Tracking electronic coherences during non-adiabatic molecular dynamics using phase-locked ultrafast spectroscopy  

Short lived coherences between molecular electronic states are conjectured to have a significant effect on the flow of charge in excited molecules and subsequently affect structural changes taking place at a later time. These coherences contribute to most experimental signals weakly, often masked by a much stronger contribution from the populations of individual states. Here we present Linear Vibronic Coupling (LVC) model simulations of a phase-locked, pump-probe measurement which suggest that such coherences can be isolated in the experimental signal. In the simulation a UV pulse resonantly excites the LVC model molecule at the one and two photon levels, following which a probe pulse with a locked relative phase ionizes the molecule at several pump probe delays. We show that repeating the experiment with different relative phase and combing the signals allows separation of coherences between the one and two photon excited electronic states, as well as between these states and the ground state, from the state populations. Further, coherences between non-adiabatically coupled surfaces can also be separately measured, though these necessarily mix with the population signal. We anticipate that this technique will be extremely useful in disentangling the role of electronic coherences in the flow of charge in molecules at early times and consequently their role in chemically relevant dynamics.