Knowledge Bank

The avoided crossing method (1,2) developed recently for studying in symmetric tops the normally forbidden transitions with $\Delta K\ne 0$ has been applied for the first time to a molecule which is not a near spherical rotor. The spectrum in $CF3H$ due to the avoided crossing ($J=1,K=0\to \pm 1,mJ=\pm 1\to \mp 1$) has been observed. In order to reach the high electric fields required while still retaining the necessary narrow line width, the electric resonance beam spectrometer has been equipped with a new pair of Stark plates capable of providing, electric fields $\sim 11$ kV/cm with a homogeneity of 1 part in $105$ over a length of 2.5 cm. The difference ($C0,-B0$) of the rotational constants for fluoroform in the ground vibrational state was determined to be 4673.36(10) MHz from the crossing field by using the measurement of the permanent electric dipole moment $\mu =1.65150(10)D$ made with conventional beam techniques. This work demonstrates that the avoided crossing method can be used to obtain the moment of inertia about the symmetry axis for any simple prolate or oblate symmetric top in which the difference in the rotational constants $\sim 5$ GHz and the dipole moment $\sim 1 $D.