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The sensitivity of the microwave-optical double resonance (MODR) technique has made it possible to record the spectra of transient chemical compounds in both their ground and excited electronic states. Although the majority of the measured microwave(or rf) transitions involve strong electric dipole allowed transitions between rotational levels, both magnetic dipole allowed and weakly electric dipole allowed A-doubling transitions have been recorded. A previous limitation in applying the technique to molecules produced in high temperature environments has been designing an effective method of simultaneously introducing the laser and low frequency microwave or rf radiation. This limitation precluded measuring A-doubling transitions in many metal containing compounds. Here we report on a method for measuring the low frequency transitions by MODR using a tri-plate transmission line for the introduction of the rf radiation. The $X2\Pi 1/2(j-1.5)$ A-doublet transition of gas-phase copper monoxide. CuO. has been recorded. The measured transition frequencies were merged with previos rotational transition measurements1 and an improved set of spectroscopic parameters were obtained. The results of these experiments will be discussed.