  In a gas of ultracold atoms whose scattering length is controlled by a
magnetic Feshbach resonance, atoms can be associated into universal dimers by
an oscillating magnetic field. In addition to the harmonic resonance with
frequency near that determined by the dimer binding energy, there is a
subharmonic resonance with half that frequency. If the thermal gas contains
dimers, they can be dissociated into unbound atoms by the oscillating magnetic
field. We show that the transition rates for association and dissociation can
be calculated by treating the oscillating magnetic field as a sinusoidal
time-dependent perturbation proportional to the contact operator. Many-body
effects are taken into account through transition matrix elements of the
contact operator. We calculate both the harmonic and subharmonic transition
rates analytically for association in a thermal gas of atoms and dissociation
in a thermal gas of dimers.
