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

Electronic structures of both the anionic and neutral triatomic species TiGe₂^-/0 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 TiGe₂^- (C_2v) was identified to be ⁴B₁, but the ²A₁ state is nearly degenerate, whereas the ³B₁ is clearly the ground state of the neutral TiGe₂ (C_2v). 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 TiGe₂^- can now be assigned. The X band of the anion photoelectron spectrum is attributed to a one-electron transition between two ground states ⁴B₁ → ³B₁. Three neutral excited states 2³A₂, 2⁵B₁, and 3⁵B₁ are energetically responsible for the B band upon one-electron photodetachement from the anionic ground state ⁴B₁. The C band is assigned to the transition ⁴B₁ → 2⁵A₁. A transition from the nearly degenerate ground state ²A₁ of the anion to the low-spin ¹A₁ 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.