Ammonia dimer studied by photoelectron spectroscopy

The He i photoelectron spectrum of (NH3)2 is shown to have only a single broad unstructured ionization band near 10 eV. This demonstrates the equivalence or near-equivalence of the nitrogen lone-pair orbitals of each NH3 unit in the dimer structure. While theory has generally predicted that (NH3)2 has a classical structure with a near-linear N--H–N linkage, such a structure would be expected to show two ionization bands separated by about 1.5 eV. However analysis of the observed low ionization energy band suggests that a maximum possible band separation would be only 0.5 eV for ionization from the nitrogen lone-pair orbitals. While this result concurs with the nonclassical structure for (NH3)2 proposed by Klemperer and co-workers, it does not exclude the symmetric C2h structure as a likely equilibrium geometry for the dimer. On the contrary, it is argued that the photoelectron spectrum is supportive of the latter structure. Split-valence basis Hartree–Fock calculations on (NH3)2 at various nonclassical geometries indicate that the dimer could have the symmetric C2h geometry but that a relatively shallow potential energy surface for asymmetric van der Waals vibrations would be responsible for the nonzero dipole moment observed by rotational spectroscopy. This conclusion corroborates the finding of the most recent ab initio calculations on (NH3)2 by Latajka and Scheiner.