A study of the valence shell absolute photoabsorption, photoionization and photodissociation cross sections, and the photoionization quantum efficiency of carbon monoxide

Abstract

The spectroscopic characteristics and the decay mechanisms of the Rydberg states arising from valence shell excitations in carbon monoxide have been investigated by using a double ion chamber and synchrotron radiation to measure the absolute photoabsorption, photoionization and photodissociation cross sections, and the photoionization quantum efficiency, in the energy range between the ionization threshold and 36 eV. Guided by earlier theoretical predictions, the prominent absorption bands due to Rydberg states belonging to series converging onto the A 2Π state ionization threshold have been assigned, in accord with previous studies, to the ndδ and the [(n + 1)s + nd]σ transitions. Apart from the (4s + 3d)σ state which decays predominantly by predissociation into neutral fragments, most of the Rydberg states belonging to these series decay preferentially by autoionization. Another Rydberg series converging onto the A 2Π state limit has been tentatively assigned to the npσ transitions, even though these transitions are predicted to be weak. The v′ = 0 and v′ = 1 vibrational levels of the 4pσ state decay preferentially by predissociation into neutral fragments. The [(n + 1)s − nd]σ transitions are also calculated to be weak but may be associated with some previously unassigned absorption bands. Structure observed in the absorption spectrum between ∼19.5 and 23.5 eV may be due to Rydberg states associated with series converging onto the D 2Π or the 3 2Σ+ state limits. This structure is discussed in relation to dispersed fluorescence spectra and negative ion yields recorded over the same energy range.