Anomalous reflection of water surface during laser ablation

The temporal behaviour of KrF laser pulses reflected from a water surface was analysed over a broad range of laser fluences and absorption coefficients. We observed that the surface reflectivity strongly changes when the volume energy density exceeds a value of 1000 J/cm³. The change in surface reflectivity, with increasing volume energy, depends on whether we have an air/water interface, where the reflectivity is increased, or a glass/water interface, where the reflectivity is decreased. We estimate the refraction index of water, which decreases from n = 1.38 to ca. 1.1 for open water surface with the increase in volume energy. The mechanism of anomalous reflectivity is discussed and we suggest that this is due to heating of the absorbing solution above the water critical point, which is followed by a process of thermal expansion of the superheated layer.