Time-resolved reaction dynamics in liquid heliumdroplets

A fundamental and driving goal in physical chemistry/chemical physics is the recording of molecular movies. An ideal molecular movie consists of “pictures” of atoms in molecules as they move around during a chemical reaction, forming new bonds and breaking old bonds. This imposes two requirements for the reaction “camera”: it must have a time-resolution that is faster than the atomic movement, which is on the femtosecond (10^-15 s) timescale; and a spatial resolution that is comparable to the atoms, which is on the Å (10^-10 m) scale.

In this project, we employ three different experimental methods to record molecular movies:

1. We use state-of-the-art femtosecond laser systems to achieve a time-resolution of ~35 femtoseconds. The laser pulses act both as the initiator of a reaction (typically by electronic excitation/ionization) and as a probe of the reaction progress (via multiple ionization of the molecules, driving a Coulomb Explosion event.)
2. We use helium nanodroplets as nanometer-sized chemical reactors. The droplets interact very weakly with molecules, are transparent to the laser light, but can still dissipate the energy released following a chemical reaction. The droplets also act as small spacers (30 Å - 55 Å) between molecules, which allows us to define an initial separation between two reactants.
3. We use an event-based ultrafast camera (Tpx3Cam), which measures all ionic species simultaneously with nanosecond time-resolution at a 5 kHz repetition rate. This allows us to look at correlations between ionic species and their momenta.

Currently, we have shown that it is possible to follow the formation of Li⁺-benzene complexes using this method, where we can control the reaction time by varying the droplet size. The current work focuses on measuring the orientation dynamics of molecules as the reaction takes place and the ion binds to the most preferable site.