  Dispersion relations and amplitudes of collective pionic modes are derived in
a pi + nucleon-hole + delta-hole model for use in transport descriptions by
means of a local density approximation. It is discussed how pionic modes can be
converted to real particles when penetrating the nuclear surface and how
earlier treatments can be improved. When the surface is stationary only free
pions emerge. The time-dependent situation is also addressed, as is the
conversion of non-physical (i.e. unperturbed delta-hole) modes to real
particles when the nuclear density vanishes. A simplified one-dimensional
scenario is used to investigate the reflection and transmission of pionic modes
at the nuclear surface. It is found that reflection of pionic modes is rather
unlikely, but the process can be incorporated into transport descriptions by
the use of approximate local transmission coefficients.
