No abstract

The free fall of atoms is observed in long molecular beams of potassium and cesium atoms. The measurement of the intensity distribution in a beam deflected by gravity represents the velocity distribution of the beam atoms and permits an accurate determination of this distribution. The results show that the measured values agree in general with those calculated for a modified Maxwellian velocity distribution in the beam. At larger deflections, i.e., for slow atoms, a deficiency of intensity was observed, which increased both with increasing deflection and with increasing pressure in the oven where the beam originates. This deficiency is explained by collisions in the immediate vicinity of the oven slit.

The spectral emissivities of graphite and carbon have been determined at a wavelength of 0.653μ in the temperature range of 1285 to 2035°K and have been found to be temperature dependent in accordance with a prediction from a relation between reflectivity and resistivity. For polished graphite of high purity having a resistivity of about 1120 microhm-cm the value obtained is 0.78; the value obtained for a polished high density graphite having a resistivity of 1740 microhm-cm is 0.78; and that for polished spectroscopic carbon having a resistivity of 5080 microhm-cm is 0.79. If graphite is sublimed from a polished surface, the latter becomes roughened, so that the emissivity increases. In one particular experiment it attained a value of 0.90.

The volatility of plutonium dioxide has been measured in the temperature range 1320°C to 1790°C by a modification of the Knudsen effusion method. The plutonium dioxide was heated in vacuum in a tantalum oven. Under these conditions the dioxide was reduced to a lower oxide, or oxides, which maintained a constant reproducible vapor pressure throughout further heating. Either an oxide was formed which was stable under the conditions of the experiment or thermal reduction continued with the formation of a second solid or liquid phase. The thermal reduction of the oxide was reversed by heating the reduced oxide in a low pressure atmosphere of oxygen.

Combining rules for parameters which characterize the short-range repulsive forces between closed-shell atoms and ions are used to determine the radius and softness for a number of alkali and halide ions and rare-gas atoms from scattering and spectroscopic data. A corresponding set of charge radii and softnesses are determined from atomic charge densities calculated in both the relativistic and nonrelativistic Hartree–Fock approximations. These characteristic atomic parameters are compared, and it is found that, for species with the same charge belonging to the same column of the periodic table, the relations between charge and force radii and between charge and force softnesses are very nearly linear. The radial distance dependence of the softness is examined, and some areas where further work is needed to follow up this empirical study are indicated.