Photoelectron spectroscopy of pyridine cluster anions, (Py)_n^-(n=4-13)

Photoelectron spectroscopy was carried out for mass-selected anion clusters of pyridine (C₅H₅N=Py) up to (Py)₁₃^-. The smallest anion cluster observed was (Py)₄^-, which exhibited two distinctly different photoelectron bands arising from dipole-bound and valence electron states. A mixed cluster of [(Py)₃(H₂O)₁]^- displayed similar features. No dipole-bound state was observed in the larger clusters of neat pyridine, (Py)_5-13^-, which were interpreted as solvated clusters of pyridine molecular anion, Py^-(Py)_4-12. Threshold electron binding energies were measured as the upper limit value of adiabatic electron affinities. They increased monotonically from 0.33 eV for the cluster size of n=4 to 1.02 eV for n=13. But their incremental change showed a large drop at n =8, as did the incremental change in vertical detachment energy, which was viewed as due to the completion of the first solvation shell at n=7. The energetics of anion solvation suggested nearly pure electrostatic interactions at play. A boundary was drawn on the adiabatic electron affinity of the pyridine molecule between -0.67 and -0.15 eV. Under a very high laser fluence condition, multiphoton processes were found to occur that lead to photofragmentation followed by photodetachment. Photofragmentation of (Py)_5,6^- yielded photofragments which revealed the same features as the dipole-bound state of (Py)₄^-. This was taken as evidence for the existence of dipole-bound excited states of diffuse orbital character in these larger clusters.