Polarization dependent femtosecond pump-probe spectra display characteristic vibrational wave packet dynamics of ClF in Ar (isotropic cage and small fragment size) and I-2 in Kr (cylindrical cage and large fragments). The intensity ratio of the signals for pumping with parallel versus crossed polarization with respect to the probe pulse is close to the value 1/3, as expected for full photoselection immediately after excitation. For ClF this ratio depolarizes to unity within tau(r)=1.2 ps, showing the ultrafast randomization of the orientation of the molecular bond due to fragment scattering off the matrix cage. The direction of the I-2 bond is geometrically fixed by the Kr matrix and the ratio remains constant.