  We report the direct observation of a novel type of rainbow caustic. In
contrast to known examples, this caustic originates from a dissipative,
stochastic process. We have observed this using cold ^{87}Rb atoms bouncing
inelastically on an evanescent-wave atom mirror. The caustic appears as a sharp
peak at the lower edge of the asymmetric velocity distribution of the bouncing
atoms. The stochastic process is a spontaneous Raman transition due to photon
scattering during the bounce. The results are in good agreement with a
classical calculation.
