bluuebunny/crossref_metadata_2025 huggingface dataset:
On the application, breakdown, and near-dissociation behavior of the higher-order JWKB quantization condition
Authors: Kirschner Sidney M. , Le Roy Robert J.
Published: February 2003

Numerical studies and analytic results show that singularities in the appropriate contour integrals cause the higher-order JWKB eigenvalue criteria to break down for energies near the dissociation limit of any potential with an attractive inverse power (Cn/Rn) tail for which n≳2. However, the expressions derived for the characteristic near-dissociation behavior of these contour integrals provide a simple way of predicting the range of binding energies over which this occurs from a knowledge of only n, the potential constant Cn, and the reduced mass μ. This analysis also shows that for more than 70% of all diatomic molecule electronic states, no vibrational levels lie in the very narrow region near dissociation where this breakdown occurs. A related near-dissociation analysis shows that for levels in the upper part of a potential well, applying the Langer–Kemble correction to the first-order quantization condition reduces the errors in calculated eigenvalues by the fraction 3/(n+1), for n?2. In contrast, for the (n=6) model problem studied numerically, use of the two-term (third-order) or three-term (fifth-order) quantization conditions reduces these errors by four or eight orders of magnitude, respectively.
Model-calculated combining rules for distance force constants and pseudocritical volumes
Authors: Pesuit David R.
Published: February 2003

Calculations based upon classical models of gas phase molecular interaction suggest that the geometric mean may be an accurate combining rule for distance force constants and pseudocritical volumes in the limit that interactions with relatively large asymmetrical molecules are considered. The analysis also suggests that the geometric mean may be too large in the limit that very large asymmetrical molecules are involved.
Determination of the proton affinity of N2 from ionization data on t
r
a
n
s-diimide
Authors: Foner S. N. , Hudson R. L.
Published: February 2003

The proton affinity of N2 was determined by mass spectrometry by measuring the appearance potential of the N2H+ from N2H2 and combining this with the heat of formation of diimide. The value for the proton affinity obtained in this study, P.A.(N2) =4.93±0.11 eV, is in good agreement with a theoretical calculation and with values derived from recent experiments on P.A.(CO2) using known differences in the proton affinity ladder to link them together. Since the P.A.(N2) measurement by this method gives a lower energy bound, the good agreement implies that very little excess kinetic energy is involved in the relevant ionization processes. Other experimental measurements that bear on the proton affinity of N2 are discussed.
Vapor pressure isotope effects in the benzene (B)–cyclohexane (C) system from 5 to 80 °C. I. The pure liquids B-d,B-d1,o
r
t
h
o-,m
e
t
a-, and p
a
r
a-B-d2, B-d6,C-d, and C-d12. II. Excess free energies and isotope effects on excess free energies in the solutions B-h6/B-d6, C-h12/C-d12, B-h6/C-h12, B-d6/C-h12, and B-h6/C-d12
Authors: Jakli G. , Tzias P. , Van Hook W. Alexander
Published: February 2003

High precision measurements of the vapor pressure differences between some deuterated benzenes B-dx (x=1, para-2, and 6) and protio benzene B-d0; between para- and ortho-, and para- and meta-dideuterobenzene; and between perdeuterocyclohexane C-d12 and protiocyclohexane C-d0 were made from near the freezing point to the normal boiling point. The data are best represented as the logarithmic ratios ln R (d6) =ln (Pd0/Pd6) =1226.5/T2−12.178/T, ln R (para/ortho) =ln (Ppara-d2/Portho-d2) =2.6/T2, and ln R (para/meta) =ln (Ppara-d2/Pmeta-d2) =−2.0/T2, together with the deviations from the law of the mean Δ (d1) =6−[ln R (d6)/lnR (d1)]=0.177−3.6×10−4t and Δ (d2-para=3−[ln R (d6)/ln R (d2-para)]=0.028 +1.0×10−4t. The isotope effects are inverse and display significant deviations from the law of the mean. The cyclohexane results are given by ln R (C-d12) =−2188.4/T2−18.587/T. New measurements of the vapor pressures of benzene–cyclohexane solutions are also reported between 5 and 80 °C. The data are in good agreement with the best earlier work. Excess free energies of the equimolar solutions B-h6/B-d6 and C-h12/C-d12 have been measured between 20 and 80 °C. Suitable fits to the data yield the following results (30 °C); Gex(B-h/B-d), Gex(C-h/C-d); Hex(B-h/B-d), Hex(C-h/C-d); 0.57,1.04;1.1,3.1 J/mole. For the isotope effects on the excess properties of benzene–cyclohexane solutions differential pressure measurements were taken as a function of temperature (20–80 °C) and concentration (XC=0.10, 0.25,0.50,0.70,0.75,0.85,0.90). The isotope effects on the excess properties at 25 °C are as follows: Gex(B-d/C-h) −Gex(B-h/C-h), Gex(B-h/C-d)−Gex(B-h/C-h); Hex(-d/C-h)−Hex(B-h/C-h), Hex(B-h/C-d) −Hex(B-h/C-h); 7.5,2.6;32,0 J/mole at Xc=0.5, but equations are presented so that the effects can be calculated at any concentration or temperature. The results are briefly discussed in the context of the theory of isotope effects in condensed phase systems.
Vapor pressure isotope effects in benzene–cyclohexane systems. III. Theoretical analysis
Authors: Jancsó Gábor , Van Hook W. Alexander
Published: February 2003

The data of Jakli, Tzias, and Van Hook (preceeding paper) on vapor pressure isotope effects in the series of deuterobenzenes, solutions of isotopic isomers of benzene and cyclohexane, and mixtures of benzene and cyclohexane are analyzed in terms of the theory of isotope effects in condensed phase systems using a cell model in the quasiharmonic approximation. The single largest contribution to the VPIE’s arises from the shift in the CH stretching force constant on condensation but other effects, including external internal coupling and the volume dependence of condensed phase internal frequencies, are also important. Molar volume isotope effects are demonstrated to be of critical importance in arriving at an understanding of the excess thermodynamic properties of solution.
A statistical theory for collisionless multiphoton dissociation of SF6
Authors: Shultz M. J. , Yablonovitch Eli
Published: February 2003

A statistical thermodynamic theory is used to calculate the dissociation efficiency of SF6 by CO2 laser photons. The dissociation efficiency calculated with the model presented is compared to that calculated with two other statistical models, one a more sophisticated model (RRKM) and the other a very crude model. We find that the calculated dissociation efficiency is insensitive to the particular statistical model used.
The effects of basis set and configuration interaction on the predicted geometries of AH2 molecules
Authors: Bell Stephen
Published: February 2003

The effects of basis set size and type on the theoretical optimum geometries calculated by ab initio SCF and CI methods is studied for the molecular species H2O 1A1, NH2 2B1, CH2 1A1, H2O+ 2B1, NH2 2A1, CH2 3B1, BH2 2A1, H2O+ 2A1, over the range from minimum to much extended basis sets. The difference in optimum geometry between SCF and CI calculations is also noted. Calculations made specially for this study have used double-zeta and double-zeta-plus-polarization Gaussian basis sets. The geometries predicted by these and many published calculations are plotted as points on diagrams of bond angle ϑ versus bond length r. It is observed that there are striking similarities between such diagrams for different species, and thus it is possible to use the diagrams in order to predict a molecular geometry with greater accuracy than by one method alone (except CI with a very extended basis). Such a prediction is made from calculations of the species NH−2 1A1.
Semiclassical theory of unimolecular dissociation induced by a laser field
Authors: Yuan Jian-Min , George Thomas F.
Published: February 2003

A semiclassical nonperturbative theory of direct photodissociation in a laser field is developed in which photon absorption and dissociation are treated in a unified fashion. This is achieved by visualizing nuclear dynamics as a representative particle moving on electronic-field surfaces. Methods are described for calculating dissociation rates and probabilities by Monte Carlo selection of initial conditions and integration of classical trajectories on these surfaces. This unified theory reduces to the golden rule expression in the weak-field and short-time limits, and predicts nonlinear behavior, i.e., breakdown of the golden rule expression in intense fields. Field strenghts above which lowest-order perturbation theory fails to work have been estimated for some systems. Useful physical insights provided by the electronic-field representation have been illustrated. Intense field effects are discussed which are amenable to experimental observation. The semiclassical methods used here are also applicable to multiple-surface dynamics in fieldfree unimolecular and bimolecular reactions.
The quadrupole moment of the H2 molecule
Authors: Poll J. D. , Wolniewicz L.
Published: February 2003

We present an improved adiabatic calculation of the quadrupole moment of the H2 molecule and of its matrix elements between various vibrational rotational states. Particular attention is paid to making a reasonable estimate of the accuracy of transition elements between widely different vibrational states. An order of magnitude estimate of the nonadiabatic contribution to the quadrupole moment is also given and we show that it is typically of the order of a tenth of a percent of the adiabatic value.
129I Mössbauer spectra of the tellurium iodides
Authors: Jones C. H. W. , Mauguin M.
Published: February 2003

The 129I spectra of Cs2TeI6, α-Me2TeI2, α-TeI and TeI4 are reported and the compound p-EtOC6H4TeI3 was also studied. The iodine coupling constant in Cs2TeI6 is comparable with that previously reported from NQR data. The other compounds each contain a number of inequivalent iodine atoms and complex spectra are observed. Computed Mössbauer parameters were obtained for all but p-EtOC6H4TeI3. A simple relationship is observed between bond length and the 129I isomer shift and coupling constant.
Ground state energy of excess electrons in n-hexane, 2,2,4-trimethylpentane, and tetramethylsilane
Authors: Kimura Toyoaki , Fueki Kenji , Kevan Larry
Published: February 2003

The ground state energies of excess electrons in n-hexane, 2,2,4-trimethylpentane, and tetramethylsilane at room temperature are calculated from a structured semicontinuum model and are found to agree well with recent experimental values. The calculations also account for localization and delocalization of electrons in these solvents.
Symmetry aspects in the investigation of scalar properties of molecules of a class with internal rotation
Authors: Meinköhn Dirk
Published: February 2003

Because of lack of symmetry, difficult classical and quantum-mechanical derivations of scalar molecular properties are characteristic for completely asymmetric molecules with an asymmetric part rotating relative to the molecular frame. The molecular class defined by the ethane skeleton is taken as an example for a demonstration of how symmetry aspects can be regained by considering suitable ’’pseudoscalar’’ properties of diastereomeric pairs of molecules.
Temperature dependence of NMR chemical shifts in cuprous halides
Authors: Becker K. D.
Published: February 2003

Measurements of NMR chemical shifts of 63Cu, 35Cl, 81Br, and 127I in the cuprous halides are reported from about −150 °C to temperatures above the melting points. From the low temperature data it is deduced by means of a simple tight binding description of chemical bonding that CuBr is the most ionic of these compounds and CuI the most covalent. In all solid and liquid phases the shifts were found to depend linearly on temperature. In the low temperature cubic phases, the cation shifts increase with temperature, while the anion shifts decrease with temperature. This unusual behavior is shown to be caused by an increase of vibrational overlap with temperature. In the high temperature cubic phases and in the melts the observed increasing diamagnetism of the cation shifts with temperature is related to the highly disordered state of the cuprous ions in these phases. Possible influences of nonstoichiometry on chemical shift are discussed in connection with measurements performed on CuI under iodine vapor.
Electric dipole moment and polarizability of the 1749 Å, 1B2 excited state of formaldehyde
Authors: Causley G. C. , Russell B. R.
Published: February 2003

The medium resolution modulated electric field (electrochromism) spectrum of the 1749 Å band of formaldehyde in the gas phase has been obtained. This differential absorption results from changes in transition probability and transition energy in the presence of a strong electric field and is related to, among other excited state properties, the square of the dipole moment change. The data, after analysis, yield the dipole moment change upon excitation to be 2.66±0.14 D. This change is thought to reduce the dipole moment to near zero in this excited state. In addition, the excited state polarizability is estimated to be 6±3×10−23 cm3 in the state under consideration. These changes coupled with previously reported experimental and theoretical results support the assignment of this transition as the 3s (a1) ←n (b2), B2←A1 molecular Rydberg. Finally, the data suggest that in this energy region there is a state of the same symmetry lying nearby.
Effect of field-dependent mobility on the escape probability. I. Electrons photoinjected in neopentane
Authors: Mozumder A. , Carmichael I.
Published: February 2003

A general procedure is described for calculating the escape probability of an electron against neutralization in the presence of an external field after it has been ejected into a dielectric liquid from a planar surface. The present paper utilizes the field-dependent electron mobility measurement in neopentane by Bakale and Schmidt. The calculated escape probability, upon averaging over the initial distribution, is compared with the current efficiency measurement of Holroyd et al. The median thermalization legnth, inferred from this comparison, depends in general upon the assumed form of initial distribution. It is less than the value obtained when the field dependence of the mobility is ignored but greater than that applicable to the high energy irradiation case. A plausible explanation is offered.
The relation between satellite peaks in x-ray emission and x-ray photoemission spectra
Authors: Martin R. L. , Kowalczyk S. P. , Shirley D. A.
Published: February 2003

A unified model is given to explain the satellite structures observed in x-ray emission spectroscopy (XES) and x-ray photoelectron spectroscopy (XPS) and the relationships between the two. This model accounts for electron correlation by the use of configuration interaction, rather than the heuristic ’’shake-up’’ approach, and actual eigenstates of the system are considered. Two situations are discussed, differing in whether the final XES states have holes in core or valence shells. For core-to-core XES transitions, the correlation-state structure is almost identical in initial and final states. Large satellite intensities may be observed in XPS, but not in XES, in accord with experiment. In core-to-valence XES transitions, the satellite structure should be similar to that observed in XPS, as observed. Detailed CI calculations for the 3s spectra of Ar are compared with measured spectra.
Study of melting and freezing in the Gaussian core model by molecular dynamics simulation
Authors: Stillinger Frank H. , Weber Thomas A.
Published: February 2003

Molecular dynamics calculations have been carried out to establish quantitative properties of the Gaussian core model near its crystal–fluid transition. Two densities have been considered, for both of which the stable crystal structure at absolute zero is body-centered cubic. Spontaneous melting and freezing events were observed at both densities. Annealing of defective crystalline phases and formation of amorphous ’’glassy’’ structures have been induced. Properties for the model at equilibrium display some surprising ’’waterlike’’ anomalies, including negative volume of melting, negative thermal expansion in the fluid, and increase in rate of self-diffusion as the system is compressed.
Electronic states of Ar2F and Kr2F
Authors: Wadt Willard R. , Hay P. Jeffrey
Published: February 2003

A b initio POL CI calculations have been performed on the low-lying states of Ar2F and Kr2F. The results support a simple diatomics-in-molecules description of Rg2F (Rg=rare gas). In each case only one ionic state (2 2B2) is found to be stable (by 0.6 eV) relative to Rg+F−+Rg. The equilibrium geometry is found to be an isosceles triangle where the Rg–Rg distance is 0.02 Å shorter than that in the 2Σ+u state of Rg+2 and the Rg–F distance is 0.14 Å greater than that in the 2 2Σ+ state of RgF. There are two dipole-allowed emissions (2 2B2→1 2A1, 2 2B2→1 2B2) of comparable strength and wavelength (267 and 274 nm, respectively, in Ar2F and 357 and 368 nm, respectively, in Kr2F). The calculated wavelengths are in good agreement with the broad bands at 290 and 400 nm observed in e-beam pumped Ar or Kr/F2 mixtures, confirming the earlier assignment of these bands to Ar2F and Kr2F, respectively. The radiative lifetime of the 2 2B2 is calculated to be 132 ns for both Ar2F and Kr2F. There are four dipole-allowed absorptions from the 2 2B2 state, one of which (2 2B2→3 2A1) is very strong and corresponds to the 2Σ+u→2Σ+g transition in Ar+2 and Kr+2. The effect of the F− on this transition is negligible, so that the calculated wavelengths (320 nm in Ar2F and 344 nm in Kr2F) and oscillator strengths are basically unchanged from the rare gas dimer ions. Therefore, Ar2F and Kr2F may be at least as important as the Ar+2 and Kr+2 absorbers in the KrF laser. Spin–orbit coupling has been included at the equilibrium geometry of the 2 2B2 state using a simple atoms-in-molecules approach. The stability of the 2 2B2 (now 4 2Γ) state is decreased slightly so that De(Rg+F−+Rg) is 0.57 eV for Ar2F and 0.49 eV for Kr2F. The emission wavelengths and radiative lifetime for the 2 2B2 state in Ar2F are unchanged, as are the wavelengths and strength of the 2 2B2→3 2A1 absorption. In Kr2F the emission wavelengths are shifted slightly to 361 and 371 nm, while the radiative lifetime becomes 133 ns. The 2 2B2→3 2A1 absorption in Kr2F shifts to 336 nm and the strength decreases by 5%. Consideration of the geometry dependence of the transition moment and emission wavelength indicates that vibrational averaging should not change the radiative lifetimes for Ar2F and Kr2F significantly. Recent experimental measurements of the Ar2F (185±45 ns) and the Kr2F (176±20 ns) lifetimes are in good agreement with the calculated values. Finally, the calculations indicate that Rg+2F− should be formed by three-body quenching of Rg+F− rather than reaction of rare gas excimers with molecular fluorine. The latter exothermic reaction produces highly excited Rg+2F−, which falls apart into Rg+F−+Rg. These conclusions are supported by recent experimental results.
A
b i
n
i
t
i
o study of structure and vibrational spectra of SN+, SN, and S2N2
Authors: Karpfen A. , Schuster P. , Petkov J. , Lischka H.
Published: February 2003

Using large Gaussian basis sets ab initio calculations on the molecules SN+, SN, and S2N2 have been performed at the SCF level. Additionally, electron correlation effects were studied for SN+ and S2N2 by the PNO–CI and CEPA–PNO methods. The following equilibrium geometries were obtained: SN+, reSN =2.64 (SCF), 2.72 a.u. (CEPA); SN, reSN=2.78 a.u. (SCF); S2N2, D2h symmetry, reSN=3.07 (SCF), 3.16 a.u. (CEPA), &NSN=90.2° (SCF), 90.5° (CEPA). Complete harmonic force fields of S2N2 and the corresponding vibrational frequencies were computed. The assignments of the experimental ir and Raman spectra for S2N2 are discussed critically on the basis of the theoretical results.
Photofragmentation of symmetric triatomic molecules: Time dependent picture
Authors: Heller Eric J.
Published: February 2003

A recent time dependent formulation of total photodissociation cross sections is exploited to give a qualitative explanation of line shapes and absorption envelopes for symmetric triatomic (XY2) vacuum uv spectra. Attention is given to the dependence of the cross section on potential surface parameters and on the nature of the initial vibrational wavefunctions. The symmetric triatomic case treated here is illustrative of techniques which can be applied to more complicated, unsymmetric polyatomic situations.
Potential energy of rare gases adatoms on alkali halide crystals
Authors: Rogowska J. M.
Published: February 2003

The potential energies of rare gas adatoms on clean distorted and undistorted surfaces of simple cubic ionic crystals were computed using the lattice summation method. The numerical computations were carried out for Ne, Ar, and Xe adsorbed on the (100) face of K, Na, and Rb halides. The dependence of binding energy on such physical properties of adsorbents and adsorbates as polarizability and optical dielectric constant was established and the formulas approximating some of these dependences within 10% accuracy were given. Similar relationships were also found for the dispersion energy and the repulsion energy of adatoms at an equilibrium distance from the crystal surface.
Radiationless transitions in a magnetic field
Authors: Stannard Paul R.
Published: February 2003

A theory of radiationless transitions for gaseous molecules in a magnetic field is presented. Two mechanisms, the direct and indirect, for the modification of the rate of decay by the field are characterized. On the basis of the theory explanations are suggested for recent observations of magnetic quenching of fluorescence in CS2, glyoxal, and NO2.
Sudden approximation calculations of reactive scattering: The H+H2 reaction
Authors: Bowman Joel M. , Lee Ki Tung
Published: February 2003

Approximate quantal calculations of the zero partial wave total indistinguishable reaction probability and the total reaction cross section for H+H2 are presented.
Electron diffraction from gaseous tellurium hexafluoride at 20, 90, and 150 °C. Molecular structure and three-atom scattering
Authors: Gundersen Grete , Hedberg Kenneth , Strand Tor G.
Published: February 2003

Electron-diffraction patterns of gaseous TeF6 obtained with 44 kV electrons at nozzle-tip temperatures of 20, 90, and 150 °C have been recorded and analyzed. Contributions to the intensity from three-atom multiple scattering and the usual two-atom scattering were calculated using scattering factors for the tellurium atom derived both from tabulated values of a relativistic HFS potential and an analytical potential. When the three-atom components of intensity were omitted the two sets of scattering factors gave about equally good agreement with experiment. Inclusion of the three-atom components based on tabulated values greatly improved the agreement and those based on the analytical form greatly worsened it. The structural results for the best model at 20 °C obtained from refinements which included corrections for ’’shrinkage’’ are ra(Te–F) =1.815(2) Å, ra(F⋅⋅⋅F) =2.564(3) Å, ra(F⋅⋅⋅F) =3.626(4) Å, l (Te–F) =0.0412(20) Å, l (F⋅⋅⋅F) =0.0814(27) Å, and l (F⋅⋅⋅F) =0.0512(55) Å; the parenthesized values are 2σ and include estimates of systematic error and data correlation.
Collisional excitation transfer between Na and Na2
Authors: Lam L. K. , Fujimoto T. , Gallagher A. C. , Hessel M. M.
Published: February 2003

We have excited Na(3s) →Na(3p) with a pulsed dye laser at Na densities of 1014–1016 cm−3 with typically ∼2% Na2. From the decay time constants, intensities and spectra of the Na and Na2 fluorescences we have identified several collisional and radiative processes in the excited sodium atom-dimer system. Comparing the solutions of coupled equations to the density dependence of the decay constants and fluorescence intensities yields a rate coefficient of (3.4+1.6−1.1) ×10−9 cm3 sec−1 for the excitation transfer process: Na* (3p)+Na2(X 1Σ) →Na(3s) +Na2* (A 1Σ and a 3Π). This indicates that some kind of long-range interaction is operative. Other rate coefficients are determined less accurately; e.g., nonradiative Na(3p) quenching rate coefficients of 10±5×10−10 cm3 sec−1 due to dimer collisions and of 3±3×10−12 cm3 sec−1 due to atom collisions are obtained.
The photoelectron spectra of the isoelectronic molecules hypochlorous acid HOCl and chloramine NH2Cl
Authors: Colbourne D. , Frost D. C. , McDowell C. A. , Westwood N. P. C.
Published: February 2003

The HeI photoelectron spectra of hypochlorous acid HOCl, and chloramine NH2Cl have been measured and interpreted. HOCl exists in the presence of a large excess of H2O, but four ionization potentials have been obtained, in good agreement with recent calculations involving perturbation corrections to Koopmans’ theorem. NH2Cl obtained in quantitative yield has five ionization potentials. The ground ionic state is probably planar as is the case of the NH+3 ion.
Kinetics of the reactions of the OH radical with HCHO and CH3CHO over the temperature range 299–426°K
Authors: Atkinson R. , Pitts J. N.
Published: February 2003

Absolute rate constants for the reactions of the OH radical with HCHO and CH3CHO have been determined over the temperature range 299–426°K by a flash photolysis–resonance fluorescence technique. The Arrhenius expressions obtained are k (HCHO) =1.25×10−11 e−(175±300)/RT cm3 molecule−1⋅sec−1, k (CH3CHO) =6.87×10−12 e(510±300)RT cm3 molecule−1⋅sec−1 with rate constants at room temperature of (9.4±1.0) ×10−12 cm3 molecule−1⋅sec−1 and (1.60±0.16) ×10−11 cm3 molecule−1⋅sec−1 for HCHO and CH3CHO, respectively.
Reactive and inelastic scattering of H2+D2 using a repulsive model potential energy surface
Authors: Brown Nancy J. , Silver David M.
Published: February 2003

Collisions between hydrogen and deuterium molecules are examined using quasiclassical dynamical trajectory calculations with the intermolecular field specified by a semiempirical potential-energy surface incorporating enough repulsive character to yield barriers to chemical exchange that are in general agreement with ab initio results. The trajectory calculations are performed at high total system energies to permit the possibility of reactions. In addition to nonreactive inelastic collisions, the reactants H2+D2 can produce four possible reactive cases with product species 2H+D2, H2+2D, HD+H+D, and 2HD. The results are presented in terms of reaction probabilities, average final state properties of the molecules, and average final state energy distributions.
Paramagnetic and optical studies of radiation damage centers in K(H1−xDx)2PO4
Authors: McMillan J. A. , Clemens J. M.
Published: February 2003

Optical and EPR studies of K(H1−xDx)2PO4 (0⩽x⩽1) single crystals irradiated at 77°K with 60Co gamma rays and observed in liquid nitrogen, argon, and oxygen, have permitted not only the identification of two radical species produced as HPȮ−4 and H2PȮ4, but also a description of the annealing processes and their correlation with the ferroelectric transition temperature. Magnetic and kinetic parameters are given.
Lattice dynamics in covalent solids: Sn in SnS2−xSex(0 ⩽x⩽2)
Authors: Herber Rolfe H. , Smelkinson Ann E. , Sienko Michell J. , Schneemeyer Lynn F.
Published: February 2003

Mössbauer effect studies using the 23.8 keV gamma transition in 119Sn have been carried out on the system SnS2−xSex (0⩽x⩽2) on both microcrystalline powders and single crystal samples. The composition dependence of the isomer shift parameter for the single line resonance spectrum shows that the covalency increases as x increases; the 5s electron population of tin in SnSe2 is ∼0.13 greater than it is in SnS2. From the temperature dependence of the area under the resonance curve it is possible to extract a Mössbauer lattice temperature ϑM which is only weakly dependent on composition for samples prepared in an identical manner. Single crystal orientation experiments with SnS2 and SnSe2 have been carried out at 295±1 K to investigate the anisotropy in the metal atom motion parallel and perpendicular to the crystallographic c axis (perpendicular and parallel to the easy cleavage plane). The vibrational anisotropy ratio, 〈x∥〉/〈x∥〉, is found to be 2.25 in SnS2 and 1.58 in SnSe2 at room temperature. The absolute values of the tin atom vibrational amplitudes have been calculated using the recoil-free fraction data for metallic (β) tin under identical conditions.
Rotational energy effect upon the branching fraction for reactive decay of the RbF+K collision complex
Authors: Zandee L. , Bernstein R. B.
Published: February 2003

The influence of the rotational energy of the RbF molecule upon its reactivity with K has been measured by the crossed molecular beam technique over a range of relative translational energy ?tr from 3.0–4.5 kcal mol−1. The experiments involve the exoergic reaction RbF+K?[RbFK]→Rb+KF+1.5 kcal mol−1, for which the reactive branching fraction FR(?tr) had been previously measured using velocity-selected but otherwise ’’thermal’’ beams. Following procedures similar to those of Stolte et al. (1977), rotationally cold (’’low-J’’) beams of RbF have been prepared (by a quadrupole state selector) and crossed with a K beam. The reactive yield (Rb flux) and thus the reactive branching fraction FR is found to be significantly greater for the ’’low-J’’ beam than for the ’’thermal’’ beam of RbF. The magnitude of this rotational energy effect, associated with a reduction of some 2.0 kcal mol−1 in the rotational energy of the reagent molecules, is similar to the effect of a comparable reduction in ?tr. Thus, within the uncertainty of the present results, rotational energy of the RbF is equivalent in importance with relative translational energy in influencing the decay of the RbFK collision complex.
Lead microclusters in the vapor phase as studied by molecular beam electron diffraction: Vestige of amorphous structure
Authors: Yokozeki Akimichi
Published: February 2003

The metal microclusters are formed in the gas phase by an isobaric cooling due to argon carrier gas, followed by adiabatic expansions through nozzle apertures. Electron diffraction patterns of these condensates show the typical Debye–Scherrer rings for the fcc Pb metal. The average size of clusters obtained, depending on the experimental conditions, ranges from 30 to 100 Å in diameter, and the cubic lattice parameters for these clusters are essentially the same, within an experimental error of 0.3%, as that for the bulk crystal. The analysis of the Bragg-peak intensities is carried out based on the usual kinematic theory and the dynamical (two-beam approximation) theory by Blackman. The former theory fails completely in accounting for small angle reflections (s≲5 Å−1), whereas the latter explains successfully the whole observe intensities, except for the second and third order reflections (400), (440), (333), and (600). From the damping of peak intensities the temperatures of these clusters are estimated to be of the order of 150 °K. A close inspection on the comparison between the observed and theoretical intensities, however, reveals periodic fluctuations in the observed peak heights as a function of the scattering angle, indicating a superposition of some diffuse scattering. The amplitudes of the fluctuations are more marked for the smaller size clusters. The characteristics of the fluctuations are accounted for by introducing liquidlike random atomic configurations into the microcrystalline. This model explains also the abnormal Bragg-peak intensities of Ar clusters reported in the literature.
Series solution of the HNC and PY equations: The simple chain recursion way
Authors: Dale William D. T. , Friedman Harold L.
Published: February 2003

The function tHNC(1,2) that appears in the HNC equation when written in the form tHNC(1,2) =ln gHNC(1,2)+βu (1,2) can be analyzed by graphical expansion and topological reduction to obtain a series solution of the form tHNC(1,2) =A1(1,2) +A2(1,2)+⋅⋅⋅, where each An is graphically represented by a simple chain sum. A similar analysis yields a solution of the same form for the function tPY(1,2) that appears in the PY equation when written in the form tPY(1,2) =gPY(1,2) eβu(1,2)−1. The graphs in a given An are formed recursively from those of lower n. Truncation of the series at different n values provides an infinite number of well-defined approximations to the pair correlation function g (1,2). The series method also represents an efficient computational method for solving the HNC and PY equations that offers some advantages over methods involving the iterative or variational solution of the integral equations. The whole procedure is readily adapted to systems with long-range forces, systems with several species of particles, and to systems suitably related to some reference system with known correlation functions.
Vibrational relaxation of N2 by H2O
Authors: Center R. E. , Newton J. F.
Published: February 2003

The vibrational relaxation of N2 by H2O has been measured behind incident shock waves in the temperature range from 1600 to 3100 °K. The H2O concentration was measured with a calibrated capacitance hygrometer. The vibrational relaxation was monitored by redundant diagnostics: interferometric measurements of the gas density and infrared emission measurements of the CO fundamental band using CO as a tracer species to reflect the N2 vibrational energy. The experimental results support the slowest published measurements of von Rosenberg et al. Taken together with existing low-temperature data, the deactivation of N2 by H2O can be expressed in the form pτN2–H2O=exp(21T−1/3−1.35) μsec-atm.
Collisionless formation and rovibronic relaxation of CH and OH from the ir multiphoton photolysis of CH3OH
Authors: Bialkowski Stephen E. , Guillory William A.
Published: February 2003

A CO2 TEA laser has been used to initiate the collisionless multiphoton dissociation of CH3OH between 1000 and 10 mtorr. The appearance of OH(X 2Πi) 50±20 nsec after the laser pulse, independent of initial CH3OH pressure (50–200 mtorr), suggests the primary dissociative channel CH3OH+nhν→CH3 +OH(X 2Πi); although CH3 could not be correspondingly confirmed. The appearance of CH(X 2Πr) 70±20 nsec after the OH(X) appearance, independent of initial CH3OH pressure (70–400 mtorr), suggests secondary collisionless dissociation. Initial rovibronic distributions of OH(X) and CH(X) were determined, as well as characteristic decay time constants. The observation of the relaxation of OH(X) over a 10 μsec time interval after radical onset, allowed the separate characterization of collisional rotational relaxation and translational diffusion.
Dynamical effects on nearly degenerate electronic levels in solids
Authors: Brawer Steven
Published: February 2003

We consider the optical absorption of a model system consisting of a two-level ’’ion’’ interacting with a continuum of lattice vibrations. The two-level system represents localized electronic energy levels such as are found on transition metal ions. The separation of the electronic energies is taken to lie within the phonon band and the interaction with the lattice vibrations is linear. For a single band of phonons the absorption is shown to consist of three peaks—a homogeneously broadened central peak flanked by two sharp lines. The existence of three absorption peaks is in contrast to the predictions of either a static crystal field or of the two-level system interacting with a single harmonic oscillator. Our results reduce to the latter case when the phonon bandwidth becomes very narrow and to the former when the phonon bandwidth becomes very large.
Effect of vibrational excitation on the molecular beam reactions of Ca and Sr with HF and DF
Authors: Karny Ziv , Zare Richard N.
Published: February 2003

The reactions of Ca and Sr with HF and DF are studied using a beam–gas configuration as a function of reagent and product vibration to obtain state-to-state reaction rates. A beam of alkaline earth metal atoms enters a scattering chamber containing hydrogen fluoride. The HF and DF reagents are pumped from v=0 to v=1 by a pulsed HF or DF laser; the alkaline earth monofluoride products are detected by a pulsed tunable probe laser. The product internal state distribution is determined from the laser excitation spectrum. While no products could be detected for reactions with DF(v=1) or DF(v=0), the rate constant for the Ca and Sr reactions with HF(v=1) are both at least four orders of magnitude larger than the reactions with HF(v=0). The implications of these results on bond energies and the fate of the excess energy of reaction are discussed.
Infrared study of diluted solutions of polyatomic molecules. I. Band shape of fundamentals associated with nondegenerate normal modes
Authors: Leicknam J. C. , Guissani Y. , Bratos S.
Published: February 2003

A theory is proposed to analyze the band shapes of the simplest class of ir band profiles of polyatomic molecule solutions. The following theoretical points are explored in detail: dependence of vibrational correlation functions on the nature of normal modes involved, calculation of vectorial correlation functions for a classical ensemble of free asymmetric rotators, construction of extended diffusion models for asymmetric tops, and use of the ordered cumulant expansion techniques to study anisotropic rotational diffusion in liquids. The theory predicts a large variety of complex, often highly irregular, band profiles. A catalogue of representative band profiles is presented which orders them according to the symmetry of the active molecule.
Molecular-dynamics investigation of solid–liquid coexistence
Authors: Hiwatari Y. , Stoll E. , Schneider T.
Published: February 2003

Molecular-dynamics simulation of a crystalline fcc solid–liquid interface has been carried out for the first time for a three-dimensional model system with interparticle interaction described by a purely repulsive pair potential: φ (r) =4ε[(σ/r)12 −(σ/r)6]+ε for r?21/6σ, and φ=0 for r ≳21/6σ. We have shown that the profile of potential energies versus layers [parallel to (100) plane] varies steeply in the interfacial region. The time variation of the profile has also been calculated for different temperatures including the overheating and supercooling states of the solid and liquid phases.
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o calculations of the vertical electronic spectra of NO2, NO+2, and NO−2
Authors: Benioff Paul A.
Published: February 2003

MCSCF/CI calculations of the vertical electronic spectra of NO2, NO+2, and NO−2 were carried out at the NO2 and NO−2 ground state experimental equilibrium conformations. These calculations extend previous calculations of the same type in that polarization functions are included in the basis set and various limitations on the number of open shells and on the number of configurations in the MCSCF and CI calculations are removed. Calculations were also carried out on NO−2 with a basis set augmented by diffuse s and p functions.
Determination of the long range nonreactive anisotropic potential of H+Cl2 and H+Br2 from nonreactive scattering experiments
Authors: Bauer W. , Shobatake K. , Toennies J. P. , Walaschewski K.
Published: February 2003

Total differential and integral cross sections have been measured for the nonreactive scattering of H atoms from Cl2 and Br2. The differential cross sections were measured with a velocity selected beam at two center of mass energies, 0.17 and 0.78 eV, and the integral cross sections covered the center of mass energy range 0.01 to 1.2 eV. The results can be reasonably well explained by a spherically symmetric Lennard-Jones (12,6) potential for which the well depth and location are determined. The experimental results are shown to be in definite disagreement with cross sections calculated for the currently accepted LEPS potential surface. The fit of the experimental data is improved by including the effect of the anisotropy, which is accounted for by using the infinite order sudden approximation. A remaining discrepancy in the integral cross sections at high collision energies can be explained by introducing a plateau in the repulsive part of the Lennard-Jones potential to take into account the distortion due to the chemical potential. This new potential model is used to predict new cross section features outside the range of the present experiments.
Fine structure due to magnetic dipole coupling in the low temperature EPR spectrum of Rb2Cu(SO4)2⋅6H2O
Authors: Hitchman Michael A.
Published: February 2003

A study of the EPR spectrum of single crystals of Rb2Cu(SO4)2⋅6H2O over the temperature range 290 to 115 K is reported. Below ∼240 K unusual fine structure occurs for certain orientations of the magnetic field H, consisting of eight equally spaced lines when H is approximately along the z molecular axis and five equally spaced lines when H lies along the y molecular axis of the Cu(H2O)62+ ion. The fine structure may be explained largely in terms of a magnetic dipole coupling with four neighboring copper (II) ions, each of which produces an approximately equal splitting of the EPR lines at the central copper (II) ion; the five line pattern occurs when the hyperfine interaction with the copper nucleus is small, while the eight line pattern results from a hyperfine interaction which is essentially equal to the magnetic dipole interactions (these each being ∼120 G). Spectra were simulated by considering the point magnetic-dipole interactions with the ten nearest Cu(H2O)62+ ions using the expression K̂=g1g2β2 (1–3 cos2ϑ)/r3, where g1 and g2 are the g values of the two interacting ions, r is the distance between them, ϑ is the angle between the internuclear vector and the magnetic field, and β is the Bohr magneton. The resulting line shapes are in reasonable agreement with experiment for most orientations of the magnetic field in the (001) and (010) crystal planes. The agreement is significantly improved if the interaction with the two nearest copper (II) ions is ∼20% greater than that calculated using the simple magnetic dipole model, and it is shown that this is consistent with the unpaired electron density being represented as a quadrupole, as is required by the shape of the dx2y2 ground state orbital, rather than as a point charge. Further improvement occurs if a weak exchange coupling of J=+30 and J=−15 G takes place with the nearest and next-nearest copper ions, respectively, and if an anisotropic component to the background half-width is introduced.
A solution to the rotational diffusion equation to describe molecular orientation due to a time dependent torque
Authors: Sun Light K. , Allen Fritz S.
Published: February 2003

When aqueous solutions of macromolecules are placed in a square, pulsed, orienting electric field they frequently become dichroic and birefrigent. By following the rise of the optical anisotropy as a function of time after application of the pulse one can frequently distinguish between permanent and induced dipole orientation modes. Occasionally, molecules which for symmetry or other reasons are known to have no permanent dipole moment mimic permanent moment orientation. A possible explanation for this phenomenon is that the induced dipole is time dependent. Here we present a formal solution to the rotary diffusion equation for a molecule subjected to a time dependent torque due to an induced dipole mechanism. A new method of solution which can have broad application has been employed. The results are seen to be a viable explanation for the subject in question.
Photoelectron spectrum of Xe2 and potential energy curves for Xe+2
Authors: Dehmer P. M. , Dehmer J. L.
Published: February 2003

The photoelectron spectrum of the Xe2 van der Waals molecule was determined at 584 Å with a resolution of 20 meV, using an apparatus which combines a supersonic molecular beam source with a hemispherical photoelectron spectrometer. This experiment gives new information on the potential energy curve for the bound Xe+2 ground state at large internuclear separation. An optimum potential was determined for this state by fitting the observed photoelectron peak to a calculated Franck–Condon distribution. The calculations show specifically that the Morse function is not an adequate approximation to the potential at large internuclear distances. This experiment also gives new information on the potential energy curves for the weakly bound Xe+2 excited states in the region of their potential minima, thus complementing Xe–Xe+ scattering experiments which probe these potentials in the region of their repulsive walls. Potential energy curves for all of the Xe+2 states dissociating to Xe(1S0)+Xe+(2P3/2 or 2P1/2) are constructed primarily from experimental results and are compared with recent theoretical calculations by Wadt. Photoelectron spectra of heavier xenon clusters were also determined and are discussed briefly.
Low temperature proton NMR in deoxymyoglobin and a new model for the low-lying Fe2+ states
Authors: Lumpkin O. , Dixon W. T.
Published: February 2003

In order to study the electronic structure of the heme Fe2+ ion we have measured proton spin–lattice relaxation rates versus applied magnetic field and temperature in deoxymyoglobin at liquid 4He temperatures. The results show that the electronic relaxation time τe satisfies 10−7<τe<10−4 s and that there is at least one excited electronic state within 2 cm−1 of the ground state. An interpretation of these findings consistent with Mössbauer and magnetic susceptibility experiments is found if the low lying electronic states derive from a nearly degenerate orbital doublet (dxy, dyz) mixed by a very weak spin–orbit interaction. In particular, contrary to the usual assumption of a rapid electronic relaxation, we propose that the temperature dependence of the Mössbauer spectrum is due to motional narrowing.
Generalized finite polynomial approximation (WINIMAX) to the reduced partition function of isotopic molecules
Authors: Lee Myung W. , Bigeleisen Jacob
Published: February 2003

The MINIMAX finite polynomial approximation to an arbitrary function has been generalized to include a weighting function (WINIMAX). It is suggested that an exponential is a reasonable weighting function for the logarithm of the reduced partition function of a harmonic oscillator. Comparison of the error function for finite orthogonal polynomial (FOP), MINIMAX, and WINIMAX expansions of the logarithm of the reduced vibrational partition function show WINIMAX to be the best of the three approximations. A condensed table of WINIMAX coefficients is presented. The FOP, MINIMAX, and WINIMAX approximations are compared with exact calculations of the logarithm of the reduced partition function ratios for isotopic substitution in H2O, CH4, CH2O, C2H4, and C2H6 at 300 °K. Both deuterium and heavy atom isotope substitution are studied. Except for a third order expansion involving deuterium substitution, the WINIMAX method is superior to FOP and MINIMAX. At the level of a second order expansion WINIMAX approximations to ln(s/s′) f are good to 2.5% and 6.5% for deuterium and heavy atom substitution, respectively.