I never use 3D Max but I am a user of maya you can add some drag field when it collide with the surface. dont use omni emitter, use volume emitter. I think in max you have reactor for creating fields. or you can slow down your time in post effect by key frame at the beginning of timeline when your explosion explodes, add another key frame at that point where you want to slow your particle and 1 last key frame at that time when your particles are at rest. then stretch last key frame as much as you want to slow down that process. In after effect this fuction is known as time-remapping. hope this will help you.