We have performed the fs pump-probe experiment on the (B) over tilde -->(a) over tilde nonadiabatic transition in the Hg-CO vdW complex. It is found that the transition occurs near the point where Hg and CO come nearest to each other on the (B) over tilde-state surface with a probability of 0.07 for their single encounter. This experiment serves as the first observation of the wave packet dynamics associated with an intermolecular energy flow in the vdW interaction.