  Revivals of optical coherence of molecular photoassociation driven by two
ultrashort laser pulses are addressed in the Condon approach. Based on textbook
examples and numerical simulation of KrF excimer molecules, a prediction is
made about an existence of photon echo on free-bound transitions. Delayed rise
and fall of nonlinear polarization in the half-collisions are to be resulted
from the resonant quantum states interference whether it be in gas, liquid or
solid phases.
