  In systems of ultracold atoms, pairwise interactions can be resonantly
enhanced by a new mechanism which does not rely upon a magnetic Feshbach
resonance. In this mechanism, interactions are controlled by tuning the
frequency of an oscillating parallel component of the magnetic field close to
the Bohr frequency for the transition to a two-atom bound state. The real part
of the s-wave scattering length $a$ has a resonance as a function of the
oscillation frequency near the Bohr frequency. The resonance parameters can be
controlled by varying the amplitude of the oscillating field. The amplitude
also controls the imaginary part of $a$ which arises predominantly because the
oscillating field converts atom pairs into molecules. For the case of a shallow
bound state in the scattering channel, the dimensionless resonance parameters
are universal functions of the dimensionless oscillation amplitude.
