Few-femtosecond sensitivity of ultrafast molecular dynamics with time-resolved photoelectron spectra*

We explore ultrafast dynamics involving non-adiabatic couplings following valence electronic excitation of small molecular systems. By measuring the time-resolved photoelectron spectra (TRPES) resulting from ionization with ultraviolet light, the excited wave packet can be tracked with state specificity. If the nuclear motion is dominated by a limited number of degrees of freedom, the TRPES also yields information about the molecular geometry. Even with limited temporal resolution, the onset times of the signal at different photoelectron energies can lead to few-femtosecond sensitivity. Applying this technique to ethylene (C$_2$H$_4$) excited to the $\pi\pi^*$ state, ultrafast motion along the twist coordinate is observed along with transient population to the $\pi3s$ state through non-adiabatic coupling.