Fine structure in the intrinsic absorption edge of TiO2

Abstract

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\ifmmode^\circ\else\textdegree\fi{}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 ${\ensuremath{\Sigma}}_{4v}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1c}$, characterized by a small doublet (3.061-3.065 eV at 1.6\ifmmode^\circ\else\textdegree\fi{}K) which constitutes the fundamental transition in polarization $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}$ parallel to $\stackrel{\ensuremath{\rightarrow}}{\mathrm{C}}$. In polarization perpendicular to $\stackrel{\ensuremath{\rightarrow}}{\mathrm{C}}$, we find the same indirect transition plus a new forbidden direct transition ${\ensuremath{\Gamma}}_{3v}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1c}$ which appears at slightly lower energy (3.031 eV at 1.6\ifmmode^\circ\else\textdegree\fi{}K). The fine structure of the forbidden exciton is resolved and the ionization energy is 4 meV.