Shank, Yen, and Hirlimann Respond:Combescot and Bok 1 have attempted to improve on the rough estimate of electron-hole plasma density given in our recent Letter 2 concerning the dynamics of the reflectivity of silicon excited with femtosecond optical pulses. Their approach is to determine a more precise effective mass in order to prescribe more accurately the density dependence of the plasma frequency and the dielectric constant. At the very high plasma density and plasma temperature encountered in this experiment this is a very complex process.Combescot and Bok use the theory of the dielectric constant and plasma frequency derived by Combescot and NoziSres 3 for germanium. Application of this theory to the case of highly excited Si is fraught with difficulty. First, while this theory may be entirely adequate for Ge at low intensities, the two-band model is inadequate to describe Si at even modest densities. The splitoff band in Si is within kT of the band edge, thus requiring a reformulation of the problem with a three-band model including the split-off band. In addition the equation for the reduced mass given by Combescot and Bok is an approximation in the high-frequency limit which does not satisfy the experimental conditions. According to Combescot and NoziSres 3 the reduced mass for holes is given by in the high-frequency approximation where (v»A = € h {m H /m L )

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We obtain gigawatt white-light continuum pulses that permit spectroscopic measurements with a time resolution of 80 fsec. These pulses extend continuously from 0.19 to 1.6 microm and have time sweeps as small as 10 fsec/1000 A. We find temporal, spatial, and spectral properties that are consistent with self-phase modulation having a prominent role in generation of the continuum.

A detailed morphological and structural analysis of CeO 2 nanoparticles has been performed using electron tomography in scanning transmission mode in high angle annular dark field. The nanoparticles have been prepared through a solvothermal synthesis assisted by microwave heating. An adequate choice of the synthesis parameters leads to particles with various well-defined morphologies: cubes, octahedrons, and nanorods. In the case of cubic CeO 2 nanoparticles, the three-dimensional analysis allowed us to precisely calculate the type and the proportion of the minor facets exposed at the nanoparticle surface. For the CeO 2 nanoparticles with an octahedron shape, it has been demonstrated that the ambiguous interpretation of the objects giving triangular views in classical transmission electron microscopy can be prevented; furthermore, precise assignments of their external shape, surface crystallography, and type of minor facets were realized. In the case of nanorods, it was shown that the external shape and the transversal symmetry are strongly dependent on the nanorod sizes. The presence of a well-defined porosity inside the rods was also evidenced thanks to the ability of the electron tomography to solve the internal structure of a nano-object.