Propagation of optical pulses in a resonantly absorbing medium is studied. Propagation time of nanosecond pulses was measured for the Rb D 1 transition. At the center of two absorption lines, delay of the pulse peak which is about ten times as large as the pulse width was observed, where zero delay is defined for the propagation with the light velocity in vacuum. On the other hand, at the peak of an absorption line, negative delay was observed for large absorption, where the advance time is as large as 25% of the pulse width. Simulation including the effect of absorption and phase shift reproduced well the experimental results.

Influence of CO 2 and H 2 O on the air oxidation reaction of Mg nanoparticles has been investigated by Mg K-edge NEXAFS technique. The water vapor in the air causes rapid progression on the air oxidation reaction of the Mg nanoparticles. The basic magnesium carbonate (4(MgCO 3).Mg(OH 2).4(H 2 O) : BMC) is formed by the long term exposure to air. The NEXAFS analysis shows an existence of several intermediates of the BMC formation reaction.

We have fabricated the Mg nanoparticles by the gas evaporation method. The AFM observation shows Mg nanoparticles fabricated under the several gas pressures of He have the average diameter of 3.6 nm to 8.0 nm. We have studied the influence of the atmospheric oxidation for the chemical state of the Mg nanoparticles by the Mg K-edge NEXAFS technique. The NEXAFS measurement shows that the chemical state of MgCO3 exists on the most lateral surface of the Mg nanoparticles and the degree of atmospheric oxidation depends on the size of Mg nanoparticles.

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