Multi-dimensional electron spectroscopy with photons

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The reaction between an electron and a neutral molecules underpins many branches of science and technology including for example, radiation chemistry and biology, interstellar chemistry, plasma etching in the semiconductor industry, and water remediation. From a basic science perspective, the reaction presents one of the simplest reactions and is a corner stone of physical chemistry. Despite this broad importance, many aspects of this seemingly simple reaction remain unknown and poorly studied. The reactions are deceivingly complex: (i) because of the inherent interplay between the changes in electronic structure (which dictates chemical bonding) and the motion of atoms in molecules, and (ii) because the incoming electron is never actually bound (i.e. energetically, it can always leave). This latter point means that inevitably, the reactions involve unstable species that are very short lived. To probe such transient species requires one to be able to capture the electronic and geometric structure of the entire reaction on the molecular timescale which is on the order of femtoseconds (1 fs = 0.000 000 000 000 001 s). However, there are no experimental methods available to do this directly. The best methods to probe the electron-molecule reaction are based on electron spectroscopic methods. In the past few years, our group has developed analogous methods that use optical excitation rather than electron excitation, with some important benefits, but also some limiting disadvantages. In this proposal, we overcome the limitations and develop methods that can probe the electronic and geometric changes in an electron-molecule reaction in real-time. The new methods are based on the idea that an excess electron can be bound to neutral molecules in an non-perturbing manner and we will exploit these electrons as an intramolecular source of electrons that is well-defined in energy and time. We will then apply these methods to probe the reaction of an electron with nucleobases, which represents a key step in biological radiation damage of DNA and for which the details of product formation and dynamics of excited states in real-time remain uncharted.