Multilayer Hf02/Si02 high reflectors (HR) and polarizers show a permanent increase in their 1(4-nm damage thresholds followinglaserconditioningatsubthreshold fluences. Threshold increasesof2-3x are typical. In an effort tobetter understand the condifioning effect we have made laser conditioning and electronic property measurements on single layers of these two materials. The laser damage threshold of 1-. un thick e-beam deposited Si02 was increased by laser conditioning for wavelengths ranging from 355 to 1064 nm. The damage threshold of 2 srnglelayers was not influenced by sub4hreshold illumination. As-deposited thin films of a-Si02 are known to contain paramagnetic electronic defects. We have used electron paramagnetic resonance (EPR) to study the concentrations and types of defects present in single layer and multilayer films of Hf02 5102. E'' and oxygen hole centers with concentrations on tI order of 107/cm3 have been measured in the Si02 layers. A previously unreported defect has been observed for Hf02. The concentrationofdefects was studied bothbeforeand afterlaserconditioningand damagewith 1064-nm photons. These electronic structure measurements are discussed in relation to an electronic defect model for laser conditioning of dielectric multilayers. 1.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.