Absorption Band Profile of the Origin Region of the b Polarized 4000 Å Anthracene Crystal Transition

Abstract

New low temperature measurements of the reflectivity of crystalline anthracene are reported. Attention is confined to the important b axis polarized absorption band in the vicinity of the 0–0 transition. In this way line broadening by ``two particle'' excitations are eliminated. It is shown how low temperature reflectivity data, even when limited to only a region of the spectrum, can provide interesting information concerning the exciton dynamics. For this crystal transition optical constants are obtained from the observed reflectance data by use of a Kramers—Kronig analysis and a General Dispersion analysis. Discussed in some detail are those methods of obtaining optical constants, the values of the various band parameters and the implications of such values, the asymmetry of the absorption band profile and what that asymmetry may indicate. The interpretation of the data is not without its difficulties especially in view of the effects which may be caused by surface irregularities and by impurities and surrounding defects. Nevertheless, strong inferences are drawn from the measurements and their subsequent analysis. These include (i) the absorption coefficient in the b axis polarized 0–0 transition of crystalline anthracene is extremely large ${\text{∼1.6× 10}}^6\hspace{0.17em}{\mathrm{cm}}^{\text{−1}}$ at 7°K; (ii) the absorption line shape is asymmetric; (iii) the coupling between the exciton and optical phonons probably exceeds that between the exciton and acoustic phonons. The quantitative aspects of these observations are consistent with the strong coupling line shape theory of Fischer and Rice and of Grover and Silbey. The consequences of the analysis are employed to suggest areas for further experimental and theoretical work.