bluuebunny/crossref_metadata_2025 huggingface dataset:
Gas-Phase Dosimetry by Use of Ionization Measurements
Authors: Meisels G. G.
Published: January 2005

A basis is provided for the use of ionization measurements to evaluate energy absorption in gas-phase dosimetry when high-energy electrons are employed as the radiation source. It is concluded that total energy absorption can be determined and that stopping powers can be employed to calculate relative energy absorption in various gases. Scattering measurements indicate that irradiation cells should be short. Relative values of the energy required to form one ion pair (W) by 1-MeV electrons are shown to be the same as those reported for lower energy electrons, and the following W values for the partial absorption of 1-MeV electrons are reported with a probable error of about ±2%: C3H8 23.4, n-C4H10 22.9, i-C4H10 23.0, i-C5H12 23.9, C3H6 24.8, 1-C4H8 24.4, trans-2-C4H8 23.9, i-C4H8 24.4, CH3CHO 26.4, (C2H5)2O 23.8, CO 32.2, SO2 30.4, NO 27.5, N2O 32.9, and NH3 26.5.

Chemical-yield measurements on ethylene suggest that the formation of acetylene can be measured with an accuracy of better than ±3%; above 400 Torr and with dose rates in the order of 1015 eV cc—1·sec—1 G(acetylene) = 3.07±0.08.
Significant Structure Theory of Physical Adsorption
Authors: McAlpin J. J. , Pierotti R. A.
Published: January 2005

The significant structure theory of liquids has been modified to describe the physical adsorption of gases on homogeneous solids. Equations have been derived for the isotherm, the isosteric heat, and the critical properties. Good agreement with experiment is obtained for the systems Ar and Kr on graphite. The lack of mathematical rigor in the foundations of the significant structure theory is compensated for to a certain extent by its good results and its arithmetical and conceptual simplicity. Its departure from a rigorous foundation, however, makes it difficult to make improvements or to understand anomalies.

The third-order dispersion energy correction for molecules adsorbed on a solid as derived by Sinanoğlu and Pitzer was used to determine the two-dimensional lattice energy and the success of the present theory in predicting the heats of adsorption and the critical temperature is a measure of the validity of their work.
On the Theory of Spin Relaxation of Gas Molecules: The Strong-Collision Limit
Authors: Freed Jack H.
Published: January 2005

A model of spin relaxation of gas molecules based upon an assumption of strong collisions is developed. A relaxation matrix formally similar to that of Redfield, but with different spectral densities, is obtained. The nuclear magnetic relaxation of gaseous orthohydrogen is discussed in terms of these results, and it is shown that, in general, molecules in different J states are expected to exhibit different relaxation times (a conclusion that is independent of the strong collision assumption) whenever the collision mechanism is effective only in reorienting J but not in changing its magnitude. Otherwise statistical averages of relaxation times are obtained. Relaxation of more complex molecules is also discussed.
On the Radial Distribution Function in Fluids
Authors: Widom B.
Published: January 2005

It is shown that if g(r) is the radial distribution function and φ(r) the pair potential, then the function F(r)=g(r) exp[φ(r)/kT]—1 has a number of properties analogous to those of the pair correlation function G(r)=g(r)—1 which are known from the Ornstein—Zernike theory. The integral of F(r) over all space is related to the fluctuations in a certain well-defined physical quantity, in the same way that the integral of G(r) is related to fluctuations in density. The integral of F(r), furthermore, is shown to be always positive, so that on the average g(r) exceeds exp[—φ(r)/kT]. Finally, it is shown that if one makes a hypothesis analogous to one which is familiar in the Ornstein—Zernike theory, then it follows that the pair correlation function G(r) cannot vanish more rapidly than the pair potential φ(r) as r→∞.
Microwave Spectrum of N-Methyl Methylenimine
Authors: Sastry K. V. L. N. , Curl R. F.
Published: January 2005

The rotational spectrum of CH3NCH2 has been studied in the region 7000–49 000 Mc/sec. Several b-type transitions have been identified. The barrier to the internal rotation of the methyl group has been found to be 1970±25 cal/mole. Quadrupole fine structure components for several transitions have been analyzed. The components of the quadrupole tensor χcc and χaa have been found to be 3.2±0.2 and 1.9±0.3 Mc/sec, respectively. The electric dipole moment has been found to be 1.53±0.020 D by measurement of Stark effects.
Occupation Probabilities in the Lattice Gas
Authors: Van Leeuwen J. M. J.
Published: January 2005

A set of relations obtained by Widom for the probabilities fr that an empty cell is surrounded by r filled cells is rederived and extended. The consequences of the relations for the partition function, the correlation functions, and partially coupled systems are discussed.
Twinning of Monoclinic Crystals as a Function of Habit Type and Centrosymmetry
Authors: Wolten G. M. , Chase A. B.
Published: January 2005

Twinning in 118 species of monoclinic crystals has been surveyed. Eighty percent of such twinning occurs on planes with a k index of zero. Some of this twinning is mechanical, but most is due to a mechanism, called the SA mechanism, described in this work. It operates only in centrosymmetric crystals and in noncentrosymmetric ones that are pseudo-orthorhombic. In most cases, SA twinning and mechanical twinning can be distinguished on the basis of the crystal's growth habit. Growth habits of monoclinic crystals can be classified into eight general types, two of which account for 75% of all monoclinic crystals. Precise criteria for pseudosymmetry are suggested.
Mass-Spectrometric Study of Ion—Molecule Reactions in Propane
Authors: Derwish G. A. W. , Galli A. , Giardini-Guidoni A. , Volpi G. G.
Published: January 2005

Ion—molecule reactions in propane have been studied up to pressure of 0.12 Torr inside the ion source of the mass spectrometer. All but four of the propane ions followed the exponential decay law remarkably well and their total cross sections of disappearance have been determined. The major reactions of these ions lead to the formation of C3H8+, C3H7+, C3H6+, and C3H5+ which, with the exception of C3H5, show very low reactivity. The ionic current due to purely secondary ions did not exceed few percent of the total ionic current at the highest pressure attained. The kinetics of most of the ion—molecule reactions occurring in propane have been investigated, and the pertinent cross sections have been determined.
Nuclear Magnetic Resonance Studies Concerning the Effect of Adjacent Hydrogen Atoms on the Hydrogen-Bond Properties
Authors: Lippincott Ellis R. , Rao A. Srinivasa
Published: January 2005

The proton-resonance absorption spectrum has been studied in several hydrogen-bonded solids with different O–H···O bond lengths ranging from 2.45 to 3.30 Å. The resonance linewidth increases with the O–H···O bond length and shows a tendency to flatten out when the bond length exceeds 2.72 Å. The proton-resonance absorption linewidth in molecular solids is due primarily to nuclear dipole—dipole interaction, and hence it is proportional to Σk(rjk)—3, where rjk is the distance between Protons j and k, where j is any particular chosen proton. Because of the inverse-cube dependence on r, appreciable contribution to the line broadening comes from nearest neighbors only. The dependence of the proton-resonance linewidth on the hydrogen-bond length, therefore, points out that the hydrogen-bond length gets smaller when the protons are far apart and larger when they are close. This result was verified independently by calculating the nearest hydrogen—hydrogen distances for compounds where the positions of hydrogen atoms are known from neutron-diffraction studies and in other cases by estimating these distances from the usual values of the bond lengths and bond angles. The observed result is explained by postulating that the interaction between the two O–H bonds involved in two adjacent O–H···O bonds is essentially electrostatic in origin; the interaction potential consists of attractive terms due to dipole—dipole, induced dipole, and dispersion interactions in addition to a repulsive term. The calculated interaction energy between two O–H···O hydrogen bonds is attractive up to O–H···O bond lengths of 2.80 Å and has a minimum value around 2.70 Å. The O–H···O bond length of 2.70 Å seems to be most preferred from the point of view of this energy. This is consistent with the fact that there are a large number of hydrogen-bonded solids with bond lengths around 2.70 Å.
Nonequilibrium Thermodynamics of Canonically Invariant Relaxation Processes
Authors: Andersen Hans C. , Oppenheim Irwin , Shuler Kurt E. , Weiss George H.
Published: January 2005

A relaxation process in which a dilute system is in weak interaction with a heat bath is termed canonically invariant if the time-dependent system distribution function maintains its canonical form throughout the relaxation process and is thus characterized by a time-dependent temperature. We discuss here the nonequilibrium (i.e., time-dependent) thermodynamics of such canonically invariant relaxation processes. Explicit analytic forms are obtained for the partition functions Q(t), the system thermodynamic properties E(t), A(t), S(t), and Cv(t), the global free energy and entropy a(t) and s(t), and their time derivatives for both classical and quantum systems. For quantum systems with a finite number of discrete energy levels, the above treatment is extended to include canonically invariant processes at negative absolute temperatures. Finally, we demonstrate that an initial canonical distribution for a fixed S(0) extremizes the entropy production of systems exhibiting canonically invariant relaxation processes.
Spin Splittings and Rotational Structure of Nonlinear Molecules in Doublet and Triplet Electronic States
Authors: Raynes W. T.
Published: January 2005

Matrix elements are presented for the Hamiltonian of a nonlinear, nonrigid polyatomic molecule in a multiplet electronic state. Their use is only appropriate for electronic and vibrational spectra since hyperfine interactions involving nuclear spins and nuclear quadrupole moments are not considered. For the most general case, nine parameters are required to take full account of spin—rotation interactions, and five are required for spin—spin interactions. For molecules of orthorhombic symmetry only three spin—rotation parameters and two spin—spin parameters are nonzero. For nonlinear molecules in doublet and triplet electronic states, explicit formulas are presented for (a) the rotational term values of symmetric rotors and (b) spin splittings of asymmetric rotors possessing orthorhombic symmetry. All these formulas reduce to well-known expressions for diatomic molecules in 2Σ and 3Σ states when K-dependent terms are ignored. Application of the above formulas to the results of Dressler and Ramsay on the 2B1 ground states of NH2 and ND2 permits the determination of the spin—rotation parameters of these molecules. All five spin parameters of formaldehyde in its lowest 3A2 state are given together with curves of spin splittings in the lower K levels. The spin parameters of HCHO, NH2, and ND2 are compared with those of NO2 and ClO2 found by recent microwave studies. For a triplet state of an orthorhombic molecule, the spin—spin constants determined by band spectroscopy are simply related to the spin constants D and E determined from zero field splittings in electron spin resonance spectroscopy. The surprisingly small value of D=0.42 cm—1 for the lowest triplet state of formaldehyde is briefly discussed in terms of a breakdown of the orbital approximation for this prototype ``n—π* state.''
Radiationless Intermolecular Energy Transfer. I. Singlet→Singlet Transfer
Authors: Bennett R. G.
Published: January 2005

Radiationless energy transfer between unlike molecules has been studied under conditions where the interaction described in the ``Förster'' formulation may be expected to dominate other effects. Primary emphasis is placed on a direct observation of the effect of energy transfer on the excited-state lifetime of the donor. These results are related through the theory with other spectroscopic measurements. Good agreement is found with the predictions of Förster based on a long-range (25–50 Å) dipole—dipole interaction leading to an R—6 distance dependence of the transition probability between donor and acceptor.
Radiationless Intermolecular Energy Transfer. II. Triplet→Singlet Transfer
Authors: Bennett R. G. , Schwenker R. P. , Kellogg R. E.
Published: January 2005

The time dependence of the de-excitation of the phosphorescent state of phenanthrene-d10 by radiationless energy transfer to the fluorescent state of rhodamine B is observed. Good agreement is found with predictions based on the long-range dipole—dipole interaction described by Förster.
Radiationless Intermolecular Energy Transfer. III. Determination of Phosphorescence Efficiencies
Authors: Kellogg R. E. , Bennett R. G.
Published: January 2005

Long-range radiationless energy transfer from the triplet states of aromatic hydrocarbons to the singlet states of fluorescent dyes (10—3 M) is used to measure the efficiencies of energy dissipation (φF, φIS, φP) for triphenylene, phenanthrene, p-terphenyl, chrysene, naphthalene, and/or their deuterated counterparts. Rhodamine B, rhodamine 6G, acriflavin, and fluorescein were found to be suitable acceptors. By measuring phosphorescence efficiencies directly, reliable estimates of T→S radiative rate constants can be obtained.
Radiationless Intermolecular Energy Transfer. IV. Triplet—Triplet Annihilation
Authors: Kellogg R. E.
Published: January 2005

Phosphorescence quenching for phenanthrene-d10 has been observed in solid solution. The quenching efficiency is dependent on the triplet concentration ηT with a threshold at ηT∼10—3 M. It is interpreted to occur by the process T1+T1→S0+T2, involving a radiationless transfer of one triplet excitation to the triplet—triplet absorption band of the other. The necessary conditions described by Förster for dipole—dipole radiationless energy transfer are shown to be satisfied.
Radiationless Intermolecular Energy Transfer. V. Singlet→Triplet Transfer
Authors: Bennett R. G.
Published: January 2005

The quenching of the fluorescent state of perylene by radiationless energy transfer into the triplet manifold of phenanthrene-d10 is observed, viz., S1D+T1A→S0D+T2A. The variation of perylene fluorescence intensity as a function of phenanthrene-d10 triplet concentration is predicted quantitatively by the Förster theory.
Absorption and Fluorescence of Europium(III) in Aqueous Solutions
Authors: Gallagher P. K.
Published: January 2005

Absorption, fluorescence, and excitation spectra of aqueous solutions of europium(III) perchlorate, chloride, nitrate, ethylenediaminetetra-acetate, and diethylenetriaminepenta-acetate were investigated. The latter three anions readily form complexes and were examined over a range of concentrations. The organic anions particularly, which are strong chelating agents, cause a general intensification of the spectra, shift to longer wavelengths, and increased crystal field splitting. No transfer of energy was observed from the ligand to the europium ion. Effects of changes in pH were determined and correlated with the weakly acidic nature of these chelates. Factors are discussed which lead to the hundredfold increase in some fluorescence after chelation without the benefit of energy transfer. The spectra are discussed with reference to the symmetry of europium species in solution and it is concluded that the relative intensities of forced electric dipole and magnetic dipole absorption or fluorescence bands are probably a better indication of the interaction of a europium ion with its surroundings than shifts of specific bands.
Electron-Exchange Reaction of Wurster's Blue in Aqueous Solution
Authors: Britt A. D.
Published: January 2005

The electron exchange between Wurster's Blue and N,N,N′,N′-tetramethyl-p-phenylenediamine dihydrochloride is investigated by ESR and NMR. The active form of the dihydrochloride is found to be the neutral diamine, and a simple relationship is derived between the apparent rate constant and the actual rate constant.
Energy Transfer in the Fluorescence of Iodine Excited by the Sodium D Lines
Authors: Brown Robert L. , Klemperer W.
Published: January 2005

Absorption of the sodium D lines excites I2 molecules predominantly to the vibration—rotation levels v′=14, J′=113; v′=15, J′=44 and 37; and v′=16, J′=106 of the B3II0+u electronic state. In the absence of foreign gases, approximately 75% of the excited molecules are in v′=15, 10% in v′=14, and 10% in v′=16. Steady-state fluorescence from these levels and neighboring levels populated by collisions with added inert molecules has been used to obtain collision cross sections for exchange of vibrational energy of excited I2 with the translational energy of the gases He, Ne, Ar, Kr, Xe, H2, HD, D2, N2, O2, and NO. Relative emission intensities were measured photoelectrically. Vibrational transfer is highly efficient; cross sections for one quantum transfer average about ⅛ kinetic-theory cross sections taken from gas viscosities and are similar to those obtained by other workers for transfer from v′=25. Information on multiple quanta transfer could not be obtained directly because of band overlapping. On transfer of a vibrational quantum by He, H2, HD, and D2, some persistence of the initial rotational state in I2 was observed, while with the heavier gases the rotational distribution was spread. In collisions with HD no evidence was found for any resonance exchange between HD rotational energy (91 cm—1 for the J=0↔1 transition) and I2 vibrational energy (100 cm—1 for level separation around v′=15). Quenching of the fluorescence by the above foreign gases and by I2 itself was found to be proportional to the product of the polarizability of the inert molecules and the reciprocal of the relative velocity of the colliding particles. Absolute radiative transition probabilities for a number of individual rotational lines in the I2 visible spectrum have also been determined.
Entropy and Related Thermodynamic Properties of Dimethoxymethane
Authors: McEachern D. Marvin , Kilpatrick John E.
Published: January 2005

The heat capacity and the heat and temperature of fusion of dimethoxymethane and the heat of vaporization of dimethoxymethane have been determined experimentally. Dimethoxymethane exhibits a melting point at 168.01°±0.01°K. The heat of vaporization at 25°C was found to be 6904±50 cal/mole. The entropy of the ideal gas at 25°C, 1 atm is 80.24±0.15 cal/deg mole.
Kinetics of Diffusion-Controlled Whisker Growth
Authors: Ruth V. , Hirth J. P.
Published: January 2005

The problem of surface diffusion-controlled whisker growth is considered in a moving frame of reference. In addition, diffusive exchange with the substrate, a problem not treated heretofore, is considered. The results are compared with experiment and with prior treatments of whisker growth. The possibility of occurrence of some prior mechanisms of whisker-growth cessation is excluded. The material parameters that may be determined from whisker-growth experiments are considered.
Thermal Conductivity of a Gas with Rotational States
Authors: McCourt F. R. , Snider R. F.
Published: January 2005

When degenerate internal states are present, the most convenient description of the average one-particle state is by means of a Wigner distribution function-density matrix which is a Wigner distribution function for translational motion and a density matrix in internal states. This matrix is assumed to be diagonal in energy but may in general be nondiagonal in degenerate states. The Boltzmann equation appropriate for this quantity is solved by the Chapman—Enskog method to obtain an expression for the thermal conductivity in terms of generalized quantum-mechanical cross sections.
Electron-Impact Studies of Aromatic Hydrocarbons. III. Azulene and Naphthalene
Authors: Van Brunt Richard J. , Wacks Morton E.
Published: January 2005

The observed ionization energies of naphthalene (8.26 eV) and azulene (7.72 eV) and the heats of formation of these compounds provides an energetic scheme consistent with the measured appearance potentials of secondary ions and the assumption of ionic intermediates common to both compounds. The occurrence of these common intermediate ions which lead to the observed fragmentation in these compounds is also demonstrated by the similarity of the mass spectra of these two compounds. Additional evidence is provided by observations made on the metastable ion transitions found in their mass spectra.
Rotation—Electronic Interaction in the Rydberg States of Diatomic Molecules
Authors: Chiu Ying-Nan
Published: January 2005

The theoretical foundation of Hund's coupling cases for the interaction between rotation and electronic motion is re-examined. The relationship between different cases is shown by angular-momentum coupling techniques. Rotational interaction terms neglected in the Born—Oppenheimer adiabatic type approximation and in the idealized Hund's cases are considered in particular. For application to Rydberg states, new and improved perturbed energy expressions of Λ doubling for spectroscopic use are derived up to the fourth order for a near Hund's Case b′ diatomic molecule and up to the second order for a near Hund's Case d′ diatomic molecule. Specific formulas are given for the p-term (L=1) and the d-term (L=2) complexes. For the approach towards cases intermediate between b′ and d′, secular determinants for this perturbation problem are formulated starting both from ideal case b′ and from ideal case d′ using two parameters, instead of one, for the d-term complex. These two approaches are shown to give equivalent results proving the consistency of the perturbations and assumptions used at both ends. The nature of these assumptions as well as the physical basis for the transition between Case b′ and Case d′ are shown.
Electron Paramagnetic Resonance of Re4+ in Single Crystals of K2PtCl6
Authors: Rahn Ronald O. , Dorain Paul B.
Published: January 2005

The electron paramagnetic resonance of Re4+ in single cubic crystals of K2PtCl6 at wavelengths of 3 cm, 1 cm, and 8 mm is reported. The experimental results were analyzed assuming the Re4+ ground state is a spin quartet with S=32 and I=52. The cubic spin Hamiltonian is given by H=gβH·S+AI·S+uβ{HxSx3+HySy3+HzSz3−15(H·S)(3S2−1)} +U{IxSx3+IySy3+IzSz3−15(I·S)(3S2−1)}.The third-order terms in S in the spin Hamiltonian were found to make a significant contribution as shown by the strong angular dependence of the resonance spectrum even in a cubic field. The main transitions were between the MS=±½ Zeeman levels. In addition to the ΔmI=0 transitions, the forbidden ΔmI=±1 transitions were observed with the transition probability of the latter showing strong field dependence. Analysis of the data gave the following values for the parameters: g=1.815±0.001,u=(−13±1)×10−3,A=(3.89±0.01)×10−2cm−1,U=(−3.8±0.1)×10−3cm−1.A brief discussion of the experimental parameters is given.
Measurement of the Energy Distribution in Field Ionization
Authors: Tsong Tien Tzou , Müller Erwin W.
Published: January 2005

The energy distribution of ions obtained by field ionization of He, Ne, Ar, and H2 near the surface of a tungsten emitter is measured with an improved retarding potential tube. Most ions are found to originate immediately beyond the critical distance determined by the condition that the electron tunnels above the Fermi level. The ion energy is determined by the ionization energy and an image force contribution, while there is no indication of a shift of the ground level of the gas atom due to the proximity of the metal surface. In the case of hydrogen the atomic ion is found to have an additional energy of ½ of the dissociation energy of the molecular ion. The half-width of the distribution is between 0.5 and 0.7 eV for all the above gases, indicating an extremely narrow ionization zone with a depth of a few tenths of an angstrom. Calculation of the energy distribution by applying the WKB method to a square well gives the proper general features and the temperature effect but a half-width too wide by a factor of 2, thereby exhibiting the limitation of a linear model.
Magnetic Characteristics of Lanthanide—Silver Compounds Having the CsCl Structure
Authors: Walline R. E. , Wallace W. E.
Published: January 2005

The magnetic characteristics of 11 intermetallic compounds represented by the formula LnAg are reported for temperatures between 2° and 300°K. YAg is a Pauli paramagnet. CeAg and PrAg are ferromagnets with Curie temperatures 9° and 14°K, respectively. The compounds with Ln=Nd, Sm, Tb, Dy, Ho, Er, or Tm exhibit pronounced susceptibility peaks, from which it is inferred that they are antiferromagnetic at low temperatures. This may also be true for GdAg whose susceptibility behavior is anomalous below 200°K. At the upper temperatures studied Curie—Weiss behavior is observed, and effective moments in good agreement with the free tripositive ion moments are obtained for all except YAg, SmAg, GdAg, and ErAg. The lack of compliance for SmAg is ascribed to limitations in the experimental procedure used. GdAg obeys the Curie—Weiss Law above 200°K, but with an effective moment of 8.57 μB. The effective moment observed for ErAg varies from about 9 μB at 298° to 6 μB at 77°K.
Faraday Rotation in Glasses
Authors: Borrelli Nicholas F.
Published: January 2005

The Verdet constants of a number of glasses have been measured with the object of studying the Faraday effect in glass.

The Verdet constant was measured for three types of glasses: (1) optical glasses, where the Verdet constant correlates well with the reciprocal dispersion; (2) diamagnetic glasses, where glasses which contain high concentrations of ions that have sp—s2 transitions give rise to large Verdet constants; (3) rare-earth glasses, i.e., paramagnetic ions, where Pr3+ and Dy3+ silicate and borate glasses have the largest Verdet constants. These latter rare-earth data were compared to the theoretical expression of Van Vleck and Hebb and to the experimental data on metaphosphates of Berger et al.
Charge-Transfer Exciton States in Aromatic Molecular Crystals
Authors: Choi Sang-Il , Jortner Joshua , Rice Stuart A. , Silbey Robert
Published: January 2005

In this paper we consider some of the consequences of the mixing of neutral exciton states (neutral excitation waves or Frenkel excitons) with charge-transfer exciton states (ion-pair excitons) in the oriented gas model of aromatic molecular crystals.

The lowest-energy charge-transfer exciton is described by suitable symmetry-adapted functions built on the basis functions of a near-neighbor positive and negative ion pair. It is then shown that the energy of this state is not greatly different from the p states of anthracene and naphthalene, and therefore that the ion-pair exciton and Frenkel exciton states can mix. For the cases of anthracene and naphthalene, such configuration interaction leads to the following contributions to the Davydov splitting of the triplet manifold: Δε=26/ΔE cm−1naphthalene,Δε=31/ΔE cm−1anthracene,where ΔE is the energy difference (in electron volts) between the triplet level and the near-neighbor charge-transfer level. In making the computations, it is necessary to consider carefully the effects of vibronic coupling and of configuration changes in the excited state. It is concluded that charge-transfer excitons can make an important contribution to the Davydov splitting of a triplet manifold. This conclusion is supported by preliminary estimates of the interaction matrix elements for crystalline benzene.
Proton Chemical Shifts of the γ-Substituted Pyridines
Authors: Wu T. K. , Dailey B. P.
Published: January 2005

The proton chemical shifts have been obtained from the NMR spectra of nine γ-substituted pyridines. The substituent-induced chemical shifts, which were calculated by subtracting from each observed value the corresponding proton shift of the unsubstituted pyridine, were found to be comparable to the shielding parameters of the analogous substituted benzenes. Moreover, these chemical shifts can be fitted to the additivity relationship of Martin and Dailey which was obtained from the proton shieldings in substituted benzenes. Therefore the shielding mechanisms in the pyridines are probably the same as those in the benzenes. The failure of the calculated chemical shifts of the α and β protons in the pyridines to agree with observed shifts, as was also the case with the corresponding substituted benzenes, is interpreted as further evidence that the shieldings at positions ortho and meta to the substituents are not determined by the pi-electron density variations alone.
Desorption from Metal Surfaces by Low-Energy Electrons
Authors: Menzel Dietrich , Gomer Robert
Published: January 2005

The effect of low-energy (15–200-eV) electrons on hydrogen, oxygen, carbon monoxide, and barium adsorbed on tungsten has been investigated by a field-emission technique. Desorption cross sections σ were determined from work function and Fowler—Nordheim pre-exponential changes and are significantly smaller than would be expected for comparable molecular processes. Marked variations in cross section with binding mode within a given system were found. Thus σH=3.5 10—20 cm2 and 5×10—21 cm2 for processes tentatively interpreted as the splitting of molecularly adsorbed H2 and desorption of H, respectively; σ0=4.5×10—19 cm2 for a loosely bound state and σ0≤2×10—21 cm2 for all other states; σBa<2×10—22 cm2 under all conditions. In the case of CO (reported in detail elsewhere), three binding modes observed previously could be confirmed and differentiated by their different cross sections: σvirgin=3×10—19 cm2; σβ=5.8×10—21 cm2, σα=3×10—18 cm2; conversion by electrons of virgin to β σvβ≥10—19 cm2. These results are interpreted in terms of transitions from the adsorbed ground state to repulsive portions of excited states, followed by de-exciting transitions which prevent desorption. Arguments are made to show that the excitation cross sections should be essentially ``normal,'' i.e., ∼10—16 to 10—17 cm2, and that the much smaller over-all cross sections observed are due to high transition probabilities to the ground state, estimated as 1014 to 1015 sec—1. A detailed calculation for the case of exponentially varying transition probabilities and repulsive upper states is presented and discussed, and the variations in cross section with binding mode made plausible. It is shown that low-energy electron impact constitutes a sensitive tool for studying chemisorption.
Electron-Impact Desorption of Carbon Monoxide from Tungsten
Authors: Menzel Dietrich , Gomer Robert
Published: January 2005

The low-energy electron impact desorption of CO adsorbed on W has been investigated by means of a field emission technique. It was found that the previously postulated binding modes of CO could be distinguished and confirmed by their differing desorption cross sections which are σvirgin=3×10—19 cm2, σvirgin to β≥10—19 cm2, σβ∼3×10—21 cm2, σα=3×10—18 cm2 for 80-V electrons. Evidence for thermal conversion of virgin states to both β and α states was obtained; electrons also seem to convert virgin to β states. In addition to desorption, dissociation with carbon formation occurred. Although quantitative estimates are difficult, the ratio of dissociation to desorption was estimated to be between 0.5 and 0.1, and seems to be least for α and greatest for β desorption. The results on carbon formation show that (at least) a large fraction of CO in the α and virgin modes is adsorbed with the C end of the molecule only. It is pointed out that electron desorption is a sensitive tool for probing chemisorption states and is able to reveal details like interconversion which are not readily detectable by other means.
Crystal-Field Splitting of Trivalent Thulium and Erbium J Levels in Yttrium Oxide
Authors: Gruber John B. , Krupke William F. , Poindexter John M.
Published: January 2005

The optical absorption and fluorescence spectra of trivalent thulium in single crystals of yttrium oxide and the absorption spectrum of pure thulium oxide, are investigated. All the 4f12 electronic energy levels except the 1S0 level are identified. The three Slater Fk parameters and the spin—orbit parameter ζ, giving the best over-all agreement between the observed spectrum and the calculated spectrum are: F2=446.70 cm—1, F4=70.491 cm—1, F6=7.534 cm—1, and ζ=2617 cm—1. A detailed crystalline-field splitting perturbation calculation is carried out for trivalent thulium and trivalent erbium ions in the C2 sites of the Y2O3 lattice. The predicted crystalline-field splitting for these ions is compared with the thulium data reported in this paper and with the erbium data reported earlier. The average deviations between the predicted and the observed splittings for 71 crystalline Stark levels in the Tm3+ spectrum and 65 levels in the Er3+ spectrum, are 13 and 11 cm—1, respectively.
Electronic Structure of Transition Metal Complexes with Nitrosyl Ligand. Mössbauer Effect Investigation
Authors: Danon J.
Published: January 2005

The Mössbauer effect was investigated with spin-paired diamagnetic pentacyanonitrosylferrate (II) (sodium nitroprusside), spin-paired paramagnetic nitroso-bis-dimethyldithiocarbamates iron (I), and with spin-free pentaquonitrosyl iron (I) complexes.

Information on the nature of bonding in the first two complexes was obtained through detailed interpretation of the isomer shift and quadrupole coupling, measured with polycrystalline and single crystal samples.

Different possible interpretations are discussed for the results with the pentaquo complex.
Simplified SCF Method for σ-Bonded Systems. I. Physical Properties of Hydrogen Halides
Authors: Pohl Herbert A. , Rein Robert , Appel Klaus
Published: January 2005

Approximate SCF LCAO—MO calculations have been carried out for the ground-state properties of four hydrogen halides; six internuclear distances were considered. In the model only the bonding electrons are considered explicitly. The effect of the inner electrons were taken account of by semiempirical methods. The electron pair is described by a single configurational wavefunction and the MO's are built from 1s and np AO's. The matrix elements are obtained utilizing the Mayer—Sklar, Mulliken, Pariser, and Pople approximations. As a secondary study the effect of the Wolfsberg—Helmholtz parameter K, in the resonance terms is studied.

The calculated binding energies, equilibrium bond distances, dipole moments, vibrational force constants, and breaking forces obey the same trends as in experimental data. Moreover the magnitudes for these calculated quantities are in substantial agreement with experiment. This agreement can be improved for a given physical property of the series by introducing the arbitrary parameter K and optimizing it for one member of the series.

It is concluded that the approximations characterizing the model, all of which had been previously used with π-bonded systems, are equally well applicable to σ-bonded systems.
Low-Temperature Observation of Oxygen-17 NMR in H2O, D2O, and Eutectic Solutions
Authors: Rabideau Sherman W. , Jackson Jasper A.
Published: January 2005

Although linewidths for the oxygen-17 resonance were recorded in supercooled H2O and D2O, it has not been possible to obtain a signal from these pure solids. Linewidth measurements were also made to —79°C in a LiI–H2O eutectic solution. Ice with ∼10% ammonium fluoride in solid solution did not display the 17O resonance below the eutectic temperature. The deuterium resonance in a 0.15M NaCl—D2O solution was used to demonstrate the presence of liquid eutectic solution in polycrystalline ice.
Nuclear Magnetic Resonance of Single Crystals of D2O Ice
Authors: Waldstein Peter , Rabideau Sherman W. , Jackson Jasper A.
Published: January 2005

Quadrupole splitting of the deuteron magnetic resonance was measured in single crystals of solid D2O at —10°C. For deuterons on the hexagonal crystal axis, eQq1/h equaled 213.2±0.8 kc/sec, and | η1 | was equal to 0.100±0.002. For the other deuterons, eQq2/h was found to be equal to 216.4±1.0 kc/sec. No change in the line separations was observed at a temperature of —70°C. No evidence was found for hindered rotation of the D2O molecules nor for intermolecular deuteron transfer at —10°C. Lower limits to the barriers hindering these motions were estimated; these limits permit estimates to be made of hydrogen-bond energies. The coupling constants in the solid are about 30% less than those reported for the free molecule. Most of this decrease must be ascribed to electronic structural changes brought about as a result of hydrogen-bond formation.
Reactions of 1D Oxygen Atoms in the Photolysis of Carbon Dioxide. II
Authors: Warneck Peter
Published: January 2005

The vacuum-uv photolysis of CO2 containing initially no oxygen was investigated in a closed system and the quantum yields of ozone and oxygen determined. The quantum yield for ozone production was found to be below Q(O3) ≤0.03, which is insufficient to explain the oxygen deficiency observed in the O2 quantum yields. The [O2]/[CO] ratio was found to depend on the light intensity, on the time interval after which an irradiated sample was subjected to analysis, and on the ratio of carbon dioxide to admixed rare gas concentration. The results concerning the [O2]/[CO] ratio can be explained by the formation of CO3 due to the attachment of 1D oxygen atoms to CO2. This reaction is in competition with the third-body recombination of 1D oxygen atoms.
Two-Photon Absorption in Crystalline Anthracene and Naphthalene Excited with a Xenon Flash
Authors: Weisz S. Z. , Zahlan A. B. , Gilreath J. , Jarnagin R. C. , Silver M.
Published: January 2005

Two-photon absorption and delayed fluorescence are observed in both anthracene and naphthalene using a 5-μsec xenon flash lamp. It is shown that coherence and monochromaticity are not important in two-photon absorption. It is also demonstrated that the two-photon absorption process does not involve triplet excitons. In anthracene the two-photon rate constant δ≃8×10—29 cm sec and is about this value in naphthalene.

Mechanical imperfections play an important role in processes involving excited states in naphthalene monocrystals. The effect of imperfections is such that only an order-of-magnitude value for the intersystem crossing constant can be evaluated at the moment: τfkst≃10—1, where τf is the fluorescence lifetime.
Binding Energy of LiH+ and the Ionization Potential of LiH: Mixed Basis Set Calculation
Authors: Browne J. C.
Published: January 2005

A rigorous lower bound of 0.038 eV for the binding energy of LiH+ has been obtained via an ab initio quantum mechanical calculation using a generalized valence-bond wavefunction with a mixed basis set of elliptic and Slater-type orbitals. The probable error in the total energy given by this calculation is of the order of 0.1 eV, so that a probable upper bound for the binding energy of LiH+ is 0.15 eV. This invalidates the conclusion of a previous SCF—MO calculation which gave a substantial minimum in the LiH+ potential curve. A rigorous upper bound of 7.91 eV for the ionization potential of LiH is found by the relation E(LiH+, calculated) — E(LiH, experimental) ≥I.P. (LiH). A probable lower bound for I.P. (LiH) is 7.8 eV. Potential curves for the system Li++H from several wavefunctions are tabulated.
Radio-Frequency and Microwave Spectra of NaF by the Molecular-Beam Electric-Resonance Method
Authors: Hollowell C. D. , Hebert A. J. , Street K.
Published: January 2005

Radio-frequency transitions of the type (J, mJ±1)→(J, mJ) have been observed for 23Na19F by the molecular-beam electric-resonance method. Spectra were observed for the first three vibrational states of the J=1 rotational level and for the first vibrational state of the J=2 rotational level. From the analysis of the radio-frequency spectra, dipole moments μv, sodium quadrupole coupling constants (eQq)v, spin—rotation interaction constants cNa and cF, and spin—spin interaction constants c3 and c4 were determined for 23Na19F. Observations of J=1→J=0 microwave transitions yielded rotational constants Bv for the v=0, 1, and 2 vibrational states. The molecular constants determined for 23Na19F are: μv=8.12349+0.06436(v+12)+0.00037(v+12)2±0.001 D,(eQq)0=−8440.1±1.5 kc/sec;(eQq)1=−8327.9±1.5 kc/sec;(eQq)2=−8220.5±2.0 kc/sec,cNa=1.4±0.5 kc/sec,cF=2.0±0.3 kc/sec,c3=3.85±0.25 kc/sec,c4=0.0+0.4 kc/sec−0.1 kc/sec,B0=13029.811±0.002 Mc/sec,B1=12894.543±0.003 Mc/sec,B2=12760.674±0.004 Mc/sec.
Crystalline Fields in Corundum-Type Lattices
Authors: Sharma R. R. , Das T. P.
Published: January 2005

Dipole moments of the deformable O2— ions in the corundum-type crystals Al2O3, Cr2O3, and Fe2O3 are calculated self-consistently. The magnitudes and signs of the dipole moments depend sensitively on the crystal parameters and vary widely over the series Al2O3, Cr2O3, and Fe2O3. Using the calculated dipole moments, the dipolar contributions to the crystalline components B10, B20, B30, B33, C33, B40, B43, and C43 have been calculated at the metal ion sites. The dipolar contributions are found to be substantial and in some cases to alter the sign of the crystalline field components obtained from point charges alone. The modified values of B20, B40, and B43 are used to obtain expressions for the optical parameters v, v′, and Dq in terms of the expectation values 〈r2〉 and 〈r4〉 for the d electrons of Cr3+ ions in Cr2O3 and ruby, and Fe3+ ions in Fe2O3 and sapphire. The coefficients of 〈r4〉 in the expressions for v and v′ are substantially altered on including the effect of the dipoles. The effect of the dipoles on the coefficient of 〈r4〉 in the case of Dq is less marked. This difference in behavior of v, v′, and Dq can be explained by the different dependences of these parameters on the trigonal departure from the cubic field. The values of Dq are, however, still seriously underestimated when Hartree—Fock values of 〈r4〉 are used. Reasonable agreement with experiment is obtained using values of 〈r2〉 and 〈r4〉 calculated with the Slater effective charge model. The comparison between theoretical and experimental quadrupole coupling constants and optical parameters is utilized to speculate on the factors which could improve agreement with experiment.
Weinstein Calculation on Hydrogen Molecular Ion
Authors: Goodisman Jerry , Secrest Don
Published: January 2005

A Weinstein calculation for the two-parameter James function for H2+ has been performed. The lower bound to the energy was maximized and a value of —1.14724 a.u. was obtained. The improvement with additional variation parameters is greater than for the upper bound.
Role of Molecular Vibrations in Optical Rotation
Authors: Hameka H. F.
Published: January 2005

It is shown that in the general case the optical rotatory power of a molecule may be evaluated on the assumption that the nuclei are at rest in the equilibrium configuration of the molecular electronic ground state and that the vibrational motions may be neglected. This rule is not valid when the frequency of the incoming light falls in one of the absorption bands of the molecule and when the frequency becomes comparable with the vibrational absorption frequencies. Especially the latter of these two cases is discussed in detail.
Semiclassical Theory of the Effects of Internal Motions on the Linewidths in Electron Spin Resonance Spectra
Authors: Freed Jack H. , Fraenkel George K.
Published: January 2005

A number of classical dynamical models are developed for describing the effects of internal rotational motions and solvent-complex formation on the ESR hyperfine linewidths of free radicals in solution. These dynamical processes can lead to linewidth effects because they cause time-dependent modulations of the isotropic hyperfine interactions of the different magnetic nuclei. An alternating linewidth effect, which has been observed in a number of recent studies, is predicted to result when there is an out-of-phase correlation between the hyperfine splittings ai(t) of equivalent nuclei. By equivalent nuclei here are meant those for which the time-average splittings 〈ai(t) 〉Av are equal, and an out-of-phase correlation is one in which an increase in the instantaneous splitting from Nucleus i, ai(t), is correlated with a decrease in the splitting aj(t) from nucleus j. This out-of-phase correlation can result from a coupling of the mechanical motions of different rotating groups and also from the effects that changes in orientations of uncorrelated rotors may have on redistributing electron spin density in the molecule. Continuous motion cases are treated by Brownian-motion theory using coupled and uncoupled internal rotors and torsional oscillators. Random jump models between discrete states are also considered. It is shown that all these models can give rise to an alternating linewidth effect provided that certain definite relationships of the proper type exist for the coupling of the motions or the variations of the spin densities. For many of these models the correlation a1(t)=a2(t) for all t, or complete equivalence of both nuclei, may also result when different relationships exist. The correlation of the splittings of more than two groups of completely equivalent nuclei will, in general, lead to more complex linewidth effects.
Accurate Computation of Vibronic Energies and of Some Expectation Values for H2, D2, and T2
Authors: Kol/os W. , Wolniewicz L.
Published: January 2005

The complete four-particle nonrelativistic Hamiltonian and 147-term wavefunctions have been used to compute the energies and the expectation values of several other operators for vibronic ground states and for the first vibrationally excited states of H2, D2, and T2. For H2 and D2 the computed dissociation energies, with relativistic corrections, are by 0.6 and 1.3 cm—1, respectively, larger than the experimental values.
Isothermal Diffusion in Systems with Glasslike Transitions
Authors: Frisch H. L.
Published: January 2005

Following Simon, we define fluid systems with a ``glasslike'' transition as systems which above the transition temperature Tξ are, and below Tξ are not in internal chemical equilibrium with respect to a set of unspecified internal parameters of even tensorial rank. We present the flow equations for the chemical components of such systems in various frames of references. In particular, in a mass-fixed frame of reference, besides a variation in the Onsager coefficients, the flow equations below Tξ are subject to a different set of subsidiary conditions (the ``new'' Gibbs—Duhem equations). We also investigate the consequences of the assumption of sorption equilibrium at the surface of our diffusion system. We find that below Tξ these systems will possess history-dependent diffusion coefficients and be subject to time-dependent surface concentrations. We can relate these time dependencies to the relaxation of the internal parameters of the system below Tξ.
Studies on Absorption Spectra of Ni++ Ion in NiCl2 in Different Organic Solvents
Authors: Chhonkar N. S.
Published: January 2005

The absorption spectra of Ni++ ion in NiCl2 in six different organic solvents were studied with a Hilger Uvispek spectrophotometer from 10 000 to 3900 Å. A comparison between the calculated and observed values of oscillator strength strengthens our view that electric dipole transition coupled with vibration give rise to these bands.

From the lowering of the term separation the covalency factor is found to vary between 0.85 and 0.89 for the different solvents. The deduced g- and magnetic moment have almost the same value in all the solvents.

The cubic field coefficient in some solvents is found to be smaller than that in aqueous solution. The cluster about the Ni++ ion appears to be more anisotropic than in aqueous solution.