Alpha-and beta-protonated naphthalenes (α-and β-HN~+) were investigated by electronic absorption and fluorescence spectroscopies in 6 K neon matrixes using a mass-selected C_(10)H_9 ~+ ion beam. The absorption spectra reveal S_1/S_2 ← S_0 transitions with onsets at 502.1 and 396.1 nm for α-HN ~+, and 534.5 and 322.3 nm in the case of β-HN~+. Wavelength-dispersed fluorescence was detected for α-HN~+, starting at 504.4 nm. Light-induced α-HN~+ → β-HN ~+ isomerization was observed upon S_2 ← S_0 excitation of α-HN~+, whereas β-HN~+ relaxed back into the more stable alpha form either upon excitation to S_1 or via thermal population of the ground state vibrational levels near the top of the energy barrier between the two isomers. The intramolecular proton transfer leading to the α-HN~+ β-HN~+ photoisomerization is fully reversible. The observations are explained with the support of theoretical calculations on the ground-and excited states of the isomers, vertical excitation and adiabatic energies, minimum-energy pathways along the relevant reaction coordinates, and conical intersections between the electronic states.
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