A reaction complex is formed from a van der Waals dimer precursor, HBr . I-2, and is monitored with picosecond time resolution using standard pump-probe spectroscopy. The reaction is initiated in a slightly attractive region of an excited electronic state with insufficient energy to fragment and will eventually undergo an internal conversion to a lower electronic state via electronic to vibration energy transfer. A resulting product, highly vibrationally excited molecular I-2, is monitored by resonance enhanced multiphoton ionization (REMPI) combined with time of flight mass spectroscopy. The HBr constituent of the precursor HBr . I-2 is photodissociated at 220 nm. The II-atom departs instantaneously, allowing the remaining electronically excited Br(P-2(1/2)) to form a collision complex, (BrI2)*, in a restricted region along the Br+I-2 reaction coordinate determined by the precursor geometry. The evolution of this complex is probed in real time by tuning the probe to the REMPI line of the I atom : 298 nm. The resulting transients include I-2(+) and I+, with lifetimes of 55(+/-5) and 40(+/-5) ps, respectively. Similar results are obtained for initiation from DBr . I-2, with risetimes of 43(+/-5) and 29(+/-5) ps measured for the I-2(+) and I+ transients, respectively. The originally formed (BrI2)* does not have enough internal energy to dissociate directly, but must undergo an internal conversion to a lower electronic state in order to continue to reactants or products. An isotope effect is also detected and explained with a simple kinetics model that is consistent with mechanism described above. Temporal discrepancies in the risetimes of I-2(+) and I+ imply that either the ground state process is also being observed or that differing vibrational states of the I-2 product are formed at differing rates and detected with differing efficiencies.