We report a novel approach to tracking nuclear motion and non-adiabatic
dynamics in molecular systems, on sub-10\,fs timescales, by combining a
detailed analysis of the onset and decay times of the features in time-resolved
photoelectron spectroscopy experiments with electronic structure calculations.
The approach is applied to photoexcitation of methanol at 156\,nm, followed by
delayed photoionization at 260\,nm, using 20\,fs laser pulses. Energy-resolved
signal onset times are extracted from the measured photoelectron spectra to
achieve high temporal resolution, beyond the pump and probe pulse duration.
When combined with ab initio calculations of selected cuts through the excited
state potential energy surfaces, this information allows the dynamics of the
transient excited molecule, which include multiple nuclear and electronic
degrees of freedom, to be tracked on their intrinsic femtosecond timescale.
Within 15\,fs of photoexcitation, we observe nuclear motion on the initially
excited bound state, through a region of non-adiabatic coupling, and along
different dissociative coordinates.