Third-order nonlinear coherent transitions are simulated for nonadiaba
tic transitions between ionic and covalent terms of KrF excimer molecu
les. The existence of delayed ultrashort coherence in the course of ph
otodissociation is viewed along with evolution of wave packets. A harp
oonlike reaction caused by photon impact is observed in numerical simu
lations, enabling us to obtain a qualitative understanding about matte
r-wave interference phenomena in bound-free transitions.