Microwave–microwave double resonance using a Fabry–Perot cavity spectrometer

The design and operation of a relatively simple, yet versatile spectrometer for microwave–microwave double resonance (MMDR) studies is described. The system consists of a high‐Q, semiconfocal Fabry–Perot cavity resonator which is crossed at right angles by a second free‐space microwave beam. We excite the cavity with a low‐power unmodulated signal source, while the free‐space irradiation source is square‐wave frequency modulated and is of moderately high power (several hundred milliwatts). Both microwave sources are scannable; the signal source is locked to the cavity while the pumping klystron is free running. The signal (cavity) frequency has been operated in the 15–40‐GHz range, and pump frequencies can be selected and easily implemented above or below the signal microwave frequency.