Figure 2. Measured dependence of the absorption edge shift as a function of the delay between pump and probe. The squares show the data, the solid line shows de cross correlation between pump and probe. The absorption edge is rapidly blue shifted and returns to an absorption level that is just smaller than the initial one. As mentioned before, for negative delays no changes in the spectrum are observed, which means that the remaining shift of the absorption spectrum observed at 500 fs time delay eventually recovers fully. Apart from remaining shift at large positive delays, the profile of the shift as a function of the time delay between pump and probe pulses follows closely that of the pump pulse. This is the behavior expected for an Optical Stark shift, where only virtual excitations are created, implying a fast response time. The residual bleaching of the absorption observed at 500 fs time delay, in opposition to what is expected from the Optical Stark effect, may be attributed to the injection of real carries into the conduction band due to the occurrence of two absorption (TPA) or residual overlap between the pump spectrum and the absorption tail. The pump pulse duration is 60 fs and for this reason the Optical Stark effect does not contribute to the signal at the 500 fs delay situation. The slow recovery component in the time response of the system must be related to band filling effects. The magnitude of this contribution to the observed signal is comparatively small.