The rotational spectrum of the CP radical was detected in the laboratory by using a source-modulated microwave spectrometer combined with a free space absorption cell. The CP radical was produced by a dc-glow discharge in a mixture of PH3, CH4, and He. Nineteen lines of five rotational transitions were observed in the frequency region of 95-334 GHz and were least-squares analyzed to determine the molecular constants: the rotational constant, the centrifugal distortion constant, the spin-rotation coupling constant, and hyperfine coupling constants for the phosphorus nucleus. Based on the measured and calculated line frequencies, astronomical searches for the interstellar CP radical were made in Orion-KL, Sgr B2, W51A, TMC-1, and IRC + 10216, for which upper limits to the column density were estimated.
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Saito et al. (1989) studied this question.