Theoretical investigation of the low-lying electronic states of TiH

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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).