DYNAMICS OF ANION FORMATION

Motivation: Electron-driven chemistry underpins science and technology ranging from plasmas in the semiconductor industry to charge-balance in interstellar molecular clouds. From a basic science perspective, the reaction of an electron and a neutral molecule to form an anion is one of the most fundamental chemical transformations. Despite this, many aspects of the underlying chemical physics remain poorly understood. The aim of the research programme is to develop new methods to realise a better understanding of the basic science underpinning electron-driven chemistry.

Philosophy: We have developed several methods to achieve the above aims. All are based in the simple premise that photoexcitation of an anion to excited states that lie in the detachment continuum populates the same resonances in the electron-molecule reaction. Hence, the basic chemistry can be probed using photons. The process is anion photoelectron spectroscopy but excitation of a resonance will lead to changes in the outgoing kinetic energies of the electrons as some of the available energy is converted into nuclear motion on the resonance potential energy surface.

2D Photoelectron Spectroscopy: To map out the dynamics of resonances in the continuum, we have developed 2D photoelectron spectroscopy. In analogy to 2D electron energy loss spectroscopy, the photon energy is scanned across the continuum and for each, a spectrum is produced to yield a 2D map of the resonances and their dynamics as encoded by the outgoing electron’s kinetic energy.

Imaging as a 3rd dimension: Photoelectron imaging adds a further dimension and provides the photoelectron angular distributions of the outgoing electron. Because this depends on the molecular orbital from which the electron is emitted, it is also sensitive to resonances and their dynamics – although the quantitative determination of such resonance-enhanced angular distributions remains unavailable.

Time-resolved electron-driven chemistry: Perhaps the most useful aspect of using light is that it is “easy” to obtain time-resolution because light pulses can be made short (femtoseconds), which is not possible for low-energy electron pulses. A pump pulse excites a resonance and a delayed probe pulse monitors the resonance population and any dynamics it undergoes. Hence, the ultrafast dynamics of the primary electron capture process can be monitored in real-time.

Effect of environment: Starting from an anion allows for mass-selection, which is not easy in electron spectroscopy as the target is neutral. Hence, incremental solvation can be studied in a controlled manner.

The “electrophore”: We have shown that certain molecules are excellent at capturing free electrons to form ground state anions. In analogy to chromophores, these can be viewed as electrophores. While broad chemical rules exist to tune a chromophore’s spectral response to light, no rules exist to tune an electrophore’s spectral response to electrons. One long-term aim is to develop such rules.