bluuebunny/crossref_metadata_2025 huggingface dataset:
The tunneling frequencies of the ammonium ion in its compounds
Authors: Smith David
Published: February 2003

The tunneling frequencies of the ammonium ion in a tetrahedral crystalline field have been computed up to a librational frequency of 320 cm−1. The tunneling frequencies determined from analyzing NMR results are used to compute the librational frequencies and barriers to rotation of a number of ammonium compounds. The librational frequencies and barriers to rotation determined from the tunneling frequencies are in reasonable agreement with the experimental results for the cubic compounds NH4I, NH4Br, and (NH4)2PbCl6. The results are less satisfactory for cubic (NH4)2SnCl6. The librational frequencies for noncubic compounds have been determined from the tunneling frequencies by assuming tetrahedral site symmetry for the ammonium ion. For the noncubic compounds NH4SCN, (NH4)2Cd(SO4)3, NH4NO3, and (NH4)2CrO4, the librational frequencies determined from the tunneling frequencies are lower than those observed experimentally, while the barriers to rotation determined from the tunneling frequencies are higher than the experimentally determined activation energies. The tunneling frequency for NH4ClO4 is discussed.
Lattice model studies of the effect of chain flexibility on the nematic–isotropic transition. I. Rigid cores having completely flexible tails
Authors: Dowell F. , Martire D. E.
Published: February 2003

A steric, mean field, three-dimensional, simple cubic lattice model using DiMarzio lattice statistics for molecules composed of rigid cores and completely flexible pendant tails is presented. The calculated trends and absolute values of the following properties at the nematic–isotropic transition for cores of length-to-breadth ratios of 5 and 10 with tails of varying length are discussed and compared with relevant experimental data: temperature, core order parameter, intermolecular tail order parameter, densities, relative density change, and entropy change. The results indicate that although completely flexible tails in this model are too flexible compared with hydrocarbon tails in real liquid crystals, tail flexibility of some degree should be important in the nematic–isotropic transition.
The low-temperature thermal conductivity of a polyoxymethylene ’’crystal’’
Authors: Greig D. , Jakeways R. , Sahota M.
Published: February 2003

The thermal conductivity κ of a polyoxymethylene (POM) ’’crystal’’ has been measured both parallel and perpendicular to the direction of fibers between 1.5 and 100 K. In the fiber direction κ∥ is very much less than might be expected from a single crystal but is instead very similar throughout the temperature range to the conductivity of extruded POM. Perpendicular to the fibers κ⊥ is rather less than in extruded material, particularly at the lowest temperatures where, even for polymers, the conductivity is unusually small. These observations lead to deductions about the specimen morphology that are substantiated by experiments involving x-ray diffraction.
Heat capacity, entropy, and Gibbs energy of technetium hexafluoride between 2.23 and 350 K; magnetic anomaly at 3.12 K; mean β energy of 99Tc
Authors: Osborne Darrell W. , Schreiner Felix , Otto Klaus , Malm John G. , Selig Henry
Published: February 2003

The heat capacity of solid and liquid technetium hexafluoride has been measured between 2.3 and 350 K. The standard entropy, S°, the enthalpy increment, (H°−H°0), and the Gibbs energy function, (G°−H°0)/T, have been calculated. At T=298.15 K, the thermodynamic functions have the following values: C°p=157.84 J K−1⋅mol−1, S°=253.52±0.25 J K−1⋅mol−1, (H°−H°0) =38331±38 J mol−1, and (G°−H°0)/T=−124.95±0.12 J K−1⋅mol−1. TcF6 has a solid–solid transition at 268.335 K associated with an enthalpy change ΔH°tr=8024.9±5.0 J mol−1. It melts at 311.14±0.05 K and has an enthalpy of fusion of ΔH°f=4618.86±1.60 J mol−1. TcF6 has a lambda-shaped anomaly in the heat capacity with a peak at 3.12 K, probably caused by a transition to a ferromagnetic state. A similar lambda-shaped transition was observed for rhenium hexafluoride at 1.30±0.05 K. The magnetic entropy of TcF6 is estimated to be 5.823 J K−1⋅mol−1. The third law value for the standard entropy of TcF6 vapor at 320 K was calculated to be S° (TcF6, g) =371.278±0.750 J K−1⋅mol−1. Molecular data for TcF6 lead to a lower value of S° (TcF6, g, 320 K) = 367.53 J K−1⋅mol−1. The entropy from molecular data can be made to agree with the calorimetric value if the frequency for the inactive vibrational mode ν6 is lowered to 131 cm−1 from the reported value of 145 cm−1. The average energy of the β decay of 99Tc was evaluated from the radioactive self-heating of the sample and was found to be 97.2±1.2 keV. This number exceeds the literature value but agrees with a more recent integration of the β spectrum.
Relativistic Xα–scattered-wave calculations for the uranyl ion
Authors: Yang C. Y. , Johnson K. H. , Horsley J. A.
Published: February 2003

Relativistic Xα–scattered-wave molecular orbital calculations have been carried out on the uranyl ion UO22+. The calculated orbital eigenvalues are in good agreement with the results of a recent x-ray photoelectron spectroscopy study of uranyl compounds. An interpretation of the optical spectrum of the uranyl ion in terms of a Hund’s case (c) (ω, ω) coupling scheme is given.
A study of the Efimov states and binding energies of the helium trimer through the Faddeev-coordinate–momentum approach
Authors: Huber H. S. , Lim T. K.
Published: February 2003

A thorough bound state analysis of the helium trimer is made using the Faddeev equations with form factors generated by a modified Kowalski–Feldman approach. The presence of Efimov states is manifested by each of the realistic potentials considered.
Temperature dependence of the laser-enhanced reaction NO+O3 (001). II. Contributions from reactive and nonreactive channels
Authors: Bar-Ziv Ezra , Moy Jerry , Gordon Robert J.
Published: February 2003

The reaction NO+O3(001) was studied over the temperature range 158–437 K. A CO2 TEA laser was used to excite ozone in a rapidly flowing mixture of O3, Ar, and NO. The total decay rate of the laser-induced chemiluminescent signal showed a strong non-Arrhenius temperature dependence. This decay rate is the sum of contributions from three processes: the laser-enhanced luminescent reaction, the laser-enhanced nonluminescent reaction, and vibrational relaxation of O3(001) by NO. A kinetic model is proposed to determine each of the rate coefficients from the size and shape of the laser-induced signal. Our analysis shows that the rate coefficient of the laser-enhanced nonluminescent reaction has an Arrhenius form, with an activation energy that is lowered by approximately 1.6±0.4 kcal/mol. The rate coefficient for relaxation of ozone has a minimum at 225 K, suggesting that attractive forces are important in the nonreactive channel.
Experiments concerning the laser-enhanced reaction between vibrationally excited O3 and NO
Authors: Hui Kin-Kwok , Cool Terrill A.
Published: February 2003

The reaction of vibrationally excited O3 with NO has been studied for the temperature range 138–410 ° K. Observations were made of chemiluminescence of vibrationally excited NO†2(2A1) and electronically excited NO*2(2B1,2) reaction products immediately following laser excitation of O3(001) molecules. The present results, combined with previous rate constant measurements, permit a determination of the separate rates for formation of NO2†(2A1) molecules and collisional deactivation of vibrationally excited O3 by NO. This latter rate constant exhibits a minimum value at 230 °K. It is shown that the stretching and bending modes of O3 make comparable contributions to reaction rate enhancement. Excitation of either the coupled stretching modes or the bending mode results in a lowering of the activation energies for reactions leading to NO*2(2B1,2) and NO†2(2A1) products of 31% (1.3 kcal/mole) and 56% (1.3 kcal/mole), respectively. The intermode coupling of vibrational energy from the stretching modes to the bending mode of O3 in collisions with NO occurs with a rate constant of (1.2±0.6) ×104 sec−1 Torr−1 at 308±3 °K.
Ground state wave vectors for the d5 (Fe3+) configuration in symmetries C4v and C2v. Application to ferrihemoglobins and related compounds
Authors: Caro P. , Faucher M. , Savy M. , Pankowska H.
Published: February 2003

Complete wave vectors for the d5 energy levels were computed on the basis of the 252‖SLJMJ〉 kets of the configuration for D4h, C4v, and C2v symmetries. The interelectronic repulsion Hamiltonian with B=800 cm−1, C=3.7B, the spin–orbit coupling Hamiltonian with ζ=400 cm−1, and a crystal field Hamiltonian written in Wybourne’s tensor operator formalism with the five parameters B40, B44, B42, B20, B22, were used in combination. The free atom parameters used correspond to Fe3+ in solids. Maps of the ground state multiplicity versus B40, B44, and B20, (C4v) were established. Two zones, where the three multiplicities (2S+1) =2, 4, and 6 meet are apparent, one corresponds to low values of B20 and medium values of B40, B44 (30 000 cm−1). The positive D factor for zero-field splitting of the 6S5/2 state was computed as a function of crystal field parameters. g∥ and g⊥ were computed for the low-spin domain in C4v; gx, gy, gz were calculated in C2v symmetry in the low-spin domain near the border with four and six multiplicities. Comparison with experimental results shows that high spin ferrihemoglobins, and related compounds with large D factors, correspond to a borderline situation with multiplicity 4. g values for low spin ferrihemoglobins can be reproduced with crystal field parameters values corresponding either to the low spin domain close to the borders, or for C2v distortion of the C4v symmetry in high or intermediate spin domains. In the border areas, the g’s are very sensitive to changes in the crystal field parameters because of the rapid variation of composition of the wave vectors in crossover situations.
Analysis of the laser-stimulated reaction: NO2*+CO→NO+CO2
Authors: Herman I. P. , Mariella R. P. , Javan A.
Published: February 2003

This paper describes the room temperature, gas phase reaction of NO2, excited by various cw visible lasers, with CO to form NO and CO2. The distilled CO2 product is detected by laser-induced fluorescence. Using this technique, and previously measured relaxation rates, the determined reaction rate constant with 4880 Å excitation is kR=3.2±1.2×10−1 5 cc/molecule sec. The observed dependences on laser intensity and reactant concentration are in agreement with a proposed mechanism in which NO2*+CO→NO+CO2 is the main step. Other possible mechanisms are found to disagree with experimental evidence. kR is found to increase smoothly by a factor of 10 as the exciting wavelength is decreased from 6125 to 4579 Å. This behavior is compared to a model in which RRK theory predicts reactivity and a step-ladder model describes relaxation. Using this RRK model fit, the observed activation energy is 1.8±0.2 eV, which appears to be 0.3–0.7 eV higher than the thermal energy barrier.
Millimeter wave spectra of the alkaline earth metal oxides: BaO, SrO, and CaO
Authors: Hocking W. H. , Pearson E. F. , Creswell R. A. , Winnewisser G.
Published: February 2003

The alkaline earth metal oxides BaO, SrO, and CaO have been produced in a gas phase flow system by the oxidation of the metal vapor, entrained in argon carrier gas, using oxygen or nitrous oxide. Rotational transitions have been measured for the isotopic species 138Ba16O, 88Sr16O, and 40Ca16O in the ground and first excited vibrational states of the X 1Σ electronic state in the region 80–220 GHz. In addition, ground vibrational transitions have been measured for the isotopic species 137Ba16O, 136Ba16O, 135Ba16O, and 87Sr16O, and 86Sr16O. The Dunham constants Y01, Y02, Y11, and Y12 have been determined from these measurements, and the vibrational constants Y10 and Y20 have been calculated. The following equilibrium internuclear distances have been derived: re(BaO) =1.939630(7) Å, re(SrO) =1.919809(95) Å, and re(CaO) =1.822203(68) Å.
Far infrared laser magnetic resonance spectrum of the oxygen atom
Authors: Davies P. B. , Handy B. J. , Lloyd Elaine K. Murray , Smith D. R.
Published: February 2003

Laser magnetic resonance transitions within the 3PJ ground state of the oxygen atom have been detected using optically pumped lasers at 147.8 μm (CH3NH2) and 145.7 μm (CH3OD). The 3P0–3P1 fine structure spacing derived for 16O is 68.71649 (±4×10−5) cm−1 and 3.5×10−4 cm−1 larger for 18O.
A memory–functions approach to vibrational relaxation in dense media
Authors: Cohen Simon S. , Wilde Richard E.
Published: February 2003

We propose a new method to study some effects concerned with vibrational relaxation in dense molecular media. Our approach makes use of the memory-functions technique to result in a clear physical description of the behavior in the system. The crucial point in this study is the introduction of a noninteracting reference system with regard to which the perturbation parameters in the real system are derived. These parameters consist of a time-modulation parameter τ and an interaction parameter labeled κ2 which accounts for the real-system spectral fourth moment. We show how the last parameter may be obtained from spectral measurements on the system. The time parameter which describes (by means of a Poisson distribution) a stochastic process for the random force acting on the molecules remains as an adjustable parameter. We compare our results with some existing simplified calculations and note the possibility of estimating the range of τ from such calculations. We apply our method to some new experimental data on AsH3 to obtain very good agreement between theory and experiment.
Pressure dependence of the lattice parameters of anthracene up to 5.4 kbar and a re-evaluation of the elastic constants
Authors: Elnahwy S. , El Hamamsy M. , Damask A. C. , Cox D. E. , Daniels W. B.
Published: February 2003

The lattice parameters of an anthracene crystal were measured as functions of hydrostatic pressure in the pressure range from 0–5.4 kbar using neutron diffraction techniques. The results were found to agree with previous measurements of the volume compressibility of anthracene and, in the a and b directions, with the values extrapolated from linear compressibility data for linear polyphenyls. The values of the linear compressibilities were used to derive an improved set of elastic constants from currently available velocity of sound data.
Destruction rates of metastable helium atoms and molecules in collisions with argon atoms
Authors: Pitchford L. C. , Deloche R.
Published: February 2003

The destruction rates of metastable helium atoms and molecules due to collisions with argon atoms have been determined at 300°K in mixtures of helium (20 to 60 Torr) and argon (10−5 to 10−1 Torr) in a pulsed afterglow. Those rates are (0.7±0.1) ×10−10 cm3 sec−1 and (1.5±0.3) ×10−10 cm3 sec−1 for the metastable atoms and molecules, respectively. In addition it was possible to confirm the value of 29.5 cm2 V sec−1 for the reduced mobility of Ar2+ in He at 20°C.
Density functional theory and molecular bonding. II. Alkali dimers
Authors: Harris J. , Jones R. O.
Published: February 2003

Binding energy curves for the 1Σ+g ground state of the alkali dimers Li2–Fr2 are calculated using the density functional formalism of Hohenberg, Kohn, and Sham. The resultant spectroscopic constants agree well with experiment and with MCSCF calculations for Li2 and Na2, in spite of the use of a local approximation for the exchange–correlation energy and of a ’’frozen-core’’ approximation. For each of the dimers, the 3Σ+u state is very weakly bound at a large internuclear separation.
Collisional broadening and spectral line shape of an entire rotational band
Authors: Koszykowski M. L. , Marcus R. A.
Published: February 2003

The impact approximation is applied to the classical binary collision operator making it possible to derive an expression for the dipole correlation function for real systems in a form which is computationally tractable and contains no adjustable parameters. Trajectory calculations are performed (in order to evaluate the microscopic expression for the relaxation parameter in the correlation function) for the system CO in dense Ar gas. Comparison is made with experimental data and excellent agreement is found for certain densities when a quantum correction is included. At higher densities (i.e., ρ−1/3< ’’the range of the potential’’) one approximation is not valid and comparison with experiment illustrates this point.
Time lag in diffusion-controlled nucleation
Authors: Peak David
Published: February 2003

Nucleation occurring in the presence of a dense solvent is accompanied by spatial correlations among the reactant clusters and monomers which affect the aggregation rates. The classical theory of nucleation, which does not take such correlations into account, overestimates the observed nucleation rate and underestimates the time lag in condensed systems. An explicit expression for the time lag in diffusion-controlled nucleation is derived. Values associated with miscibility gap experiments differ from the classical predictions by factors of 5 to 10.
Molecular dynamics study of clustering. I
Authors: Zurek Wojciech H. , Schieve William C.
Published: February 2003

A study of the cluster formation in a two-dimensional dilute gas of hard sphere square well discs is presented. The temperature range is chosen close to the gas-liquid phase transition. The evolution of the system is discussed. Abundances of clusters with different topologies are estimated.
Experimental determination of relative signs of dipole moment derivatives: HCN and DCN
Authors: Bruns Roy E. , de Barros Neto Benicio
Published: February 2003

An alternative numerical analysis for the experimental determination of the relative signs of the dipole moment derivatives with respect to the normal coordinates is presented. Use is made of the polar tensor and effective charge equations and the G–intensity sum rule of Crawford. The relative signs are determined by sufficiently accurate individual infrared fundamental intensities of one molecule and only the sum of these intensities for an isotopically related molecule. The intensity data of HCN and DCN are used to illustrate the procedure. The as yet unreported values of the atomic polar tensors and effective charges of these molecules are also reported.
An analysis of the NMR line shapes of the ammonium ion undergoing composite tunneling and reorientational motions at low temperatures
Authors: Lalowicz Z. T. , McDowell C. A. , Raghunathan P.
Published: February 2003

An analysis is made of the NMR line shapes of the rotationally tunneling ammonium (NH4+) ion which also undergoes rapid reorientataional jumps around its C3 or C2 symmetry axis. The proposed composite motion, believed to be brought about by the coupling of rotor states of the ion with lattice phonons, leads to line narrowing and offers an interpretation of experimental line shape and second moment data not explained previously. In particular, the proton resonance line shapes of (NH4)2SnCl6, NH4BF4, NH4ClO4, (NH4)2GeF6, (NH4)2PbCl6, and (NH4)2PtCl6 at liquid helium temperatures are discussed. The analysis suggests that proton magnetic resonance line shape measurements on single crystals may provide an assessment on the state of ordering of the tunneling and reorienting NH4+ ions.
Lifetimes of IBr(B3π+) by laser-excited time-resolved fluorescence
Authors: Wright J. J. , Havey M. D.
Published: February 2003

Fluorescence lifetimes for the v′=2,3, and 4 levels of the B3π+0 state of IBr have been measured by means of laser-excited time-resolved fluorescence. A technique is described which reduces the presence of Br2 and I2 to an acceptable level, allowing an unambiguous determination of the IBr lifetime. The observed lifetime for the three vibrational levels studied is essentially the same, 0.54±0.05 μs.
Time resolved measurements in high pressure mass spectrometry: An analysis of assumptions
Authors: Meisels G. G. , Illies A. J. , Stradling R. S. , Jennings K. R.
Published: February 2003

The assumptions underlying the use of time resolved measurements for the determination of ion–molecule reaction rate constants in high pressure chemical ionization sources have been examined and problems introduced by diffusion and drift are presented; these require conversion from ion currents to ion concentrations in the presence of applied fields when ion current ratios of different ions are used. Moreover, the existence of an irresolvable difference in the significance of arrival times of product and reactant ions is demonstrated, but it is shown how this error can be minimized. A method for the measurement of total disappearance rate constants for association reactions is demonstrated and applied to the reaction CO+2+2CO2→ (CO2)+2+CO2; the third order rate constant for this process is 1.5±0.1×10−28 cm6 sec−1 at 409° K.
Infrared-to-visible conversion in (Y, Yb, Er)F3 and (Y, Yb, Ho)F3
Authors: Ettinger D. G. , Niemczyk T. M.
Published: February 2003

Visible emission spectra from (Y0.80Yb0.19Er0.01)F3 and (Y0.80Yb0.19Ho0.01)F3 were obtained upon excitation at 970 nm. Emission bands at 656, 548, 520, 409, and 380 nm are observed in the Er3+ phosphor, and at 746, 645, 544, and 489 nm in the Ho3+ phosphor. The origin of each of these bands is discussed. Data presented for the 656 nm emission in the Er3+ phosphor show that it can be the result of only one of three previously proposed mechanisms. The 746 nm band from the Ho3+ system has not been previously discussed in the literature even though it is the most intense emission band. Data presented for the temperature dependence of the upconversion process in these two phosphors show that generalizatons made about the mechanism of energy transfer in the Ho3+ system based on the more widely studied Er3+ system are not valid.
Mean dipole moment derivatives, atomic anisotropies, and effective charges of diatomic hydrides
Authors: Bruns Roy E. , Brown Richard E.
Published: February 2003

Mean dipole moment derivatives, atomic anisotropies, and effective charges for the first and second row diatomic hydride molecules have been calculated from the theoretical CEPA values of the equilibrium dipole moment, dipole moment derivative, and internuclear distance reported by Meyer and Rosmus. The behavior of these values as a function of atomic number, both across and down the periodic table, is extensively analyzed. The theoretical values are compared with those obtained from experiment. The ratio of the equilibrium dipole moment to the internuclear distance and the dipole moment derivative show linear dependencies on the atomic number for the Group III through Group VII first and second row diatomic hydrides. This information is used to estimate these quantities for the metal diatomic hydrides of the third and fourth row.
Energy transfer cross sections in relation to generalized oscillator strengths for the donor–acceptor pair benzene–acetone
Authors: Klump Kirby N. , Lassettre Edwin N.
Published: February 2003

Generalized oscillator strengths have been determined for the ? 1B2u←? 1A1g transition in benzene and the ? 1A2←? 1A1 transition in acetone. From the data, and the spectroscopic data and analysis of other investigators, the generalized oscillator strengths for the reverse transition in benzene have been calculated approximately. Huo’s theory of energy transfer cross sections [J. Chem. Phys. 66, 3572 (1977)] and the generalized oscillator strengths have been used to calculate the cross section for the resonant energy transfer process benzene (? 1B2u) + acetone (? 1A1) → benzene (? 1A1g)+acetone (? 1A2). The energy transfer cross section calculated in this way is found to be 39 Å2 with an estimated uncertainty of ±5 Å2. The experimental value is 33±2 Å2 [K. Janda and F. S. Wettack, J. Am. Chem. Soc. 94, 305 (1972)]. Huo’s theory is based on the Born approximation, neglecting electron exchange. The agreement between calculated and observed cross sections shows that the Born approximation (without exchange) is holding even though the molecules are colliding with relative velocities in the thermal range.
Threshold behavior of endoergic bimolecular reactions: A statistical phase space approach
Authors: Chesnavich W. J. , Bowers M. T.
Published: February 2003

A statistical phase space theory is developed for threshold and near threshold behavior of endoergic bimolecular reactions. A simple, exact threshold law for the cross section is derived for collisions between atomic, diatomic, or polyatomic species in all combinations. An approximate expression is derived for the cross section that is valid between threshold and about 0.5 eV above threshold. Analytical expressions are obtained in both cases that allow cross section versus energy curves to be generated using only hand-held electronic calculators. The utility of the theory is demonstrated by comparing the theoretical predictions with experiment for the reactions C+⋅ (D2,D⋅)CD+, H2O+⋅ (H2,OH⋅)H+3, and D3O+(D2,D2O)D+3. The first reaction behaves statistically in the threshold region while the second and third do not. A correlation is made between the statistical behavior of the reaction and the stability of the collision complex: The intermediate CD+2⋅ is very stable (98 kcal/mole) while the ions H4O+⋅ and D5O+ are unstable. The utility of the approximate threshold theory for exploring the molecular origins for the cross-sectional changes with energy is emphasized.
Ionizing collisions of cesium and potassium atoms with water
Authors: Warmack R. J. , Stockdale J. A. D. , Compton R. N.
Published: February 2003

Crossed beam studies of K and Cs collisions with H2O and D2O have been made for laboratory alkali beam energies from 10 to 200 eV. The only ionic products observed were K+, Cs+, OH−, and OD−. The positive and negative ion thresholds are equal to within ∼0.1 eV and correspond to the H(2S)+OH−(1Σ) limit of H2O−. The positive and negative ion energy and angular distributions are discussed including the observation of an OD− ’’pickup’’ reaction.
Assignments in the ultraviolet spectra of symmetrically substituted sulfides
Authors: McDiarmid Ruth
Published: February 2003

The ultraviolet absorption spectra of (SiH3)2S, (GeH3)2S, [Si(CH3)3]2S, and [C(CH3)3]2S were recorded from 2300–1200 Å. These and the published spectra of other symmetrically substituted sulfides were collectively analyzed. The results were interpreted to demonstrate (1) the presence of a n→3d transition in each of the molecules and (2) that of the two allowed 4p-Rydberg transitions, the 1B1 transition is the one observed in H2S and (CH3)2S. Some of the published assignments of H2S, (CH3)2S, and [C(CH3)3]2S are revised.
Statistical properties of steps on crystal surfaces
Authors: Gilmer George H. , Weeks John D.
Published: February 2003

We examine the equilibrium properties of steps on crystal surfaces using mean field theory and the more accurate pair approximation. The solid-on-solid model of surface orientations vicinal to the simple cubic (100) face is employed in all calculations. The pair approximation for the excess energy of an isolated step is in good agreement with recent Monte Carlo simulations. Also presented for the isolated step are excess values of entropy, density of atoms with the coordination of kink sites, and the free energy. In each case two step orientations are considered. Interactions between steps on vicinal surfaces are discussed, and a section of the γ plot (surface free energy as a function of surface orientation) is calculated.
The (b 1Σ+) state of PH in the vacuum ultraviolet photolysis of phosphine
Authors: Di Stefano G. , Lenzi M. , Margani A. , Xuan C. Nguyen
Published: February 2003

PH (b 1Σ+) has been observed as a primary product in the vacuum ultraviolet photolysis of PH3. The identification has been made from the analysis of some emission bands at 697.3 nm attributed to the transition PH (b 1Σ+→X 3Σ−) and from the measured energy threshold for the excitation of the above bands. The primary photolysis process PH3 (X̃ 1A1) →PH(b 1Σ+) +2H (2S) is consistent with the experimental threshold. The (b 1Σ+) state of PH and the ’’forbidden’’ transition to the ground state have been observed for the first time. The T0 value of PH (b 1Σ+) is 1.78 eV.
Photoabsorption in molecular nitrogen: A moment analysis of discrete-basis-set calculations in the static-exchange approximation
Authors: Rescigno T. N. , Bender C. F. , McKoy B. V. , Langhoff P. W.
Published: February 2003

Theoretical investigations of photoexcitation and ionization cross sections in molecular nitrogen are reported employing the recently devised Stieltjes–Tchebycheff moment-theory technique in the static-exchange approximation. The coupled-channel equations for photoabsorption are separated approximately by identifying the important physically distinct excitation processes associated with formation of the three lowest electronic states of the parent molecular ion. Approximate Rydberg series and pseudospectra of transition frequencies and oscillator strengths are constructed for the seven individual channel components identified using Hartree–Fock ionic core functions and normalizable Gaussian orbitals to describe the photoexcited and ejected electrons. Detailed comparisons of the theoretically determined discrete excitation series with available spectral data indicate general accord between the calculated and observed excitation frequencies and oscillator strengths, although there are some discrepancies and certain Rydberg series have apparently not yet been identified in the measured spectra. The total Stieltjes–Tchebycheff vertical photoionization cross section obtained from the discrete pseudospectra is in excellent agreement with recent electron–ion coincidence measurement of the cross section for parent–ion production from threshold to 50 eV excitation energy. Similarly, the calculated vertical partial cross sections for the production of the three lowest electronic states in the parent molecular ion are in excellent accord with the results of recent electron–electron coincidence and synchrotron–radiation branching ratio measurements. The origins of particularly intense resonancelike features in the discrete and continuum portions of the photoabsorption cross sections are discussed in terms of excitations into valencelike molecular orbitals. Small discrepancies between theory and experiment are attributed to specific autoionization processes and channel couplings not included in the calculations. In contrast to previously reported model or local potential studies, the present results employ the full nonlocal and nonspherical molecular Fock potential in ab initio photoabsorption calculations. The excellent agreement obtained between theory and experiment in molecular nitrogen suggests that highly reliable photoabsorption cross sections for diatomic molecules can be obtained from Hilbert space calculations and the Stieltjes–Tchebycheff method in the static-exchange approximation under appropriate conditions.
O(1S) interactions—the product channels
Authors: Slanger T. G. , Black G.
Published: February 2003

The first measurements are reported of the reaction pathways for the interaction between oxygen atoms in the 4.19 eV 1S state, and four molecules, N2O, CO2, H2O, and NO. Distinction is made between three possible paths—quenching to O(1D), quenching to O(3P), and chemical reaction. With N2O, the most reasonable interpretation of the data indicates that there is no reaction, in sharp contrast with the interaction between O(1D) and N2O, which proceeds entirely by reaction. Similarly, there is no reaction with CO2. With H2O, the reactive pathway is the dominant one, although electronic quenching is not negligible. With NO, O(1D) is the preferred product.
Products of the O(1S) –O2 interaction
Authors: Slanger T. G. , Black G.
Published: February 2003

The branching ratio for the production of O(1D) and O(3P) from the deactivation of O(1S) by O2 has been measured. The fraction going to O(1D) was found to be 0.31±0.07, the remainder going to O(3P). The quenching of O(1S) by O2 does not lead to O2(b 1Σ+g) formation, as occurs in the O(1D) –O2 interaction.
Representations of molecular force fields. V. On the equilibrium structure of methane
Authors: Bartell L. S. , Kuchitsu Kozo
Published: February 2003

The correction from mean (rg) to equilibrium (re) bond lengths in methane is reinvestigated in response to suggestions that the original Kuchitsu–Bartell (KB) corrections were excessive by as much as 0.006 Å. It is found that the KB model anharmonic force field with its original parameterization does overcorrect, relative to the new Pulay ab initio force field, but only by 10% of the original (rg−re) corrections, or 0.0022 Å for CH4 and 0.0015 Å for CD4. The KB model with the more recent MUB-2 nonbonded parameterization gives results in close accord with the ab initio results, supporting the utility of the augmented anharmonic Urey–Bradley representation of force fields. Reported discrepancies between experimental and theoretical methane bond lengths cannot be due solely to the form of or original parameterization of the KB model field.
Complex refractive indices in the infrared for ZnCl2 in water
Authors: Tyler I. L. , Querry Marvin R.
Published: February 2003

Near-normal incidence reflectance spectra relative to water were measured throughout the 270?ν?5000 cm−1 spectral region for 20.3%, 29.4%, and 40.2% aqueous solutions of ZnCl2. Spectral values of the complex refractive index N (ν) =n (ν)+ik (ν) of the ZnCl2 solutions were computed by use of subtractive Kramers–Kronig techniques. Values of N (ν) were obtained in a form suitable for future computations of Mie scattering in a cloud composed of aqueous ZnCl2 droplets. The spectra were further analyzed to determine the central frequencies, half-widths, and strengths of infrared bands due to the fundamental intra- and intermolecular vibrational modes of liquid water.
Density response theory of nonbonded interactions
Authors: Payne Philip W.
Published: February 2003

A model is proposed whereby nonbonded interaction energies are related to the behavior of electron density. Modification of Hartree–Fock perturbation theory permits derivation of the density response δρ (1) caused by effective potential variation δU (2)−δμ. The effective potential is the electrostatic potential U referenced against chemical potential μ. The density response is modulated by the density matrix and is exponentially damped according to the approximate formula δρ (1) =−1/(2π) ℱ{d3r2ρ (1,2) e−εr12 [δU (2)−δμ]}/r12, where ρ (1,2) is the density matrix and ε is an average valence shell orbital energy. Density functional methods are employed in deriving a relationship between the density modulation δρ (1) and the nonbonded energy change. The latter is the sum of a purely electrostatic component and a charge reorganization energy associated with δρ (1): ΔE=ΔEes+ℱd3r1δρ (1) δU (1). This model is used to clarify the origin of closed shell repulsion and barriers to internal rotation.
Calculations of resonance Raman cross sections in forbidden electronic transitions: Scattering of the 992 cm−1 mode in the 1B2u band of benzene
Authors: Korenowski G. M. , Ziegler L. D. , Albrecht A. C.
Published: February 2003

Strictly dipole forbidden electronic states are shown to be possible sources of Raman cross section for the scattering of totally symmetric fundamentals. In particular, the absolute scattering cross section and depolarization ratio of the 992 cm−1 a1g mode in benzene are calculated in the region of the electronically forbidden 1B2u transition. The molecular polarizability in this region is derived from two principle sources. One is the preresonance contribution of an effective electronic transition in the deep vacuum ultraviolet. The other is due to contributions from the vibrational substructure of the 1B2u transition which vary in sign. Interference effects lead to interesting resonance profiles for both the scattering cross section and the depolarization ratio. The in-resonance contributions to the polarizability are determined by a semiempirical approach to the summation over all vibrational subspaces. The damping constant need not serve as an artifical breadth parameter when that breadth is not lifetime limited. Once the complete vibrational subspace has been included in the theory much smaller values of the damping constant than previously used can be accommodated.
The influence of hindered molecular rotation on the dielectric constants of alcohols: A comment upon a little recognized error of Oster and Kirkwood
Authors: Lesikar Arnold V.
Published: February 2003

It is shown that the calculation of the alcohol correlation parameter g made by Oster and Kirkwood is in error as the result of an averaging procedure since proved to be invalid. A correct calculation is carried out on the basis of the same model of the alcohol self-association as used by Oster and Kirkwood, yielding a value of 2.0 for g. Literature dielectric data is discussed in relation to this result. It is suggested that the large g values observed in the less sterically hindered alcohols are due mainly to the increment of the dipole moment occurring as a result of hydrogen bond formation. Rotation around the hydrogen bond in these materials is probably almost free.
Design of natural collision coordinates to describe dissociation of polyatomic molecules
Authors: Band Yehuda B. , Freed Karl F.
Published: February 2003

We introduce a set of conditions to be satisfied by natural collision coordinates in order to enable the nonseparable multidimensional bound-continuum Franck–Condon integrals, arising in the quantum theory of collinear polyatomic dissociation processes, to be reduced to one-dimensional integrals. Because these conditions are not satisfied by available natural collision coordinate schemes, we investigate, in detail, the nature of one set of coordinates which satisfies these conditions. It is demonstrated that these coordinates can be chosen to faithfully represent the asymptotic motions of the fragments as well as the motion of the fragments in the Franck–Condon region on the repulsive electronic surface. The kinetic energy operator is displayed as an explicit function of the repulsive surface coordinates. The half-collision boundary conditions are described in a fashion to enable numerical computations of the continuum functions along the reaction coordinate.
Multiphoton dissociation products from halogenated hydrocarbons
Authors: Sudbo Aa. S. , Schulz P. A. , Grant E. R. , Shen Y. R. , Lee Y. T.
Published: February 2003

Multiphoton dissociation of various halogenated hydrocarbons by laser beams is reported. The statistical theory of unimolecular reactive is found to be adequate to explain the process.(AIP)
Reconstruction of optical absorption band shapes from their moments
Authors: Kelley C. Stuart
Published: February 2003

The formalism for reconstructing an absorption band shape from its moments is presented. Examples of reconstructing low-temperature, unstructured shapes indicate that only the four lowest moments are important. These moments are related to the area under the band, its location, its half-width, and its skewness. Inclusion of skewness is critical to an accurate reconstruction. These conclusions are appropriate at all temperatures.
Analytical potentials from a
b i
n
i
t
i
o computations for the interaction between biomolecules. IV. Water with glycine and serine zwitterions
Authors: Carozzo L. , Corongiu G. , Petrongolo C. , Clementi E.
Published: February 2003

The glycine zwitterion, in two different conformations, and the serine zwitterion, in three different conformations, have been analyzed in their interaction with a molecule of water placed at 281 different orientations and distances. For these orientations and distances SCF–LCAO–MO computations have been performed on the glycine–water and serine–water complexes, using a Gaussian basis set. The computed interaction energies have been fitted with a sum of pair potentials. Two simple forms of the pair potentials have been considered: the first is the one used in the previous paper and is of the form -Aij/r6ij +Bij/r12ij +Cijqiqj/rij; the second contains a more flexible description of the electrostatic interaction, and is of the form -Aij/r6ij +Bij/r12ij +Cijqiqj/rij +Dijqiqj/r2ij, where A, B, C, and D are numerical constants, and qi and qj are the atomic charges of the ith atom of M and of the jth atom of H2O at infinite separation. The pair potentials are discussed by computing potential curves and contour maps of isoenergy levels.
Perturbed Hartree–Fock calculations: Electric and magnetic properties of water in static fields
Authors: Lazzeretti Paolo , Zanasi Riccardo
Published: February 2003

Coupled Hartree–Fock perturbation theory has been applied to obtain accurate predictions of electric and magnetic properties of the water molecule employing a large basis set expansion consisting of contracted Gaussian functions. Particular emphasis is laid on electric dipole polarizability, shielding factors, magnetic susceptibilities, and magnetic shielding constants of oxygen and hydrogen atoms. The gauge dependence of computed magnetic properties is analyzed in a number of ways. Uncoupling of the Hartree–Fock perturbed equations is also discussed. The near Hartree–Fock wavefunction obtained here permits excellent theoretical determinations of the quantities under consideration and a gratifying agreement with experiment.
Scaling theoretic deconvolution of bulk relaxation data: State-to-state rates from pressure-broadened linewidths
Authors: DePristo Andrew E. , Rabitz Herschel
Published: February 2003

Inversion of certain types of relaxation data is shown to be possible by application of two new developments. First, a recently developed scaling theory of rotationally inelastic cross sections is utilized in deriving the corresponding formulas for the state-to-state rate constants kjj′. Second, practical means are presented for assessing the number of independent pieces of information contained in the experimental data. Application of this work to simple relaxation times (i.e., T1 or T2) provides an essential reduction in the number of unknowns and allows for the determination of the rates k0Δ by inversion of a set of algebraic equations subject to the constraints k0Δ?0. The dimensionality of this set of equations is related to the number of independent pieces of information contained in the experimental data. A stable and fast method for solving the equations is given. The inversion of pressure broadening data to yield state-to-state rate constants is illustrated for the CO–, O2–, N2–rare gas systems.
Experimental confirmation of renormalization—group prediction of critical concentration fluctuation rate in hydrodynamic limit
Authors: Chen S. H. , Lai C. C. , Rouch J.
Published: February 2003

Our independent measurements of shear viscosity, scattered light intensity, and Rayleigh linewidths of the critical mixture of nitrobenzene–n-hexane system support the recent renormalization-group calculation that shows that in the hydronamic limit the concentration diffusion coefficient is consistent with D?kBT/5πζsξ, within the experimental error.
Mechanisms of central O-atom abstraction reactions: A molecular beam, laser-induced fluorescence study of Group IIIB + ROH systems
Authors: Liu K. , Parson J. M.
Published: February 2003

The laser-induced fluorescence method has been used to determine vibrational and rotational energy distributions of metal oxides formed in reactions of Group IIIB atoms with H2O, D2O, CH3OH, C2H5OH, iso-C3H7OH, and n-C3H7OH. The observation of metal oxide rather than metal hydroxide in these reactions can be explained using orbital symmetry arguments. Although no vibrational inversions are observed, the reaction of Y with D2O produces considerably less vibrational excitation of YO than the reaction with H2O, while alcohols give less vibrational and rotational excitation in YO for larger alkyl groups. The internal energy distributions are generally not in quantitative agreement with statistical expectations. In the case of the ethanol and propanol reactions agreement with statistical vibrational results is improved if fragmentation to three products is assumed, presumably via a concerted ring mechanism.
Statistical thermodynamics of chain-folded polymer crystals
Authors: Ishinabe Takao
Published: February 2003

A simple molecular theory is developed in order to discuss the thermodynamic stability of chain-folded crystals. The partition function for a crystal containing different fold loops is formulated for a monodisperse polymer in terms of the lattice model; the analytical evaluation is made by using the maximum term method. The free energy calculated decreases linearly with decreasing the number of fold loops, i.e., its minimum corresponds to the extended-chain-type crystal. However, a lamellar-type crystal with long fold period and tight fold loops is also actually stable owing to the existence of high activation free energy barriers between them. The average number of monomer units comprised in the loop is estimated to be 5–7 irrespective of the lamellar thickness. The irreversible lamellar thickening and melting behavior of folded-chain crystals can be explained from this model. The results are also valid for the polydisperse system having a chain length distribution; it gives the crystal of nonuniform but well-defined lamellar thickness.
Rate constants for the reactions of the OH radical with (CH3)2NH, (CH3)3N, and C2H5NH2 over the temperature range 298–426 °K
Authors: Atkinson R. , Perry R. A. , Pitts J. N.
Published: February 2003

Rate constants for the reactions of the OH radical with (CH3)2NH, (CH3)3N, and C2H5NH2 have been determined over the temperature range 298–426°K by a flash photolysis–resonance fluorescence technique. The Arrhenius expressions obtained are with rate constants at room temperature of (6.54±0.66) ×10−11, (6.09±0.61) ×10−11, and (2.77±0.28) ×10−11 cm3 molecule−1 sec−1 for (CH3)2NH, (CH3)3N, and C2H5NH2, respectively. Possible mechanisms and atmospheric significance of these reactions are discussed.
Lead whisker growth from the metallic vapor phase
Authors: Thompson W. A.
Published: February 2003

Vapor growth conditions for small (<10 nm) lead whiskers have been studied in a sealed cryogenic chamber with base pressure below 10−14 torr. Using a field emission technique developed by Gomer for mercury whiskers we have investigated the temperature range for exponential whisker growth; the crossing temperature (250 K) where two dimensional nucleation is thought to occur, and lower temperatures where nonexponential growth is observed.
The photoabsorption cross section of O2 from 55–350 Å
Authors: Mehlman G. , Ederer D. L. , Saloman E. B.
Published: February 2003

The photoabsorption cross section of O2 was measured photoelectrically in the wavelength range 55–350 Å. The 250 MeV storage ring at the National Bureau of Standards provided the background continuum. The absolute accuracy of the measurements is ±3% between 55–140 and 170–300 Å and ±5% in the wavelength range 140–170 and 300–350 Å. The cross section decreased monotonically with increasing photon energy. No thresholds or new structure was observed in the total cross section.
Raman spectrum of a Lennard-Jones glass
Authors: McTague J. P. , Mandell M. J. , Rahman A.
Published: February 2003

A normal mode analysis of Lennard-Jones glass, generated by molecular dynamics methods, has been made. The first order Raman spectrum of such a system of polarizable atoms is then calculated. Results agree well with the observed collision-induced light scattering spectra of liquid rare gases. Comparison is also made with a model which assumes a single mode-independent scattering matrix element, and significant differences are found.