We outline the methodology of negative-ion photoelectron imaging and general aspects of interpretation of the results using the CS2 - and S2 - 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 1Σg +, 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 3Σg -, a 1Δg, b 1Σg +, c 1Σu -, and A‘ 3Δu 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 electron 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 (i) 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.

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We report the formation of heterogeneous OCS–water cluster anions [(OCS)n(H2O)k]− (n⩾1,n+k⩾2), of which OCS−⋅H2O is the most interesting species in view of the near absence of unhydrated OCS− in the same ion source. The presence of OCS−⋅H2O indicates that the intra-cluster formation of OCS− does occur as part of the [(OCS)n(H2O)k]− formation mechanism. In this light, the near absence of unhydrated OCS− anions points towards their metastable nature, while the abundance of the hydrated anions is attributed to the stabilizing effect of hydration. These conclusions are supported by the results of an extensive theoretical investigation of the adiabatic electron affinity (EA) of OCS. We conclude that the EA of OCS is either negative or essentially zero. The best estimate based on the Gaussian-3 theory calculation puts the EA at −0.059±0.061 eV. A study of the structure and energetics of OCS−⋅H2O predicts the existence of four structural isomers. Using the coupled-cluster theory, we find that the most stable structure is stabilized by 0.543 eV relative to the separated OCS−+H2O limit.

The negative ion photoelectron imaging technique is illustrated using two relatively simple atomic and molecular anion systems, and then applied to the study of a cluster system. Photoelectron images of I- and CS2- at 267 nm and 800 nm respectively are presented. Photoelectron spectra and angular distributions are obtained from the images and the concepts underlying these and their interpretation are outlined. The imaging technique is then applied to (CS2)n - (n = 2-4) cluster anions, for which 400 nm images are presented. Features of these images are highlighted and discussed with reference to solvation effects and structural properties of the cluster anionic moiety. Photoelectron signatures of different forms of the cluster core are discussed. These core structures are anionic monomer units solvated by the remaining n - 1 CS2 molecules or covalent dimer units solvated by the remaining n - 2 molecules. Images of the n = 2 anion at 400, 530 and 800 nm reveal information about the electron detachment processes within the different cluster types and both direct detachment and autodetachment are seen. The direct transitions are seen from clusters with either core type, while autodetachment is only seen from clusters with the covalent dimer core. The imaging work also reveals evidence of a previously unreported electronic transition within the direct detachment band due to the covalently bound core type.

Photoelectron images are recorded in the photodetachment of two series of cluster anions, (CO(2))(n)(-), n=4-9 and (CO(2))(n)(-).H(2)O, n=2-7, with linearly polarized 400 nm light. The energetics of the observed photodetachment bands compare well with previous studies, showing evidence for switching between two anionic core structures: The CO(2)(-) monomer and covalent (CO(2))(2)(-) dimer anions. The systematic study of photoelectron angular distributions (PADs) sheds light on the electronic structure of the different core anions and indicates that solvation by several CO(2) molecules and/or one water molecule has only moderate effect on the excess-electron orbitals. The observed PAD character is reconciled with the symmetry properties of the parent molecular orbitals. The most intriguing result concerns the PADs showing remarkable similarities between the monomer and dimer anion cluster-core types. This observation is explained by treating the highest-occupied molecular orbital of the covalent dimer anion as a linear combination of two spatially separated monomeric orbitals.