Insight into the Rydberg states of CH

Ab initio electronic structure calculations of a relatively large number of Rydberg states of the CH radical were carried out employing the multireference single and double excitation configuration interaction ( MRD-CI ) method. A Gaussian basis set of cc-pV5Z quality augmented with 12 diffuse functions was used together with an extensive treatment of electron correlation. The main focus of this contribution is to investigate the 3 d Rydberg complex assigned by Watson [ Astrophys. J. 555 , 472 ( 2001 )] to three unidentified interstellar bands. The authors' calculations reproduce quite well the absolute excitation energies of the three components of the 3 d complex, i.e., Σ + 2 ( 3 d σ ) , Π 2 ( 3 d π ) , and Δ 2 ( 3 d δ ) , but not the energy ordering inferred from a rotational assignment of the 3 d ← X Π 2 laboratory spectrum. The computation of the 4 d complex is reported for the first time along with a number of other higher lying Rydberg species with an X Σ + 1 core. The lowest Rydberg states belonging to series converging to the a Π 3 and A Π 1 excited states of C H + are also calculated.