Sudden onset of strong absorption followed by forced recovery in KrF laser-irradiated fused silica

A high-purity synthetic fused silica sample (Suprasil 2) was irradiated by a KrF laser at 248 nm, 300 Hz, and 500 mJ/cm(2). Transmission at 248 nm, transmission at 210 nm, and fluorescence at 650 nm were monitored in real time. The sample starts out in a weakly absorbing state. Then, after several million pulses, it experiences a sudden increase in 248-nm absorption with accompanying dramatic changes in its relaxation and fluorescence behavior. Further irradiation leads to (partial) bleaching of the UV absorption.