We investigated the coherent motion of vibrational wave packets in the \B> (1) Pi(u) state of the potassium dimer applying two color pump/probe spectroscopy with a sub 100 fs time resolution. Special interest was paid to the ionization probe step which was analyzed carefully by varying the probe energy over a wide range. Time-dependent quantum calculations explain the experimental outcomes by introducing a nonconstant transition dipole moment between the \B> and the ionic state \X+> and by taking into account the excitation of long lived autoionizing Rydberg states. (C) 1997 American Institute of Physics. [S0021-9606(97)02647-0].