Absorption spectra of crystalline fullerite 60 at pressures up to 19 GPa

(Received 2 July 1992, accepted 30 July 1992)

Abstract
The absorption spectra of the thin fullerite crystals, C₆₀, have been measured in the energy range 1.1-3.1 eV at room temperature and pressures up to 19 GPa. The mean rate of the spectral pressure shift d has been estimated for strong- and weak-absorption regions. It has been found that in the strong-absorption region its value at normal pressure is -0.15 eV/GPa, and then its absolute value decreases to 0.019 eV/GPa at 12 GPa. The weaker-absorption region located, under normal pressure, at the red edge shifts much more slowly (  eV/GPa) and coalesces with the strong-absorption region at pressures exceeding 2.0 GPa. The T=4.2 K measurements indicate the presence of a fine structure in the weak-absorption region of fullerite 60, that transforms to a step at room temperature. The dependence of the weak-absorption edge shift on the intermolecular distances has been determined and shown to be governed by the Van der Waals forces of the intermolecular interaction.