Nonadiabatic and Time-Resolved Photoelectron Spectroscopy for Molecular Systems.

We quantify the nonadiabatic contributions to the vibronic sidebands of equilibrium and explicitly time-resolved nonequilibrium photoelectron spectra for a vibronic model system of trans-polyacetylene. Using exact diagonalization, we directly evaluate the sum-over-states expressions for the linear-response photocurrent. We show that spurious peaks appear in the Born-Oppenheimer approximation for the vibronic spectral function, which are not present in the exact spectral function of the system. The effect can be traced back to the factorized nature of the Born-Oppenheimer initial and final photoemission states and also persists when either only initial or final states are replaced by correlated vibronic states. Only when correlated initial and final vibronic states are taken into account are the spurious spectral weights of the Born-Oppenheimer approximation suppressed. In the nonequilibrium case, we illustrate for an initial Franck-Condon excitation and an explicit pump-pulse excitation how the vibronic wavepacket motion of the system can be traced in the time-resolved photoelectron spectra as a function of the pump-probe delay.