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

A single shot measurement of subpicosecond time resolution was developed to investigate the transient surface reflection changes of excimer laser abla ted liquids during one UV pulse. Polysilicone oil, methyl(metacrylate), sty rene, and water were irradiated by a 500 fs KrF excimer laser. The irradiat ed area was illuminated by an optically delayed 496 nm, 500 fs probe dye la ser pulse at an incident angle of 45 degrees. As the tilted probe beam scan ned along the ablating surface, the temporal increase in the reflected inte nsity appeared as a spatial modulation of the probe beam. The maximum refle ctivities 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 subpicose cond excimer laser ablation. In the case of styrene it was found that highe r fluence resulted in a higher increase of reflectivity. The reflectivity e nhancement was observed, both via its effect on the absorption coefficient as well as through its own photoionization by 2-photon absorption, while in creasing the UV absorption of methylmetacrylate by naphthalene doping.