  In a system of ultracold atoms near a Feshbach resonance, pairs of atoms can
be associated into universal dimers by an oscillating magnetic field with
frequency near that determined by the dimer binding energy. We present a simple
expression for the transition rate that takes into account many-body effects
through a transition matrix element of the contact. In a thermal gas, the width
of the peak in the transition rate as a function of the frequency is determined
by the temperature. In a dilute Bose-Einstein condensate of atoms, the width is
determined by the inelastic scattering rates of a dimer with zero-energy atoms.
Near an atom-dimer resonance, there is a dramatic increase in the width from
inelastic atom-dimer scattering and from atom-atom-dimer recombination. The
recombination contribution provides a signature for universal tetramers that
are Efimov states consisting of two atoms and a dimer.
