B. K. Annis

Hydration of the Dy3+ ion in dysprosium chloride solutions determined by neutron diffractiona)

The coordination of water molecules about the Dy3+ ion in a 2.38 m DyCl3 in D2O solution has been determined by neutron scattering. The information was obtained using samples that were identical except for the isotopic species of the Dy3+ ions. The experiment yields the distribution of the deuterium and oxygen atoms in the first hydration shell. Each Dy3+ ion is surrounded by 7.4±0.5 water molecules. The dysprosium–oxygen separation is 2.37 A and the dysprosium–deuterium separation is 3.04 A. The cation–water molecule orientation is closer to planar than that which has generally been observed in other aqueous solutions.

Thermal Creep in Gases

The development of a pressure gradient in a gas‐filled capillary due to the application of a temperature gradient is known as thermal transpiration. In the near‐continuum limit (small Knudsen number) the mechanism for the transpiration effect is a creeping motion of the gas in a thin layer adjacent to the surface. Measurements performed in the near‐continuum limit are compared with a number of theoretical calculations for the thermally induced creep velocity. The experimental data for He, Ne, Ar, Kr, and N2 agree best with the recent theoretical work of Loyalka and indicate little or no dependence on gas type, in contrast with another recent theory which indicates a marked dependence on the thermal accommodation coefficients. In addition, the effect of the thermal creep coefficient upon the evaluation of rotational collision numbers from thermal transpiration measurements is discussed.

Rotational Collision Numbers of N2, O2, CO, and CO2 from Thermal Transpiration Measurements

Thermal transpiration results which were determined at an (assumed) characteristic temperature of 504°K are reported for all of the noble gases and for N2, O2, CO, and CO2. The helium data are found to be anomalous when viewed in terms of the dusty‐gas model for thermal transpiration. Application of the model in the zeroth differential approximation yields the following rotational collision numbers for the polyatomic molecules: N2, 6.0; O2, 2.2; CO, 1.3; CO2, 1.9.

Temperature Dependence of Rotational Collision Numbers from Thermal Transpiration

The temperature dependence of the rotational collision numbers for O2, N2, CO, and CO2 was investigated in the range 475–676°K by the thermal transpiration technique with the use of a novel apparatus design. In all cases the collision number was found to increase with temperature. For O2 and N2, where some comparison with theory is possible, the experimental rate of increase with temperature was found to be faster than the predicted behavior. The results for CO and CO2 are in general agreement with other transpiration measurements and with acoustic absorption results which were obtained at lower temperatures.

Molecular beam study of the reactions of K and Cs with UF6, WF6, MoF6, TeF6, and SeF6

The reactions of K and Cs with UF6, WF6, MoF6, TeF6, SeF6, and SF6 at thermal energies have been investigated using crossed molecular beams. Except for SF6, these hexafluorides have remarkably high electron affinities. In fact, the electron affinities of MoF6, WF6, and UF6 exceed the ionization potentials of K and Cs so that spontaneous ion pair formation is energetically possible. In no case was a significant ion current observed. Instead, exchange reactions to form MF (and possibly MAF5) are observed. The measured product angular distributions were in all cases found to be consistent with the model of complex formation. However, substantial differences in the nature of the complex disintegration were noted.

Ion pair formation and atom abstraction in collisions of Cs and UF6

The ion pair formation reaction Cs+UF6→Cs++UF6− and the atom abstraction reaction Cs+UF6→CsF+UF5 have been studied over the relative kinetic energy range 0.2 to 1.3 eV using a seeded nozzle source for UF6. The cross section for ion pair formation was found to increase with the kinetic energy and a value of 32 A2 was observed at 1.3 eV. Unlike previous results at thermal energies, the angular distributions of CsF provided no indication of complex formation but were found to be peaked in the backward (c.m.) direction. Estimates of the abstraction reaction cross section show it to be the dominant channel in this energy range.

Collisional decomposition of UF−6

Absolute cross sections for the collisional decomposition of UF−6 into its three lowest asymptotic channels in collisions with the rare gases have been measured for collision energies ranging from below the threshold for decomposition up to a laboratory collision energy of 500 eV. The product velocity spectra have also been measured for one of the decomposition channels at the highest collision energy. The experimental results are found to be consistent with the predictions of a two‐step collision model where the unimolecular decomposition of excited UF−6 ions is described in a statistical framework.

Composition Dependence of the Thermal‐Diffusion Factor of a Dusty Gas

The composition dependence of the thermal‐diffusion factor αT of a dust–gas mixture is derived. In the case of monatomic gas, (αT)−1 is shown to vary linearly with mole fraction, but nonlinear contributions arise in the case of a polyatomic gas as a result of inelastic gas–gas collisions. Application of the result to a description of thermal transpiration in terms of the “dusty‐gas” model yields a correction to the previously derived equation for thermal transpiration and may have a significant effect on the determination of rotational collision numbers from thermal transpiration measurements.

Neutron scattering from solutions: the hydration of lanthanide and actinide ions

The neutron scattering difference method is described and applied to investigations of the aqua rare-earth ions, Nd/sup 3 +/ and Dy/sup 3 +/. Metal-water distances and hydration numbers have been unambiguously determined for these ions inner coordination spheres. The values of the hydration number, n, of 8.5 +- 0.2 for Nd/sup 3 +/ and 7.4 +- 0.5 for Dy/sup 3 +/, directly support the claim of Spedding et al. that n decreases by one unit across the lanthanide series. The possible application of this method to actinide ions in solution is also discussed. 7 refs., 4 figs.

Collisions of UF−6 ions with Ar, Xe, SF6, and UF6

Electrostatic energy analysis has been performed on the products of collisions of up to 200 eV (lab) UF−6 ions with Ar, Xe, SF6, and UF6. Negative ions of UF6 containing internal excitation energy up to ∼5 eV have been observed and their distributions as functions of scattering angle and internal excitation recorded. The behavior of the fragment ions F− and UF−5 was investigated and found to be in accord with a simple statistical model of the collision dynamics.