Avoided-crossing molecular-beam experiments on fluoroform (CF3H) and fluoroform-d (CF3D)

Abstract

The avoided‐crossing molecular‐beam method for studying normally forbidden (ΔK≠O) transitions in symmetric tops has been applied to fluoroform (CF₃H) and fluoroform‐d (CF₃D), thus marking the extension of the method to systems which are not near spherical rotors. In order to reach the high electric fields required while still retaining the necessarily narrow linewidth, the electric resonance spectrometer has been equipped with a new pair of Stark plates capable of providing electric fields up to about 20 kV/cm with a homogeneity of 1 part in 10⁵ over a length of 3 cm. The anticrossing (J,K) = (1,0)↔(1,±1) has been studied for both CF₃H and CF₃D. In each case, the rotational constant C₀ along the symmetry axis has been obtained to 0.002%. From anticrossing spectra observed in combined electric and magnetic fields, the signs of the rotational g factors g_∥ and g_⊥ have been shown to be negative. From a conventional molecular beam study for each isotopic species, a value of the permanent electric dipole moment accurate to 60 ppm was determined and improved values of g_∥ and g_⊥ were obtained. The direction of the electric dipole moment is shown by two methods to be +HCF₃−. A brief discussion of the difficulties in these methods is given.