Anomalous reflection of water surface during laser ablation

The temporal behaviour of KrF laser pulses reflected from a water surface w as analysed over a broad range of laser fluences and absorption coefficient s. We observed that the surface reflectivity strongly changes when the volu me energy density exceeds a value of 1000 J/cm(3). The change in surface re flectivity, with increasing volume energy, depends on whether we have an ai r/water interface, where the reflectivity is increased, or a glass/water in terface, where the reflectivity is decreased. We estimate the refraction in dex of water, which decreases from n = 1.38 to ca. 1.1 for open water surfa ce with the increase in volume energy. The mechanism of anomalous reflectiv ity is discussed and we suggest that this is due to heating of the absorbin g solution above the water critical point, which is followed by a process o f thermal expansion of the superheated layer. (C) 2000 Published by Elsevie r Science B.V.