Dry laser cleaning threshold fluence - How can it be measured accurately?

Dry/damp laser cleaning studies are reported that used a XeCl excimer laser to remove alumina particles from glass. These have compared the laser cleaning of samples with medium and low particle densities. Earlier work at Macquarie University used an UV-copper vapour laser (wavelength 255 nm, pulse length 35ns) to investigate single pulse, dry/damp laser cleaning of alumina particles from microscope slides and fused silica surfaces. Medium densities of particles were used. A threshold fluence for dry laser cleaning of 90 mJ/cm² was measured. Subsequent work using a XeCl excimer laser (wavelength 308 nm, pulse length 8 ns) is reported. The threshold fluence for single pulse laser cleaning of medium density particles increases by a factor of ∼4 at this longer wavelength. Using a sample preparation technique involving laser ablation to prepare lower particle density samples, with reduced particle agglomeration, the results appear qualitatively and quantitatively different than for samples with a medium particle density on the surface. However, careful analysis shows that the dry laser cleaning threshold is the same in each case. This supports the use of the medium particle densities as an experimentally reliable method of determining the laser cleaning threshold fluence.