Rarely occurring single-photon electronic transitions in the Raman-driven q
uantized motion of a trapped ion are considered. Using a quantum trajectory
approach it is demonstrated that these transitions, which are usually igno
red, may cause significant quantum decoherence effects. In addition, we fin
d an asymmetric evolution in the Wigner function due to induced transitions
of a nonsymmetric type. [S1050-2947(99)51511-4].