Electron impact excitation of allowed and forbidden transitions between fine-structure levels in Ti III.*

We present an elaborate and extensive theoretical investigation on electron impact excitation of doubly-ionized titanium (Ti III) to meet the needs of spectral analysis and plasma modelling. The calculation is carried out in the close-coupling approximation using the B-spline  R-matrix  method. The multi-configuration Hartree–Fock method in combination with B-spline  configuration interaction expansions and the non-orthogonal orbitals technique is employed for accurate descriptions of the target wave functions and adequate accounts of the various interactions between the target states. Relativistic effects are treated at the level of the semirelativistic Breit-Pauli approximation. The present close-coupling expansion includes 138 fine-structure levels of Ti III belonging to the 3d², 4s², 4s4p, 3d4l (l = 0 − 3), 3d5l (l = 0 − 3), 3d6s, and 3d6p configurations. Comprehensive sets of radiative and electron collisional data are reported for all possible transitions between the individual levels. Thermally averaged collision strengths are determined using a Maxwellian distribution for a wide range of temperatures from 10² K to 10⁵ K. The accuracy of the calculated radiative parameters is validated by comparing with available values from the NIST database and previous literature. Given the lack of sufficient currently available experimental and theoretical data, the electron impact excitation cross sections of the Ti III fine-structure levels presented here are systematic, extensive, and internally consistent, thus making them suitable for many modelling applications.