Microwave spectroscopy of the CCl radical

The CCl radical was generated by a dc glow discharge in CCl4, and rotational transitions of C35Cl in both 2 Π 1 2 and 2 Π 3 2 were observed in the mm-wave region by using a source modulation microwave spectrometer. From an analysis of the observed spectra the magnetic and electric quadrupole hyperfine coupling constants of the 35Cl nucleus were determined precisely. The Λ-doubling constant p0 was determined to be positive, indicating that unknown 2Σ− excited electronic states are making dominant contributions to the Λ doubling in the ground state. The previous diode-laser result [J. Mol. Spectrosc.85, 416–426 (1981)] was reanalyzed by using improved wavelength standards and by fixing the ground-state parameters to the present microwave values including the B0 rotational constant 20 797.1725(38) MHz. The vibration-rotation constant αB and the equilibrium bond length re were thus determined to be 0.006 768 2(48) cm−1 and 1.645 218(16) A, respectively, with 2.5 times standard deviations in parentheses. The spin density on the chlorine atom was estimated to be about 0.20 from the observed hyperfine coupling constants.