Titanium Digermanium: Theoretical Assignment of Electronic Transitions Underlying Its Anion Photoelectron Spectrum.

Electronic structures of both the anionic and neutral triatomic species TiGe were theoretically studied employing single-reference (DFT and RCCSD(T)) and multiconfigurational (CASSCF/CASPT2 and CASSCF/NEVPT2) methods with large basis sets. The ground state of TiGe (C) was identified to be B, but the A state is nearly degenerate, whereas the B is clearly the ground state of the neutral TiGe (C). On the basis of the computed ground and excited states of both neutral and anionic structures, all electronic transitions giving rise to experimental anion photoelectron bands in the spectrum of TiGe can now be assigned. The X band of the anion photoelectron spectrum is attributed to a one-electron transition between two ground states B → B. Three neutral excited states 2A, 2B, and 3B are energetically responsible for the B band upon one-electron photodetachement from the anionic ground state B. The C band is assigned to the transition B → 2A. A transition from the nearly degenerate ground state A of the anion to the low-spin A of the final neutral state can be ascribed to the A band. Furthermore, the first two bands' progressions, whose normal vibrational modes were accessible from CASSCF/CASPT2 calculations, were also simulated by determination of multidimensional Franck-Condon factors.