  We present a study of spin 1/2 Kondo singlets in single electron transistors
under a microwave frequency bias excitation. We compare time-averaged
conductance $G$ to predicted universal response with respect to microwave
frequency, oscillation amplitude and the Kondo temperature and find a
non-adiabatic response when the microwave photon energy $hf$ is comparable to
the Kondo temperature $k_B T_K$. We show that our measurements are
qualitatively consistent with the predictions for the radiation-induced
decoherence rate of the Kondo spin.
