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

This paper presents an attempt of examining the irregularities appearing in a complicated AΠ state of the CH molecule with their reasons provided. By using the experimental data for the AΠ−XΣ bands system of the CH 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 AΠ state cannot be determined by means of the equilibrium constants calculated in the previous works. Due to so far unidentified AΠ 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 rovibronic structure of regular lower state XΣ of A−X system independent of possible perturbations appearing in the upper state of AΠ of this system. It also enabled one to calculate for the first time the real (perturbed) term values for the AΠ (v′ = 0, 1, 2, and 3) state of the CH ion molecule. These values suggest that rotational irregularities in the AΠ state examined are negligibly small. In order to confirm the nonexistence of rotational perturbations in the AΠ (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.