No abstract

Experiments are reported which lead to the conclusion that electronic excitation energy in solid and liquid Ne, Ar, Kr, and Xe can be trapped by the formation of excimers. The consistency of this conclusion with other electronic properties of these substances is briefly discussed. Atomic lines (gas) 135 500cm-1 136320 cm-1 93 730 cm-1 95 420 cm-1 80900cm-1 84930cm-1 68000cm-1 77 190 cm-1 Atomic lines (solid) d 96 790 cm-1 99 200 cml 81 870 cm-1 87 100 cm-1 68 800 cm-1 77 590 cm-1 14 H. T. Davis, S.

X-ray diffractometer data from polycrystalline samples of alpha-oxygen frozen from the liquid are accounted for by a monoclinic structure: space group C2/m, b axis unique, a = 5.403±0.005, b = 3.429±0.003, c = 5.086±0.005 Å; β = 132.53±0.04°; volume = 69.44 Å3; calculated density = 1.530 g/cm3. The cell contains two molecules centered at the lattice points 000 and ½½0, which are symmetry centers. The atoms are at positions x, 0, z; x̄, 0, z̄; ½+x, ½, z; ½—x, ½, z̄, with x = 0.089±0.01, z=0.153±0.015. The nearest center-to-center distances between molecules is 3.20 Å and the 0–0 bonds are found to be perpendicular to the ab plane within an experimental accuracy of a few degrees. The published neutron diffraction data, interpreted on the basis of the structure, imply that the antiferromagnetic structure consists of antiparallel moments at 000 and ½½0.

Lattice constants for face-centered cubic argon determined on a precision x-ray diffractometer were found to be 5.3118 Å at both 4.2 and 15°K, 5.3187 at 21°K, 5.4033 at 64°K, with estimated errors of ±0.0005 Å, and 5.458 at 81°K. Least-squares fitting to a polynomial expression gives, for T≥21°K, a=5.29980+ 5.6389×10—4T+1.4231×10—5T2+3.552×10—8T3 (an expression not intended to be used below 21°K). Lattice constants for the close-packed hexagonal phase that sometimes coexists with the cubic may be taken as ahex=0.7071 acubic and chex=1.633ahex throughout the temperature range of solid argon. Evidence of rapid diffusion was seen near the melting point; at lower temperatures, cold work produced strain hardening. Agreement with published density measurements is good; values from some prior x-ray work lie slightly below these values. Techniques were developed which permit plastic deformation and annealing of specimens to produce fine-grained strain-free samples suited to powder diffractometry. When grain growth could not be avoided (above about 65°K) a method of single crystal diffractometry was used.

The crystal structure of α-fluorine, stable below 45.6°K, has been investigated from x-ray powder-diffraction patterns. Alpha fluorine is monoclinic, probably space group C2 / m, but space group C2 / c cannot be ruled out completely. There are four molecules per unit cell. At 23°K, the cell dimensions are: a = 5.50 ± 0.01 Å, b = 3.28 ± 0.01 Å, c = 10.01 ± 0.01 Å, β = 134.66° ± 0.02°. The unit-cell volume at 23°K is 128.4 Å3, the calculated density is 1.97 g/cm3, and the calculated molar volume is 19.3 cm3/mole. The structure is very similar to α-O2, the main difference being that the molecules are tilted, probably only by about 11° to alternate sides of the normal to the basal plane in alternate layers, leading to a doubling of the unit cell in the c direction as compared to α-O2. The fact that the unit cell contains four molecules admits the possibility of space group C2 / c with the molecules tilted out of the ac plane.