Knowledge Bank

The microwave absorption spectrum $(K=0\to 1)$ of the CN radical in the $v=0$ and 1 levels of the electronic ground state has been observed in glow discharges in nitrogen-cyanogen mixtures. A phase-locked, computer controlled Zeeman modulation system is used with the cell at room temperature. The zero field frequencies of the seven major hyperfine components for each vibrational stare are extracted from the complicated Zeeman modulation patterns observed by a careful least squares analysis of the line-shapes. The molecular constants obtained for $v=0$ are in good agreement with, but more accurate than, the previous astrophysical values, and for $v=1$ the present work constitutes the first determination of the hyperfine constants. The precision of the determined molecular parameters in both states is sufficient to obtain reliably the variation of the coupling constants with vibrational quantum number.