In these experiments we used time-of-flight energy analysis to determine the kinetic processes occurring at laser damage on surfaces. A triaxial charge collector of relatively large area was designed for use in the target vacuum chamber. It could be biased in several ways to detect either positive or negative charge, and it could resolve charged particle time-of-flight over a field free region of about 6 cm. The laser system was a Q-switched Nd:YAG laser with 10 ns pulses at 1064 nm. Measurements were made on single crystal germanium wafers and on half-wave delectric thin film samples of ThF2 and MgF2 on fused silica.
On germanium, ion emission was only detected when the surface morphology indicated boiling. This occurred at about 1 J/cm2 or a little over twice the melt or damage onset fluence. For thin film samples, ion emission correlated with surface damage. For all samples, ion velocity distributions and the observed correlation between kinetic energy and ion flux were characteristic of laser heated plasma processes following charge emission.