A detailed analysis has been made of atomic positional coordinates and thermal motions in a-quartz. Complete data in three dimensions were obtained at room temperature using filtered Mo K radiation; (hie0) and (00/) reflections were also measured at 223 and 155 °K. Intensities were measured with a scintillation counter. Fourier and least squares techniques were used to determine positional coordinates and the magnitude and orientation of atomic thermal ellipsoids.Data previously reported by Brill et al. (1939, 1942) have been refined by least squares methods and the new results have been found to be in good agreement with ours. The temperature dependence of the atomic coordinates and of the thermal parameters have been investigated by least squares analyses of data obtained at different temperatures and by direct comparison of electron density maps based on room and low temperature data. The applicability of the I)ebye approximation to the thermal data for a-quartz has also been investigated.Bond lengths at room temperature are: Si-O: 1.603 ± 0.003 A and 1.611 ± 0.002 /~. The Si-O-Si angle is 143.9 °. Two of the four tetrahedral O-Si-O angles are close to 109°; the others are 110 °.
Lyman-alpha production in collisions between protons and hydrogen atoms has been measured in a crossed beam experiment, within the energy range 600 eV to 30 keV. The Lyman-alpha photons result both from excitation of thermal target atoms -H++H(fs) -& H++H(2p)and from electron capture into excited states of the projectiles -H++H(1s)~H(2p)+H+. These two processes are experimentally distinguishable in terms of the Doppler shift in the capture radiation. Measurements of the angular distribution of the Lyman-alpha photons, made with an oxygen-6ltered iodine-filled counter, enable the signals from the two processes to be separately determined. Relative cross sections so obtained are normalized with the aid of earlier measurements of Lyman-alpha production in e -H(1s} collisions. The results are discussed in the light of recent theoretical developments.
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