Fine structure in the intrinsic absorption edge of TiO2

The polarization-dependent absorption spectra and wavelength-modulated absorption spectra of Ti${\mathrm{O}}_2$ have been measured under very-high-resolution conditions between 1.6 and 300°K. For the first time the fine structure of the fundan ental absorption edge is resolved and a detailed investigation is presented. We find an indirect allowed transition ${\Sigma }_{4v}\to {\Gamma }_{1c}$, characterized by a small doublet (3.061-3.065 eV at 1.6°K) which constitutes the fundamental transition in polarization $\overrightarrow{\mathrm{E}}$ parallel to $\overrightarrow{\mathrm{C}}$. In polarization perpendicular to $\overrightarrow{\mathrm{C}}$, we find the same indirect transition plus a new forbidden direct transition ${\Gamma }_{3v}\to {\Gamma }_{1c}$ which appears at slightly lower energy (3.031 eV at 1.6°K). The fine structure of the forbidden exciton is resolved and the ionization energy is 4 meV.