bluuebunny/crossref_metadata_2025_split huggingface dataset:
Crystal Fields at Rare-Earth Ions
Authors: Burns Gerald 
Published: January 2005

The object of this paper is to find out why the calculated rare-earth crystal-field parameters do not agree with those determined experimentally. As is well known, in explaining the 4f spectra in the rare-earth compounds, one needs relatively few crystal-field constants, Vnm's. These Vnm's have been measured for many rare-earth ions in LaCl3, ethylsulfates and other lattices. One can study the Vnm's in a given lattice as the nuclear charge is varied (starting from Ce, Z=58 to Yb, Z=70). For a given n and m one finds that the Vnm's vs Z vary fairly smoothly. However, when the crystal fields are calculated using an ionic model one cannot get good agreement with the Vnm's. In general, neither the magnitude nor the Z dependence of the Vnm's can be reproduced. The calculated crystal-field parameters are written as: (1−αn)〈rn〉Anm, where αn is the shielding factor (the outer 5s2p6 electrons to some extent shield the inner 4f electrons from the crystal field), 〈rn〉 is the expectation value of rn over the 4f electrons, and Anm is a lattice sum over the charges in the lattice that produce the crystal field. To try to determine why the agreement is bad, the effect of the lattice in altering the values of αn and 〈rn〉 was calculated. However, the principle trouble appears to come from the Anm values. Anm∝̃1/Rn+1, where R is the distance between the rare-earth ion and the charges in the lattice that produce the crystal fields. For the important n=4 and 6 lattice sums one gets very rapid convergence. The Anm sums are usually calculated assuming the lattice can be replaced by point charges, point dipoles, etc., at the nucleus of the ions surrounding the rare-earth ions. Thus, R is taken as the various internuclear distances. However, for lattice sums that depend on such high powers of R, one must consider the extended nature of the charge distribution of the ions surrounding the rare earths. Another way of stating the trouble is: Since the distance between the observer (rare-earth ion) and the charge distribution is not large compared to the extent of the charge distribution it is not a good approximation to replace the charge distribution by point moments. From numerical examples it is easy to see, for n=4 and 6, that most of the contribution to the Anm sums comes from the parts of the surrounding ions that are closest to the rare-earth ions. Thus it becomes difficult to calculate the Anm's accurately. However, using this idea, agreement is obtained with the dependence of the Vnm's on Z and some of the interrelationships between the different n terms.
For the lattices usually used as hosts one cannot calculate the A20 sum because of its sensitivity to the x-ray data and charge distribution parameters. However, by using nuclear quadrupole resonance data to eliminate calculating the actual lattice sum, one gets good agreement between the measured V20's and (1−α2)〈r2〉A20.
Self-Consistent Group Calculations on Polyatomic Molecules. I. Basic Theory with an Application to Methane
Authors: Klessinger M. , McWeeny R. 
Published: January 2005

In polyatomic molecules it is usual to recognize physically distinct groups of electrons (e.g., inner shells, σ bonds, π electrons). The π electrons of conjugated molecules have received most attention, other groups of electrons being recognized only as a ``framework'' with empirically determined characteristics. In this series of papers, all groups of electrons are admitted, each group (localized or nonlocalized) being made self-consistent in the field of the others. In this way bond properties, π-electron parameters, etc., may be determined from first principles, taking into account the whole molecular environment. Excited states may be determined by admitting ``excitons'' (i.e., localized excitations) within the various groups.
The theory is illustrated by means of a completely nonempirical calculation on the methane molecule. The ground state is somewhat better than that given by approximate SCF theory, the excited states are somewhat worse. One advantage of the approach is that it lends itself admirably to semiempirical development.
Diffusion Coefficients of the Systems He—T2 and He–TH
Authors: Amdur I. , Malinauskas A. P. 
Published: January 2005

Binary mutual diffusion coefficients have been measured for the systems He—T2 and He–TH by a radioactive tracer method at four temperatures in the range 195° to 374°K. The results, reduced to one atmosphere and normalized to the system He–H2, can be expressed within an average deviation of slightly less than 1% by the empirical relations He−T2:  log10D=1.6599log10T−3.9230,He–TH:  log10D=1.6960log10T−4.0140.
The ratio of the normalized coefficients for He—T2 and He–TH show a small departure from unity, starting at 1.02 at the lowest temperature and decreasing smoothly to 0.996 at the highest. On the assumption that deviations from classical mechanics, contributions from internal degrees of freedom, and the effects of asymmetric force fields are the same, or negligibly small, for both systems, different sets of Lennard-Jones (12–6) potential parameters have been obtained which correlate the experimental results as well as the empirical equations.
Diffusion Coefficients of Hydrogen Isotopes
Authors: Amdur I. , Beatty J. W. 
Published: January 2005

Binary mutual diffusion coefficients have been measured by a radioactive tracer method for the following systems: H2−T2 and D2−T2 at 195°, 273°, and 353°K; H2–TH and D2−TH, at 195° and 273°K.
The ratios of the normalized coefficients for H2−T2 relative to H2–TH and for D2−T2 relative to D2−TH show the same trend as that previously reported for He–T2 relative to He–TH. In the present case both ratios are slightly greater than unity at 195° and slightly less than unity at 273°K. A larger isotope effect, produced by the substitution of D2 for H2, is reflected in the normalized coefficients for D2−T2 relative to H2−T2 and for D2−TH relative to H2–TH. The average values of these ratios are about 2% greater than unity at the lowest temperature and about 5% greater than unity at the higher temperatures.
Energy-Transfer Processes in Monochromatically Excited Iodine Molecules. I. Experimental Results
Authors: Steinfeld Jeffrey I. , Klemperer William 
Published: January 2005

The fluorescence excited in iodine by the green emission line of mercury, in the presence of foreign gases, has been examined by high-resolution photoelectric photometry. Cross sections for quenching, vibrational, and rotational energy transfer for the v′ = 25, J′ = 34 level of the B 3Π0u+ state of iodine have been obtained, for collisions with 3He, 4He, Ne, Ar, Kr, Xe, H2, O2, CO2, SO2, CH3Cl, and NH3. Emission bands have been rotationally analyzed to yield partial total cross sections for the inelastic processes. These have been corrected for multiple scattering by a computer simulation procedure.
The correlation of quenching efficiency with the mass and polarizability of the collision partner, found by Brown for the quenching of the v′ = 15 level of iodine, holds for the v′ = 25 level as well. There is no contribution to quenching efficiency from a permanent electric dipole moment. The data suggest that a complex set of interactions, including electrostatic polarization, spin—orbit forces, and specific chemical effects, are responsible for quenching, indicating a multiplicity of repulsive states contributing to the induced predissociation process. The vibrational energy-transfer efficiency is maximum when the mean collision time equals the period of oscillation of the molecule, which corresponds to a collision reduced mass ≃40 for T = 370°K, v′ = 25 of I2; vibrational transfer is largely independent of the internal structure of the collision partner. The efficiency of pure rotational energy transfer (Δv′ = 0) shows a similar smooth dependence on reduced mass of the collision system, with the exception of the molecules SO2, CO2, and CH3Cl, which are more efficient. Quantitative measurements have been made on partial components of total inelastic cross sections for ΔJ′ up to ±20 angular momentum units, with evidence for a total spread of ΔJ′ up to 30 or 40. The width of the distribution of partial cross section components decreases in the order Δv′=+2&gt;−2≳+1&gt;−1≫0; also, the ΔJ′ distribution is broader for collisions with heavier particles.
Mass-Spectral Fragmentation of Phenyl-n-Alkanes
Authors: King A. Bruce 
Published: January 2005

The fragmentation mechanisms for the 1-phenyl and 2-phenyl alkanes were found to differ from those having substituted two or more carbon atoms from the terminal carbon atom. The latter series of compounds have two competing primary fragmentation reactions involving loss of either alkyl group. The loss of the smaller alkyl group has the lower appearance potential, but the competing reaction to lose the larger alkyl group increases in yield more rapidly at the higher electron energies, and it has greater intensity at 70 eV. The ratio of intensities of these two ions correlates with the relative position of the phenyl group from the center of the chain for phenyl alkanes of different carbon numbers. Subsequent decomposition of these primary ions is by successive loss of olefins of C3 through C6.
The 2-phenyl compounds differ in that subsequent skeletal decomposition of the CH3C+HC6H5 is less probable, thus making this ion the major ion instead of C7H7+. The 1-phenyl alkanes decompose directly to C7H7+ for the lower-carbon-number compounds, while the larger molecules form toluene ion followed by hydrogen-atom loss to C7H7+ and other fragmentation reactions. Partial isomerization to the 4-phenyl compounds is suggested.
Attempts to explain the higher yield of the higher-energy primary path for the internally substituted compounds within the framework of current theoretical models were unsuccessful. It is suggested that the usual method of applying the quasiequilibrium assumption is not valid for molecules of this size and degree of localization of charge and energy.
Adsorption of Potassium on Tungsten
Authors: Schmidt Lanny , Gomer Robert 
Published: January 2005

The adsorption of potassium on thermally annealed and field-evaporated tungsten surfaces was studied by field emission. The φ̄- vs-θ̄ curve has a minimum of 1.78 eV at θ̄=0.83, n̄=3.2×1014 atom/cm2 (which coincides with the minimum of φ110), and approaches φK at θ̄=2–3. Distinct minima also occur for other close-packed planes, like 211. At θ̄=1, n̄=3.9×1014 atom/cm2, φ̄=1.95 eV the emission pattern is indistinguishable from that of clean tungsten. The average charge per K atom, q̄, varies from 0.27 at low θ to 0.2 electron charges at θ̄=0.8. The value of q on the close-packed W planes at low coverage may be considerably higher.
The activation energy of neutral desorption varied from 2.67 to 2.4 eV over the interval 0&lt;θ̄&lt;0.4 and fell rapidly at higher coverage, approaching the heat of sublimation of K, 1.0 eV, at θ̄=1.0. The activation energy of ionic desorption was measured over the interval 0&lt;θ̄≤0.07, and indicates that ionic desorption occurs on regions of φ=5.3 eV. dφ/dθ̄ on these regions is very high, suggesting that the K-atom density there exceeds the average by a factor of 3 or more, and that adsorption on these regions may be nearly ionic at low θ̄.
The activation energy of surface diffusion was measured quasidifferentially as a function of θ̄. Ediff and the pre-exponential term of the diffusion coefficient increased with θ̄ in the range 0&lt;θ̄≤0.6–0.7, Ediff rising from 0.3 eV at θ̄=0 to 0.8 eV at θ̄=0.6. This is tentatively explained in terms of activated hopping into the almost empty second layer, followed by rapid diffusion, terminated by recombination of the excited atom with a hole in the first layer. At θ̄=0.8–1.0, Ediff falls sharply, corresponding to diffusion in the second layer.
It is suggested that K adsorption sites are relatively weakly structure dependent, so that a displacement of atoms from the initially occupied, optimal sites occurs at θ&gt;∼0.4 in order to accommodate more adsorbate in some direct contact with the substrate. The experimental results are shown to be consistent with a model based on delocalized bonding, i.e., metallic adsorption.
Relations between Raman Spectroscopy and Nuclear Spin Relaxation
Authors: Gordon R. G. 
Published: January 2005

Raman spectra and nuclear spin relaxation experiments are both affected by molecular rotation. The information about molecular reorientation furnished by these experiments is shown to be related under certain assumptions, most important of which are the following: (1) The substance is a molecular liquid or gas. (2) The molecules have either linear, symmetric-top, or spherical-top symmetry. (3) The spin relaxation is controlled by the intramolecular electric-quadrupole interaction or intramolecular nuclear magnetic dipole interactions. The relations are independent of the usual crude dynamical models for molecular reorientation, and hold for both classical and quantum systems. For liquid nitrogen, the Raman and spin relaxation experiments are shown to be in satisfactory agreement.
Raman experiments should be of value in interpreting spin relaxation experiments in simple molecules, since Fourier transformation of a Raman spectrum furnishes the detailed time dependence of a rotational correlation function, whereas spin experiments observe only the area under such a correlation function.
Paramagnetic Resonance Absorption of Copper Ions in Porous Crystals
Authors: Nicula A. , Stamires D. , Turkevich J. 
Published: January 2005

The EPR absorption of Cu II ions in porous crystals was studied. The effect of Cu concentration, adsorbed phases, and structure of the crystal on the EPR of Cu+ + ions is discussed in detail.
For small Cu II concentrations, the EPR spectrum obtained is in accord with the theory of Sands for polycrystalline Cu-containing solids. Further, by choosing the appropriate crystal type and physical state it was possible to resolve the hyperfine structure due to the Cu nuclear moment, both in the g∥ and g⊥ bands, and thus obtain values for A and B. Our data fit the following Hamiltonian: H=β0[g‖HzSz+g⊥(HxSx+HySy)]+A SzIz+B(SxIx+SyIy).Using the analysis of Maki and McGarvey, we obtained values of α, β1, β and ``K'' which characterized the type of bonding of Cu II ions in the crystal lattice.
From the variation of A, g, β, and ``K0'' parameters, the mechanism and process of adsorption of water on these crystals were ascertained. Some results obtained on Cu II containing hydrated crystals are compared to some Cu II ions in different solutions.
Gas-Phase Photolysis of Cyclohexane in the Far Ultraviolet: Modes of Decomposition of the Neutral Excited Cyclohexane Molecule and Reactions of the Parent Cyclohexane Ion
Authors: Doepker R. D. , Ausloos P. 
Published: January 2005

The photolysis of cyclohexane—cyclohexane-d12 (1:1) mixtures and of cyclohexane-1,1,2,2,3,3-d6 was investigated using xenon (8.4 eV) and krypton (10, 10.6 eV) resonance radiation. It is demonstrated that the formation of hydrogen by molecular detachment is the major primary process cyclo−C6H12*→C6H10+H2. In this process, the hydrogen is mainly eliminated from a single carbon atom. The resulting C6H10 diradical rearranges mainly to cyclohexene, a fraction of which decomposes to form 1,3-butadiene and ethylene. It is shown that, because of collisional deactivation, the probability of the latter mode of decomposition decreases with an increase in pressure or an increase in wavelength. Other primary processes such as cyclo−C6H12*→3C2H4 and, to a lesser extent, cyclo−C6H12*→2C3H6 as well as processes leading to the formation of H atoms and CH3 radicals are also shown to occur.
In the photolysis at 1236 and 1165 Å, C6H12+ ions are formed which undergo H2-transfer reactions such as C6H12++CD3CDCD2→C6H10++CD3CDHCD2HC6H12++(CD2)3→C6H10++CD2HCD2CD2H. The efficiency of these ion—molecule reactions does not depend on the concentration of C3D6 or oxygen, but is reduced by NO because of the occurrence of a charge transfer from the cyclohexane ion to NO. The same processes have been reported to occur in the gas-phase photolysis of similar hydrocarbon mixtures.
Spectral Intensities of the Trivalent Lanthanides and Actinides in Solution. I. Pr3+, Nd3+, Er3+, Tm3+, and Yb3+
Authors: Carnall W. T. , Fields P. R. , Wybourne B. G. 
Published: January 2005

We have correlated the experimentally measured intensities of the solution absorption spectra of Pr3+, Nd3+, Er3+, Tm3+, and Yb3+ in three different solvents with a theoretical expression derived by Judd. The expression, P=Σ lim λTλσ(fNψJ‖∪(λ)‖fNψJ′′)2λ=2,4,6is derived from a theory in which the ligand-field interactions with the central 3+ ion cause a mixing of higher configurations of opposite parity into the fN configuration and gives rise to induced electric-dipole transitions. A prerequisite for the intensity calculations is the computation of complete intermediate coupling eigenvectors for the energy levels investigated. Other possible mechanisms such as magnetic-dipole and electric-quadrupole transitions are considered. The oscillator strengths of important magnetic-dipole transitions are tabulated.
The results give the first experimental verification of the ability of the theory to account for large changes in certain bands of the same lanthanide observed in different solvents. Conclusions are drawn respecting the Tλ's obtained in a given medium for all the lanthanides studied, and also for those obtained for a given lanthanide in different media. The J dependence of the Tλ's is removed to facilitate their intercomparison, and the resulting parameters are tabulated. The Tλ's for Yb3+ are obtained by an extrapolation of the values computed for Tm3+ and Er3+. The nature of the model required by the theory is discussed.
Polymer NMR Spectroscopy. XI. Polystyrene and Polystyrene Model Compounds
Authors: Bovey F. A. , Hood F. P. , Anderson E. W. , Snyder L. C. 
Published: January 2005

The high-resolution NMR spectra of isotactic and ``atactic'' polystyrene in tetrachloroethylene and o-dichlorobenzene are compared to the spectra of the model compounds, meso- and dl-2,4-diphenylpentane. Analysis shows that despite marked differences in the appearance of the polymer and model spectra—particularly in the β-methylene region—isotactic polystyrene has the same local conformation as the meso pentane. This conformation is a segment of the 31 helical conformation which isotactic polystyrene is reported to have in the crystalline state. It is probable that this conformation is quite strongly preferred for all isotactic vinyl polymers. This does not necessarily mean, however, that such polymers exist as 31 helices in solution (although polystyrene probably does) because the chains may deviate widely from a 31 helix by undergoing mirror inversions of the local conformation with little net expenditure of energy. Thus, NMR does not at present allow us to make any larger statement concerning the over-all polymer conformation (e.g., the end-to-end distance) in solution.
For the dl-pentane, two conformations are present, their proportions varying with temperature. This finding cannot, however, be readily related to the conformation of syndiotactic polystyrene (or of the syndiotactic sequences in ``atactic'' polystyrene) because the NMR spectrum of ``atactic'' polystyrene, unlike that of the isotactic, is too poorly resolved to give the necessary information. It appears that there may be other, more favorable cases for which this can be done.
Production of Aluminum Hydride by Hydrogen-Ion Bombardment
Authors: Appel M. , Frankel J. P. 
Published: January 2005

An ultrapure aluminum target specimen was bombarded with 12.5-MeV deuterons for 300 μA·h. The specimen was sufficiently thick so that all the hydrogen was trapped within it. At no time did the temperature exceed 300°F.
X-ray diffraction studies of the specimen indicated the formation of a new phase with the hexagonal structure predicted for aluminum hydride and d spacings which coincide with those given for aluminum hydride in the literature. Least-squares analysis of 21 diffraction peaks shows a hexagonal structure with c/a=1.57±0.05, c=4.55±0.08 Å, and a=2.90±0.05 Å.
Crossed-Beam Investigation of N2D+ Production in N2+—D2 Collisions
Authors: Turner B. R. , Fineman M. A. , Stebbings R. F. 
Published: January 2005

The reaction N2++D2→N2D++D has been studied in a crossed-beam experiment. The angular distribution of the secondary ions was determined with a movable quadrupole mass filter. Differential cross sections have been obtained within the energy range 7.5 to 57.5 eV.
Total cross sections for this reaction have been determined using another crossed-beam apparatus in which the product ions are collected in toto. Total cross sections were also derived from the angular measurements, but less reliance is placed in these values.
Orbital Radii of Atoms and Ions
Authors: Waber J. T. , Cromer Don T. 
Published: January 2005

Radii corresponding to the principal maxima in the radial distribution functions r2ψi2(r) have been obtained for the ground states of all the atoms in the periodic table. The relativistic wavefunctions employed were solutions of the Dirac equations. Slater's ρ⅓ method for including exchange and Latter's method for the self-interaction correction were used in ``creating'' the potential.
The radii corresponding to these maxima are used as measures of the size of each orbital. The relation between the orbital radii of the valence electrons is shown as plots for the first, second, and third series of transition elements.
In the elements below nobelium, only four maxima occur in the total charge-density function. The relation of these maxima to the shell structure of the atom and the effect of shielding by outer electrons are discussed. The effect of the relativistic contraction on orbitals of low angular momentum is shown.
Matrix Elements for Vibration—Rotation Transitions in the HBr Overtone and Hot Bands
Authors: Babrov Harold J. , Shabott Abraham L. , Rao B. Sesh 
Published: January 2005

The strengths of the vibration—rotation lines of the first overtone, first hot band, and fundamental band of HBr have been measured by a curve of growth method, applying a correction for two overlapping lines devised by Sakai. The squares of the electric-dipole matrix elements |M02(m) |2 and |M12(m) |2 for the lines have been calculated, and have been fitted, respectively, to a cubic and quadratic polynomial in m, using the method of least squares. The experimental results for all three bands are compared with the theory of Herman and Wallis. The fact that the experimental |M02(0) |2 is less than that calculated from a linear dipole-moment function clearly requires a positive value of M2, the second derivative of the dipole-moment function.
The squares of the matrix elements |M01(0) |2 and |M02(0) |2 are used to calculate M1 and M2, the dipole-moment coefficients, for Morse and anharmonic oscillators. Of all the possible sets of M1 and M2 obtained in each case, the one giving results in better agreement with |M12(0) |2 is chosen. The chosen values of the dipole-moment coefficients are M1=+4.56×10−11 esu and M2=+0.69×10−3 esu cm−1 for the Morse oscillator and M1=+4.63×10−11 esu and M2=−0.70×10−3 esu cm−1 for the anharmonic oscillator. Since the sign of M2 is positive, the Morse oscillator results are preferred.