Reflection spectra of natural faces of crystalline anthracene

Abstract

Polarized reflection spectra, taken at 1.5 K, off faces (001), (201̄), (1̄01), (110), (1̄1̄1), and (1̄1̄2) of vapor grown anthracene crystals are reported for the wavelength range 400-350 nm. This range spans most of the first singlet electronic transition of the crystal. The faces (001), (201̄), and (1̄01) contain the crystallographic b axis and, according to the Fresnel formulas, should have identical normal incidence b-polarized reflection spectra. However, the spectra of the (201̄) and (1̄01) faces do not show the reflection minima assigned to surface transitions that are prominent within the stop band of the 0-0 transition on face (001). On the other hand, the higher vibronic transitions of (001), (201̄), and (1̄01) appear to be identical for b polarization in agreement with the Fresnel formulas. The absence of surface transitions on (201̄) and (1̄01) is attributed to coupling between surface and volume excitons. Spectra of faces (1̄1̄2), (1̄1̄l), and (110) display no feature attributable to surface transitions. A resonance prominent in the b-polarized spectra off (001), (201̄), and (1̄01) persists in the spectra of the other faces even when the Coulomb exciton lies at higher energy. This feature is tentatively assigned to a two-particle process in which a volume polariton and lattice phonon are simultaneously created. © 1978 American Institute of Physics.