  The interlayer gallery between two adjacent sheets of van der Waals materials
is expected to modify properties of atoms and molecules confined at the atomic
interfaces. Here, we directly image individual hydrogen atom intercalated
between two graphene sheets and investigate its dynamics by scanning tunnelling
microscope (STM). The intercalated hydrogen atom is found to be remarkably
different from atomic hydrogen chemisorbed on external surface of graphene. Our
STM measurements, complemented by first-principles calculations, show that the
hydrogen atom intercalated between two graphene sheets has dramatically reduced
potential barriers for elementary migration steps. Especially, the confined
atomic hydrogen dissociation energy from graphene is reduced to 0.34 eV, which
is only about a third of a hydrogen atom chemisorbed on graphene. This offers a
unique platform for direct imaging of the atomic dynamics of confined atoms.
Our results suggest that the atomic interfaces of van der Waals materials may
provide a confined environment to tune the interfacial chemical reactions.
