  We demonstrate optical sideband spectroscopy of a single $^9$Be$^+$ ion in a
cryogenic 5 Tesla Penning trap using two-photon stimulated-Raman transitions
between the two Zeeman sublevels of the $1s^{2}2s$ ground state manifold. By
applying two complementary coupling schemes, we accurately measure Raman
resonances with and without contributions from motional sidebands. From the
latter we obtain an axial sideband spectrum with an effective mode temperature
of (3.1 $\pm$ 0.4)~mK. This results are a key step for quantum logic operations
in Pennings traps, applicable to high precision matter-antimatter comparisons
tests in the baryonic sector of the standard model.
