  We explore the many body physics of a Bose condensed atom gas at finite
temperature through the Raman transition between two hyperfine levels. Unlike
the Bragg scattering where the phonon-like nature of the collective excitations
has been observed, a different branch of thermal atom excitation is found
theoretically in the Raman scattering. This excitation is predicted in the
generalized random phase approximation (GRPA) and has a gapped and parabolic
dispersion relation. The gap energy results from the exchange interaction and
is released during the Raman transition. The scattering rate is determined
versus the transition frequency $\omega$ and the transferred momentum $\vc{q}$
and shows the corresponding resonance around this gap. Nevertheless, the Raman
scattering process is attenuated by the superfluid part of the gas. The
macroscopic wave function of the condensate deforms its shape in order to
screen locally the external potential displayed by the Raman light beams. This
screening is total for a condensed atom transition in order to prevent the
condensate from incoherent scattering. The experimental observation of this
result would explain some of the reasons why a superfluid condensate moves
coherently without any friction with its surrounding.
