Vaporization Kinetics During Pulsed Laser Heating of Liquid Hg

There is a growing body of experimental evidence showing that the kinetics of nascent vapor produced during pulsed laser heating of metals cannot always be ascribed to the surface thermal conditions. Some investigators have proposed that the discharge of energetic (nonthermal) atoms from metals can involve light coupling to surface plasmons. This requires surface roughness to facilitate wave vector matching of laser light with surface electromagnetic excitation modes. If true, superthermal vaporization kinetics should disappear from time-of-flight measurements when an optically smooth surface is used. Unfortunately, maintaining such an ideal surface is infeasible on a solid target because each laser pulse introduces nanometer sized roughness through the process of melting and resolidification. We have investigated the nature of vaporization from a liquid Hg surface using a nanosecond laser emitting 5 eV photons. Surface tension of the liquid provides an optically smooth surface for this experiment. Nevertheless, we observe superthermal vaporization kinetics from liquid Hg. Yet, the shape of the energy distribution is Boltzmann (the thermal expectation), and the energy distribution does not demonstrate any quanta characteristic of vaporization mediated by an electronic excitation. [S0022-1481(00)01602-9]