Broadband AC Conductivity of XUV Excited Warm Dense Gold

The properties of ultrafast laser excited warm dense gold have been extensively studied in the past decade [1, 2]. In those studies, a 400nm ultrashort laser pulse was used to excite the 5$d$ electrons in gold to 6s/p state. Here we will present our recent study of warm dense gold with 245eV, 70fs pulses to selectively excite 4$f$ electrons using the XUV-FEL at FLASH. The AC conductivity of the warm dense gold was measured at different wavelengths (485nm, 520nm, 585nm, 640nm and 720nm) to cover the range from 5$d$-6$s/p$ interband transitions to 6$s$/$p$ intraband transitions. Preliminary result suggests that the onset of 5$d$-6$s/p$ band transition shifts from 2.3eV to $\sim$ 2eV, which is in agreement with the study of 400nm laser pulse excited warm dense gold [3]. More detailed analysis of our data will also be presented.\\[4pt] [1] Z. Chen \textit{et. al}, \textit{PRL }\textbf{110}, 135001 (2013)\\[0pt] [2] T. Ao \textit{et. al}, \textit{PRL }\textbf{96}$, $055001 (2006)\\[0pt] [3] Y. Ping \textit{et. al}, \textit{PRL} \textbf{96}, 255003 (2006)