Dynamical changes in the structure factor of liquid water, S(Q,t), are measured using time-resolved x-ray diffraction techniques with 100 ps resolution. On short time scales following femtosecond optical excitation, we observe temperature-induced changes associated with rearrangements of the hydrogen-bonded structure at constant volume, before the system has had time to expand. We invert this data to extract transient changes in the pair correlation function associated with isochoric heating effects, and interpret these in terms of a decrease in the local tetrahedral ordering.

X-ray diffraction topographs of extremely pure and perfect silicon single crystals are made using low-energy undulator radiation from a positron storage ring. Typical defect images observed are rather large round images having a black-white contrast and a diameter of about 40 µm. Applying the dynamical theory of x-ray diffraction, the defect contrast is explained by tensile strain in the lattice around voids close to the exit surface. This discovery of void-like microdefects explains, at least in part, the reduced density of the crystal intended to be used for a redefinition of the unit of mass, the kilogram.

No abstract

No abstract

The performance of CsI photocathodes has been characterized for use with grazing incidence soft x rays. The total electron yield and pulsed quantum efficiency of a CsI photocathode has been measured in a reflection geometry as a function of photon energy (100 eV to 1 keV), angle of incidence, and the electric field between the anode and photocathode. The total electron yield and pulsed quantum efficiency increase as the x-ray penetration depth approaches the secondary electron escape depth. Unit quantum efficiency in a grazing incidence geometry is demonstrated. A weak electric-field dependence is observed for the total yield measurements; while no significant dependence is found for the pulsed quantum efficiency. The effect of the pulse height distribution on the detective quantum efficiency is discussed. Theoretical predictions agree accurately with experiment.