Two approaching atoms see a potential similar as shown in the Figure. For two ground state atoms (S+S) the long range potential has the form of R^-6. For atoms in the states (S+P) the long range potential scales with R^-3. The outer turning points of the excited molecular states are located where the S+S potential is essentially flat. By inducing a laser field resonant to a free-bound transition excited state molecules can be generated. The excited molecules can decay into free atoms or ground state molecules. Usually the molecules are measured via trap loss or photoionization.

This characteristic behavior of such a coherent system has been observed by measuring this sub natural linewidth quantum interference feature in a thermal gas.

One way to describe the origin of the EIT dip in the lambda system is to say that the system has been put into a dark state, i.e., a coherent superposition of the two ground states |a> and |g> with the proper relative phase so that the system no longer absorbs light. Applying this particular view to the photoassociation system leads to the interpretation that we have created a rather unusual kind of dark state: a coherent superposition of two colliding atoms and a bound molecule. In other word this is like a superposition between an apple and a orange. The laser tunings are only appropriate to create a dark state for one particular collision energy; for other collision energies, the superposition state is only “gray.”