Measurement of the intermolecular vibration–rotation tunneling spectrum of the ammonia dimer by tunable far infrared laser spectroscopy

Over 150 lines in six tunneling subbands of an intermolecular vibration located near 25 cm−1 have been measured with partial hyperfine resolution and assigned to (NH3)2. The transitions sample all three types of tunneling states (A, G, E) and are consistent with the following assumptions: (1) G36 is the appropriate molecular symmetry group; (2) the equilibrium structure contains a plane of symmetry; (3) interchange tunneling of inequivalent monomers occurs via a trans path; (4) the 2C3+I limit of hydrogen exchange tunneling is appropriate; (5) tunneling and rotational motions are separable. A qualitative vibration–rotation tunneling energy level diagram is presented. Strong perturbations are observed among the states of E symmetry. This work supports the conclusions of Nelson et al. [J. Chem. Phys. 87, 6365 (1987)].