We demonstrate that broadband impulsive stimulated Raman scattering with transient grating experimental geometry is capable of recording vibrational spectra in time and frequency domains to reveal the dissociation mechanism of liquid CH3I. The repulsive (3)Q(0) potential energy surface is modified to a quasi-bound surface in the liquid CH3I due to the solvent cage, making the wave packets to get trapped. A small portion of the wave packet passes through the solvent cage, resulting in the C-I bond breakage with the product I-2. This work provides a first glimpse of immense potential of BB-ISRS in studying photodissociation dynamics in liquids. (C) 2015 Elsevier B.V. All rights reserved.