  The velocity distribution of a hot ionic beam can be filtered with a narrow
stimulated Raman process to prepare a colder subensemble, as substantiated in
this theoretical analysis. Using two counter-propagating, far-detuned lasers,
we can define a $\pi$-pulse for the resonant velocity to transfer atoms within
the linewidth of the Raman resonance between the ground-states of a
$\Lambda$-system. Spontaneous emission from the two single-photon resonances,
as well as the ground-state decoherence induced by laser noise, diminishes the
efficiency of the filter. From a comprehensive master equation, we obtain
conditions for the optimal frequency pair of the lasers and evaluate the filter
performance numerically, as well as analytically. If we apply this analysis to
current $^{40}$Ca$^+$ ion experiments, we obtain a sensitivity for measuring
high ion acceleration voltages on the ppm level or below.
