  We report efficient microwave-optical two photon excitation of Rb Rydberg
atoms in a magneto optical trap. This approach allows the excitation of
normally inaccessible states and provides a path toward excitation of high
angular momentum states. The efficiency stems from the elimination of the
Doppler width, the use of a narrow band pulsed laser, and the enormous electric
dipole matrix element connecting the intermediate and final states of the
transition. The excitation is efficient in spite of the low optical and
microwave powers, of order 1 kW and 1 mW, respectively. This is an application
of the large dipole coupling strengths between Rydberg states to achieve two
photon excitation of Rydberg atoms.
