Rotate a particle on collision?

Hi! I need a way to rotate my mesh particles to be totally flat with the ground when they collide with a plane. I've got the particles colliding with my plane and I have stopped their movement on collision, but I can't seem to figure out how to set each particle's rotation on collision...

I've tried implementing OnParticleCollision() (tried on both the plane and the particle system itself), but it's never getting that message. "Send Collision Message" is turned on in the particle system's properties.

How can I make my particles lay flat on the plane when they collide with it? Thanks in advance!