  The relative orientation of colliding molecules plays a key role in
determining the rates of chemical processes. Here we examine in detail a
prototypical example: rotational quenching of HD in cold collisions with H2. We
show that the rotational quenching rate from j=2 -> 0, in the v=1 vibrational
level, can be maximized by aligning the HD along the collision axis and can be
minimized by aligning the HD at the so called magic angle. This follows from
quite general helicity considerations and suggests that quenching rates for
other similar systems can also be controlled in this manner.
