Electronic Excited States of Naphthalene: Perspectives from Synchrotron Radiation-Based Photoabsorption and TDDFT Studies

We report a synchrotron radiation-based study of the simplest Polycyclic Aromatic Hydrocarbon (PAH) molecule, naphthalene, in the energy region 35,000-88,000 cm^-1 (4.4-10.9 eV). A complete spectral analysis of the VUV region is carried out for the first time, and several new bands are reported, while reassignments are made for many of the bands reported earlier. An extensive Rydberg series converging to the first seven ionization energies of naphthalene are observed, interspersed with several valence transitions. Rydberg series of ns, np, and nd types are assigned based on quantum defect analysis and correlated with theoretical calculations. Time-dependent density functional calculations performed using several functionals and basis set combinations helped in verifying and consolidating spectral assignments. From the absolute absorption cross-section data, the UV-VUV photolysis rates at different altitudes are estimated. It is found that the lower limit to the photolysis lifetime varies from ∼1 h at 20 km to ∼4 s at 50 km. Potential energy curves of the first few singlet and triplet excited states with respect to C-H bond length do not show any evidence of direct dissociation, thus implying that the H loss channel may not be very prominent in neutral naphthalene, in contrast to cationic naphthalene.