The primary electron-attachment process in electron-driven chemistry represents one of the most fundamental chemical transformations with wide-ranging importance in science and technology. However, the mechanistic detail of the seemingly simple reaction of an electron and a neutral molecule to form an anion remains poorly understood, particularly at very low electron energies. Here, time-resolved photoelectron imaging was used to probe the electron-attachment process to a non-polar molecule using time-resolved methods. An initially populated diffuse non-valence state of the anion that is bound by correlation forces evolves coherently in ∼30 fs into a valence state of the anion. The extreme efficiency with which the correlation-bound state serves as a doorway state for low-energy electron attachment explains a number of electron-driven processes, such as anion formation in the interstellar medium and electron attachment to fullerenes.

Nonvalence states of neutral molecules (Rydberg states) play important roles in nonadiabatic dynamics of excited states. In anions, such nonadiabatic transitions between nonvalence and valence states have been much less explored even though they are believed to play important roles in electron capture and excited state dynamics of anions. The aim of this Feature Article is to provide an overview of recent experimental observations, based on time-resolved photoelectron imaging, of valence to nonvalence and nonvalence to valence transitions in anions and to demonstrate that such dynamics may be commonplace in the excited state dynamics of molecular anions and cluster anions.

The anion-π bond has emerged as an important non-valence interaction in supramolecular and biological structure. Although recognized as a strong non-covalent interaction, driven by electrostatic charge-quadrupole moment and correlation interactions, benchmark experimental and computational studies on the intrinsic anion-π bond strength are scarce. Here, we present a gasphase photoelectron spectroscopic study on the archetypical iodidehexafluorobenzene anion-π bonded complex. In combination with high-level electronic structure calculations, the anion-π bond strength is found to be 0.53 eV (51 kJ mol −1). The interaction arises for a large part from correlation forces (~40%), with electrostatic quadrupole-anion and polarization making up most of the remainder.

Frequency-resolved photoelectron spectra are presented for (CF) with n = 1-5 that show that CF is solvated by neutral CF molecules. Direct photodetachment channels of CF are observed for all n, leaving the neutral in the S ground state or triplet states, T and T. For n ≥ 2, an additional indirect electron loss channel is observed when the triplet-state channels open. This indirect emission appears to arise from the electron capture of the outgoing photoelectron s-wave by a neutral solvent molecule through an anion nonvalence state. The same process is not observed for the S detachment channel because the outgoing electron wave is predominantly a p-wave. Our results show that anion nonvalence states can act as electron-accepting states in cluster environments and can be viewed as precursor states for diffuse states of liquid CF.

Frequency-resolved (2D) photoelectron (PE) spectra of the anionic clusters (C6F6)n − , for n = 1-5, and time-resolved PE spectra of I − C6F6 are presented using a newly built instrument and supported by electronic structure calculations. From the 2D PE spectra, the vertical detachment energy (VDE) of C6F6 − was measured to be 1.60 ± 0.01 eV and the adiabatic detachment energy (ADE) ≤ 0.70 eV. The PE spectra also contain fingerprints of resonance dynamics over certain photon energy ranges, in agreement with the calculations. An action spectrum over the lowest resonance is also presented. The 2D spectra of (C6F6)n − show that the cluster can be described as C6F6 − (C6F6)n−1. The VDE increases linearly (200 ± 20 meV n −1) due to the stabilising influence on the anion of the solvating C6F6 molecules. For I − C6F6, action spectra of the absorption just below both detachment channels are presented. Timeresolved PE spectra of I − C6F6 excited at 3.10 eV and probed at 1.55 eV reveal a short-lived non-valence state of C6F6 − that coherently evolves into the valence ground state of the anion and induces vibrational motion along a specific buckling coordinate. Electronic structure calculations along the displacement of this mode show that at the extreme buckling angle, the probe can access an excited state of the anion that is bound at that geometry, but adiabatically unbound. Hence, slow electrons are emitted and show dynamics that probe predominantly the outer-turning point of the motion. A PE spectrum taken at t = 0 contains vibrational structure, assigned to a specific Raman and/or IR active mode of C6F6.