The method of femtosecond velocity-gating in complex systems is reported with an application to the caging problem in a pure solvent cluster of iodobenzenes (n = 2-8). Two structural forms of iodine atoms in the clusters are identified based on the two distinct product recoil velocity distributions and their temporal behaviors : One is the "uncaged" iodine atoms that exhibit ultrafast dissociations (similar to 1.4 ps) and high recoil velocities; the other is the "caged" iodine atoms that escape very slowly (up to 300 ps) from the clusters with very low kinetic energies. These solvation dynamics relate to the nature of the microscopic molecular structures.