Propagation of femtosecond light pulses in a resonant absorption medium is studied. The propagation time of the light pulses was measured in a dye solution by the optical-Kerr-gate method. Nonadherence to the conventional group velocity domega/dk , which is defined in weak absorption region, was found. The observed wavelength dependences of the group delay and the spectral change in the anomalous dispersion region are qualitatively in good agreement with the theoretical prediction obtained from the new definitions of group velocity, which can be clearly defined even in strong absorption region.

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Adsorbed structures of barium on Si(110) surfaces have been studied by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Several structures of "10 × 1," "12 × 2," 11 × 6 and streaky 4 × n were observed by LEED at annealing temperatures of about 775°C, 800°C, 900°C and 1100°C, respectively, after a-few-monolayer deposition of Ba at room temperature. In the STM experiment periodic structures of "10 × 6" and 11 × 6, and a rearrangement of pentagon pairs which is the unit structure seen on the terrace for the clean Si(110) "16 × 2" surface, were observed.

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