The emission of ion bursts from single crystal ZnO during 248-nm irradiation

Exposing chemical-vapor-transport grown, single crystal ZnO to 300 mJ/cm$^{2}$ pulses of 248-nm radiation (KrF excimer) produces occasional bursts of ion emission. This fluence is well below the threshold for optical breakdown near 500 mJ/cm$^{2}$, which we identify by the presence of 473-nm emission due to the 5s$^{2 3}$S$_{1}$ to 4s$^{1}$4p$^{1 3}$P$_{1}$ transition of excited neutral zinc. The principal ion was identified as Zn$^{+}$ by quadrupole mass spectroscopy. These ions are often accompanied by electrons to form a tenuous plasma. The bursts are not correlated with pulse-to-pulse fluctuations in laser intensity. We attribute these bursts to the accumulation of laser-produced defects during prolonged irradiation. When the defect density reaches a critical value, a burst is observed.