Negative ion photoelectron spectroscopy of (benzene)n− (n=53–124) and (toluene)n− (n=33–139): Solvation energetics of an excess electron in size-selected aromatic hydrocarbon nanoclusters

We present a negative ion photoelectron spectroscopic study on the solvation energetics of an excess electron in the size-selected aromatic hydrocarbon nanoclusters, (benzene)n− (n=53–124) and (toluene)n− (n=33–139). The formation and stability of these negatively charged clusters were investigated using mass spectrometry with two different sources: (1) low-energy (≈0.3 eV) electron attachment and (2) high-energy electron impact. The results reveal that very large coordination numbers (n∼25) are necessary for the formation of stable benzene and toluene cluster anions. This suggests that the second solvation layer (effect) is essential for stable binding of the excess electron in these clusters. The energetics of the cluster anions were also explored by negative ion photoelectron spectroscopy. The photoelectron spectra obtained were related to bulk parameters, e.g., solvent reorganization energy and conduction band minimum, via an analysis of vertical detachment energies versus n−1/3. The bulk solvent reorganization energies thus estimated have revealed the excess electron trapping levels in bulk benzene and toluene. The applicability of the generalized cluster size equation is examined in these nondipolar aromatic hydrocarbon systems for the first time.