Condensation of laser-produced gold plasma during expansion and cooling in a water environment

Physical processes involved in laser ablation in liquid (LAL) are studied using a gold target irradiated through transparent water. During and after irradiation, the heated material from the surface of a target produces a plume that expands into liquid-forming nanoparticles (NPs). The LAL method of NP production is ecologically much cleaner than others. A better understanding of the processes associated with complicated hydrodynamic phenomena leading to LAL is important for controlled manufacturing. We consider laser pulses with different durations tau(L) covering fifth orders of magnitudes ranging from 0.1ps to 0.5ns and large absorbed fluences F-abs near optical breakdown of liquid. It is shown that the trajectory of the contact boundary with liquid at the middle and late stages after passing the maximum intensity of the longest pulse is rather similar for very different pulse durations if energies F-abs are comparable. We trace how hot (in a few eV range) dense gold plasma expands, cools down, intersects a saturation curve, and condenses into NPs appearing first inside the water-gold diffusively mixed intermediate layer where gold vapour has the lowest temperature. Later, the pressure around the gold-water contact drops down below the critical pressure for water. As a result, the nanoparticles find themselves in gaseous water bubble where density of water gradually decreases to 10(-4)-10(-5)g/cm(3)at maximum bubble expansion.