First-principles calculations of solid and liquid aluminum optical absorption spectra near the melting curve: Ambient and high-pressure results

We present ab initio calculations of the linear optical conductivity of heated Al at ambient pressure and at the conditions relevant for shock melting $\left(P\sim 125\text{GPa},T\sim 5000\text{K}\right)$. It is shown that the visible and near-UV optical spectrum is very sensitive to the phase (fcc solid versus liquid) of Al for both $P=0$ and 125 GPa. The ambient-$P$ results confirm an earlier prediction and the results of a recent experiment while the high-$\left(P,T\right)$ results allow us to conclude that in situ measurements of optical constants should be able to diagnose the shock melting of Al.

Figure 1

(Color) The optical conductivity, ${\sigma }_1\left(\omega \right)$, of Al at various $\left(P,T\right)$ conditions. For each $\left(P,T\right)$ point, maximum and minimum values for ${\sigma }_1$ were taken from ten ionic configurations separated from each other by $\delta t=1.5\text{ps}$. With the liquid at $\left(P=125\text{GPa},T=5000\text{K}\right)$, the variation in ${\sigma }_1\left(\omega \right)$ between different ionic configurations is negligibly small, therefore the lines corresponding to the minimum and the maximum practically coincide.

Figure 2

(Color) The optical conductivity, ${\sigma }_1\left(\omega \right)$, of fcc Al at $T=950\text{K}$ (purple lines) is compared to the those at $T_{ion}=300\text{K}$ with elevated electron temperatures, $T_{ele}=10000\text{K}$ and $T_{ele}=20000\text{K}$, to demonstrate that the smearing of the interband transition peak due to an elevated electron temperature is different from that due to thermal lattice disorder. The ${\sigma }_1\left(\omega \right)$ for $T_{ion}=T_{ele}=300\text{K}$ is also plotted as a reference. The different results for $T_{ion}=300\text{K}$ were calculated from the same ionic configuration. The small configuration dependence, seen in Fig. 1, would not substantively affect these results.