We have carried outinsitutransient absorption and fluorescence spectroscopy measurements in two lsquo;lsquo;wetrsquo;rsquo;(OH content sim;0.1percnt;) fused silica samples (Suprasil II from Heraeus Amersil and Phyphen;30 from Shinhyphen;Etsu Quartz Product) during KrF laser irradiation. Both samples exhibit an absorption peak at 210 nm corresponding to theErsquo;center. For Suprasil II, there is also a 265 nm absorption peak, and both peaks increase with the number of irradiated pulses showing little relaxation after the laser was turned off. The region irradiated with three million pulses at 400 mJ/cm2fluence ten months ago has a residual absorption of about 10percnt;/cm at 210 nm. On the other hand, the Phyphen;30 shows a rapid increase in the 210 nm absorption in both the unirradiated and previously irradiated regions during the initial irradiation and levels off after a few thousand pulses. There is no residual absorption at the spot irradiated for 63 million pulses ten months ago. However, the initial rate of increase in the previously irradiated spot is twice as high as compared to the unirradiated spot. This suggests the density of the precursor state for theErsquo;center is higher in the previously irradiated region. The fluorescence intensity at 650 nm increases with the induced absorption for Suprasil II, but is almost independent of the number of irradiation pulses in Phyphen;30. The quasilinear repetitionhyphen;rate dependence suggests the fluorescence is transient in nature and relaxes partially between successive laser pulses.
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