Investigation of Anion Resonances Using Photoelectron Spectroscopy and Density Functional Theory

Abstract

The structure and dynamics of temporary excited states of anions (resonances) has been probed using a combination of photoelectron (PE) spectroscopy and computational methods. Such resonances are important in understanding electron-driven chemistry. Here, photoexcitation from an anion to an excited state of the anion that lies in the electronic continuum is closely related to the analogous electron impact resonances. A particular emphasis is placed on how non-covalent interactions in clusters affect these dynamics. Two-dimensional (2D) PE spectroscopy is employed which provides fingerprints of resonance dynamics. Additionally, computational methods are used to aid the interpretation of experimental results and to lay a foundation for future studies.

To demonstrate the applicability of these methods, we have probed a range of different anionic systems of relevance to astro-, bio-, and plasma-chemistry as well as fundamental chemical reaction dynamics. We studied the dynamics of anthracene resonances, showing that resonances overall decay by electron detachment. Time-dependent density-functional theory (TDDFT) calculations in conjunction with a stabilisation method could assign all observed resonances. For the para-benzoquinone radical anion, the addition of a single water molecule was found to lead to a dramatic enhancement in the ability for resonances to form the ground-state anion. Larger water clusters similarly showed that groundstate formation was facile. Clusters of the para-benzoquinone radical with other parabenzoquinone molecules showed dissociation dynamics following the excitation of a resonance.

Finally, the cluster of the iodide anion and trifluoromethyl iodide was studied as a reactive intermediate in an SN2 reaction, in which the stereochemistry has been reversed from the traditional backside attack to a frontside attack pre-reaction complex. Overall, the interplay between TDDFT and 2D PE spectroscopy is shown to provide exquisite insight into the electronic structure of complex anionic clusters and their resonances, despite the complex structure of many of these clusters. This provides a stepping stone to studying larger and more complex anionic systems.