  Tunneling of Bloch electrons through a vacuum barrier introduces new physical
effects in comparison with the textbook case of free (plane wave) electrons.
For the latter, the exponential decay rate in the vacuum is minimal for
electrons with the parallel component of momentum ${\bf k}_\parallel=0$, and
the prefactor is defined by the electron momentum component in the normal to
the surface direction. However, the decay rate of Bloch electrons may be
minimal at an arbitrary ${\bf k}_\parallel$ (``hot spots''), and the prefactor
is determined by the electron's group velocity, rather than by its
quasimomentum.
