D2(v=1) by D2
Authors: Drozdoski Walter S. , Young Robert M. , Bates Richard D. , Hancock J. K.
Published: February 2003

Rate constants for the vibrational deactivation of CO in collisions with HD, D2, and He have been measured as a function of temperature using the laser excited vibrational fluorescence technique. Throughout the 109–630 °K range, CO–He and CO–HD samples exhibit a single exponential decay, dominated by V–T,R transfer, with rates increasing rapidly with temperature. Typical collision deactivation rate constants at 630 °K are 22.7 sec−1 torr−1 for He and 82.5 sec−1 torr−1 for HD, and, at 109 °K, 0.067 sec−1 torr−1 for He and 0.27 sec−1 torr−1 for HD. At low temperatures, diffusion and radiative decay become important contributions to the observed rates. In CO–D2 mixtures, double exponential decay of CO fluorescence at large D2 mole fractions is obtained, corresponding to rapid V–V transfer between the (v=1) vibrational levels of CO and D2, followed by coupled V–T,R deactivation. The V–V transfer rate (ΔE=−850 cm−1) increases from 0.26 sec−1 torr−1 at 202 °K to 69.4 sec−1 torr−1 at 633 °K. The V–T,R deactivation rate constant for excited CO by D2 is significantly slower than that by HD and H2, going from 0.089 sec−1 torr−1 at 156 °K to 3.7 sec−1 torr−1 at 450 °K, indicating the decreased importance of the rotational states of D2 in the deactivation of CO than hypothesized for H2 and HD. The deactivation of excited D2 by collisions with D2 also increases with temperature, ranging from 0.080 sec−1 torr−1 at 202 °K to 6.9 sec−1 torr−1 at 450 °K.
Negative ion electron impact studies of arsenic trihalides: AsF3, AsCl3, and AsBr3
Authors: Pabst R. E. , Bennett S. L. , Margrave J. L. , Franklin J. L.
Published: February 2003

Low energy electron impact of AsF3, AsCl3, and AsBr3 gases gave many ions by dissociative resonance capture. Appearance potentials have been determined by deconvolution of the capture curves, and translational energies of the majority of the ions have been measured through the resonance processes. The X− ions are formed along with electronically excited neutral AsX2, and the results allow the electronic transition energies to be determined: 7.3, 4.3, and 2.6 eV for AsF2, AsCl2, and AsBr2, respectively. From the processes giving AsX−2+X, the experimental data allow the heats of formation of the negative ions ΔHf(AsX−2) to be derived: −129.7, −65.3, and −72.5 kcal mole−1 for AsF−2, AsCl−2, and AsBr−2, respectively. The ions AsCl− and AsBr− appear to be formed along with electronically excited X2 neutral: ΔHf(AsCl−) =−2.2 kcal mole−1 and ΔHf(AsBr−) =1.7 kcal mole−1 are derived. The ions X−2 appear to be formed with electronically excited AsX neutral, allowing the electronic transition energies to be derived: E*el(AsBr) ≳0.6 eV and E*el(AsCl) =2.5 eV.
Isotope shifts in methane near 6000 cm−1
Authors: Fox K. , Halsey G. W. , Jennings D. E.
Published: February 2003

Isotope shifts for well resolved vibration-rotation spectra of 12CH4 and 13CH4 in the 6000 cm−1 region are reported. (AIP).
Theoretical studies of molecular ions: Be−2
Authors: Jordan K. D. , Simons Jack
Published: February 2003

The ground 2Σg state of the Be−2 anion is predicted to be stable with respect to electron loss and dissociation, with a vertical electron detachment energy at Re=4.62 a.u. of 0.29 eV. The Be−2 potential curve is found to cross the repulsive Be2 curve near 5.8 a.u., which corresponds closely to the outer turning point of the v=3 vibration of Be−2.
The screened INDO (INDO/S) model: Application to photoelectron spectrum of benzonitrile
Authors: Krogh-Jespersen Karsten , Ratner Mark
Published: February 2003

A screened INDO model, which we previously developed for analysis of optical spectra and excited-state properties of organic compounds, is applied to the prediction of photoelectron spectra. Results are quite satisfactory for benzene, and an assignment based on Koopmans’ theorem allows complete assignment, including previously unassigned peaks, in the benzonitrile molecule.
Relative quenching rates of NO(B 2Π, v′=9)
Authors: Hikida T. , Nakajima S. , Ichimura T. , Mori Y.
Published: February 2003

The quenching rates of NO(B 2Π, v′=9) have been measured for He, H2, CO, CF4, N2, and CO2, relative to NO self-quenching rate, by noting the reduction in fluorescence intensity of the β (v′=9) bands. The values obtained are 1.0:0.12:0.13:0.13:0.14:0.17:0.18 for NO, He, H2, CO, CF4, N2, and CO2. Absolute quenching rates are also estimated to be close to hard-sphere collision rate constants and the quantum yield of the β (v′=9) emission to be of the order of 10−2.
Laser photoelectron spectrometry of PO−, PH−, and PH−2
Authors: Zittel P. F. , Lineberger W. C.
Published: February 2003

Fixed-frequency laser photoelectron spectrometry has been used to study the ions PO−, PH−, and PH−2. The electron affinities of PO, PH, and PH2 were determined to be (1.092±0.010), (1.028±0.010), and (1.271±0.010) eV, respectively. A bound excited state (ã 1Δ) of PO− was observed with a term energy of (0.556±0.010) eV. The vibrational frequencies of X̃ 3Σ− PO−, ã 1Δ PO−, X̃ 2Π PH−, and 2B1 PH2 (ν1) were found to be (1000±70), (1020±80), (2230±100), and (2270±80) cm−1, respectively. A Franck–Condon factor analysis of the PO− and PH− spectra gave the equilibrium internuclear separations re(X̃ 3Σ− PO−) = (1.540±0.010) Å and 1.414⩾re(X̃ 2Π PH−) ⩾1.400 Å. The geometries of 1A1 PH−2 and 2B1 PH2 were found to be nearly the same. The PH− photoelectron spectrum includes transitions to the PH X̃ 3Σ− and ã 1Δ states, allowing determination of the intercombination separation of (0.950±0.010) eV.
On microwave discharge sources of new chemical species for matrix-isolation spectroscopy and the identification of charged species
Authors: Wight Charles A. , Ault Bruce S. , Andrews Lester
Published: February 2003

The mechanism for trapping new chemical species by condensing the products of a microwave discharge with inert matrices has been investigated. Variation of geometrical, electrical, and chemical parameters of the Ar, HCl, Cl2 system indicated that the major product species—HCl2 radical or anion—was formed under conditions where neither ions nor atomic species produced in the discharge were condensed in the matrix. The mechanism for forming the product species is vacuum ultraviolet photolysis of the sample during deposition with radiation from the microwave discharge, since a coaxial orifice discharge tube provided photolysis and produced the product species, while studies with an off-axis orifice discharge tube, which could not serve as a photolysis source, did not produce the product. The H atom–Cl2 reaction gave HCl using both discharge tubes, but the HClx2 species was produced only with the coaxial tube. Hence, this species requires vacuum ultraviolet light in addition to H and Cl atoms for its production, and its identification as HCl−2 is strongly supported. Similar observations of ArnD+ and O−3 with the coaxial tube, but not with the off-axis tube, confirm their identification as charged species.
Penning ionization of He(2 3S)+Ar
Authors: Wang Z. F. , Hickman Albert P. , Shobatake Kosuke , Lee Y. T.
Published: February 2003

A new measurement of the differential cross sections for He(2 3S)+Ar has been carried out at 65 and 132 meV. A potential V0−iVi is found which fits our data as well as the total ionization cross section results of Illenberger and Niehaus. Some sources of uncertainties inherent in our analysis are discussed. The first order semiclassical approximation, which has been employed by several groups, is critically examined. A guideline is proposed to define the region within which this approach can be safely adopted.
A semiclassical theory for spectral line broadening in molecules
Authors: Smith Earl W. , Giraud M. , Cooper J.
Published: February 2003

A semiclassical S-matrix theory is developed and applied to spectral line broadening in linear molecules perturbed by atoms. This theory uses curved classical trajectories determined by the isotropic part of the atom–molecule interaction and the S-matrix is treated to all orders in the interaction. Numerical calculations can be made rather easily even for high quantum numbers. The theory is least accurate for very low quantum numbers, but even then calculations agree to within 10% with close coupling results where comparisons could be made. Comparisons were also made with other theoretical approaches using model potentials and with experiment using ab initio potential surfaces.
Low-temperature vibrational relaxation in gaseous deuterium fluoride: Monomer and polymer deactivation effects
Authors: Hancock J. K. , Saunders A. W.
Published: February 2003

Deuterium fluoride vibrational energy transfer measurements have been performed at 198, 209, 232, 264, and 296°K using the laser excited vibrational fluorescence technique. Rate contants for the following processes have been determined: HF(v=1)+DF(v=0) →ke+k12 HF(v=0)+DF(v=1,0) ÷ΔE=1055,3962 cm−1, DF(v=1)+DF(v=0) →k222DF(v=0)+ΔE=2907 cm−1, DF(v=1)+Ar→k2mDF(v=0)+Ar+ΔE=2907 cm−1. Experimental results taken at 198, 209, and 232°K exhibited rate enhancements for increasing DF pressure which is most easily explained as HF(v=1) and DF(v=1) quenching by (DF)n. Relaxation rates in this regime were best described by the empirical equation τ−1=C exp(DPDF), where C and D are temperature dependent coefficients and PDF is the total DF pressure, [DF+(DF)n]. The variation τ−1 with exp(PDF) cannot be reconciled in terms of a monomer–dimer model only. Higher order terms (n=4 and 6) are expected to dominate the collisional quenching of HF(v=1) and DF(v=1). The characterization of (DF)n and its effect upon these studies are discussed in detail. Measurements in undiluted DF were also performed at 200 and 296°K. The room temperature results clearly indicate a slower DF self-quenching rate than measured in DF–argon mixtures. This relaxation time lengthening can be explained by assuming that vibrational and rotational relaxation occurs on a similar time scale in undiluted DF.
Wigner phase space method: Analysis for semiclassical applications
Authors: Heller Eric J.
Published: February 2003

We investigate the suitability of the Wigner method as a tool for semiclassical dynamics. In spite of appearances, the dynamical time evolution of Wigner phase space densities is found not to reduce to classical dynamics in most circumstances, even as h→0. In certain applications involving highly ’coherent’ density matrices, this precludes direct h-expansion treatment of quantum corrections. However, by selective resummation of terms in the Wigner–Moyal series for the quantum phase space propagation it is possible to arrive at a revised or renormalized classicallike dynamics which solves the difficulties of the direct approach. In this paper, we review the Wigner method, qualitatively introduce the difficulties encountered in certain semiclassical applications, and derive quantitative means of surmounting these difficulties. Possible practical applications are discussed.
Valence bond calculation of nuclear spin–spin coupling constants. I. The distinguishable electron method
Authors: Kirtman Bernard , Kirtley Kathryn Barr , Hasman David A.
Published: February 2003

The uncoupled valence bond ’’distinguishable electron’’ method (DEM) is extended to the nuclear spin–spin coupling property. Provision is made in the theory for the triplet spectrum to have a different zero order potential from that of the ground state singlet. The case of a strong interaction with a low lying triplet is treated explicitly. As an example, we carry out a calculation on HD in the simple screening approximation using identical singlet and triplet screening constants. For this zero-order potential the one electron DEM equations are exactly soluble. Furthermore, the major effect due to splitting of the screening constants is readily determined. Our value for the HD coupling constant is in good agreement with experiment.
Energy flux of hot atoms
Authors: Wotzak G. P. , Kostin M. D.
Published: February 2003

The process in which hot atoms collide with thermal atoms of a gas, transfer kinetic energy to them, and produce additional hot atoms is investigated. A stochastic method is used to obtain numerical results for the spatial and time dependent energy flux of hot atoms in a gas. The results indicate that in hot atom systems a front followed by an intense energy flux of hot atoms may develop.
The heats of transition of solid ethane
Authors: Roder Hans M.
Published: February 2003

The existence of a solid–solid transition in ethane is now generally accepted. A value for the heat of transition, 2437±35 J/mol at 89.774 K, together with a new value for the heat of fusion, 279±6 J/mol at 90.337 K, is reported in this work. The sum of the new values is in excellent agreement with the earlier measurements, provided that the heat required to raise the temperature of the solid between transitions is included. The transition temperatures are in agreement with the most recent determinations of other authors.
Rotational diffusion of spheroidal molecules
Authors: Warchol Mark P. , Vaughan Worth E.
Published: February 2003

Berne’s formalism for calculation of the correlation function of the permanent dipole moment of a sphere of linear molecules imbedded in its own medium is applied to spheroidal molecules. It is found that distinct parts of the correlation function are associated with the individual dipole moment components but the constraint imposed by the geometry does not prevent an interaction between components which affects the dynamic response. The calculation provides an algorithm for the ellipsoidal case (generalization of the Perrin model) and gives limiting cases relevant to dielectric experiments to which the ellipsoidal calculation must conform.
The question of energy randomization in the decomposition of chemically activated C2H4F
Authors: Farrar J. M. , Lee Y. T.
Published: February 2003

The question of whether energy randomization occurs in the decomposition of chemically activated C2H4F is further examined with the crossed molecular beam technique by measuring the collision energy dependence of the C2H3F product recoil energy distributions for the reaction F+C2H4. The symmetric angular distributions of product molecules indicate that the lifetime of C2H4F remains longer than a rotational period even at the highest collision energy studied. The effect of the exit channel barrier becomes less important at higher collision energies since the fraction of the total energy associated with the barrier becomes smaller as the collision energy increases. The experimental recoil velocity distributions yield average product translational energies which remain almost constant at ∼50% of the total available energy: These results are shown to be inconsistent with phase space theory and the recent ’’Tight Transition State’’ theory of Marcus. Our experiments and the infrared chemiluminescence experiments of McDonald and co-workers indicate that the C2H3F product energy distribution is nonstatistical, a conclusion seemingly at variance with Rowland’s determination of lifetimes from stabilization of chemically activated C2H4F produced in hot-atom experiments. We discuss possible explanations for differences among the experimental data on this system.
Energy-dependent cross sections for quenching of Na(3p 2P) by several gases
Authors: Barker John R. , Weston Ralph E.
Published: February 2003

The lifetime of Na(3p 2P) atoms, in the presence of various quenching gases (H2, D2, N2, CO, CO2, and C2H4), has been directly determined by the single-photon time-correlation method. Na(2P) atoms were produced with varying amounts of translational energy by photodissociation of NaI with light of wavelengths 215–240 nm. The large cross sections obtained, and their inverse dependence on relative collisional energy, are best explained on the basis of a ’’harpooning’’ mechanism, in which a strongly attractive ionic state provides the coupling between upper and lower covalent states.
Infrared matrix study of reactions of UF4 with fluorine
Authors: Kunze K. R. , Hauge R. H. , Hamill D. , Margrave J. L.
Published: February 2003

Chemical reactions between UF4 and fluorine to produce UF5 and UF6 were studied by the technique of matrix isolation and observation of infrared spectra of reacting species in the matrix.(AIP)
Magnetic susceptibility of 249Cf in an octahedral environment
Authors: Karraker D. G. , Dunlap B. D.
Published: February 2003

The magnetic susceptibility of Cf3+ doped into Cs2NaYCl6 is reported over the temperature range 2.2 to 100°K.(AIP)
Depolarized Rayleigh scattering and backbone motion of polypropylene glycol
Authors: Jones Dane R. , Wang C. H.
Published: February 2003

The depolarized Rayleigh scattering spectra of polypropylene glycol in optically isotropic cyclohexane were studied as a function of polymer molecular weight, concentration, and temperature. The relaxation times were obtained by curve fitting the observed spectra to Lorentzian functions convoluted with the instrumental spectra. By studying the dependence of relaxation times on viscosity, temperature, and concentration and also comparing the results with theoretical predictions of molecular and sidegroup reorientations, we have associated the depolarized Rayleigh scattering spectra of polypropylene glycol with the segmental motion of polymer backbone. The relaxation times for dilute solutions were found to be larger than those for concentrated solutions. The relaxation times were also found to be independent of the molecular weights of the polymer. The depolarized Rayleigh scattering results were also compared with other types of relaxation studies.
Calculation of the positronium hyperfine pressure shift in buffer gases
Authors: Bearman Gregory H. , Mills Allen P.
Published: February 2003

An approximate quantitative theory of the positronium hyperfine pressure shift (HPS) is discussed. Adopting the approach of Adrian, the fractional shift Δ (R) is calculated as a function of the positronium–buffer gas separation R; the measured HPS is obtained by a thermal average of Δ (R) over R. A quantum mechanical averaging technique is used, rather than classical methods, in which the Boltzmann factor exp[−V (R)/kT], where V (R) is the intermolecular pair potential, is replaced by the positronium center-of-mass wavefunctions. The fractional shift Δ (R) is divided into two parts; a negative long range van der Waals contribution and a short range positive exchange contribution. It is argued that in the positronium center-of-mass approximation the short range contribution to Δ (R) is the same for both hydrogen and positronium and therefore existing calculations on hydrogen are utilized. Perturbation theory is used to obtain the long range contribution of Δ (R) and two models are used to evaluate the matrix elements that occur. In either model the long range contribution is related to the van der Waals interaction and can be obtained from a power series in R−1. A hard core potential model of the problem is solved exactly and predicts the HPS to be nearly temperature independent, a result essentially due to the long deBroglie wavelength of thermal positronium. Finally, the wave equation is numerically integrated with realistic potentials to find the positronium center-of-mass wavefunctions and the pressure shifts. Results of the numerical calculation also indicate a relatively small temperature coefficient of the HPS. The calculations of the positronium hyperfine pressure shift have substantially better agreement with measured pressure shifts than do calculations on hydrogen.
The infrared and Raman spectra of matrix isolated binary and mixed mercury halides. II. Extension to low frequencies
Authors: Givan A. , Loewenschuss A.
Published: February 2003

The vibrational investigation of all matrix isolated HgX2 and HgXY (X,Y=Cl, Br, I) molecules has been extended to the bending mode region. The use of an auxiliary third monochromator (TTM) in Raman experiments reduced background scattering down to low frequencies, previously inaccessible. All ν2 frequencies are reported, standard entropies evaluated, and bending force constants calculated. The possibility of slightly bent structures, especially for HgBrI, as formerly discussed on the basis of stretching force constant trends, is reinforced. Irregularities in relative intensities of the stretching mode bands of the mixed dihalides are interpreted in terms of differences in electronegativity of the halogens. Several observations are also made on nonmonomeric bands.
Ice crystal growth in water vapor at high saturation
Authors: Bartley David L.
Published: February 2003

A simple technique is presented for estimating the energy of formation of monolayer icelike clusters at ice–vapor interfaces. Under the assumptions that the ice surfaces are smooth and sparsely covered with monomers, dimers, etc., in near equilibrium with the vapor, and that the bond energies and configurational entropy dominate the energy of formation, it is found that the basal surfaces prefer triangular embryos with an orientation which reverses from layer to layer, whereas the most stable clusters on the prism surfaces are rectangular in configuration. The preferred prism clusters are determined to have a significantly lower critical energy of formation than the basal clusters due to differences in both corner free energy and configurational entropy. This phenomenon provides a mechanism for strongly anisotropic crystal growth at high saturations.
Infrared laser enhanced reactions: Chemistry of NO(v=1) with O3
Authors: Stephenson J. C. , Freund S. M.
Published: February 2003

Vibrationally excited nitric oxide, produced by absorption of carbon monoxide laser radiation, was found to react significantly faster at T=298 °K than does thermal NO in the reaction with ozone: NO+O3→k1NO2* (2B1)+O2, (1) NO(v=1)+O3→k′1NO*2(2B1)+O2, (1′) NO+O3→k2NO2(2A1)+O2, (2) NO(v=1)+O3→k′2NO2(2A1)+O2. (2′) The modulation in visible fluorescence from the NO2* product, following laser excitation of NO, was measured. The exponential decay time of NO(v=1) infrared fluorescence following the laser excitation was also monitored and related to the vibrational deactivation process NO(v=1)+MkNO–M→NO+M. The results obtained are k′1/k1=4.7±2.61.4, (k′2+kNO–O3)/k2?18, and kNO–O2=920±80 sec−1⋅Torr−1. This rate enhancement occurs even though the excited NO bond does not break during the reaction.
Potential surface for the collinear collision of Ne and H2+
Authors: Hayes E. F. , Siu A. K. Q. , Chapman F. M. , Matcha R. L.
Published: February 2003

A potential energy surface for the Ne–H2+ reaction has been obtained in the LCAO–MO–SCF approximation. Analysis of the surface indicates that the reaction Ne+H2+→NeH++H should proceed with an endoergicity of 12 kcal/mole, in agreement with the experimental results of Chupka and Russell. Several procedures for parameterizing a diatomics-in-molecules (DIM) representation of the NeH2+ surface are considered. The results show that an accurate representation of the SCF surface can be obtained from the DIM model using a minimum of diatomic and triatomic data.
Wideline NMR studies of ammonia intercalation complexes. I. TaS2 (NH3) and TiS2 (NH3)
Authors: Silbernagel B. G. , Dines M. B. , Gamble F. R. , Gebhard L. A. , Whittingham M. S.
Published: February 2003

Wideline NMR observations of the 1H and 15N nuclei in the intercalation complexes TaS2(NH3) and TiS2(NH3) have been made at temperatures from 5 to 300 K. Changes in the dynamics of the intercalated NH3 molecules have been traced by determining the temperature dependence of the linewidths and second moments of the 1H resonance. Above 160 K, molecular motions average the proton dipolar interactions to zero, which requires that molecular diffusion and reorientation occur at rates in excess of 105 s−1. The residual proton shifts reflect the magnetic properties of the complexes. Around 125 K, a strong line broadening indicates that translation and reorientation have slowed to this rate. Although translation and reorientation are correlated, the data suggests that they are not tightly coupled to one another. The molecules continus to spin about their threefold (C3) symmetry axes at 5 K, and saturation studies of the resonance line suggest that the rate for this spinning is still ∼108 s−1 at this temperature. In contrast to solid NH3, the NH3 molecules in these intercalation complexes are much more mobile in spite of comparable molecular densities. Apparently the two dimensional character of the NH3 array precludes the establishment of the directed hydrogen bonds that inhibit motion in solid ammonia.
A new algorithm for molecular geometry optimization with illustrative application to methylamine
Authors: Payne Philip W.
Published: February 2003

The Hessian matrix needed for Newton–Raphson optimization can be directly evaluated without recourse to repeated calculations of the energy gradient. This affords a substantial improvement over optimization methods currently used for molecular calculations. Application to methylamine shows that torsion and inversion are strongly coupled.
Correlation energy difference models for first and second row hydrides
Authors: Zittel P. F. , Lineberger W. C.
Published: February 2003

Experimental measurements of electron affinities and intercombination splittings for first- and second-row diatomic hydrides are used with accurate, near Hartree–Fock calculations to determine the change in electron correlation energy (δEcorr) on negative ion formation and intercombination excitation. Values of δEcorr for the diatomic processes are found to be nearly identical to δEcorr for the analogous united atom and separated atom processes. The results are interpreted in terms of the similarity between the π molecular orbitals of the diatomic and the p orbitals of the atomic species. The analysis is extended to some triatomic dihydrides for which the relevant experimental and theoretical data are available.
A study of nitrogen dioxide (22B2) photodecomposition to O(1D) and NO(2Π) in its second predissociation region 2500−2139 Å
Authors: Uselman William M. , Lee Edward K. C.
Published: February 2003

A photochemical decomposition study of NO2(22B2) from specified vibronic states is reported. The yields of excited oxygen atoms O(1D), below and abovce the 2440 Å dissociation limit (40973 cm−1), were measured in the presence of neopentane through the C−H insertion product neopentyl alcohol. The sharp onset of O(1D) formation below 2450 Å suggests a change in the predissociation mechanism probably involving a vibrational energy redistribution mechanism at energies above this limit. Vibrational mode specificty is vibrational energy redistribution mechanism at energies above this limit. Vibrational mode specificity is examined, and the asymmetric stretch vibration (ν3′) is considered important in the O(1D) formation. It is apparent from the O(1D) quantum yield of 0.5±0.1 at high energies that the formation of O(3P) products is equally competitive. The electronic symmetry correlations of various NO2 species with different possible dissociation products are clarified. The assistance of the ground state rotationl energy is affecting the predissociation by supplementing tyhe photon energy is less apparent with the 22B2 state than at the lower 3979 Å predissociation limit [E.K.C. Lee and W. M. Uselman, Faraday Discuss. Chem. Soc. 53, 125, (1972)] due to the larger rotational barriers resulting from the shallower potential well depth of the 22B2 state.
Relativistic effects in diatomic molecules. II. Relativistic contributions to binding energy in alkali halide molecules
Authors: Matcha Robert L.
Published: February 2003

First order perturbation theory, treating the relativistic Breit–Pauli operators as perturbations and LCAO–SCF–MO solutions to Hartree–Fock equations as zeroth order wavefunctions, is used to investigate changes in relativistic related portions of the energy accompanying molecular formation. The dissociation energy associated with these interactions Derel is found to vary from 0.94 kcal for LiF to 7.7 kcal for LiBr. The latter represents 7.8% of the dissociation energy. The principal contributor to this quantity is the spin–orbit energy change.
Measurement of thermal electron dissociative attachment rate constants for halogen gases using a flowing afterglow technique
Authors: Sides G. D. , Tiernan T. O. , Hanrahan R. J.
Published: February 2003

A flowing afterglow apparatus was constructed and used to measure thermal electron attachment rate constants in the halogen gases fluorine, chlorine, and bromine. The operation of the afterglow system and the mathematical models applied in treating the data were tested by measuring the thermal attachment rate constant for electrons in sulfur hexafluoride. The average value obtained for this rate constant is 4.2±1.1×10−8 cm3 molecule−1 sec−1 when a microwave discharge was used as the electron source and 3.6±1.8×10−8 cm3 molecule−1 sec−1 when a filament was used as the electron source. The average electron temperature was estimated to be approximately 600 °K for the microwave discharge source and approximately 350 °K for the filament source. A charge transfer reaction between sulfur hexafluoride and the ion O2− was also investigated in the present study to further assess the operation of the flowing afterglow apparatus in the microwave discharge source configuration. The average rate constant obtained for this reaction is 3.7±0.4×10−11 cm3 molecule−1 sec−1 for an estimated ion temperature of 300–325 °K. The average rate constants obtained for the dissociative attachment of electrons in fluorine are 4.6±1.2×10−9 cm3 molecule−1 sec−1 for an electron temperature of approximately 600°K and 3.1±1.2×10−9 cm3 molecule−1 sec−1 for an electron temperature of approximately 350 °K. The average rate constants obtained for dissociative electron attachment in chlorine and bromine are 3.7±1.7×10−9 and 1.0±0.9×10−11 cm3 molecule−1 sec−1, respectively, for an electron temperature of approximately 350 °K. The rate constant for the three-body attachment reaction Br−+Br2+Ar to form Br3− was also measured and found to be 1.9±0.5×10−28 cm6 molecule−2 sec−1.
A Mössbauer study of aftereffects of Auger events in cobalt chelates with varying degree of conjugation
Authors: Misroch Mark B. , Schramm Charles J. , Nath Amar
Published: February 2003

The electron-capture decay of a cobalt-57 atom triggers an Auger event resulting in the loss of several electrons from the molecule in which it is incorporated. The 14.4 keV Mössbauer emission conveys information regarding the chemical forms in which the daughter iron-57 is stabilized within 10−7 sec following electron capture. The electronic relaxation occurs very rapidly and several tens of eV electronic excitation energy is deposited on the molecule during charge neutralization. Emission Mössbauer spectra of cobalt-57 labeled complexes of the strong field ligands phenanthroline and ethylenediamine, namely [57Co(III)(phen)3](ClO4)3⋅2H2O, [57Co(III)(phen)2(en)]Cl3⋅H2O, [57Co(III)(phen)(en)2] (NO3)3, and [57Co(III)(en)3]Cl3 were studied. The chelate molecules possess varying degree of conjugation. The presence of strong field ligands in the chelates made it easier to identify the high spin ionic iron species Fe2+ and Fe3+, which can form only as a result of fragmentation. Molecules which have no conjugation, e.g., [57Co(III)(en)3]3+, or which possess small degree of conjugation like [57Co(III)(phen) (en)2]3+, fragment in practically 100% of the Auger events. On the other hand, the molecules of [57Co(III)(phen)2(en)]3+, which possess relatively higher degree of conjugation, escape fragmentation in about 20% of the events; while the highly conjugated molecules of [57Co(III) (phen)3]3+ seem to escape fragmentation in almost 100% of the events. One can perhaps conclude that the probability of fragmentation of a molecule as a consequence of the Auger event diminishes with higher degree of conjugation in the molecule. It is speculated that the aftereffects of the Auger event result in collective excitation of the π-electrons in a highly conjugated system. The de-excitation of the ’’plasmon’’ occurs rapidly, presumably through emission of a single electron or an energetic photon. The mixed ligand chelates, [57Co(III)(phen)2(en)]3+ and [57Co(III)(phen) (en)2]3+ show an interesting phenomenon. The cationic iron species formed as a result of fragmentation recombines with electron(s) to form an electronically excited cation, presumably 57Fe+2 which interacts with neighboring cobalt chelate molecules. The phenanthroline ligands are preferentially transferred onto the electronically excited cation forming [57Fe(III)(phen)3]3+ quite efficiently. The transfer of ligands occur in less than 10−7 sec., as is evidenced by the manifestation of the tris-phenanthroline chelate in the emission Mössbauer spectra.
Endoergic ion–molecule-collision processes of negative ions. I. Collision of I− on SO2
Authors: Refaey Kamel M. A. , Franklin J. L.
Published: February 2003

A new method has been developed for the investigation of endoergic ion–molecule-collision processes of negative ions. This method is, in effect, an extension of an earlier one developed by Refaey and Chupka used in connection with positive ions. An ion source with separate ionization and collision chambers was constructed and mounted on a Bendix time-of-flight mass spectrometer. I− ions produced in the ionization chamber by electron resonance capture of HI are pulsed to the desired energy where they enter a collision chamber containing a different gas. Cross sections for various endoergic ion–molecule-collision processes are measured as a function of the incident ion (I−) energy. Collision of I− on SO2 gives the secondary ions SO2−, IO−, O−, SO−, and S− and/or O2− in the energy range 2–25 eV (cm). Experimental thresholds were used to determine heats of formation as well as the electron affinities of various species. The electron affinities of SO2, IO, SO, and S were determined as 1.05, 2.3, 1.2, and 2.0 eV. The onset for the formation of O− indicates that it is accompanied by the neutral ISO. This onset leads to ΔHf(ISO) =−16 kcal mole−1. IS has been suspected as a product in two separate cases. A fairly good estimate of ΔHf(IS) can be set as 74 kcal mole−1.
Endoergic ion–molecule-collision processes of negative ions. II. Collisions of I− on H2S, CS2, and COS
Authors: Refaey Kamel M. A.
Published: February 2003

Investigations of the cross sections for the formation of various secondary ions by endoergic collisions of I− on H2S, CS2, and COS have been made with I− ions having laboratory kinetic energies ≃0–100 eV. H2S gives the secondary ions HS−, S−, IS−, H−, and possibly IHS−. Only the secondary ions S− and IS− are observed in the case of CS2. Collision of I− on COS gives the secondary ions S−, ICO−, and O−. Because of the fact that peaks due to secondary ions are broader than normal peaks, the ions HS− and S− appear as one peak. Similarly, the ions IS− and IHS− appear as one peak. Experimental thresholds are used to deduce the heats of formation as well as the electron affinities of various species. The electron affinities obtained are: E.A.(HS) =2.2 eV, E.A.(IS) ≃2.7±0.3 eV, E.A.(IHS) ≃1.1 eV, and E.A.(ICO) ≃3.15 eV. In addition, ΔHf(ICS) has been determined as 56.6 kcal mole−1.
Nonlinear resonance and stochasticity in intramolecular energy exchange
Authors: Oxtoby David W. , Rice Stuart A.
Published: February 2003

The dynamics of intramolecular vibrational energy transfer is studied, with particular reference to its effect on the unimolecular dissociation rates of isolated molecules. The molecule is represented as a set of classical, coupled, nonlinear (anharmonic) oscillators which transfer energy through resonant interactions. We suggest that isolated nonlinear resonances (which predominate at low energies) lead to trapping of the vibrational energy of the system, hence to slow vibrational relaxation, while resonance overlap at higher energies leads to rapid energy exchange and the random lifetime distribution assumed by RRKM theory. Results are presented for some simple model systems, and the approach is compared with other recent theoretical descriptions of vibrational relaxation in isolated molecules.
Electronically excited states of CO2+ formed in collisions of heavy ions with CO2: Bending vibrations produced in the ? 2Πu state of CO2+
Authors: Sim William , Haugh Michael
Published: February 2003

The emission from CO2+ produced by collisions of He+, Ne+, Ar+, and Kr+ has been examined under single collision conditions at 0.1 nm spectral resolution using a beam technique with energies from 0.2 to 4 keV. A fine structure is observed that extends through the 300 to 500 nm region. It has been identified as bending transitions in the ? 2Πu→? 2Πg system of CO2+. The bending transitions in this system have not been analyzed previously. Collisions of the inert gas ions with CO2 produce CO2+ (? 2Πu) with extensive bending vibrations excited, while collisions with H2+, electrons and photons produce the ? 2Πu state with no observable bending. For the heavy ion collisions (except H2+) up to six quanta of bending vibrations are excited. The fraction of ? state produced with bending modes excited range from 0.40 to 0.60 of the total Ã state produced in the collisions, depending on velocity and particle identity. To explain these observations we propose formation of a quasimolecule analogous to HCO2 which gives a bent CO2 intermediate that is the source of the bending vibrations in the CO2+ ? state product. The emission spectrum of the isotope 13CO2+ is presented for the first time. The 13CO2+ spectrum verifies the vibrational assignment of bands involving the asymmetric stretch and allows identification of a perturbation in the ? 2Πg (100) level as a Fermi resonance. The emission cross sections for several He lines have been measured at 4 keV.