bluuebunny/crossref_metadata_2025 huggingface dataset:
Moderation Processes in Hot Atom Chemistry
Authors: Baer Michael , Amiel Saadia
Published: February 2004

The various moderation processes involved in hot atom chemistry are discussed. Two main types are considered: (i) moderation processes of hot atoms before entering into a chemical combination, (ii) moderation processes of hot molecules formed with various degrees of translational and internal energies. For the first case, it is proved rigorously that under certain conditions the ratio αm1 / αm2 related to two moderators is equal to (1 − β1) / (1 − β2,) where αmi, i = 1, 2, is the average energy loss per collision and βi = [(Mi − m) / (Mi + m)]2, with m and Mi the masses of the projectile and two different moderators. For the second case, the medium dependence of the survival probability, or alternatively of the decomposition probability, is derived. As an application of this treatment the reaction CH4+80Br→CH380Br+H, studied experimentally in different noble gases, is discussed.
Spectral Intensities of Some Nd3+ β-Diketonates
Authors: Mehta P. C. , Tandon S. P.
Published: February 2004

The oscillator strengths of the absorption bands of Nd3+ acetylacetonate, benzoylacetonate, dibenzoylmethide, and thenoyltrifluoroacetonate in solvents (a) methanol, (b) methanol plus ethanol (1:3), (c) DMF, and (d) DMF plus solvent (b) (1:4) have been determined spectrophotometrically. The results are in remarkably good agreement with the theories of Judd. Only two parameters τ4 and τ6 are required to predict the oscillator strength of the absorption bands. In the case of hypersensitive transitions an additional parameter τ2 is necessary.
Energy Levels of Pr+3 in Pr(NO3)3·6H2O
Authors: Wensky D. A. , Moulton W. G.
Published: February 2004

The energy levels of Pr+3 in Pr(NO3)3·6H2O have been determined from optical absorption experiments on single crystals at 77°K. With the exception of three bands, all of the electronic intra-f1 transitions from the ground state multiplet to higher multiplets were observed. The absorption spectrum along with the determined Stark level schemes of the multiplets and their centers of gravity are reported. An expanded “free-ion” Hamiltonian which includes configuration interactions along with spin–other-orbit and spin–spin interactions was used in the analysis of the multiplet centers of gravity. In a least-squares fit calculation the best-fit free-ion parameters found were F2 = 315.44 cm−1, F4 = 51.17 cm−1, F6 = 5.19 cm−1, ζ4f = 741.12 cm−1, α = 18.76 cm−1, β = −756.77 cm−1, γ = 88.05 cm−1, M0 = 0.43 cm−1. The average deviation between the calculated best fit and the experimental “free-ion” energy levels is 27 cm−1. A first-order perturbation calculation was carried out for the Stark levels of the various observed multiplets of the Pr+3(4f2) ion in a crystal field of C1 symmetry. Because of the large number of crystal field parameters involved in the potential, only a tentative set of best-fit B kq parameters were determined from a least-squares calculation.
Calculation of the He–H Interaction Energy
Authors: Fischer C. R. , Kemmey P. J.
Published: February 2004

The He–H interaction energy has been calculated by an SCF open-shell procedure due to Roothaan. Results obtained are in good agreement with the interaction deduced from atomic-beam measurements of Amdur and Mason. The present calculation has yielded lower values for the short-range potential than other comparable theoretical calculations.
Focusing and Orienting Asymmetric-Top Molecules in Molecular Beams
Authors: Jones Eugene M. , Brooks Philip R.
Published: February 2004

Asymmetric-top molecules can be deflected in an inhomogeneous electric field, and the deflected molecules can be oriented by allowing them to pass adiabatically into a uniform field. It is possible to account quantitatively for the transmission in a hexapole field if only first order and intermediate (or fast) Stark effects are considered. Rigid rotors with the dipole moment parallel to the a axis and internal rotors with low barriers to rotation have many levels which so interact and these types of molecules are easily deflected. Rigid rotors with the dipole parallel to the b axis have far fewer levels which interact; for these molecules the deflection is weak and may be undetectable. For rigid rotors and free internal rotors it is possible to obtain dipole moments by matching theoretical curves with experimental points.
Measured Lifetimes of Rotational and Vibrational Levels of Electronic States of N2
Authors: Johnson A. Wayne , Fowler R. G.
Published: February 2004

The lifetimes of vibrational levels of the B 3πg and C 3πu electronic states of molecular nitrogen and of the B 2Σu+ electronic states of the positive molecular nitrogen ion were measured by photometrically monitoring the excited level number densities in a pulsed invertron. Heavy-particle pressure damping cross sections were obtained from experimental studies of the pressure dependence of the reciprocal lifetimes. Lifetimes of individual rotational levels from K = 14 to K = 35 of the B 2Σu+(υ = 0) levels were measured. The lifetimes of all rotational levels were identical. However, our results suggest a dependence of the line strength on the rotational level different from that predicted by the Born–Oppenheimer approximation with zero rotational–vibrational interaction and zero electron spin–rotational interactions.
Ab Initio Study of the Geometries, Jahn–Teller Distortions, and Electronic Charge Distribution in the CH4+ Ion
Authors: Arents John , Allen Leland C.
Published: February 2004

The CH4+ ion is known to be involved in many ion–molecule reactions, presumably including those leading to the precursors of life, but little experimental information about it is available. In the present investigation, the energy of CH4+ in various geometries is calculated by the restricted SCF MO method using a Gaussian basis set. The ion is distorted from the regular tetrahedron because of the Jahn–Teller effect. Two stable geometries are found: tetragonal (D2d), RCH = 2.0943 bohr, dihedral angle = 53.3°, E = −39.74593 hartree; trigonal (C3υ), RCH1 = 2.5859, RCH2,3,4 = 2.0121, H1CH2 angle = 98.28°, E = − 39.74156 hartree. Other geometries investigated are, in order of decreasing energy, regular tetrahedron, square plane, and CH3+ + H. The relations among these geometries are discussed in terms of orbital energies, energy components, and population analysis.
Further Selection Rules for Excitation by Electron Impact at Small Momentum Changes
Authors: Lassettre Edwin N.
Published: February 2004

At kinetic energies in the range 300–500 eV, there is an accumulation of experimental evidence that the Born approximation does not hold when term symbols are the same in the initial and final states. In the present research, it is shown that deviations from the Born approximation at small momentum changes are proportional to a series of matrix elements of operators, the first (and dominant) term involving an operator which is invariant under the operations of the symmetry group of the scatterer. This accounts for the observation in the first sentence of this Abstract. Selection rules for the next term in the series are also discussed.
Absolute Electron Collision Cross Sections for Two Forbidden Transitions in Nitrogen at Kinetic Energies of 300–500 eV
Authors: Skerbele Ausma , Lassettre Edwin N.
Published: February 2004

Absolute differential electron collision cross sections and generalized oscillator strengths have been measured for two forbidden transitions, a 1Πg ← X 1Σg+ and a″ 1Σg+ ← X 1Σg+, in N2. These measurements were carried out at electron kinetic energies of 300, 400, and 500 eV and at small scattering angles (θ = 2°–6°) to facilitate extrapolation to zero momentum change. The Born approximation apparently holds for the excitation of the first transition but not for the second. Generalized oscillator strengths, however, extrapolate to the optical oscillator strength (zero) in both cases in accord with the limit theorem previously demonstrated.
Studies in Molecular Dynamics. VIII. The Transport Coefficients for a Hard-Sphere Fluid
Authors: Alder B. J. , Gass D. M. , Wainwright T. E.
Published: February 2004

The diffusion coefficient, the shear and bulk viscosity, and the thermal conductivity have been evaluated by means of their Einstein expressions by molecular dynamics computation over the entire fluid region. The autocorrelation functions for the different transport coefficients as well as their various components (such as kinetic and potential) have been obtained also. The results are compared to the predictions of the Enskog theory which involves a nearly exponential autocorrelation function. The observed deviations from an exponentially decaying autocorrelation function persist for many mean collision times, indicating that highly collective effects must be involved. The largest deviations for the transport coefficients occur near solid densities for the viscosity which is about twice as large as the Enskog prediction and for the diffusion coefficient which is about a factor of 2 smaller. In conformity with the Stokes relation, the product of diffusion and viscosity is found to be nearly constant over the entire fluid density range and in nearly quantitative agreement with the theoretically predicted constant using slipping boundary conditions. The deviations from the Enskog theory for the thermal conductivity are barely perceptible within the few percent accuracy of the data. The same is true for the bulk viscosity with its larger inaccuracies.
Direct Flow Measurement of O2(b1Σg+) Quenching Rates
Authors: O'Brien R. J. , Myers G. H.
Published: February 2004

A relaxation method is developed which permits a direct measurement of O2(b1Σg+) quenching rate constants. An Al wire spiral is used to selectively quench O2(1Σ) which is normally present in discharged oxygen as a steady-state intermediate. The characteristic time for the reestablishment of the O2(1Σ) steady state is used to determine wall and bimolecular quenching rates. With this method, absolute rate constants are obtained from relative intensity measurements of the O2(1Σ) emission at 7619 Å. The rate constant for quenching by H2O is found to be 4.0 ± 0.6 × 10−12 cc sec−1. Rate constants for a number of other quenchers are reported.
Ionic Collision Processes in Gaseous Ammonia
Authors: Ryan K. R.
Published: February 2004

Rate coefficients for reactions resulting from collisions of NH3+ and NH2+ with NH3 have been measured as a function of primary ion translational energy over the energy range 0–8.0 eV. NH4+ was the only observed product of NH3+/NH3 collisions. The rate coefficient drops sharply with increasing energy from 18.1 × 10−10 cm3 molecule−1·sec−1 for thermal ions to 10.2 × 10−10 cm3 molecule−1·sec−1 for ions with a maximum translational energy (KE) of 1 eV. Above 3 eV an essentially constant value of 8 × 10−10 cm3 molecule−1·sec−1 is obtained. Collisions of NH2+/NH3 result in the production of both NH4+ and NH3+. With increasing ion energy below 1 eV the relative number of collisions which proceed via the less exoergic mechanism increases. Above 3-eV KE the rate coefficient for the removal of NH2+ increases which is taken to indicate that in this energy range the cross section for electron transfer is greater than that for ion–molecule reaction. The rate coefficient for NH4+ production drops to an undetectable level at about 2-eV KE rising again above 3-eV KE. It is proposed that this new onset can be attributed to the endoergic reaction in which the neutral products are N+H and not NH as is supposed for the proton-transfer process at lower ion energies.
Results of Ab Initio Calculations on Formamide
Authors: Christensen D. H. , Kortzeborn R. N. , Bak B. , Led J. J.
Published: February 2004

Ab initio calculations on formamide, HCONH2, have been carried out for 44 nuclear configurations with a (s, p) = (11, 7) basis set for C, O, and N and a (s, p) = (5, 3) set for the hydrogens, and for three nuclear configurations with a (s, p, d) = (11, 7, 1) basis set for C, O, and N and a (s, p) = (5, 3) set for the hydrogens. Primarily, a structure from microwave work was assumed. Within the range of experimental errors of the C=O, the C–N, and the N–H distances the molecular energy was shown to pass through minima. The calculated dipole moment and the barrier to internal rotation around the C, N bond are in satisfactory agreement with experiments. Also, the changes of molecular geometry occurring if HCONH2 is twisted around its amide CN bond are consistent with qualitative expectations. The calculated barrier to NH2 inversion in HCONH2 of 30–40 cm−1 is too low by a factor of ∼ 10.
Natural Orbital Valence Shell CI Studies of Diatomic Molecules. II. Potential Energy Curves and Spectra of Boron Carbide
Authors: Kouba Joseph E. , öhrn Yngve
Published: February 2004

A method of using natural orbitals and valence shell configuration mixing is applied to obtain spectral properties within the clamped nuclei approximation for diatomic boron carbide. This molecule, known to exist from mass spectroscopic studies but not further studied, is shown to have a 4Σ− ground state and a dissociation energy greater than 2.9 eV.
Rotational Constants of Torsionally Excited Molecules
Authors: Meyer R. , Wilson E. Bright
Published: February 2004

Molecular torsion is described by a model which has only one internal degree of freedom but allows for distortions depending on the torsional angle τ. Evidence for such distortions is found by comparing observed and calculated rotational constants associated with ground and excited states of skeletal torsion in symmetric conformers of ethyl formate, propionyl fluoride, and fluoroacetyl fluoride. For these cases, possible torsional modes involving only skeletal angular distortions are discussed. Effective rotational constants are calculated from exact energy levels for J = 0 and J = 1. A convenient numerical method, based on discretization of the torsional angle and trigonometric interpolation, is applied and tested with respect to over-all numerical convergence. An internal axis method is found to converge faster than the principal axis method and shows that the effective rotational constants are approximated to considerable accuracy by the expectation values of instantaneous rotational constants if the respective torsional wavefunctions are localized. In such cases it is easy to evaluate corrected instantaneous rotational constants which may be related in the usual way to a τ-dependent molecular structure, i.e., to the torsional mode.
Valence-Excited States of Carbon Monoxide
Authors: O'Neil Stephen V. , Schaefer Henry F.
Published: February 2004

Precise quantum mechanical calculations have been performed on the 72 states of carbon monoxide which dissociate to a 3P, 1D, 1S, or 5S carbon atom plus a 3P, 1D, or 1S oxygen atom. A minimal basis set of Slater-type orbitals (optimized for the C and O atoms) was used, and holding the atomic 1s orbitals doubly occupied, full configuration interaction calculations were carried out for all molecular states at no fewer than nine internuclear separations. Seventeen bound states (De ≥ 0.27 eV) were obtained, eight of which have been observed experimentally. The theoretical ordering of known bound states agrees with experiment except for the a 3Π and A 1Π states. This fact is rationalized by noting that these two states have much smaller re values than do the other excited states. The most interesting of the unobserved predicted bound states are the 5Σ+ and 5Π states, which dissociate to 3PC+3PO, and the third 1Π state, with a relatively large calculated dissociation energy. A dominant molecular orbital configuration is associated with each of the first eleven bound states. Calculated spectroscopic constants are compared with available experimental data. Potential curves are presented and discussed for both bound and repulsive molecular states. The A 1Π state has a calculated maximum of 1135 cm−1, in good agreement with the experimentally based estimate (950 ± 150 cm−1) of Simmons, Bass, and Tilford. Calculations were also carried out on the X 1Σ+, a 3Π, and a′ 3Σ+ states using 1s atomic Hartree–Fock orbitals and 2s and 2p STO's optimized for the X state at the experimental re. These calculations correctly placed the a 3Π state below the a′ 3Σ+ and resulted in improved De and re values for all three states.
Electron Spin Resonance Spectra of α-Nitronylnitroxide Radicals; Solvent Effects; Nitrogen Hyperfine Tensor; g Anisotropy
Authors: D'Anna Joseph A. , Wharton James H.
Published: February 2004

This paper describes a detailed electron spin resonance study of three α-nitronylnitroxide doublet radicals: α-methylnitronylnitroxide (α-MNN), α-phenylnitronylnitroxide (α-φNN), and α-0-tolylnitronylnitroxide (α-0TNN). The average g values and the isotropic hyperfine coupling constants for the paramagnetic nuclei have been measured. A solvent-induced variation in the isotropic coupling constants has been observed and some evidence is given for slight changes in g value with solvent. Studies of polycrystalline samples of α-MNN indicate a molecular anisotropy is associated with the g tensors for these molecules. Estimates of the diagonal elements of the g tensors have been obtained from spectra of the radicals in solid solutions of Duco Cement. The principal value of the nitrogen hyperfine tensor along the π direction has been determined from the solid solution spectra. The other principal values of the nitrogen hyperfine coupling tensors are estimated from observed linewidths. Spin densities have been calculated from the hyperfine coupling constants. The unpaired electron distribution is found to be essentially the same in all of the doublet radicals studied and is found to be primarily localized in the nitronylnitroxide ring. The nitronylnitroxide ring is indicated to be of C2V symmetry with the unpaired electron occupying a π molecular orbital that transforms as the B1 representation in the C2V point group. The nitronylnitroxides are found to be similar in many respects to the simple nitroxide radicals.
Vapor Pressures of 36Ar and 40Ar. Intermolecular Forces in Solid and Liquid Argon
Authors: Lee Myung W. , Fuks Salomon , Bigeleisen Jacob
Published: February 2004

The vapor pressures of 36Ar and 40Ar have been measured in the temperature range 62–102°K. Measurements in the liquid agree quantitatively with the previous measurements of Clusius et al. which cover the limited range 84–88°K. A significant difference is found between our equilibrium measurements, which have a precision of 0.5% in the logarithm of the vapor pressure ratio, and dynamic measurements of the isotope fractionation in solid–vapor systems. The reduced partition functions of the solid and liquid are calculated from the observed vapor pressure ratios. The reduced partition ratio of the solid can be represented by the equation T2ln(fs / fg) = 63.39 ± 0.34 − (0.124 ± 0.005)T − (95 / 63)(63.4 − 0.124T)2 / T2. The experimental values of ln(fs / fg) are in good agreement with calculations from the improved self-consistent phonon theory of an anharmonic lattice with a 13–6 potential. Since the quantum effect is small, 〈∇2U〉 is obtained directly from the reduced partition functions. In the liquid range the experimental results agree quantitatively with previous estimates based on 12–6 potential, with calculations from molecular dynamics and with the Rowlinson correlation function. It is shown that the reduced 〈∇2U*〉 is the same for neon and argon both in the liquid and solid. The change in 〈∇2U〉 on melting is discussed in terms of the change of the number of nearest neighbors on melting and the “significant structure theory of liquids.”
Molecular Structure of CCl3PF4 from Infrared and Laser Raman Spectroscopy. Pentacoordinated Molecules. XV
Authors: Holmes Robert R. , Fild Manfred
Published: February 2004

The molecular structure of CCl3PF4 indicated by the gas-phase infrared spectrum recorded between 33 and 4000 cm−1 and the laser Raman spectrum of the liquid is a trigonal bipyramid with the CCl3 group located in an equatorial position. Polarization measurements have been made as well as the recording of the infrared spectrum of the solid at 77°K as aids in spectral analysis. The spectra agree with the selection rules for C2υ symmetry. Complete assignments are given. Structural comparisons are made with other trigonal bipyramids containing the PF4 group.
Laser-Raman Spectrum (10–4500 cm−1) of Potassium Dicyanoargentate(I)
Authors: Loehr Thomas M. , Long Thomas Veach
Published: February 2004

The laser-Raman spectrum of KAg(CN)2 is measured in the region 4500–10 cm−1, and ν2, the symmetric silver–cyanide stretch, is unambiguously discriminated for the first time. The laser-Raman spectrum indicates that the Ag(CN)2− ion lies on a site of C2 rather than Ci symmetry. The resolution of 13C and 15N isotopic bands (in natural abundance) in the CN stretching region provides further evidence that metal–cyanide attachment is through the carbon atom in this complex. The low-frequency region is studied at high resolution, and 10 Raman-active lattice modes are resolved.
Hindered Internal Rotation and Dielectric Relaxation
Authors: Klug Dennis D. , Kranbuehl David E. , Vaughan Worth E.
Published: February 2004

A model calculation of the dielectric behavior of a liquid whose molecules have conformation-dependent dipole moment components is presented. The molecules contain two groups of equal size which interact according to parabolic and cosine potentials of varying symmetry. The calculated dielectric behavior is discussed in terms of the model parameters. Measurements of dielectric permittivities and losses of 4-bromobiphenyl in dilute solution with carbon tetrachloride at 20, 30, and 40°C are interpreted in light of the model calculation. Measurements of dielectric permittivities and losses of 2,2′-bipyridine and 2,2′-dibromobiphenyl in dilute solution with carbon tetrachloride at 10–40°C are interpreted in light of a model calculation involving hindered internal rotation reported previously. The advantages and limitations of the dielectric method for characterizing hindered internal rotation in molecules in the liquid state are discussed.
Derivation of a Master Equation for the Relaxation of Internal Degrees of Freedom
Authors: Keizer Joel
Published: February 2004

Using projection operator techniques and the Liouville formalism, a derivation of an exact evolution equation for the internal degrees of freedom of a molecule in a temperature bath is presented. This non-Markovian “master equation” gives the time evolution of the diagonal part of the reduced density matrix of the internal degrees of freedom. Since the collision term in this equation depends explicitly on the intensive variables of the reservoir, the simultaneous limits of low reservoir density and long time may be used to reduce the exact equation to a Markovian master equation. Using an identity which connects the Liouville formalism to scattering theory, it is shown that the collisional transition probabilities which occur in this master equation can be written in terms of scattering cross sections. Finally, it is demonstrated that the transition probabilities satisfy the condition of detailed balance and that the master equation agrees with that obtained by the usual physical derivations.
Picosecond Pulse Radiolysis Studies. I. The Solvated Electron in Aqueous and Alcohol Solutions
Authors: Bronskill M. J. , Wolff R. K. , Hunt J. W.
Published: February 2004

Observations of the solvated electron in water and the common aliphatic alcohols have been made in the time region ∼ 20–350 psec. Molar concentrations of acid have been used to scavenge eaq− and k(eaq−+Haq+) has been measured as (1.2 ± 0.2) × 1010 liter/mole·sec for [Haq+] from 0.5 to 5.0M. The factors limiting the time response of our picosecond pulse radiolysis system are discussed. The time of formation of esol− is estimated to be less than 10 psec for all solvents studied. The yield of solvated electrons in alcohols has been measured to be approximately one-third of the yield in water at picosecond times.
Picosecond Pulse Radiolysis Studies. II. Reactions of Electrons with Concentrated Scavengers
Authors: Wolff R. K. , Bronskill M. J. , Hunt J. W.
Published: February 2004

The decay of solvated electrons in various aqueous solutions was studied using a stroboscopic pulse radiolysis technique with a time resolution of 24 psec. High concentrations of solutes such as H2O2, acetone, CdCl2, NaNO2, and NaNO3 decreased the initial solvated electron yield, while high acid concentrations did not. Reasons for this reduction in yield are discussed, and the conclusion reached is that the scavengers must be reacting with a precursor to the solvated electron. We believe that this precursor is probably a low-energy electron which reacts prior to solvation.
Search for the First A1g−3 ← B2u+3 Transition in Naphthalene
Authors: Pavlopoulos T. G.
Published: February 2004

The polarization of the triplet–triplet absorption spectrum of naphthalene-d8 in the 3250–6000-Å region was measured by the photoselection method. The spectrum 3250–4300 Å, which consisted of three strong and five weak lines, evidenced uniform polarization and therefore seemed to result solely from the transition B3g−3 ← B2u+3. The weak triplet–triplet absorption band in the 4300–6000-Å range showed four strong and several weak bands of mixed polarization, where some corresponded to that expected for the A1g−3 ← B2u+3 transition. The energy separation of this transition from the B2u+3 level and its oscillator strength were considerably smaller than were predicted from theoretical calculations.
Analytical Evaluation of Multicenter Integrals of r12−1 with Slater-Type Atomic Orbitals VI. Asymptotic Expansions for Large Internuclear Distances
Authors: Kay Kenneth G. , Silverstone Harris J.
Published: February 2004

Asymptotic expansions valid for large internuclear distances are derived for the four-center integral of r12−1 with Slater-type atomic orbitals. In certain cases, the dominant part of the complete expansion is a multipole-moment-type expansion. The asymptotic formulas are simpler than the convergent formulas given previously, but some are not directly applicable when certain combinations of orbital exponents vanish.
Determination of the Spin–Orbit and Trigonal-Field Splittings of the 5E State of Fe2+ in ZnO, ZnS, CdS, ZnSe, CdSe, ZnTe, and CdTe Crystals by Magnetic Susceptibilities
Authors: Mahoney J. Patrick , Lin Chun C. , Brumage William H. , Dorman Franklin
Published: February 2004

The magnetic susceptibilities of the Fe2+:ZnSe, Fe2+:CdSe, Fe2+:ZnTe, Fe2+:CdTe, Fe2+:ZnS, Fe2+:CdS, and Fe2+:ZnO crystals have been measured over the temperature range of 2–300°K in order to determine the spin–orbit and trigonal-field (for CdS and ZnO) splittings of the ground orbital state 5E of Fe2+ in the crystal lattice. The first five crystals are magnetically isotropic with no detectable trigonal-field effects, and their susceptibilities tend toward constant values at very low temperature. From the temperature dependence of the susceptibility, we obtain the spacings of the A1 and T1 component of the 5E group of the first five crystals as 12, 12, 16, 18, and 15 cm−1, respectively. These results give spin–orbit coupling constants | λ | = 85, 81, 96, 99, and 99 cm−1, respectively, when used in conjunction with the optical data of the 5E−5T2 transitions. The Fe2+:CdS crystals show a small magnetic anisotropy but otherwise similar susceptibility data which yield 16 cm−1 for the A1−T1 spacing, 1 cm−1 for the trigonal-field splitting of T1, and | λ | = 97 cm−1. For the case of Fe2+:ZnO, the magnetic anisotropy is much larger, and the T1 levels split into two components A2 and E which are 8 and 18 cm−1, respectively, above the A1 ground state as determined from the magnetic data.
Production of Positive Ions and Electrons in Collisions of 1–25-keV Protons and Hydrogen Atoms with CO, CO2, CH4, and NH3
Authors: McNeal R. J.
Published: February 2004

Cross sections are reported for the production of slow, positive ions (σ+) and electrons (σ−) in collisions of 1–25-keV protons and hydrogen atoms with CO, CO2, CH4, and NH3. For H+ bombardment σ− is the total ionization cross section and σ+ − σ− is the total electron capture cross section, σ10. For H0 bombardment σ+ is a good approximation to the total ionization cross section. The results are compared with other results in different, overlapping energy ranges, and their application to the physics of planetary atmospheres is discussed.
Hindered Rotation in the Ground and Excited Electronic States of o-Xylene
Authors: Ingham Kenneth C. , Strickler S. J.
Published: February 2004

A study has been made of the hot bands in the ultraviolet absorption spectrum of ortho-xylene and some deuterated derivatives. It is shown that these hot bands arise from excited levels of torsional vibrations of the methyl groups. The spectrum can be explained fairly well assuming independent oscillators with a barrier to internal rotation of 700 ± 50 cm−1 in the ground state and 240 ± 35 cm−1 in the excited state. The origin of the barrier and the applicability of the independent oscillator model are also discussed.
On the Existence of Local Segmental Order in Undiluted Bulk Polymers
Authors: Tonelli Alan E.
Published: February 2004

The mean-square end-to-end distance and its temperature coefficient, the change in the intramolecular conformational entropy on melting, and the optical anisotropy, as manifested by the strain birefringence, and its temperature coefficient are calculated in the rotational isomeric-state approximation for unperturbed linear polyethylene chains containing short stretches of skeletal bonds exclusively in the all trans or planar zigzag conformation. The results are compared with the corresponding quantities calculated for a polyethylene chain free of such conformational constraints and with experimental data taken from the literature which were obtained on undiluted bulk polyethylene. This comparison shows that the optical properties of polyethylene are more sensitive to the local segmental order, as defined in this study, than are its dimensions and their temperature coefficient or the change in intramolecular conformational entropy on melting. An estimate of the upper limit of local segmental order in bulk polyethylene, which is consistent with experimental and calculated results, is presented, and the conclusion that little, if any, segmental order exists in bulk polymers is drawn. The effect of diluents on the strain birefringence of polymer networks is commented upon in this connection.
Nonequilibrium Kinetics: Explicit Time Dependence of Perturbed Distribution Functions and Autocorrelation Expressions
Authors: Kapral Raymond , Hudson Suzanne , Ross John
Published: February 2004

The explicit time dependence of perturbed distribution functions is considered for a chemically reacting dilute gas mixture. The solutions obtained for the distribution functions approach in time the normal solutions of a Chapman–Enskog perturbation treatment within a few mean free times and provide an autocorrelation expression for the first nonequilibrium correction to the rate coefficient for a bimolecular gas-phase reaction. To illustrate the results, explicit solutions are obtained for two model systems.
Light Scattering from Chemically Reacting Fluids: Coupled Chemical Reactions
Authors: Weinberg Michael , Kapral Raymond
Published: February 2004

The total light scattering intensity from a fluid where chemical relaxation takes place via a sequence of coupled reactions is calculated and examined. The fast reaction limiting case where the rate coefficients in one of the steps are much greater than those in the other step is studied in considerable detail. It is found that only if the reaction is fast and the effective polarizabilities of the species participating in the fast step are equal will the spectrum be simplified. In this case, the broadening of the Rayleigh peak will be due principally to the slow chemical reaction. When one of the reactions is very fast, the spectrum can be analyzed by assuming chemical equilibrium between the species undergoing rapid reaction.
Molecular Momentum Distributions and Compton Profiles. I. General Theory and Boron Hydrides
Authors: Epstein Irving R. , Lipscomb William N.
Published: February 2004

A method is described for the calculation of momentum distributions and Compton profiles of polyatomic molecules. Distributions, profiles and momentum expectation values are calculated for the boron hydrides B2H6, B4H10, B5H9, B5H11, and B6H10. Contour maps of total and difference densities in momentum space are presented for diborane. The results are analyzed in terms of localized orbitals. The analysis shows that traditional concepts of two- and three-center electron pair bonds may find great utility in interpreting momentum space distributions.
Radiationless Transitions in the Eu3+ Center in LaAlO3
Authors: Blasse G. , Bril A. , de Poorter J. A.
Published: February 2004

Three different types of radiationless transitions play a role in the radiationless decay of the luminescent Eu3+ center in LaAlO3: (a) a temperature-independent process from the charge-transfer state, (b) a temperature-dependent process (ΔE = 0.15 eV) from this state, (c) a temperature-dependent process (ΔE = 1.1 eV) from the 5D0 level. All three processes are of importance because the charge-transfer state of Eu3+ is situated at low energies in LaAlO3. If, however, this state is situated at higher energies, as it is in other host lattices, only process (b) will be observed in the conventional range of temperatures.
Small-Angle Scattering of X Rays from Carbon Dioxide in the Vicinity of Its Critical Point
Authors: Chu B. , Lin J. S.
Published: February 2004

Intensity measurements of x rays scattered at small angles by carbon dioxide are reported for various equilibrium states in the critical neighborhood of the temperature–density diagram. From our studies we have observed that (1) the K2 dependence in a plot of reciprocal relative scattered intensity (Is−1) versus scattering angle is obeyed over the following ranges: 2.66 × 10−4 Å−2 < K2 = (16π2 / λ2) sin2(θ / 2) < 8.06 × 10−3 Å−2, 0.343° ≤ T − Tc ≤ 5.640°K, 87.5 ≤ ρ ≤ 356.5 amagat, and along the liquid side of the coexistence curve and (2) the temperature dependence of the isothermal compressibility (βT) and the long-range correlation length (ξs) have the forms: βT = fε−γ and ξs = ξ0ε−ν, where ε = (T − Tc) / Tc; f, γ and ν are constants with respect to temperature. We have obtained γ = 1.32−0.10+0.09 and ν = 0.67 ± 0.05 at the critical isochore, and νliquid ≈ 0.58 along the liquid side of the coexistence curve. Using an empirical equation of the form Is−1∝[τ − τsp(ρ)]γ with τsp = Tsp / Tc and Tsp being the pseudospinodal temperature, we have obtained γ = 1.20 ± 0.09 at ρ = 268.3 amagat and γ = 1.13±0.09 at ρ = 192.4 amagat. Moderate success is obtained in testing a more general scaling equation for Is.
Kinetics of Electron and Ion Reactions during the γ Radiolysis of Liquid Neopentane; Dependence of Charge Scavenging Kinetics on Hydrocarbon Molecular Structure
Authors: Horacek K. , Freeman G. R.
Published: February 2004

The large free ion yield in neopentane has been confirmed by chemical measurement: for nitrous oxide solutions at 23°C g(N2)fi = 0.95 and g(N2)max ≈ 4.7. The average number of nitrogen molecules formed per electron scavenged appears to be 1.1 ± 0.1 at all nitrous oxide concentrations. Thus gfi = 0.9 in neopentane, which is sevenfold larger than that in n-hexane. The median range of the secondary electrons in liquid neopentane is about fourfold greater than that in n-hexane. The greater range in neopentane correlates with the more sphere-like shape of the molecules. In spite of the larger median electron range in neopentane, the ease with which electrons and ions can be scavenged in spurs in that liquid is similar to that in hydrocarbons with distinctly nonspherical molecules. Furthermore, the ratio of efficiences of electron to positive ion scavenging (β− / β+) is 17 in neopentane, similar to the value 13 in the “nonspherical” methylcyclopentane. During the radiolysis of pure neopentane two types of ions are formed: G(PH+) = 1.7 and G(N+) = 3.0. PH+ yields hydrogen when neutralized by an electron, but not when neutralized by a massive negative ion. PH+ can also donate a proton to ammonia. N+ does not yield hydrogen upon neutralization, nor does it donate a proton to ammonia. In pure neopentane G(H2) = 2.5 ± 0.1 and G(CH4) = 3.9 ± 0.2 at a dose of 1.5 × 1018 eV/g. Scavenging the free ions by either N2O or ND3 reduces g(H2) by about 0.35, but has no effect on the methane yield.
Study of Equilibria in Nitrogen Dioxide
Authors: Blend Harvey
Published: February 2004

The equilibria of a gaseous mixture of nitrogen dioxide and nitrogen tetroxide is studied by measuring the velocity of a sound wave propagated through the gas. The degree of dissociation α and the equilibrium constant Kp are determined as functions of temperature from 0 to 50°C, and pressure from 0 to 1 atm. At 30°C, Kp is found to be 0.214 atm. The heat of reaction ΔH is determined to be 14.59 kcal mol−1 for the second-law value, and 13.65 kcal mol−1 for the third-law value.
Statistical Mechanics of Rodlike Particles. III. A Fluid of Rigid Spherocylinders with Restricted Orientational Freedom
Authors: Cotter Martha A. , Martire Daniel E.
Published: February 2004

A scaled particle treatment is applied to a fluid of rigid spherocylinders of radius a and cylindrical length l, with cylindrical axes permitted to point in only three mutually perpendicular directions. Singling out one of these, by means of some arbitrary device such as an infinitesimal applied field, and denoting by s the fraction of rods in each of the directions perpendicular to this “preferred” direction in any given configuration of the system, an expression is derived for the configurational Helmholtz free energy Ac as a function of s, and an equation determining s̄, that value of s which minimizes Ac, is obtained. At densities below some critical value ρc, the only solution to the latter equation is s̄ = 13, but for ρ > ρc, an anisotropic solution is also possible. A first-order anisotropic–isotropic transition is observed and the transition densities, free volume fractions, and entropy change are determined at two values of a and for ratios l / a from 2 through 18. Thus, that transition which Zwanzig observed in a fluid of very long rods (l / a → ∞) is also observed in fluids of relatively short rigid rods. The transition free volume fractions, the entropy of transition, the degree of order S in the anisotropic phase at the transition, and the relative density change Δρ / ρaniso are all seen to increase with increasing l / a, and values obtained for the latter two quantities are compared with experimentally determined values. The isothermal compressibility and the expansivity of the system are also calculated at free volume fractions (densities) near the anisotropic→isotropic transition, but no pretransition effects are observed.
Response to Double Irradiation of a Nuclear Spin System: General Treatment and New Multiple Quantum Transitions
Authors: Bucci P. , Martinelli M. , Santucci S.
Published: February 2004

A general theory has been developed to calculate the response, in NMR conditions, of a spin system irradiated by two nearly resonant waves having different frequencies and intense enough to produce an appreciable saturation. The absorption signal has been calculated employing the statistical operator technique, treating the electromagnetic field by the second quantization formalism. Two kinds of transitions are expected: The first one arises from processes in which n photons of one wave are emitted and n photons of the other wave are absorbed, the second one arises from processes involving the emission of n photons of one wave and the absorption of n ± 1 photons of the other. In the first case all the transitions coalesce in a single dispersionlike signal, which occurs when the frequencies of the two waves are nearly the same; in the second case a many-line spectrum occurs in which emission and absorption lines are present. The experimental results here reported strictly agree with theoretical previsions.
Water Molecule Interactions
Authors: Hankins D. , Moskowitz J. W. , Stillinger F. H.
Published: February 2004

Accurate SCF calculations have been carried out to investigate the potential of interaction for pairs and triplets of water molecules. The most stable pair configuration involves a linear hydrogen bond of length ROO = 3.00 Å and strength 4.72 kcal/mole. Three-molecule nonadditivities are large in magnitude and vary in sign according to the hydrogen-bond pattern involved. In both aqueous liquids and solids, the net trimer nonadditivity effect amounts to increased binding energy, decreased neighbor distance, and slightly enhanced tendency toward perfect tetrahedral coordination symmetry. The nonadditivity furthermore is inconsistent with the phenomenology of simple mutual electrostatic polarization between neighboring molecules.
Electronic Structures of Polymers Using the Tight-Binding Approximation. II. Polyethylene and Polyglycine by the CNDO Method
Authors: Fujita Hiroko , Imamura Akira
Published: February 2004

Formulas for the calculation of the electronic structure of polymers were derived by the tight-binding approximation using the CNDO/2 method. This formalism was applied to the calculation of the electronic structure of polyethylene in the all-trans conformation and polyglycine in the α-helix form. For polyethylene, conformation analysis was carried out to obtain a fairly good agreement with the experimental results. The results of our conformation analysis were also in fair agreement with the calculation by the extended Hückel method which had already been published in this series of papers. The electronic structure of polyglycine was found to reflect the effect of the hydrogen bond formation in the α-helix form.
Electronic Raman Effect. VI. Enhancement of the Intensity of Electronic Raman Transitions in Special Cases
Authors: Grünberg P. , Koningstein J. A.
Published: February 2004

Crystal fields, with no center of inversion, mix configurations of different parity into an eigenfunction of a rare earth ion in a crystal. Also, different J values are mixed. It is shown that configuration mixing can be detected in the resonance electronic Raman effect, whereas J mixing allows the occurrence of the irreducible tensor component α00. Experimental examples are given.
Breakdown Curves for Polyatomic Negative Ions
Authors: Lifshitz Chava , Peers A. MacKenzie , Grajower R. , Weiss M.
Published: February 2004

Negative-ion breakdown curves were obtained at low (0–1 eV) energies for SF6 and a number of fluorocarbons. The dissociation rate constants, as a function of internal energy of the parent negative ions, were computed for the processes observed and used to estimate kinetic shifts and thermochemical appearance potentials, at 0°K, for the fragments. The results (e.g., metastable curve widths and temperature effects) were found to be in agreement with the quasiequilibrium theory (QET) of mass spectra. It is predicted that most of the parent negative ions studied (e.g., C7F14− from perfluoromethylcyclohexane) would not fragment if formed by the capture of a thermal electron by the vibrationally cold molecule.
Lattice Dynamics of Sodium Azide. A Group-Theoretical Analysis
Authors: Rao K. R. , Trevino S. F.
Published: February 2004

The symmetry properties of normal modes of the lattice vibrations of sodium azide are investigated. The formalism suggested by Maradudin and Vosko, which is relevant for the present study, is summarized. Symmetry vectors at several points of the Brillouin zone are determined. Interrelationships between the elements of the dynamical matrix have been evaluated. The compatibility relations among the dispersion curves for various wave vectors are also derived.
Generalized Linear Trajectory Approximations and the Constant Acceleration Approximation
Authors: Hynes James T. , Deutch J. M.
Published: February 2004

The evaluation of correlation functions by means of approximation of the time evolution operator is discussed. It is shown that different approximations may be obtained depending upon the particular factorization of the equilibrium distribution function in the averages to be computed. With the approximation of free particle dynamics, generalized linear trajectory approximations for correlation functions are obtained. The circumvention in the generalized approximation of the separation of the intermolecular potential employed in the linear trajectory approximation introduced by Helfand is discussed. For low density, it is demonstrated that the constant acceleration approximation introduced by Bloom and Oppenheim is exactly equivalent to a generalized linear trajectory approximation. An explicit expression for the deviation of the constant acceleration approximation result from the exact correlation function expression is obtained. The differences between the constant acceleration and generalized linear trajectory approximations at higher densities are discussed.
Theoretical Analysis of the Electronic Structure and Molecular Properties of the Alkali Halides. V. Potassium Chloride and Lithium Bromide
Authors: Matcha Robert L.
Published: February 2004

The results of recent self-consistent field calculations on potassium chloride and lithium bromide are presented. Special emphasis is given to an analysis of molecular properties including nuclear quadrupole coupling constants, isotope shifts, spectroscopic constants, relativistic interactions, molecular force constants, and the electronic dipole moment.
Rotation-Vibration Energy Transfer in Collisions between OH(A 2Σ+) and Ar and N2
Authors: Welge K. H. , Filseth S. V. , Davenport J.
Published: February 2004

Collisional relaxation of OH(A 2Σ+, υ′, K′) in high rotational levels of υ′ = 0 and 1 has been investigated with respect to transitions from rotational levels in υ′ = 0 to levels in υ′ = 1. Initial nonequilibrium rotational distributions of OH(A 2Σ+) in υ′ = 0 and 1 were produced by monochromatic photodissociation of H2O with the radiation of a krypton resonance lamp at 1236 and 1165 Å. The effect of added foreign gases (Ar and N2) on the population of individual levels in υ′ = 0 and 1 has been studied under steady-state conditions by observing the emission intensities of individual lines in the (0, 0) and (1, 1) bands of the OH(A 2Σ+→X 2Π) transition. The essential observation was made on the population of the rotational level K′ = 15 in υ′ = 1. The population of this level increased significantly in the presence of Ar and N2 beyond the initial population produced from H2O alone. In comparison, the population of adjacent levels remained relatively unchanged or decreased when foreign gas was added. The effect on the (υ′ = 1, K′ = 15) level is attributed to the collisional transfer process OH(A 2Σ+, υ′ = 0, K′ = 20) + Ar, N2 → OH(A 2Σ+, υ′ = 1, K′ = 15) + Ar, N2 + ΔE, where the energy difference, ΔE = 27 cm−1, between the two levels is small compared to the heat bath energy, kT = 250 cm−1. For this process and its reverse, a rate constant of the order of 10−11 cm3 molecule−1·sec−1 has been derived. According to angular-momentum conservation, the process involves, in the case of Ar as collision partner, an increase in the impact parameter which is estimated to be about 0.3 Å.
Approximate Evaluation of the Second-Order Term in the Perturbation Theory of Fluids
Authors: Smith W. R. , Henderson D. , Barker J. A.
Published: February 2004

The second-order term in the free energy, A2, as given by the Barker–Henderson perturbation theory is considered for the square-well and 6:12 potentials. The second-order term is written in terms of integrals of the distribution functions of the unperturbed hard-sphere system. If the superposition approximation is used for the third- and fourth-order distribution functions, these integrals may be evaluated fairly easily and values for A2 may be obtained. Comparison of these values with either the exact second-order term Monte Carlo calculations of the exact second-order term gives exact agreement for the lattice gas, fairly good agreement for the square-well potential, and good agreement for the 6:12 potential.