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.