  Atomically thin two-dimensional crystals have revolutionized materials
science. In particular, monolayer transition metal dichalcogenides promise
novel optoelectronic applications, due to their direct energy gaps in the
optical range. Their electronic and optical properties, however, are
complicated by exotic room-temperature excitons, whose fundamental structure
and dynamics has been under intense investigation. While interband spectroscopy
probes energies of excitons with vanishing centre-of-mass momenta, the majority
of excitons has remained elusive, raising questions about their unusual
internal structure, symmetry, many-body effects, and dynamics. Here we report
the first direct experimental access to all relevant excitons in single-layer
WSe2. Phase-locked mid-infrared pulses reveal the internal orbital 1s-2p
resonance, which is highly sensitive to the shape of the excitonic envelope
functions and provides accurate transition energies, oscillator strengths,
densities and linewidths. Remarkably, the observed decay dynamics indicates a
record fast radiative annihilation of small-momentum excitons within 150 fs,
whereas Auger recombination prevails for optically dark states. The results
provide a comprehensive view of excitons and introduce a new degree of freedom
for quantum control, optoelectronics and valleytronics of dichalcogenide
monolayers.
