  Frontier orbitals, i.e., the highest occupied and lowest unoccupied orbitals
of a molecule, generally determine molecular properties, such as chemical
bonding and reactivities. Consequently, there has been a lot of interest in
measuring them, despite the fact that, strictly speaking, they are not
quantum-mechanical observables. Yet, with photoemission tomography a powerful
technique has recently been introduced by which the electron distribution in
orbitals of molecules adsorbed at surfaces can be imaged in momentum space.
This has even been used for the identification of reaction intermediates in
surface reactions. However, so far it has been impossible to follow an
orbital's momentum-space dynamics in time, for example through an excitation
process or a chemical reaction. Here, we report a key step in this direction:
we combine time-resolved photoemission employing high laser harmonics and a
recently developed momentum microscope to establish a tomographic, femtosecond
pump-probe experiment of unoccupied molecular orbitals. Specifically, we
measure the full momentum-space distribution of transiently excited electrons.
Because in molecules this momentum-space distribution is closely linked to
orbital shapes, our experiment offers the extraordinary possibility to observe
ultrafast electron motion in time and space. This enables us to connect their
excited states dynamics to specific real-space excitation pathways.
