Formation of negative ions from pyridine clusters in collision with high-rydberg rare-gas atoms and slow electrons

Gas-phase clusters of pyridine were allowed to collide with high-Rydberg rare-gas atoms, Rg**, or slow electrons having kinetic energies of 1-15 eV. The negative cluster ions of pyridine, (C₅H₅N)_n^-, observed by mass spectrometry had a threshold size of 4 for the Rg**-impact ionization and 3 for the electron-impact ionization. These ionization processes are interpreted in terms of vertical electron capture accompanied by intra- and intercluster relaxation of the excess energy associated with the ionization. No significant evaporation occurs in the Rg**-impact ionization if the excess energy is distributed statistically among the intermolecular degrees of freedom. It is therefore concluded that the vertical electron affinity of the pyridine cluster is positive at n ≥ 4. In contrast, the electron-impact ionization causes significant evaporation because of the release of the electron kinetic energy, as indicated by the difference in the size distribution of (C₅H₅N)_n^- and the shift in the threshold size from 4 to 3; in this case, the (C₅H₅N)₃^- ion is produced from larger cluster ions by evaporation, and hence, the adiabatic electron affinity of (C₅H₅N)₃ must be positive. This conclusion can be explained by a simple calculation of the electron affinity based on the electrostatic interaction in a negative cluster ion.