Structural dynamics reveal molecular behaviour that can be used to direct reactions or create chemical devices. They are conventionally measured using absorption spectroscopy, which characterizes molecules according to the light they absorb at different frequencies. Spectra are observed over time to follow a reaction. However, molecules do not necessarily absorb light at easily accessible frequencies, meaning that important reaction steps are often inaccessible.
Coulomb explosion imaging (CEI) opens the exciting possibility of observing such chemistry directly. CEI uses an intense and ultrafast laser pulse to quickly remove binding electrons from a molecule through field ionization, leaving a highly charged ion that explodes into fragments due to Coulomb's law. Measuring and correlating the relative velocities of these fragments as a function of time allows the shape of the molecule before the explosion to be reconstructed at different stages of a reaction.
Projects in this area focus on initiating photochemistry and probing the resulting dynamics by CEI. Figure 1 illustrates a typical example where the C-I bond distance of CH2BrI is probed following its photolysis at 272 nm. The I and CH2Br co-fragments are initially close together, leading to significant electrostatic repulsion following ionization. The greater separation at later times results in less kinetic energy, creating a 'Coulomb curve' that represents the bond dissociation. Modeling this curve reveals the excitation of the CH2Br fragment and the C-I bond length.
This research is aimed at observing complete reactions for more complex processes, particularly as they pass through short-lived intermediates and transition states that are of fundamental importance for controlling reactivity.