Using a transient-grating diffraction technique, we have compared the ultrafast dynamics of the linear reflectivity from a GaAs surface at the fundamental and second-harmonic frequencies with the dynamics of the second-harmonic generation (SHG) in refiection at an excitation level exceeding the melting threshold. It is shown that the ultrafast (within 100 fs) drop in the SHG efficiency cannot be accounted for by changes in the linear dielectric susceptibility that take place on a longer time scale. This fact indicates a fast change of the long-range crystalline symmetry of GaAs within a semiconductorlike phase, preceding a transition to the metallic phase as the crystal is melted.

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An abrupt (less than 100 fs) decrease in the second-harmonic intensity reflected from the surface of a GaAs (110) wafer has been observed experimentally. The linear reflectivity was found to increase on a time scale of ~1 ps. Thus the concept of fast atomic disorder induced by electronic excitation within a relatively cold lattice is given new experimental support.

Between January and December 1989, 123 patients with distal ureteral stones were treated with in situ extracorporeal Shockwave lithotripsy (ESWL) using a modified HM3 Dornier lithotriptor. One hundred and three patients (83.7%) were stone-free, 20 patients (16.3%) had small residual stones ( < 1 mm²) that passed spontaneously. In situ ESWL treatment alone was successful in 88 of the 103 stone-free patients (85.4%). Fifty-six (54.4%) needed 1 ESWL session, 26 (25.2%) needed 2 ESWL sessions. Nineteen of the 123 patients (15.4%) needed auxiliary endourological measures. The results of ESWL treatment were correlated to pretherapeutically identified parameters such as stone size, radiopacity, outer contour, shape, inner structure, biochemical analysis of the stones and grade of dilatation of the upper urinary tract. The results prove that size ( > 75 mm²), radiopacity and grade of dilatation have a direct correlation to the difficulty to disintegrate ureteral stones.

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