STARK-EFFECT AND DIPOLE-MOMENTS OF THE AMMONIA DIMER IN DIFFERENT VIBRATION-ROTATION-TUNNELING STATES

In this paper we present Stark measurements on the G:K=-1 vibration-ro tation-tunneling (VRT) transition, band origin 747.2 GHz, of the ammon ia dimer. The observed spitting pattern and selection rules can be exp lained by considering the G(36) and G(144) symmetries Of the inversion states involved, and almost complete mixing of these states by the ap plied electric field. The absolute values of the electric dipole momen ts of the ground and excited state are determined to be 0.763(15) and 0.365(10) D, respectively. From the theoretical analysis and the obser ved selection rules it is possible to establish that the dipole moment s of the two interchange states must have opposite sign. The theoretic al calculations are in good agreement with the experimental results: T he calculated dipole moments are -0.74 D for the lower and +0.35 D for the higher state. Our results, in combination with the earlier dipole measurements on the G:K=0 ground state and the G:K=1 transition with band origin 486.8 GHz, confirm that the ammonia dimer is highly nonrig id. Its relatively small and strongly K-dependent dipole moment, which changes sign upon far-infrared excitation, originates from the differ ence in dynamical behavior of ortho and para NH3. (C) 1996 American In stitute of Physics.