Time-resolved investigation of the transient surface reflection changes of subpicosecond excimer laser ablated liquids

A single shot measurement of subpicosecond time resolution was developed to investigate the transient surface reflection changes of excimer laser ablated liquids during one UV pulse. Polysilicone oil, methyl(metacrylate), styrene, and water were irradiated by a 500 fs KrF excimer laser. The irradiated area was illuminated by an optically delayed 496 nm, 500 fs probe dye laser pulse at an incident angle of 45 degrees. As the tilted probe beam scanned along the ablating surface, the temporal increase in the reflected intensity appeared as a spatial modulation of the probe beam. The maximum reflectivities were 1.5-2.5 times higher than those of the initial samples. The reflectivity increase is caused by a plasma mirror induced by the subpicosecond excimer laser ablation. In the case of styrene it was found that higher fluence resulted in a higher increase of reflectivity. The reflectivity enhancement was observed, both via its effect on the absorption coefficient as well as through its own photoionization by 2-photon absorption, while increasing the UV absorption of methylmetacrylate by naphthalene doping.