Analysis of the A1Π State on the Basis of the Douglas–Herzberg Bands System in the CH+ Ion Molecule

This paper presents an attempt of examining the irregularities appear-ing in a complicated A1Π state of the CH+ molecule with their reasons provided. By using the experimental data for the A1Π−X1Σ+ bands system of the 12CH+ ion radical, it was proved that the vibrational and rotational quanta of the upper state reveal the same unusual behaviour, i.e. very clear nonlinear dependence on vibrational quantum number (v ′ ≥ 3) of the upper state. Therefore, upper vibrational levels (v ′ ≥ 3) of the A1Π state cannot be determined by means of the equilibrium constants calculated in the previous works. Due to so far unidentified A1Π state perturbations, the reduction of the wave numbers to the rovibronic parameters was carried out by means of individual, band-by-band analysis method, using with this end in view the nonlinear least squares method introduced by Curl and Dane, and Watson. This method allowed one to make already calculated constants of the rovi-bronic structure of regular lower state X1Σ+ of A−X system independent of possible perturbations appearing in the upper state of A1Π of this system. It also enabled one to calculate for the first time the real (perturbed) term values for the A1Π (v ′ = 0, 1, 2, and 3) state of the 12CH+ ion molecule. These values suggest that rotational irregularities in the A1Π state exam-ined are negligibly small. In order to confirm the nonexistence of rotational perturbations in the A1Π (v ′ = 0, 1, 2, and 3) state, up to the observed Jmax level, appropriate graphs of functions fx(J) and gx(J) introduced by Gerö and Kovács, where x = Q, PR, and PR, were drawn. Also, their course was analysed in detail. PACS numbers: 33.20.Kf 1