Anisotropy of the Singlet Transitions of Crystalline Anthracene

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Abstract
The reflection spectra of three faces of crystalline anthracene in the region 5000–1820 Å have been observed at room temperature, and the corresponding absorption spectra have been derived by a Kramers–Kronig analysis. The results for face (001) agree with previous direct absorption work. For faces (201̄) and (1̄10) the Davydov splittings of the 3800‐Å transition are found to be substantially different from those observed for the (001) face. The strong 2500‐Å transition gives rise to reflection bands which are 1.5–2.0 eV wide. These bands are examples of metallic reflectivity which had previously been observed only for some organic dye crystals. For radiation incident upon the (201̄) and (1̄10) faces the splittings of the strong transition are smaller by nearly an order of magnitude relative to that observed in the case of the (001) face. These results are interpreted as strong evidence for the dependence of dipole sums on the relative orientation of the wave vector k to the transition moment d. The experimental observations are compared with the dipole theory of exciton states in molecular crystals. Qualitative agreement between dipole calculations and experiment is good, and quantitative differences are attributed to the presence of higher multipole and electron exchange effects. It is estimated that these nondipolar interactions contribute approximately − 1600 cm−1 to the total Davydov splitting of the intense 2500‐Å transition.