When two atoms with different masses collide, the atom with the lower mass will typically change its energy significantly (a “hard collision”), whereas the change in the energy of the heavier atom will be relatively small (a "soft collision"). When the two atoms are identical, we may expect that the “softness” of the collision would be neither hard nor soft, but somewhere in between.

Velocity-changing collisions between cold trapped atoms average the inhomogeneous broadening and thereby modify both their spectral width and line shape. We show that a precise measurement of the decay of atomic coherence can yield information on the “softness” of the atomic collisions. We further show that spin echo techniques are even more sensitive to the collisions "softness" and allow a quantitative measurement of it.