Baldini, E.Chiodo, L.Dominguez, A.Palummo, M.Moser, S.Yazdi-Rizi, M.Aubock, G.Mallett, B. P. P.Berger, H.Magrez, A.Bernhard, C.Grioni, M.Rubio, A.Chergui, M.2017-05-012017-05-012017-05-01201710.1038/s41467-017-00016-6https://infoscience.epfl.ch/handle/20.500.14299/136749WOS:000399054700001Anatase TiO2 is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron-hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier-Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy.text::journal::journal article::research article