Rotational structure and dissociation of the Rydberg states of CO investigated by ion-dip spectroscopy

In a series of spectroscopic work of the Rydberg states of CO, we present the rotational analysis of the v = 0 and 1 levels of the singlet ns, np, nd and nf-Rydberg states (n = 4-7). The spectra were measured by ion-dip spectroscopy with triple resonance excitation via the 3sσ:B ¹Σ ⁺ or the 3pσ:C ¹Σ⁺ state. All the spectra were rotationally well resolved and the term value, quantum defect and the rotational constant were obtained for each state. Through the analysis of the rotational structure, the coupling between the Rydberg electron and the ion core has been investigated. For the np -Rydberg states, a switching from Hund's case (b) to (d) was clearly observed with the increase of n. A significant perturbation was observed in the 6pπ ¹Π and 7pπ ¹Π states and it is suggested that these states are perturbed by the state with the same symmetry. For the nf-Rydberg states, the observed electronic energy was well analyzed by the long range force model and the precise ionization potential was obtained. The Rydberg↔valence and inter-Rydberg states interactions were also investigated. For the ns-Rydberg states, the interaction matrix element with the repulsive state was estimated from the measurement of linewidth of the rotational levels. The potential curve of the repulsive state to which ns-Rydberg states predissociate was also determined. Selective predissociation was found for the e-symmetry levels both in the v = 0 and 1 levels of the nf-Rydberg state. A strong interaction between the v = 0 levels of the 6d- and 7s-Rydberg states was observed.