We report an investigation of the direct forbidden absorption edge of Ti${\mathrm{O}}_2$. For the first time we have resolved the weak quadrupolar $1s$ exciton and measured its binding energy. Taking into account polaron effects, we estimated the bare electron effective mass in the ${\Gamma }_1$ minimum of the conduction band and obtained a fairly reasonable value of $3m_0$.

• Received 21 July 1977

DOI:https://doi.org/10.1103/PhysRevLett.39.1490