We present an experimental and a theoretical study of nonadiabatic wave packet dynamics in the intermediate coupling regime as exhibited by the IBr molecule. Using a femtosecond pump–probe molecular beam technique, we generated a wave packet which evolves on the electronically excited B3P 0+ /Y(0+) coupled states. The wave packet dynamics was detected by a time-delayed probe pulse which induced two photon ionization to the ground state of the IBr + ion. The study consisted of a systematic variation of the pump laser wavelength from the crossing point of the two coupled states to the dissociation limit of the bound diabatic state. The theoretical study is based on the convolution of the products of the energy resolved X1 å+ ®B3P0+ /Y(0+) bound–free dipole matrix elements and the free–bound two-photon ionization amplitudes (calculated exactly using the artificial channel method) with the profiles of the pump and probe pulses. The theoretical calculations reproduce the general decay, recurrence, and revivals observed experimentally. The importance of treating nonadiabatic dynamics beyond the Landau–Zener approximation, as well as the utility of femtosecond pump–probe techniques in probing simultaneously short and long lived resonances is demonstrated.