Lattice stability in ultrafast laser excited gold

Central to the calculation of structural, dynamical and thermal properties of matter is the interatomic potential that is little known particularly for non-simple metals under extreme conditions. A direct manifestation of interatomic potential is lattice stability that can readily be determined by observing the disassembly of an excited sample. Here we report the study of fs-laser excited Au. In our experiment, 30nm-thick freestanding gold foils are irradiated by 400nm, 45fs (FWHM) laser pulses to produce isochoric heated warm dense states with excitation energy densities ranging from ∼0.5–7.5MJ/kg (1010-1.5×1011J/m3). The disassembly of the sample is monitored with Frequency Domain Interferometry (FDI) using an 800nm, chirped laser probe pulse. The first finding is the dependence of disassembly time on excitation energy density. This provides the first test of predictions obtained from MD simulations1 using a glue model interatomic potential2. An equally significant finding is the appearance of surface melting that precedes bulk melting at energy densities above 3MJ/kg. Interestingly, the melting of two uppermost surface layers in Au has also been predicted in the glue model simulations2. The melting occurs at ∼100K below the bulk melting temperature and remains confined to the two layers. It should be noted that our results are not only of fundamental importance for understanding and modeling the properties of warm dense matter, but also key to the understanding of high intensity laser ablation of solids.