Theoretical investigation of the low-lying electronic states of TiH

Abstract

The vertical excitation energies of thirty electronic states of TiH as well as the potential curves of all quartet and doublet states correlating with the first two channels Ti + H have been investigated by using extensive MRD-CI calculations. Based on the results for the transition energies and dipole transition moments, the known absorption bands extending from 18 200 to 21 300 cm^-1 (a strongest band near 18 870 cm^-1 assumed to be a X ⁴Φ → ⁴Δ transition) are assigned to transitions from X ⁴Φ into 1 ⁴Γ (calculated T _vert = 17 420 cm^-1) and into 4⁴Δ (calculated T _vert = 19 440 cm^-1), with possible contribution of the excitation 1 ⁴Σ^- → 5 ⁴Π (calculated T _vert = 17 500 cm^-1). These strong (perpendicular) 8σ → 4π bands lie rather close to the measured excitation energies assigned to the dipole allowed high-intensity transition 4s → 4p of the Ti atom. In addition the present study shows that two TiH bands of relatively high-intensity lie near 11 000 cm^-1 (i.e. X ⁴Φ → 2⁴Φ and 1 ⁴Σ^- → 2 ⁴Σ^-), a prediction awaiting its experimental verification. The low-energy (parallel) 8σ → 6σ(3d) bands have T _e values about 4500 cm^-1 higher than the equivalent 4s → 3d atomic transition. The term values for the first TiH Rydberg members ⁴Φ and ⁴Π (both 8σ → 5s states lying slightly above 32 000 cm^-1) compare well with the experimental 4s → 5s term values of the Ti atom. The analysis of the bonding in the 1 ²Δ and 1 ²Π states shows that these states have contributions from 3dσ (Ti), and as a consequence have shorter equilibrium distances than their quartet counterparts. States with sextet multiplicity are repulsive. The calculated dipole moment μ at R (TiH) = 3·47 a.u. for all bound states from channels I and II reveal a strong dependency of μ on the relative occupation of 6σ(3dσ) and 8σ, as pointed out by a low μ of 0·92 D for 1 ⁴Δ (1δ6σ7σ²8σ) and a high μ of 4·61 D for 2 ⁴Δ (1δ3π²7σ²). Between both extreme values, other examples are provided by μ = 1·90 for X ⁴Φ (1δ3π7σ²8σ) and μ = 2·83 D for 2 ⁴Φ (1δ3π6σ7σ²).