Photoionization and infrared laser spectroscopy of molecular complexes: the transition from the molecular to the nanoscale

The microscopic nature of neutral alkaline earth metal-ammonia complexes, namely Ca(NH3)n, Sr(NH3)n, and Ba(NH3)n have been investigated in a molecular beam. Ab initio calculations undertaken in support of the experimental results provided valuable insight into the structures of these complexes. In addition, calculations have also been carried out on other complexes, namely, Mg(NH3)n+ and Eu(NH3)n. Ionization energies have been determined by photoionization for Ca(NH3)n, and Sr(NH3)n complexes and combined with ab initio calculations to assist the interpretation of the photoionization data. Good agreement between the experimental photoionization threshold and theoretical ionization energies based on the lowest energy structures of the complexes is found, indicating that predicted lowest energies structures dominate in the molecular beam. Infrared spectra in the N-H stretching region have been recorded for the Ca(NH3)n, Sr(NH3)n and Ba(NH3)n complexes for the first time using mass-selective photodissociation spectroscopy. The IR spectra of all of these complexes are red-shifted from those of the free ammonia molecule, suggesting that the M-N interaction weakens the N-H bond. The IR spectra and ab initio calculations confirm that the interior structure, in which a metal is surrounding by a number of ammonia molecules is favoured over a structure in which the metal atom sits upon a cluster of ammonia molecules (a ‘surface’ structure). Ab initio calculations predict that the first solvation shell of the Mg(NH3)n+ complexes can hold up to six NH3 molecules. In the case of the Eu(NH3)n complexes, the theoretical calculations predict that the interior structure is energetically favoured, similar to the neutral alkaline earth metals. Calculations show that the first solvation shell is found to close with eight ammonia molecules around the central europium atom.