  We use time-resolved momentum microscopy at a free-electron laser (FEL) and
extend orbital tomography into the time domain to image the electronic wave
functions of excited molecular orbitals. This technique provides unprecedented
insight into the ultrafast interplay between structural and electronic
dynamics. In this work we prove general applicability and establish the
experimental conditions at FEL sources to minimize space charge effects and
radiation damage. We investigate a bilayer pentacene film on Ag(110) by optical
laser pump and FEL probe experiments. From the momentum microscopy signal, we
obtain time-dependent momentum maps of the excited-state dynamics of both
pentacene layers separately. Combining experimental observations with a
theoretical study, we interpret the observed signal for the bottom layer as
resulting from the charge redistribution between the molecule and the substrate
induced by excitation. We identify that the dynamics of the top pentacene layer
resembles excited-state molecular dynamics.
