  In spin- and angle-resolved photoemission spectroscopy (SARPES) the
energy-momentum dispersion of electronic states in crystalline solids is
measured along with the spin direction of the photoemitted electrons. The
technique therefore allows for mapping out a material's band structure in a
spin resolved fashion. By conducting SARPES measurements using low-energy
photons, the spin sensitivity of the technique can be combined an increased
bulk probe depth, provided by the large electron inelastic mean-free path at
these kinetic energies, to directly access the spin structure of electronic
states at buried interfaces. Here, we demonstrate this capability by using
SARPES to determine the spin polarization of photoelectrons emitted from a
6-nm-thick film of the topological insulator Bi$_2$Se$_3$ using photons with an
energy of 8.5 eV. By modelling the expected spin structure in the film, we show
that the complex spin polarization that is observed is the integrated spin
signal from spin-polarized states at the surface, bulk and buried interface
(bottom surface) of the topological-insulator film. Our results therefore
allows us to directly determine the spin texture of the buried Dirac interface
state. This capability is highly attractive for state-of-the art spectroscopic
measurements of the spin-physics at play in quantum-material based or
spintronic devices where spin-polarized interface states define the operational
principle of the devices.
