Volume 58, Issue 5, 01 March 1973

Magnetic structure of NdZn_{2}
View Description Hide DescriptionNeutronpowderdiffraction measurements reveal that NdZn_{2}, having the body‐centered orthorhombic crystal structure (CeCu_{2}‐type), undergoes magnetic ordering below . The magnetic structure can be described by the composite of a sinusoidal transverse wave moment alignment polarized in the b axis direction and propagating along the c axis, and a ferromagnetic component along the b axis. The propagation wavelength and amplitude of the sinusoidal component at 4.2°K are, respectively, 2.35 in units of c and . The ferromagnetic component equals . Results of magnetization measurements are consistent with the above findings.

Production of metastable singlet O_{2} photosensitized by NO_{2}
View Description Hide DescriptionEnergy transfer from electronically excited NO_{2} to O_{2} has been studied. The is produced by irradiation with light having . Observations of light emission from , and have been made, and from these measurements at various pressures, rates have been measured for production of and by energy transfer from , for quenching of by O_{2}, NO_{2} and wall collisions, and for quenching of by NO_{2}. The efficiency for production is , per collision quenching , and for production, . The production efficiency reported here is considerably greater than our preliminary value of 1%. The metastable O_{2} generated in an urban atmosphere by energy transfer from is as effective as ambient O(^{3} P) in initiating the oxidation of alkylated aromatic hydrocarbons and 0.1% or less as effective in the case of small, unsubstituted olefins.

Vibrational relaxation of HCl with inert gas collision partners
View Description Hide DescriptionThe vibrational relaxation of HCl has been measured behind incident shock waves in the presence of He, Ne, Ar, and Kr over a temperature range of 800–4100°K and a pressure range of 10–400 torr. The data show a decrease in the efficiency of energy transfer with increasing reduced mass. This tendency appears to be much stronger than predicted by present theories even when the reduced mass is assumed to be that for H atom‐inert gas collisions rather than HCl‐inert gas.

Temperature dependence of electron attachment at low energies for polyatomic molecules
View Description Hide DescriptionThe cross sections for production of negative ions by attachment of low‐energy (<0.2 eV) electrons to selected polyatomic (SF_{6}, CCl_{4}, CFCl_{3}, CH_{2}Br_{2}, CH_{3}I, CHCl_{3}, CF_{3}Br, CH_{3}Br) molecules have been determined as a function of target gas temperature in the range from 300 to 1200°K. For the case of SF_{6} it is found that the total cross section for negative ion production is independent of gas temperature, although it is known from the experiments of Chen and Chantry that the relative abundances of different species of negative ions produced from SF_{6} are strongly temperature dependent. In contrast to SF_{6}, the total cross sections for negative ion production in some halogenated hydrocarbons are found to be strongly temperature dependent. It is found that the total cross section for negative ion production in many polyatomic molecules tends towards an absolute maximum as the gas temperature is increased.

Deuterium isotope exchange between carboxylic acids and hydrogen
View Description Hide DescriptionThe catalytic exchange of deuterium between carboxylic acids and hydrogen according to , where R=H or CH_{3}, has been studied with the acids in the pure state at various temperatures, and in solutions of carbon tetrachloride at 25°C. The hydroxyl hydrogen of the acids is about 3–4 times more enriched in deuterium than the hydrogen gas. The measured isotope effects are discussed in terms of molecular force field calculations for formic and acetic acid.

Penning ionization optical spectroscopy: Metastable helium (He 2 ^{3} S) atoms with nitrogen, carbon monoxide, oxygen, hydrogen chloride, hydrogen bromide, and chlorine
View Description Hide DescriptionThe generation of triplet metastable helium atoms (19.8 eV) by a hollow cathodedischarge coupled to a flow apparatus is described. With this apparatus the Penning ionizationreactions of He(2 ^{3} S) with six diatomic molecules were investigated at pressures of 1–5 torr of helium and less than 1 mtorr of reagent at 300, 225, and 77°K. The reactions were monitored by analysis of the visible and ultraviolet emission spectra. The following band systems were observed: and . For the last four cases atomic emissions were observed indicating the presence of reaction channels in competition with Penning ionization. The vibrational populations of and were not in accord with populations expected for vertical transitions from the molecular ground state. However, the , and vibrational populations were very similar to those expected for a vertical ionization. Rotational populations of and were found to be very similar to those of the neutral precursor molecule, although at 77°K a non‐Boltzmann distribution was observed for high N′ states of . The results are discussed in terms of the two state potential model of Penning ionization.

On the variation principle in electron‐atom scattering
View Description Hide DescriptionThe variation principle in electron‐atom scattering is analyzed in an attempt to explain the difficulties encountered in even the simplest e‐H expansion type calculations. Different variational methods are compared and an approach is introduced which will eliminate the occurrence of spurious singularities found in other methods. This method is applicable to both elastic and inelastic collisions.

Annihilation of positrons in hydrocarbons and halogenated hydrocarbons adsorbed on silica gel
View Description Hide DescriptionPositron lifetime spectra of a series of hydrocarbons and halogenated aromatic compounds adsorbed on silicagel have been studied as a function of adsorbate concentration. The fact that the anomolous long lifetime component found in pure silicagel is not affected by adsorbate concentration up to 1.5–1.7 mmole/g SiO_{2} indicates that thermalized positronium cannot react chemically with these compounds. The decrease of the intensity of the long lifetime component with the adsorbate concentration can be interpreted in terms of positron capture by these compounds or by chemical reactions of highly energetic Ps atoms with the adsorbate molecules.

On the Mott‐Cabrera oxidation rate equation and the inverse‐logarithmic law
View Description Hide DescriptionIt is shown that the often cited inverse‐logarithmic oxidation law, , where X is the oxide thickness and t the observation time, is not an asymptotic solution of the rate equation derived by Mott and Cabrera in their theory of low temperature oxidation. Thus whether or not the inverse‐logarithmic law is experimentally verified has no bearing on the validity of this theory. A correct solution is presented, and it is shown that for thin oxide films a plot of X ^{−1} vs should yield straight lines. Vermilyea's data on Ta is reexamined according to this analysis and yields reasonable results for the activation energy of defect solution (1.59 eV) and for the potential across the oxide (1.79 V).

and states of manganese hydride
View Description Hide DescriptionAb initio self‐consistent‐field calculations have been carried out close to the Hartree‐Fock limit for the high spin and electronic states of MnH. Five different hydrogen‐centered basis sets of Slater functions are tested for H^{−} and MnH. Predicted spectroscopic constants (with experimental values in parentheses) for the ground state are . And for the state, . The electronic structure of the two states is discussed with respect to the contributions of the different molecular orbitals to the electric dipole moment and population analysis. Although the population analysis predicts the state to be more ionic than the state, the dipole moments are computed to be 1.6 and 4.1 D, both +MnH−. The small dipole moment of the state is seen to be due to a large −MnH+ contribution from the nonbonding 8σ orbital (which is replaced by 4π in the state). The manganese atom 3d ^{5} configuration survives essentially unchanged in both states of MnH, and it is suggested that the electronic structure of other transition metal hydrides may be quite similar, involving a single ionic bonding σ orbital.

Vibrational intensities of F_{2}CO, Cl_{2}CO, and Br_{2}CO
View Description Hide DescriptionThe complete neglect of differential overlap (CNDO) approximate wavefunctions for Cl_{2}CO have been applied to the calculation of the derivatives of the dipole moment with respect to the symmetry coordinates. Although it was not possible to choose unique sets of derivatives in the A _{1} and B _{1} species, the acceptable experimental derivatives are very close in magnitude permitting a comparison and interpretation of these derivatives with those found previously for F_{2}CO. Differences in the derivatives of F_{2}CO and Cl_{2}CO are used to predict the preferred experimental derivatives for Br_{2}CO. The effect of d orbitals in the calculation of the Cl_{2}CO derivatives is examined. The experimental results for the out‐of‐plane CCl and CBr bends indicate that these derivatives are positive and opposite in sign to the in‐plane derivatives, although the Cl_{2}CO out‐of‐plane bending derivative is very close to zero.

Vibrational intensities of F_{2}CS and Cl_{2}CS
View Description Hide DescriptionThe CNDO approximate molecular wavefunctions for F_{2}CS and Cl_{2}CS have been applied to the calculation of the derivatives of the dipole moment with respect to the symmetry coordinates. Unique sets of experimental derivatives in the A _{1} symmetry species of these molecules are definitely preferred according to these calculations. Two experimental sets of derivatives in the B _{1} symmetry species of each molecule are consistent with the calculated results. All the preferred experimental derivatives for the stretching coordinates, and , and the in‐plane bending coordinate, , have negative signs. The CF out‐of‐plane bending derivative in F_{2}CS appears to be negative although the corresponding CCl derivative is probably positive. The theory qualitatively reflects trends in the magnitudes of these derivatives in the different X_{2}CY molecules and in different symmetry species of the same molecule. Empirical equations of the type , relating corresponding derivatives for F_{2}CO, Cl_{2}CO, F_{2}CS, and Cl_{2}CS, are discussed.

Ultrasonic investigation of the nematic‐isotropic phase transition in MBBA
View Description Hide DescriptionAn ultrasonic investigation of the relaxation processes associated with the nematic‐isotropic phase transition has been carried out on the liquid crystalp‐methoxybenzylidine‐p‐n‐butylaniline (MBBA). Simultaneous measurements of the sound velocity and absorption in samples of MBBA without any preferred orientation have been made at eight frequencies from 0.3 to 23 MHz. The temperature range from 23 to 77°C was studied, with special attention given to the region near the transition at . Far from the transition the absorption and velocity dispersion can be well described by a single relaxation process for which the relaxation time and the relaxation strength both increase substantially as the transition is approached. In the immediate vicinity of the transition the acoustic properties are characteristic of a multiply relaxing fluid.

DW‐variational study of collinear atom‐diatom collisions
View Description Hide DescriptionSchwinger's variational principle in its fractional form is applied to the study of vibrational excitations in collinear atom‐harmonic oscillatorcollisions. Trial wavefunctions used are those of distorted waves derived from distortion potentials based on (a) a free molecule approximation, (b) an internal average, and (c) an optical potential recently proposed by Roberts. It is found that for nearest neighbor transitions in most of the systems studied a simple variational treatment yields transition probabilities superior to those of a perturbation expansion such as the distorted‐wave Born series.

Tables of collision integrals for the Barker‐Pompe, Barker‐Fisher‐Watts, and Parson‐Siska‐Lee inert gas potentials
View Description Hide DescriptionThis paper presents tables of collision integrals , collision integral ratios, higher order correction factors for viscosity,thermal conductivity and diffusion, and the isotopic thermal diffusion factor for the Barker‐Pompe, Barker‐Fisher‐Watts, and Parson‐Siska‐Lee potentials. These potentials could be quite accurate in predicting dilute gas transport properties of Ar, Kr, Xe, and Ne.

Generalized van der Waals theories for surface tension and interfacial width
View Description Hide DescriptionAn exact relation between surface tension and the direct correlation function (DCF) for the inhomogeneous region is developed which may be viewed as an exact formulation of the van der Waals density gradient theory for inhomogeneous fluids. A reasonable approximation to the DCF is employed to obtain characteristic features of the interfacial density gradient over an extended temperature range. This theory predicts that the interfacial width increases as at the ``low'' temperatures considered, and these results are in good agreement with the corresponding predictions of our earlier capillary wave treatment of interfaces. Comparison is also made with the predictions of an alternate, exact van der Waals‐like formulation which is obtained from the expression for surface tension in terms of the pair number density and the intermolecular force. A combination of the DCF formulation and the capillary wavetheory provides the possibility of predicting both interfacial thicknesses and surface tension, a route also available from pair capillary wavetheory. Numerical examples corresponding to argon at 90°K are given.

Accurate ab initio calculation of the BeH molecule. I. The and states
View Description Hide DescriptionResults of calculations using a configuration interaction LCAO‐MO method are presented for the and states of BeH. Apart from the 1s shell of Be, the correlation energy of the remaining three electrons is calculated quantitatively using an extended basis set and complete configuration interaction including all single, double and triple replacements. The calculated spectroscopic constants are in excellent agreement with available experimental data. The present calculations also predict a value of , and dismiss the possibility of a potential maximum in the ground state potential energy curve.

Wide‐line nuclear magnetic resonance study of solid methanols
View Description Hide DescriptionA protonmagnetic resonance study of dry CH_{3}OH and CH_{3}OD between 1.8°K and the melting points shows rotation of the CH_{3} group over the complete temperature range, a well‐defined decrease in line‐width at the temperatures of the monoclinic to orthorhombic transformation, and absence of the narrow line found in earlier NMR studies. Inconsistencies between previous NMR results are ascribed to the effects of saturation, inadequate thermal conditioning, and the presence of water. Second moments of the monoclinic form show adjacent CH_{3} groups to be further apart than given by Tauer and Lipscomb's x‐ray structure. Second moments of the orthorhombic form agree with dielectric results in showing large‐amplitude rotatory motion. The ``second transition'' found in earlier volumetric and dielectric studies of CH_{3}OH and CH_{3}OD probably marks the eutectic temperature where monoclinic methanol, methanol monohydrate, and liquid phase coexist. The second moment of CD_{3}OH is almost invariant between 4 and 155°K although there is a significant change in line shape.

Ionic soft sphere parameters from Hartree‐Fock charge densities
View Description Hide DescriptionSoft sphere parameters (R, ρ) that describe the charge density ρ_{ c } in the outer region of an ion through the expression are obtained for 32 ions from charge densities given by good Hartree‐Fock (HF) calculations. These are compared to those previously obtained for 178 closed shell and transition ions from Hartree‐Fock Slater (HFS) charge densities. Systematic differences are noted, and it is recommended that the soft sphere parameters from HF calculations be used for the halide ions and noble gas atoms and those from HFS for the remaining ions.

Trajectory studies of atomic recombination reactions. III. Energy transfer in bromine‐inert gas systems
View Description Hide DescriptionEnergy transfer in collisions of helium, argon, and xenon atoms with highly vibrationally excited bromine molecules is studied by computing 3D classical trajectories. Dissociation probabilities P _{diss} are obtained as a function of the energy barrier to be overcome. They vary approximately as , and where μ is the reduced collision mass. Mean vibrational and rotational energy changes are calculated for systems in which the bromine molecules are initially in thermal rotational equilibrium but have varying degrees of vibrational excitation. The relative importance of vibration‐rotation coupling at different vibrational energies is demonstrated. The order of collision numbers for rotational relaxation is He>Ar>Xe. The relative efficiencies of He, Ar, and Xe atoms in producing vibrational relaxation of Br_{2} vary with the initial degree of vibrational excitation.