The microwave spectrum of the NCl radical in the electronically excited (a1Δ) state

The pure rotational spectrum of the 14N35Cl radical in the first electronically excited a 1Δ state was detected by microwave spectroscopy. The NCl radical was produced by a dc‐glow discharge of an N2 and Cl2 mixture between 175–210 K. Seven rotational transitions for υ=0 and five for υ=1, showing hyperfine structures due to the nitrogen and chlorine nuclei, were observed in the 162–404 GHz region. The rotational, centrifugal distortion, and hyperfine coupling constants including nuclear spin–rotation coupling constant of the chlorine nucleus were determined accurately by a least‐squares analysis of the measured frequencies. The equilibrium structural parameters were derived and discussed.