Site-Selective Excitation of Ti Ions in Rutile TiO via Anisotropic Intra-Atomic 3d → 3d Transition.

Site-selective excitation (SSE), which is usually realized by tuning the wavelength of absorbed light, is an ideal way to study bond-selective chemistry, analyze the crystal structure, investigate protein conformation, etc., eventually leading to active manipulation of desired processes. Herein, SSE has been explored in (110)-, (100)-, and (011)-faced rutile TiO, a prototypical material in both surface science and photocatalysis fields. Using ultraviolet photoelectron spectroscopy and photon energy-, substrate orientation-, and laser polarization-dependent two-photon photoemission spectroscopy (2PPE), intra-atomic 3d → 3d transition from the split Ti 3d orbitals, i.e., band gap states and excited states at ∼1.00 eV below and ∼2.40 eV above the Fermi level, respectively, has been proven for all of the samples, suggesting that it is a common property of this material. The distinct structure of rutile TiO results in the anisotropic 3d → 3d transitions with the transition dipole moment along the long axes ([110] and [11̅0]) of TiO blocking units. This anisotropy facilitates the selective excitation of Ti ions in the two types of TiO, which cannot be realized by conventional wavelength tuning, via polarization alignment of the excitation source. Discovery in this work builds the foundation for future investigation of site-selective photophysical and photochemical processes and eventually possible active manipulation in this material at the atomic level.