Knowledge Bank

The excited {\it E} internal rotor state of He-CH$3$F is studied using microwave spectroscopy and {\it ab initio} calculations. Bound state calculations on a high-level {\it ab initio} intermolecular potential surface} predict a T-shaped ground state and two low-lying excited states with the He localized either at the C end (linear'') or the F end (anti-linear'') of the C-F bond, similar to that found for the {\it A} internal rotor state presented previously.} There are several distinct differences between the {\it A} and {\it E} states. Whereas the ground state rotational transitions of the {\it A} state could be fit with a standard asymmetric rotor Hamiltonian, the {\it E} state exhibits a much more complex energy level structure due to the internal rotation of the CH$3$F. In addition, the {\it E} state is calculated to be bound by 11.709 \wn, 0.249 \wn greater than the {\it A} state relative to their respective dissociation products, {\it K} = 1 or {\it K} = 0 CH$3$F, and the ground state to linear state energy gap is predicted to be 4.148 \wn, 0.156 \wn greater than in the {\it A} state. At present six rotational transitions between five different energy levels have been observed and assigned with the aid of double resonance experiments and the {\it ab initio} calculations.