bluuebunny/crossref_metadata_2025 huggingface dataset:
Localized orbitals and short-range molecular interactions. IV. Nonadditivity in quartet and quintet states of hydrogen atoms
Authors: Magnasco V. , Musso G. F. , Figari G.
Published: February 2003

The separation of the short-range interaction energy into Coulombic and penetration components introduced in Paper I of this series is extended to open-shell systems of maximum spin multiplicity. Ab initio calculations with a minimal set of 1s Slater orbitals are reported for the quartet state of H3 (isosceles triangles) and for the quintet states of tetrahedral and square H4. Nonadditivity of short-range interactions is seen to arise from the penetration of nonorthogonal localized open-shell orbitals into the core provided by nuclei and unperturbed electrons of all other groups, a result already found for closed shells. The validity of representing nonadditive effects in terms of powers of the overlap integrals is shortly discussed.
Fokker–Planck equations for simple non-Markovian systems
Authors: Adelman S. A.
Published: February 2003

Exact generalized Fokker–Planck equations are derived from the linear Mori–Kubo generalized Langevin equation for the case of Gaussian but non-Markovian noise. Fokker–Planck equations which generate the momentum and phase space probability distribution functions (pdf’s) for free Brownian particles and the phase space pdf for Brownian oscillators are presented. Also given is the generalized diffusion equation for the free Brownian particle pdf in the zero inertia limit. The generalized Fokker–Planck equations are similar in structure to the corresponding phenomenological equations. They, however, involve time-dependent friction and frequency functions rather than phenomenological constants. Explicit results for the frequency and friction functions are given for the Debye solid model. These functions enter as simple multiplicative factors rather than as retarded kernels. Further the phase space Fokker–Planck equations contain an extra diffusive term, a mixed phase space second partial derivative, not occurring in the phenomenological equations. For short times the generalized Fokker–Planck equations reduce to the appropriate Liouville equations. For systems with long time tail decay, e.g., the hydrodynamical Brownian particle and an oscillator in a harmonic lattice, the generalized equations do not asymptotically reduce to the phenomenological equations since these latter predict exponential decay. Moreover the exact generalized equations are not equivalent to the familiar approximate generalized Fokker–Planck equations with retarded kernels except when both types of equations reduce to the phenomenological form. The value of the approximate equations with retarded kernels as an improvement upon the phenomenological equations is thus subject to question.
Some similarities between ionic states and excited states in the reaction potential surface
Authors: Chu San-Yan
Published: February 2003

Recent ab initio calculations on the Li+He interaction potential by Krauss et al. show that the interaction between the excited Li atom in its 2P state and the ground state He atom is attractive and quite similar to the interaction potential between the cation Li+ and He. The idea of similar potential curves between a σ→π* type excited system and an ionic system is further applied to the ketone system for a qualitative study of the relative activation barriers for n,π* and π,π* states towards enolization reactions. The breakdown of the concepts of net charge dependence of the reaction activity and the deficiency of the CNDO method for the present type of interaction potential are discussed.
Raman spectrum of solid nitrogen at high pressures and low temperatures
Authors: Medina F. D. , Daniels W. B.
Published: February 2003

The Raman spectrum of solid nitrogen has been studied at high pressures and low temperatures using a method of sample preparation that allows the separation of effects due to change of molar volume and of temperature, respectively. Two lines have been observed in the lattice region of the γ phase which are identified as Eg and B1g librational modes on the basis of frequency and relative intensity calculations. An asymmetrical line has been observed in the stretching region of this phase. In the α phase, the measured Grüneisen parameters indicate that neither the quadrupolar nor the 6–12 atom–atom interaction potential has the correct volume dependence. The temperature dependence of the frequency and linewidth of the Eg librational mode is proposed to be due to libron–phonon interactions. Two very broad Raman lines are observed in the lattice region of the β phase. The low- and high-frequency lines are identified with translational and librational modes, respectively. The observations are consistent with a precessing molecule model for the β phase.
The Hohenberg–Kohn theorem
Authors: Epstein S. T. , Rosenthal C. M.
Published: February 2003

As a contribution to understanding how the Hohenberg–Kohn theorem ’’works,’’ an analogous theorem is derived for matrices and explicated in detail for the cases of 2×2 Hermitian and 3×3 real symmetric matrices.
The chemisorption of carbon monoxide on the iridium (III) surface
Authors: Comrie C. M. , Weinberg W. H.
Published: February 2003

The chemisorption of CO on the (111) surface of Ir at room temperature and below has been investigated using both low-energy electron diffraction (LEED) and thermal desorption mass spectrometry. The CO is adsorbed rapidly onto the (111) Ir forming a well-ordered (√3×√3) R30° overlayer structure after an exposure of approximately 2 to 3 Langmuirs. For larger exposures the overlayer compresses continuously as additional CO is adsorbed until a (2√3×2√3) R30° structure is formed. The surface coverage of CO corresponding to the (√3×√3) R30° structure is one-third monolayer, and mass spectrometry has shown that the surface coverage corresponding to the (2√3×2√3) R30° structure is 7/12 monolayer, where monolayer coverage is based on the number of surface Ir atoms. The isosteric heat of adsorption at one-third monolayer coverage was measured by monitoring the intensity of the overlayer LEED beams as a function of CO pressure and surface temperature, and it was found to be 35±1 kcal/mole. Thermal desorption revealed the existence of two apparent binding states of the CO at saturation coverage. At low coverage (ϑ≲0.4), the CO desorbs in a single peak, whereas the second state builds in at a higher coverage. The appearance of the second state in the thermal desorption spectra coincides with the transformation between the two overlayer structures. The probability of adsorption of the CO is nearly constant for ϑ≲0.2 and has a value of essentially unity, whereas the adsorption probability at larger surface coverages is much smaller, e.g., the value has dropped by more than an order of magnitude by the time ϑ=0.4 has been reached. Models are suggested for the observed surface structures, and the results are compared with other data of CO chemisorption on the close-packed surface of other Group VIII transition metals.
The influence of crystal structure on radical formation in x-irradiated amino acids
Authors: Shields Howard , Hamrick Phillip J.
Published: February 2003

ESR spectra from the L and DL optical isomers of thirteen amino acids have been observed after x-irradiation at 77 K. For most of the acids, spectra from the L and DL isomers are different at corresponding temperatures in a range from 77 to 250 K. These differences are attributed to differences in crystal structure, and specifically to the conformation of the amino group with respect to the adjacent carboxyl group. The temperature at which deamination occurs is described by the equation T=13+137 sin2ϑ, where ϑ is the dihedral angle between the C–N bond and the pz orbital occupied by the unpaired electron.
Thermal transitions between collisionally and tight-complex limited kinetics in the gas phase. Reactions involving carbonium ions
Authors: Meot-Ner M. , Field F. H.
Published: February 2003

H− transfer reactions of C2H5+ and s-C3H7+ with C4–C8 tertiary hydrocarbons were found to exhibit transition from slow kinetics at high temperatures to fast kinetics at low temperatures. Above the transition temperature, characteristic to each reaction, the reactions proceed with less than unit efficiency and the rate constants exhibit significant negative temperature dependencies with the functional form k=AT−1.5 to k=AT−2.5. Below its transition temperature, each reaction becomes a typical fast ion–molecule reaction. In these lower temperature regions the rates are comparable to the calculated ion–molecule collision rates and are independent of further lowering of the temperature. The transition between temperature dependent and temperature independent behavior is interpreted on the basis of a mechanism proceeding through two stages each of which may be rate limiting. The first step is an ion–molecule collision whose rate is independent of temperature. The second step involves the formation of a tight complex. On the basis of transition state theory considerations, the second process is expected to exhibit a negative temperature dependence. Effects of structure on reaction kinetics were investigated in H− transfer reactions from C4–C8 tertiary hydrocarbons to C2H5+, s-C3H7+, t-C4H9+ and t-C5H11+. The reactions of t-C4H9+ and t-C5H11+ are slower by about two orders of magnitude than analogous reactions of s-C3H7+ and show large negative temperature dependencies up to k=AT−5.5.
Threshold electron photoionization study of H2: Vibration–rotation states of H+2(X 2Σ+g)
Authors: Peatman Wm. B.
Published: February 2003

The measurement of the rotational energy levels of H+2 (X 2Σ+g) for v=0–7 on ordinary hydrogen at room temperature is reported. The transitions are directly observed by means of a newly developed threshold electron analyzer which has an observed resolution (FWHM) of 0.003 eV and for which, unlike previous analyzers, the transmission probability goes identically to zero for energetic electrons. The molecular constants obtained are in agreement with values obtained via Rydberg series extrapolations and with theory. The adiabatic ionization potential of hydrogen was determined to be 124 424±9 cm−1.
Resonance Raman spectroscopy of Mn(III) etioporphyrin I at the π→π* and charge transfer bands: The use of charge transfer bands to monitor the complexation state of metalloporphyrins
Authors: Asher Sanford , Sauer Kenneth
Published: February 2003

Electronic transitions of Mn(III) etioporphyrin I (MnETP) are assigned by the use of resonance Raman spectroscopy (RRS). Dramatic differences are found in the RR spectra of MnETP upon excitation within different absorption bands. RRS supports the assignment of the strong absorption band of Mn(III) porphyrins between 460–490 nm to a charge transfer transition. The two bands between 540–600 nm are assigned to vibronic components of a π→π* transition. The RR spectra of MnETP–X (X=F−,Cl−,Br−,I−, or butanol) show large differences in the low frequency Raman spectrum (100–500 cm−1) depending on the axial ligand. Pure Mn–halide vibrations are assigned. An explanation is proposed to account for the differences between the RR spectra excited in the Q bands and the charge transfer band. Some of the vibrational Raman bands in the low frequency region may serve as probes for the degree of out-of-plane distortion of the metal from the porphyrin plane.
Collisional relaxation of electronically excited copper: 3d9 4s2 (2D5/2)
Authors: Trainor Daniel W.
Published: February 2003

A flash photolysis experiment whereby optically metastable copper atoms (3d9 4s2 2D5/2) were produced in a high temperature (600 °K) reaction cell by the photodecomposition of copper halides (CuCl and CuI) is described. The subsequent decay of these low-lying electronically excited copper atoms was monitored by time resolved atomic resonance absorption techniques to provide collisional quenching rate constants for relaxation of the 2D5/2 state by He, Ar, Xe, N2, CO, CO2, SF6, and O2.
Recombination of atomic hydrogen on low temperature surfaces
Authors: Schutte A. , Bassi D. , Tommasini F. , Turelli A. , Scoles G. , Hermans L. J. F.
Published: February 2003

The interaction of H and H2 thermal beams with low temperature inert surfaces has been studied under ultrahigh vacuum (uhv) conditions. The sticking coefficient and the recombination rate for H atoms have been found to depend strongly on the H2 coverage of the surface. In all cases studied when the recombination occurred the greatest part of the recombination energy went to the surface. The present results and those from previous works are interpreted on the basis of a simple model.
Rates of OH radical reactions. II. The combination reaction OH+NO+M
Authors: Overend Ralph , Paraskevopoulos George , Black Catherine
Published: February 2003

The combination of OH+NO+M at 295 °K was studied over the pressure range 25–770 torr of various third bodies M, using the flash photolysis resonance absorption technique. Absolute second order rate constants kII for the reaction were measured at various pressures of He, N2, SF6, CF4, and H2O, and at 720 torr of Ar. The data were treated by a simple two step Lindeman scheme. The limiting value of kII at high pressure, kII∞, was found to be kII=1.1×1013 cm3 mol−1⋅s−1. The efficiencies of the third bodies for collisional energy transfer, η, relative to the efficiency of H2O taken as one were calculated to be H2O=1, CF4=0.41, SF6=0.37, N2=0.12, Ar=0.05, and He=0.02.
A CI study of the classical and nonclassical structures of the vinyl cation and their optimum path for rearrangement
Authors: Weber J. , Yoshimine M. , McLean A. D.
Published: February 2003

A b initio calculations including configuration interaction (CI) are reported for the classical (linear) and the nonclassical (bridged) structures of the vinyl cation (C2H3+), and for the minimum energy path for the rearrangement from one to the other. A ’’double-zeta plus polarization’’ one-particle basis of contracted Gaussian functions was used to construct an n-particle space for the CI calculation consisting of the ground state self- consistent field (SCF) configuration and all single and double excitations from it. Critical evaluation of the calculations leads to the chemical predictions that (i) the two structures have the same energy, to within ∼1–2 kcal/mole with probably the bridged structure more stable, (ii) that the minimum energy path for rearrangement keeps a planar configuration of the atomic nuclei, and (iii) that the barrier to rearrangement is low, less than ∼1–3 kcal/mole. The calculations include geometry optimization along the rearrangement path as well as the effects of electronic correlation, and we demonstrate that inclusion of both these factors is essential for reliable estimates of the energetics of the rearrangement. A discussion is made of the electronic structure changes during rearrangement, and of the resulting significant differences in the electronic spectra of the linear and bridged forms of the cation.
Stability of the bromate–cerium–malonic acid network. I. Theoretical formulation
Authors: Clarke Bruce L.
Published: February 2003

The steady state stability problem for the detailed mechanism of the bromate–cerium–malonic acid system (Belousov–Zhabotinskii system) is set up mathematically. Justification for examining a reduced network with four independent reactants and eight reactions is based on recently developed theorems on the stability of topologically similar chemical networks. The reduced network’s possible steady state currents are found to lie in a convex cone whose cross section is a three dimensional ’’current polytope’’ Πc with seven vertices—each vertex being one of the extremal ’’framing current loops’’ of the network. The number of steady states is proven to be either one or three and in several special cases is proven to be one. The graphs of possible arrows are then constructed for each of the framing current loops and the feedback cycles which can destabilize are discussed. The network is proven to be stable in each of several simplices of a decomposition of Πc. The stability problem is subdivided into units which will be analyzed in detail in subsequent publications.
Stability of the bromate–cerium–malonic acid network. II. Steady state formic acid case
Authors: Clarke Bruce L.
Published: February 2003

The stability of the nonequilibrium steady states of the reduced network for the complete mechanism of the bromate–cerium–malonic acid (Belousov–Zhabotinskii) system is examined for the case where formic acid does not appear in the overall reaction (i.e., is not added to the system initially). The stability analysis involves constructing, by computer, the third Hurwitz determinant, which is a 2575 term polynomial. The system is unstable when the polynomial is negative, and the asymptotes of the negative region are found by computer analysis of the terms in the polynomial. The asymptotic unstable region determined by this polynomial is the union of 19 convex polyhedral cones which are defined by 152 linear inequalities in eight parameters. The network is also unstable in the region where a second polynomial is negative. The asymptotes of this region are described by eleven inequalities. The terms in these polynomials are interpreted diagrammatically as products of feedback cycles of the reaction network. The terms which destabilize arise from three essential feedback cycles: (1) a positive feedback cycle of mutual inhibition between Br− and HBrO2; (2) a negative feedback cycle in which HBrO2 activates HOBr, HOBr activates Br−, and Br− inhibits HBrO2; and (3) a negative feedback cycle in which HBrO2 activates Ce(IV), Ce(IV) activates Br−, and Br− inhibits HBrO2. The network will stabilize if [Br−] becomes large; however, the boundary of stability for large bromide ion is made very irregular by the second and third feedback cycles above. About eighteen important inequalities characterize the irregularities. The stability is found to be very sensitive to small currents in the nondominant flux loops. The reduced network may in certain circumstances be taken as a model of the detailed network. The model is not valid for low concentrations of bromate, cerium, or malonic acid because the reverse reactions which are missing in the reduced network then stabilize the detailed mechanism and the stability inequalities obtained here must be supplemented with additional inequalities. The reduced network is the core of the stability problem for the full network. All simplifications made in the reduced network should have a sufficiently predictable effect on the stability analysis to be removable theoretically. This paper extracts new insights into reaction network stability from the computer results. A detailed theory of destabilization by positive feedback cycles is developed. A general principle for selecting all the destabilizing cycles of a network is discussed with reference to this example.
Predicted lifetimes, oscillator strengths, and wavelengths of highly ionized many-electron heavy atoms (P XI to Sn XLVI), with both relativistic and correlation effects
Authors: Sinanoğlu Oktay , Luken William
Published: February 2003

Wavelengths and oscillator strengths of in-shell transitions in highly ionized heavy atoms are treated including both relativistic and correlation effects. The 2s22p→2s2p2 transitions reported in the boron isoelectronic sequence for atomic numbers up to 50 (Sn), typify these four to nine electron heavy ions. The results indicate that nonrelativistic orbital theory (RHF) results for these transitions may be significantly in error for atomic numbers as low as 14 (Si). The predicted lifetimes are found to be in the range measurable using beam foil spectroscopy coupled with modern heavy ion accelerators.
Statistical theory of angular distributions and rotational orientation in chemical reactions
Authors: Case D. A. , Herschbach D. R.
Published: February 2003

For A+BC→AB+C exchange reactions governed by a statistical collision complex, a large class of directional properties dependent on the rotational orientation of reactant or product molecules are shown to be determined by only two parameters, Λ=〈l/(l+j) 〉 and Λ′=〈l′/(l′+j′) 〉. Here l and l′ are the initial and final orbital angular momenta and j and j′ the rotational momenta of the reactant and product molecules. The angular distribution and the spatial orientation or polarization of the rotational momentum of product molecules are treated in detail. A classical form of statistical theory is used in which the directional properties are determined by Legendre moments involving averages over various unobserved angles between angular momentum vectors. Numerical formulas that give these averages in terms of Λ and Λ′ are derived from phase space calculations for a wide variety of reaction systems. Comparisons with previous treatments of product angular distributions involving other parameters show these can likewise be expressed in terms of Λ and Λ′. Methods are discussed for estimating these parameters and relating them to other reaction properties.
Improved closed form wavefunction for H+2
Authors: Claro Francisco
Published: February 2003

A possible closed form wavefunction for the hydrogen molecular ion H+2 is presented and the expectation values for the grand state energy are calculated by solving the wave equation using the closed form wave functions. (AIP)
A note on vibrational excitation of HCl by electron impact
Authors: Caledonia G. E. , Center R. E.
Published: February 2003

Previously determined rate constants for the electron impact vibrational excitation of HCl are commented upon. When analysis of experimental data is presented. (AIP)
Resonant electronic-to-rotational energy transfer: Quenching of F (2P1/2) by H2 (j=0)
Authors: Rebentrost Frank , Lester William A.
Published: February 2003

Coupled-channel calculations of cross sections for quenching of F (2P1/2) by H2(J=0) are carried out over an energy range of 0.135–1.565 kcal/mole. (AIP)
Comparison of model calculations of the dipolar autocorrelation function
Authors: Anderson Janet S. , Vaughan Worth E.
Published: February 2003

Dielectric dispersion data from the literature are reanalyzed using Berne’s self-consistent theory of rotational diffusion. It is found that the Berne theory provides a significantly better representation for pure polar liquids and for mixtures with one polar and one nonpolar component. No conclusions could be drawn from analysis of mixtures with more than one polar component.
Crossed molecular beam-tunable laser determination of velocity dependence of intramultiplet mixing: K(4p2P1/2)+He →K(4p2P3/2)+He
Authors: Anderson R. W. , Goddard T. P. , Parravano C. , Warner J.
Published: February 2003

The velocity dependence of intramultiplet mixing, K(4p2P1/2) +He→K(4p2P3/2)+He, has been measured over the relative velocity range v=1.3–3.4 km/sec. The cross section appears to fit a linear function Q (v) =A (v−v0), where a=6.3×10−4 Å2 and v0= 7.9×104 cm/sec. The value of A is obtained by normalization to the literature thermal average cross section. The intramultiplet mixing theory of Nikitin is modified to yield Q (v) for the process. The modified theory correctly exhibits detailed balancing, and it is normalized to provide a very good fit to the observed Q (v). The magnitude of the normalization factor, however, is larger than that predicted from recent pseudopotential calculations of the excited state potentials. The temperature dependence of intramultiplet mixing is predicted. The use of laser polarization to determine the mj dependence of the process K(4p2P3/2+He→K(4p2P1/2)+He and other collision processes of excited 2P3/2 states is examined.
Structures, binding energies, and charge distributions for two to six atom Ti, Cr, Fe, and Ni clusters and their relationship to nucleation and cluster catalysis
Authors: Anderson Alfred B.
Published: February 2003

In a low spin molecular orbital approximation, binding energies and optimized structures for various two to six atom clusters of Ti, Cr, Fe, and Ni are calculated. Ti, with few d electrons, shows a preference for tightly packed clusters with positively charged corner atoms while Ni favors open and ringlike clusters with negatively charged corner atoms; Cr and Fe, with nearer to half-filled d shells, prefer tightly packed configurations, but charge distributions are less predictable. The parameterization and spin models used in the theoretical procedure are considered. The binding energies, structures, and charge distributions are discussed in relation to transition metal cluster catalysis and experimental Fe nucleation studies. The photoemission spectra for O and CO on a nine atom Fe (100) model cluster are calculated and are found to be similar, within calculational and experimental resolution, to those for the Fe(100) surface.
The dynamics of the O(1D2)+CO(X 1Σ+, v=0) reaction
Authors: Shortridge R. G. , Lin M. C.
Published: February 2003

The quenching of the O(1D) atom by CO(v=0) has been studied by means of a flash-photolytic CO-laser resonant absorption method. The complete CO vibrational population distribution (0⩽v⩽7) was determined at four temperatures between 246 and 323 °K. The results of both isotope-labeled experiments, 16O(1D)+C18O, and simple statistical model calculations indicate that the quenching reaction takes place via a CO2 complex. The good agreement between observed and calculated CO vibrational population distributions implies that the 45.4 kcal/mole electronic energy carried by the O(1D) atom has been completely randomized among all vibrational modes of the CO2 complex.
Time-resolved light scattering from a collisionally perturbed molecular resonance
Authors: Mukamel Shaul , Ben-Reuven Abraham , Jortner Joshua
Published: February 2003

Time-resolved near-resonance photon scattering in a gas is expressed in terms of a triple convolution of the photon counting rate from a single resonance (characterized by a total width which contains additive contributions from radiative decay, spontaneous predissociation, and collisionally induced predissociation) with a Lorentzian profile, which incorporates the effects of collisionally induced intrastate cross relaxation and phase shifts, and with a Doppler–Gaussian profile. This theory provides a quantitative picture for the recent experimental data of Rousseau, Patterson, and Williams [Phys. Rev. Lett. 34, 1306 (1975)] on photon scattering from collisionally perturbed molecular iodine.
Quantum yields for the production of Se(1S) from OCSe (1100–2000 Å)
Authors: Black G. , Sharpless R. L. , Slanger T. G.
Published: February 2003

The quantum yield of Se(4 1S0) production from OCSe has been measured over the wavelength interval 1100–2000 Å. A large yield (≳0.75) is observed between 1640 and 1800 Å, which supports the assignment 1Σ+←1Σ+ to the strong absorption observed in this region. The major emission observed is the B (3Σ−u) →X (3Σ−g) band system of Se2, the upper state being produced by the reaction of Se(1S) with OCSe. This reaction has a rate coefficient of (1.6±0.2) ×10−10 cm3 molecule−1⋅sec−1 but ? (17±3) % of the reaction produces Se2B (3Σ−u). From the short wavelength threshold of the molecular emission, a value of (2.69±0.05) eV was obtained for the dissociation energy of the OC–Se bond.
Quenching of electronically excited selenium atoms Se(1S0)
Authors: Black G. , Sharpless R. L. , Slanger T. G.
Published: February 2003

The Se2 B 3Σ−u→X 3Σ−g emission produced by OCSe photolysis in the vacuum ultraviolet is due to the reaction of Se(4 1S0) with OCSe. The decay of this emission has been analyzed to give rate coefficients for removal of Se(4 1S0) by a number of simple gases. Half-quenching pressures for Se2 B 3Σ−u have also been determined. A comparison is made with removal rate coefficients for S(3 1S0) and O(2 1S0).
The dielectric properties of H2O and D2O ice Ih at MHz frequencies
Authors: Johari G. P.
Published: February 2003

The permittivity and loss of ice Ih have been measured between 0.5–100 MHz in the temperature range 243–273 °K. An analysis of the temperature dependence of the limiting high-frequency permittivity ε∞ shows that the known decrease in frequency of lattice vibrations with increasing temperature predicts values of ε∞ much higher than those measured. It is suggested that the absorptivity at the infrared frequencies should decrease with increasing temperature. Deuteration of ice decreases its ε∞ by ∼3%. This is interpreted to indicate less absorption of infrared frequencies in D2O than in H2O ice. Ice shows a secondary relaxation centered in the MHz frequency range with an amplitude of 0.08 at 268 °K. The ε∞ of single crystal ice, measured perpendicular to the c axis, is indistinguishable from that of polycrystalline ice, thus indicating little anisotropy of the high-frequency polarization.
Lifetime studies of the NH molecule: New predissociations, the dissociation energy, and interstellar diatomic recombination
Authors: Smith Wm. Hayden , Brzozowski Jerzy , Erman Peter
Published: February 2003

Previously unknown predissociations in the NH radical have been discovered via high resolution lifetime measurements using the high-frequency deflection technique. Measurements were made for individual rotational levels for v′=0 and v′=1 for the NH A 3Π and the c 1Π states. The origin of the observed predissociation is indicated to be the 5Σ− level arising from the ground state asymptote. These data allow a determination of the dissociation energy of the ground state and the singlet–triplet manifold separation. Implications of these results for the astrophysical abundance of NH formed by radiative recombination processes are discussed.
Separation of core and valence regions in atoms
Authors: Politzer Peter , Parr Robert G.
Published: February 2003

It is proposed that the minimum in the radial density function R (r) = 4πr2ρ (r) defines a physically meaningful boundary surface separating core and valence regions of first-row atoms. The point rm at which this minimum occurs has been found to fall within the interval in which the linear lnρ (r) vs r plots of these atoms undergo a significant change in slope. When the valence region is defined in the proposed manner, a valid estimate of its electronic energy can be obtained with the formula E=−12π/7 (Z−−i−i) ℱ∞rmρ (r) r dr, in which Ni is the number of electrons in the core region of an atom with nuclear charge Z. This equation is a modified form of an expression previously found to be accurate for atoms. For Be, C, O, F, and Ne, Ni ranges from 2.05 to 2.20 electrons. The core-valence separation in larger atoms may be treated in a similar manner; rm is then the outermost minimum in the radial density function.
Reactions of chlorine and bromine with hydrogen astatide
Authors: Grover J. R. , Iden C. R. , Lilenfeld H. V.
Published: February 2003

Angle-differential cross sections are reported for the Reactions (2), Cl+Hat→HCl+At and (3), Br+Hat→HBr+At. Total reaction cross sections at 6.5 kcal/mole initial translational energy are 25.6±4.2 Å2 for (2) and 1.48±0.34 Å2 for (3). Mean energies in product translation are 7.2±0.6 kcal/mole for (2) and 1.0±0.2 kcal/mole for (3). The center-of-mass angular distribution of (2) is forward–backward peaked and almost symmetric about 90°. For (3), about as much product scatters into the forward hemisphere as into the backward. The measured cross sections for both (2) and (3) are close to the limiting value, which is calculated by correcting an assumed total cross section for inelastic plus reactive processes of 50 Å2 for kinematic restrictions independent of spatial configuration and for the effects of electronic adiabaticity. This means that the smaller cross section for (3) is consistent with its smaller mean product translational energy. It also means that for the direction and position of either attack or retreat (or both) no limitations can be claimed if electronic adiabaticity is assumed, and only loose restrictions if such adiabaticity is relaxed. The data thus do not require a model that specifies attack and retreat at small impact parameters exclusively. Reactions of Hat with Cl and Br are broadly similar to the corresponding reactions of HI, although there are obvious differences of detail. The information obtained in this experiment demonstrates the feasibility of using reagents labeled with short-lived nuclides for the study of reactive scattering in crossed molecular beams.
A modified pseudopotential approach to the heavy-atomic molecular systems: Application to the X 2Σ1/2+, A 2Π1/2, and the A 2Π3/2 states of the HgH molecule
Authors: Das G. , Wahl A. C.
Published: February 2003

A new approach to the problem of quantitatively describing interactions in heavy-atomic molecular systems is presented. In this approach the core electrons are removed from the Hamiltonian by using the exact numerical relativistic Hartree–Fock core potential. Both the local and nonlocal terms of this potential are represented in terms of a given basis set suitable to the valence electrons. Two categories of core-orthogonality constraints are distinguished: those pertaining to outer core orbitals which can be qualitatively described by the given basis set and those pertaining to the inner core orbitals which cannot be thus described. While explicit descriptions for the former are used as orthogonality constraints, approximate ’’pseudopotential’’-type terms are introduced into the Fock operators to represent the latter’s orthogonality effects. Estimates of the core distortion and relativistic contributions are considered. Application to the lowest 2Σ1/2+, 2Π1/2, and 2Π3/2 states of the HgH molecule is described.
Exact enumeration of self-avoiding walks on the cubic lattice terminally attached to an interface
Authors: Middlemiss Keith M. , Whittington Stuart G.
Published: February 2003

Self-avoiding walks on the cubic lattice which are terminally attached to an interface are enumerated exactly up to n=13. The data is analyzed to investigate the rate at which the numbers of tails approach their limiting behavior and to locate the transition point at which adsorption begins to occur when the walks interact with the interface by an attractive force.
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o evaluation of the fine structure and radiative lifetime of the 3A2(n→π*) state of formaldehyde
Authors: Langhoff Stephen R. , Davidson† Ernest R.
Published: February 2003

The spin–orbit contribution to the zero-field splitting (ZFS) in the CH2O 3A2(n→π*) state is evaluated using the full Breit–Pauli Hamiltonian. All calculations are carried out at the planar ground state geometry using a double-zeta plus polarization basis of contracted Gaussian-lobe functions augmented with diffuse s and p functions. Configuration–interaction wavefunctions, constructed using the 3A2 canonical orbitals, are used to describe the 3A2 state and all states coupling to it via the spin–orbit Hamiltonian. The excitation energies and oscillator strengths obtained from these wavefunctions are in good agreement with other theoretical calculations and with experiment. Of the 12 states considered in the second-order perturbation theory treatment of the spin–orbit interaction, the 1A1 ground and the nearby 3A1(π→π*) states were the most important. Rydberg states were observed to have very small spin–orbit matrix elements and consequently to have little effect on the ZFS. The spin–orbit contributions to the ZFS parameters D and E were −0.224 and 0.009 cm−1, respectively, which when added to the spin–spin contribution obtained in an earlier paper [S. R. Langhoff, S. T. Elbert, E. R. Davidson, Int. J. Quantum Chem. 7, 999 (1973)] give total values of D=0.314 cm−1 and E=0.04 cm−1. These results are larger than the best experimental results of D=0.141 cm−1 and E=0.02 cm−1, determined by Birss et al. [F. W. Birss, R. Y. Dong, and D. A. Ramsay, Chem. Phys. Lett. 18, 11 (1973)] from a rotational analysis of the 0+←0 bands of the 3A2←1A1 transition. An extensive calculation was also undertaken to assess the degree of convergence in the second-order perturbation theory treatment. The contribution of the lowest 100 singlet and triplet states of A1, B1, and B2 symmetry were considered where each state was described by a 100-term CI wavefunction. This calculation gives a spin–orbit contribution to D of −0.221 cm−1 essentially identical to the previous result providing evidence that the second-order treatment has indeed converged. The radiative lifetimes of the three sublevels of the triplet state were determined using the same representations for the manifold of electronic states. In the high temperature limit, the radiative lifetime was determined to be between 0.02 and 0.06 sec, somewhat longer than the estimated experimental value of 0.01 sec. The mutual perturbation of the 1A1 ground and 3A2 states and the perturbation of the 3A2 state by the 1A1(π→π*) state were determined to be most important in determining the lifetime. These results ensure that the emitted light is polarized primarily along the carbon–oxygen bond in agreement with experiment. A critical examination of the quantitative validity of the numerical results is presented to assess the reliability of the theoretically determined lifetimes.
Infrared–microwave double resonance studies of collison-induced transitions and energy transfer processes between vibration–rotation–inversion levels of NH3
Authors: Kano Satoru , Amano Takayoshi , Shimizu Tadao
Published: February 2003

When the 14NH3 molecules in the upper inversion level of the J=8, K=7 (para) rotational state are pumped by using the N2O P (13) laser line, many microwave inversion lines change their intensities because of collision-induced populational transfer. Pressure dependence of this effect has been studied for several tens of inversion transitions in the ground vibrational state. The non-Boltzmann distribution introduced into the pumped levels is transfered to other levels by two mechanisms: (i) collision-induced transitions into or out of the pumped levels and succeeding cascading processes and (ii) selective energy transfer mechanisms to collision partners. In the former mechanism the transfer of populational change is caused by the transition of pumped molecules whereas in the latter it is caused by collision partners. Although the first mechanism is more efficient than the second, the second mechanism is not negligible and causes the populational change in also ortho rotational state. We used mixtures of 15NH3 and 14NH3 and monitored the 15NH3 inversion lines to discriminate between the two mechanisms. Since 15NH3 is not pumped by the laser line, the double resonance signal is caused only by the second process. The result on 15NH3 is similar to that for the ortho-14NH3, which clearly indicates that the signal previously observed for the ortho-14NH3 is not due to the collision-induced ortho–para conversion but due to the second mechanism. Double resonance signals for para-14NH3 show different pressure dependence from those of ortho-14NH3 and 15NH3. The results are analyzed consistently by taking account of various collision mechanisms. In the course of this analysis it was found that the V–V energy transfer becomes much more inefficient when the energy discrepancy becomes several tens of wavenumbers.
The photoelectron spectra of diazene, diazene-d2, and t
r
a
n
s-methyldiazene
Authors: Frost D. C. , Lee S. T. , McDowell C. A. , Westwood N. P. C.
Published: February 2003

The photoelectron spectra of the transient species diazene, N2H2; diazene-d2, N2D2; and trans-methyldiazene, MeNNH have been recorded and interpreted. Four ionization potentials in the HeI energy range (21.2 eV) have been detected in the case of N2H2, where the orbital energies are found to follow the sequence n+, π, n−, and σ. N2H2 exhibits a highly structured photoelectron spectrum which has been fully resolved and analyzed in conjunction with its deutero analog. Several features of the spectrum are discussed, in particular the magnitude of the n+/n− separation, which is the largest yet observed, a comparison with N2F2 and the perfluoro effect, and the position of the π ionization. Our results also suggest that the 1700 Å band in the absorption spectrum of N2H2 is a n+→3p Rydberg transition. In addition, using the experimental Franck–Condon factors, we have deduced the geometry of the N2H2 and N2D2 ground state ions and compared the photoelectron spectra with the isoelectronic molecules B2H6, C2H4, CH2NH, and CH2O. The results for N2H2 and MeNNH are compared with the known data for MeNNMe, and the effect of increasing methylation on the orbital energies is discussed.
Theoretical study of stable negative ions of polar molecules: NaH−, LiH−, LiF−, BeO−
Authors: Jordan K. D. , Griffing K. M. , Kenney J. , Andersen E. L. , Simons J.
Published: February 2003

In order to investigate the nature of the binding of electrons to polar molecules, we have performed ab initio calculations on LiH, LiF, NaH, BeO, and their anions. The anions are predicted to be stable and our calculated vertical electron affinities of the parent molecules are 0.30, 0.46, 0.36, and 1.77 eV for LiH, LiF, NaH, and BeO, respectively. It is demonstrated that electron correlation effects are small in the binding of electrons to these polar molecules since the extra electron occupies a predominantly nonbonding orbital on the electropositive atom.
A photoionization mass spectrometer study of CFCl3, CF2Cl2, and CF3Cl
Authors: Ajello J. M. , Huntress W. T. , Rayermann P.
Published: February 2003

A photoionization mass spectrometer has been used over the wavelength range 650–1100 Å (19.07–11.27 eV) to determine the appearance potentials, heats of formation, and photoionization efficiencies of the following ions: CFCl+2, CCl+3,CFCl+, and CCl+2 from CFCl3; CF2Cl+2, CFCl+2, CF2Cl+, CFCl+, and CF+2 from CF2Cl2; and CF3Cl+, CF3+, and CF+2 from CF3Cl. The parent ion CFCl+3 of CFCl3 was not observed.
Viscous liquids and the glass transition. V. Sources of the excess specific heat of the liquid
Authors: Goldstein Martin
Published: February 2003

The possibility that the excess ’’configurational’’ specific heat ΔCp of liquids above the glass transition temperature Tg might contain contributions either from changes in vibrational frequencies with configurational state, from changes in the anharmonicity of vibrations, or from the influence of changes in configurational state on the thermodynamic contributions of molecular rearrangement processes that also give rise to secondary relaxations below Tg is investigated with the use of calorimetric data on six quenched and annealed glasses. It is concluded that at least half of ΔCp arises from these sources, the greater part of this either from anharmonicity or from the molecular rearrangement processes.
Isotope chemistry and molecular structure. Deviations from the first rule of the mean
Authors: Ishida Takanobu , Bigeleisen Jacob
Published: February 2003

The method of the finite orthogonal polynomial expansions of the Bigeleisen–Mayer function is used to study deviations from the first rule of the mean of isotope chemistry. It is shown by sample calculations for hydrogen isotope substitution in water, methane, and ethylene and carbon isotope substitution in ethylene that the second order corrections, (h//kT)4Σδλ2i account for all but ±0.001 to ±0.01 of the natural logarithm of the symmetry corrected exact harmonic disproportionation constant at 300 °K. The structure of the second order corrections have been related to the molecular structure through the use of internal coordinates. It is shown that subrules local to sets of equivalent coordinates exist in addition to the sum rules formed over the common symmetry classes or the totality of molecular eigenvalues. The local subrules are conveniently expressed in matrix form, D matrix. The latter can be used as a basis for the derivation of the sum rules. The first and second order D matrices are derived for isotopic disproportionation reactions involving both end atom and central atom isotopic substitutions. For end atom isotopic substitution, stretching coordinates cannot lead to deviations from the rule of the mean without kinetic or potential energy coupling from other coordinates, i.e., the local sum rule for pure stretching coordinates holds to all orders. It is shown that the second order deviations from the rules of the mean depend only on the square of differences in isotopic reciprocal masses and only those force constants relating to atoms for which there is isotopic substitution. Detailed analyses of the deviations from the rules of the mean are given for the disproportionation reactions studied, in terms of the molecular geometries, forces, and masses.
On the kinetic meaning of the second law of thermodynamics
Authors: Keizer Joel
Published: February 2003

A kinetic-molecular theory which connects dissipation and fluctuations is used to examine the second law of thermodynamics. Considerations are restricted to systems with stable equilibrium states and are based on a conservation condition satisfied by transport processes which obey microscopic reversibility. The conservation condition leads to a statement about the accessibility of equilibrium states which is comparable to the Carathéodory statement of the second law. Insofar as the transport of heat into a system is the only process which violates microscopic reversibility, this statement is equivalent to the second law. The present treatment also gives a simple kinetic proof of the Clausius inequalities TRdS/dt?dQ/dt and dS/dt?0 for the entropy. Using the statistical aspects of the fluctuation–dissipation postulates, a class of state functions related to the equilibrium statistical distribution are defined, and it is verified that the entropy is one of these functions. A brief discussion is given of how to extend these results to systems with multiple phases or at nonequilibrium steady states.
Raman and depolarized Rayleigh scattering studies of molecular motions of liquid 1,2,5-thiadiazole
Authors: Jones D. R. , Wang C. H. , Christensen D. H. , Nielson O. F.
Published: February 2003

The polarized and depolarized Raman components of the ν3, ν4, ν5, and ν6 totally symmetric vibrations of pure liquid 1,2,5-thiadiazole were measured as a function of temperature. The half-widths of the polarized spectra for the ν3, ν4, and ν5 modes significantly decreased as temperature was increased. The complications affecting the separation of rotational motion from the depolarized anisotropic spectra of asymmetric top molecules were analyzed. Raman correlation times, which correspond to the weighted contribution from rotation about various axes, were obtained. Depolarized Rayleigh reorientational relaxation times for pure 1,2,5-thiadiazole were also measured. The depolarized Rayleigh results agreed with only the ν6 Raman results. The temperature dependence of the Raman depolarization ratios was also investigated.
Dielectric properties of ice VI at low temperatures
Authors: Johari G. P. , Whalley E.
Published: February 2003

The limiting low-frequency permittivity of ice VI at a pressure of about 9 kbar has been measured between 301 and 128 °K to seek evidence for an ordering transformation. In the experimental range, the quantity (ε0 −ε∞) T, where ε0−ε∞ is the orientation polarization, increased by a factor of 1.49, and follows a Curie–Weiss law with an apparent ordering temperature of 47 °K. Ice VI appears, therefore, to be approaching a ferroelectrically ordered state at low temperature. However, neutron diffraction measurements (Ref. 14) indicate that ice VI recovered at ∼100 °K and atmospheric pressure is partly ordered in an antiferroelectric manner, and so there is presumably an unobserved transformation between the conditions of 128 °K and 9 kbar and 100 °K and 1 bar. The relaxation time follows an Arrhenius law with constant activation energy down to the lowest temperatures, in contrast to ice Ih, in which the activation energy drops to about a half below a temperature that depends on the sample. No doubt the impurities that cause the drop in ice I are too insoluble in ice VI to affect its relaxation time.