Probing the electronic structure of small molecular anions by photoelectron imaging

We outline the methodology of negative-ion photoelectron imaging and general aspects of interpretation of the results using the CS₂^- and S₂^-1 anions as model systems. The CS₂^- images are recorded using 800, 530, 400, and 267 nm photons. The observed transitions result in the formation of CS₂ in the X ¹Σ₁⁺, a ³B₂, b ³A₂, and A ¹A₂ states. The S₂^- measurements are carried out at the same wavelengths with the exception of 800 nm. The resulting images reveal the detachment transitions assigned to the X ³Σ_g^-, a ¹Δ_g^-, b ¹Σ_g⁺, c ¹Σ_u^-, and A′ ³Δ_u states of the neutral. The choice of detachment wavelengths serves as a "zoom" selectively focusing on chosen transitions, in some cases allowing the observation of their vibrational structure. The photoelectron spectra and angular distributions obtained from the images are used to discuss the electronic structure and detachment dynamics. In particular, two approaches to interpreting the angular distributions are discussed. One method employs the Cooper-Zare central-potential model adapted to the molecular case. It considers an expansion of the parent orbital in the basis of single-center atomic-orbital functions, for which the partial waves comprising the ejected are determined. The application of this model to molecular anions is straightforward, if the parent molecular orbital resembles an atomic orbital, which is the case for S₂^-, but not CS₂^-. In the latter case, a different qualitative approach is proposed, which (1) relies upon the electric-dipole approximation and group theory for the determination of the detached electron wave function symmetry, (ii) restricts the analysis to symmetry (electric dipole) allowed s and p partial waves, and (iii) qualitatively treats the orientation averaging by considering only a few "principal" molecular orientations. The results provide a foundation for the qualitative interpretation of anion photoelectron images.