Microhydration effects on the electronic spectra of protonated polycyclic aromatic hydrocarbons: [naphthalene-(H 2 O) n = 1,2 ]H

Vibrational and electronic spectra of protonated naphthalene (NaphH + ) microsolvated by one and two water molecules were obtained by photofragmentation spectroscopy. The IR spectrum of the monohydrated species is consistent with a structure with the proton located on the aromatic molecule, NaphH + -H 2 O. Similar to isolated NaphH + , the first electronic transition of NaphH + -H 2 O (S 1 ) occurs in the visible range near 500 nm. The doubly hydrated species lacks any absorption in the visible range (420-600 nm) but absorbs in the UV range, similar to neutral Naph. This observation is consistent with a structure, in which the proton is located on the water moiety, Naph-(H 2 O) 2 H + . Ab initio calculations for [Naph-(H 2 O) n ]H + confirm that the excess proton transfers from Naph to the solvent cluster upon attachment of the second water molecule.