  Atomically uniform Sb(111) films are fabricated by the method of molecular
beam epitaxy on an optimized Si(111) surface. Two dimensional quantum well
states and topological surface states in these films are well resolved as
measured by angle-resolved photoemission spectroscopy. We observe an evolution
of direct transition resonances by varying the excitation photon energy (and
thus the perpendicular crystal momentum). The experimental results are
reproduced in a comprehensive model calculation taking into account
first-principles calculated initial states and time-reversed
low-energy-electron-diffraction final states in the photoexcitation process.
The resonant behavior illustrates that the topological surface states and the
quantum well states are analytically connected in momentum space in all three
dimensions.
