Nonadiabatic wave packet dynamics: Experiment and theory in IBr

We present an experimental and a theoretical study of nonadiabatic wave pac ket dynamics in the intermediate coupling regime as exhibited by the IBr mo lecule. Using a femtosecond pump-probe molecular beam technique, we generat ed a wave packet which evolves on the electronically excited B (3)Pi(0+)/Y( 0(+)) coupled states. The wave packet dynamics was detected by a time-delay ed probe pulse which induced two photon ionization to the ground state of t he IBr+ ion. The study consisted of a systematic variation of the pump lase r wavelength from the crossing point of the two coupled states to the disso ciation limit of the bound diabatic state. The theoretical study is based o n the convolution of the products of the energy resolved X (1)Sigma(+) --> B (3)Pi(0+)/Y(0(+)) bound-free dipole matrix elements and the free-bound tw o-photon ionization amplitudes (calculated exactly using the artificial cha nnel method) with the profiles of the pump and probe pulses. The theoretica l calculations reproduce the general decay, recurrence, and revivals observ ed 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. (C) 1999 American Institute of Physics. [S0021-9606(99)003 05-0].