Hyperfine Structure of the CN Radical Determined from Astronomical Observations

We report the determination of the hyperfine, $\rho$-doubling and rotational constants of the CN molecule, by observation of its $N=1\mathrm{}\to 0$ line emission from an interstellar molecule cloud. The $N=1\mathrm{}\to 0$ multiplet has seven allowed transitions, all of which were measured to an accuracy of 0.5 MHz or better. This astronomical measurement is the first sucessful attempt at determining the CN hyperfine structure in the free molecule, since the appropriate transitions have yet to be observed in the laboratory.