  We report on the first observation of stimulated Raman scattering from a
Lambda-type three-level atom, where the stimulation is realized by the vacuum
field of a high-finesse optical cavity. The scheme produces one intracavity
photon by means of an adiabatic passage technique based on a counter-intuitive
interaction sequence between pump laser and cavity field. This photon leaves
the cavity through the less-reflecting mirror. The emission rate shows a
characteristic dependence on the cavity and pump detuning, and the observed
spectra have a sub-natural linewidth. The results are in excellent agreement
with numerical simulations.
