Ultrafast melting of copper studied with femtosecond electron diffraction.*

Understanding ultrafast melting of metals is crucial for applications ranging from laser micro-machining to warm dense matter (WDM) experiments. Here we report results of using femtosecond electron diffraction to study structural evolution of Cu as it underwent ultrafast laser-induced solid-liquid phase transition. In our experiments, 40-nm-thick polycrystalline Cu films were irradiated by 400nm, 130fs laser pulses to produce WDM states. Structural evolution of irradiated target was measured with 3.2MeV, 350fs electron pulses. We observed homogeneous melting that occurs within 10ps at absorbed energy densities of \textasciitilde 1.2-2.5 MJ/kg. The measured melting times are understood with two-temperature model simulations. The experimental results are consistent with TTM simulations using electron-ion coupling strength G$_{\mathrm{ei}}$ that is inferred from the T$_{\mathrm{i}}$ evolution from Laue peak dynamics. The inferred G$_{\mathrm{ei}}$ has a much weaker T$_{\mathrm{e}}$ dependence and is about a factor of four lower than the T$_{\mathrm{e}}$-dependent value calculated by density functional theory [Z. Lin, PRB (2008)].