Using a chirped pulse probe technique, we have obtained single-shot measurements of temporal evolution of ac conductivity at 1.55 eV (800 nm) during electron energy relaxation in nonequilibrium warm dense gold with energy densities up to $4.1\mathrm{MJ}/\mathrm{kg}$ ($8\times {10}^{10}\mathrm{J}/{\mathrm{m}}^3$). The results uncover important changes that have been masked in an earlier experiment. Equally significant, they provide valuable tests of an ab initio model for the calculation of electron heat capacity, electron-ion coupling, and ac conductivity in a single, first principles framework. While measurements of the real part of ac conductivity corroborate our theoretical temperature-dependent electron heat capacity, they point to an electron-ion coupling factor of $\sim 2.2\times {10}^{16}\mathrm{W}/{\mathrm{m}}^3\mathrm{K}$, significantly below that predicted by theory. In addition, measurements of the imaginary part of ac conductivity reveal the need to improve theoretical treatment of intraband contributions at very low photon energy.

An abrupt change in energy transport has been observed in femtosecond laser heated gold when the absorbed laser flux exceeds $\sim 7\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. Below this value, the absorbed flux is carried by ballistic motion of nonthermal electrons produced in interband excitation. Above this value energy transport appears to include ballistic transport by nonthermal electrons and heat diffusion by thermalized hot electrons. The ballistic component is limited to a flux of $\sim 7\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. This offers a unique benchmark for comparison with theory on nonequilibrium electron transport.

We present a new measurement of lattice disassembly times in femtosecond-laser-heated polycrystalline Au nanofoils. The results are compared with molecular dynamics simulations incorporating a highly optimized, embedded-atom-method interatomic potential. For absorbed energy densities of $0.9–4.3\mathrm{MJ}/\mathrm{kg}$, the agreement between the experiment and simulation reveals a single-crystal-like behavior of homogeneous melting and corroborates the applicability of the interatomic potential in the nonequilibrium warm dense matter regime. For energy densities below $0.9\mathrm{MJ}/\mathrm{kg}$, the measurement is consistent with nanocrystal behavior where melting is initiated at the grain boundaries.

Using recently obtained ac conductivity data we have derived dc conductivity together with free electron density and electron momentum relaxation time in two-temperature warm dense gold with energy density up to 4.1 MJ/kg ($0.8\times {10}^{11}\mathrm{J}/{\mathrm{m}}^3$). The derivation is based on a Drude interpretation of the dielectric function that takes into account contributions of intraband and interband transitions as well as atomic polarizability. The results provide valuable benchmarks for assessing the extended Ziman formula for electrical resistivity and an accompanying average atom model.