Discerning vibronic molecular dynamics using time-resolved photoelectron spectroscopy

Dynamic processes at the molecular level occur on ultrafast time scales and are often associated with structural as well as electronic changes. These can in principle be studied by time-resolved scattering(1-3) and spectroscopic methods, respectively. In polyatomic molecules, however, excitation results in the rapid mixing of vibrational and electronic motions, which induces both charge redistribution and energy flow in the molecule(4,5). This 'vibronic' or 'non-adiabatic' coupling is a key step in photochemical(6) and photobiological processes(7) and underlies many of the concepts of molecular electronics(8), but it obscures the notion of distinct and readily observable vibrational and electronic states. Here we report time-resolved photoelectron spectroscopy measurements that distinguish vibrational dynamics from the coupled electronic population dynamics, associated with the photo-induced internal conversion, in a linear unsaturated hydrocarbon chain. The vibrational resolution of our photoelectron spectra allows for a direct observation of the underlying nuclear dynamics, demonstrating that it is possible to obtain detailed insights into ultrafast non-adiabatic processes.