  Moir\'e superlattices in atomically thin van-der-Waals heterostructures hold
great promise for an extended control of electronic and valleytronic lifetimes,
the confinement of excitons in artificial moir\'e lattices, and the formation
of novel exotic quantum phases. Such moir\'e-induced emergent phenomena are
particularly strong for interlayer excitons, where the hole and the electron
are localized in different layers of the heterostructure. In order to exploit
the full potential of correlated moir\'e and exciton physics, a thorough
understanding of the ultrafast interlayer exciton formation process and the
real-space wavefunction confinement in the moir\'e potential is indispensable.
However, direct experimental access to these parameters is limited since most
excitonic quasiparticles are optically dark. Here we show that femtosecond
photoemission momentum microscopy provides quantitative access to these key
properties of the moir\'e interlayer excitons. We find that interlayer excitons
are dominantly formed on the sub-50~fs timescale via interlayer tunneling at
the K valleys of the Brillouin zones. In addition, we directly measure
energy-momentum fingerprints of the moir\'e interlayer excitons by mapping
their spectral signatures within the mini Brillouin zone that is built up by
the twisted heterostructure. From these momentum-fingerprints, we gain
quantitative access to the modulation of the exciton wavefunction within the
moir\'e potential in real-space. Our work provides the first direct access to
the interlayer moir\'e exciton formation dynamics in space and time and reveals
new opportunities to study correlated moir\'e and exciton physics for the
future realization of exotic quantum phases of matter.

Mesoscale and Nanoscale Physics;Other Condensed Matter