Features of the optical absorption of crystals of the fullerene C-60 in the region of the orientational phase transition

The absorption coefficient of perfect single crystals of the fullerene C-60 is measured in the energy range 1.6-2.1 eV at temperatures from 4.2 to 300 K. An absorption fine structure is discovered in the and is assigned to el ectronic and vibronic transitions with the production of free excitons and excitons localized on structural defects. It is shown that in the region of the structural phase transition from a face-centered cubic structure to a simple cubic structure the absorption coefficient undergoes a jump, which i s associated with an energy shift of the free exciton line toward lower ene rgies. It is discovered that spatial inhomogeneity, which is associated wit h the growth of the new phase from a finite number of nuclei, appears in th e crystal at the time of this transition. (C) 1998 American Institute of Ph ysics. [S1063-7761(98)02512-8].