Dissociation energy and vibrational predissociation dynamics of the ammonia dimer

Experiments using infrared excitation of either the intramolecular symmetric N-H stretch (nu(NH,S)) or the intramolecular antisymmetric N-H stretch (nu(NH,A)) of the ammonia dimer ((NH3)(2)) in combination with velocity-map ion imaging provide new information on the dissociation energy of the dimer and on the energy disposal in its dissociation. Ion imaging using resonance enhanced multiphoton ionization to probe individual rovibrational states of one of the ammonia monomer fragments provides recoil speed distributions. Analyzing these distributions for different product states gives a dissociation energy of D-0 = 660 +/- 20 cm(-1) for the dimer. Fitting the distributions shows that rotations are excited up to their energetic limit and determines the correlation of the fragment vibrations. The fragments NH3(nu(2) = 3(+)) and NH3(nu(2) = 2(+)) have a vibrational ground-state partner NH3(nu = 0), but NH3(nu(2) = 1(+)) appears in partnership with another fragment in nu(2) = 1. This propensity is consistent with the idea of minimizing the momentum gap between the initial and final states by depositing a substantial fraction of the available energy into internal excitation. (C) 2011 American Institute of Physics. [doi:10.1063/1.3625634]