  We develop a theoretical model to describe the radio-frequency (rf) induced
coupling of a pair of colliding atoms to a Feshbach molecule when a magnetic
field arbitrarily far from the Feshbach resonance is modulated in time. We use
the dressed atom picture, and show that the coupling strength in presence of rf
is equal to the Feshbach coupling strength multiplied by the square of a Bessel
function. The argument of this function is equal to the ratio of the atomic rf
Rabi frequency to the rf frequency. We experimentally demonstrate this law by
measuring the rate of rf-association of molecules using a Feshbach resonance in
d-wave collisions between ultra-cold chromium atoms.
