Interaction of vacuum ultraviolet excimer laser radiation with fused silica. I. Positive ion emission

We report time- and mass-resolved measurements of Si+ and O+ emission from ultraviolet-grade fused silica during exposure to pulsed 157 nm excimer laser radiation at fluences below the threshold for optical breakdown. The emission intensities are increased by treatments that increase the density of surface defects, such as abrasion, and are reduced by treatments that reduce the density of surface defects, such as annealing. Ion emission is a sensitive probe of mechanical damage on silica surfaces. The mean ion kinetic energies are typically several eV: 8–9 eV for Si+ and about 4 eV for O+. Hartree–Fock studies of candidate defect sites suggest that antibonding states excited by 157 nm photons play a critical role in the release of these ions. We propose that positive ion emission from fused silica under these conditions is best explained by a hybrid mechanism involving (a) the excitation of an antibonding chemical state (Menzel–Gomer–Redhead mechanism) and (b) the acceleration of the positive ion by repulsive electrostatic forces due to the photoionization of nearby electron traps.