Electron transmission spectroscopy (ETS) is one ofthe most suitable means for detecting the formation of gas-phase temporary anions (resonance processes), that is, for measuring negative electron affinity values. The decay of these unstable anions can follow a dissociative channel, which generates long-lived fragment anions. Dissociative electron attachment spectroscopy (DAS) measures the yield of these fragments as a function of the incident electron energy.

A brief description of the two techniques is given, together with selected results obtained in two fields of research: i) the systematic study of the electron-acceptor properties in saturated and unsaturated hydrocarbons containing group 14, 15, 16 and 17 elements; ii) the characterisation of the energy and nature of the lowest-lying anion states and the study of the metal-ligand bonding in transition metal complexes. The electron transmission data have often been supported by multiple scattering Xα (MS-Xα) calculations, using a stabilisation procedure, which have proven to be reliable and accurate in reproducing experimental data.

Substituents containing third-row (or heavier) elements, such as Si, P, S and Cl, increase the electron-acceptor properties of adjacent π systems and give rise to low-energy σ* resonances not present in the ET spectra of the corresponding second-row (C, N, 0 and F) derivatives. The presence of heteroatom-heteroatom bonds, such as in polysulfides and polysilanes, lads to even lower-energy σ* orbitals. Temporary anions in group 17 derivatives often follow a dissociative decay channel, with production of negative halogen fragments. Their abundance in different compounds, however, vary over several orders of magnitude.

The ETS studies of transition metal complexes are reviewed, together with the description of the localisation properties of the various anion states and the charge distributions in the neutral states provided by MS-Xα calculations. Both the experimental and theoretical results indicate a net transfer of negative charge from the ligands to the central metal atom.