Effect of pressure on the optical absorption in aluminium

The optical reflectivity of Al below 3 eV has been investigated in the 300 kbar pressure range (22% volume change) at T=300K. The dominant optical interband absorption at h(cross) omega =1.5 eV associated with the splitting between parallel bands shifts to higher proton energy at an initial rate of 4.35+or-0.3 meV kbar -1 . The volume dependence of the corresponding pseudopotential Fourier coefficients is dU(200)/d ln V=-1.58 eV. The second parallel band absorption feature at energy 2U(111) ( approximately=0.4 eV at 1 bar) exhibits a similar pressure shift and becomes clearly observable in high-pressure reflection spectra. The optical response of Al under pressure basically follow the predictions derived from an empty-core pseudopotential in combination with the Ashcroft-Sturm expression for optical interband absorption in polyvalent nearly-free-electron metals, but with a significantly larger width of the low-energy absorption band at energy 2U(111). X-ray diffraction studies support the stability of the FCC phase up to 300 kbar.