Photofragmentation spectrum of protonated acridine compared to the simulated spectrum: a) the experimental spectrum plotted as the fragment ion signal (arbitrary units) as a function of the excitation energy; b) the simulated spectrum plotted as the Franck-Condon factors as a function of the excitation energy, calculated using the calculated ground and excited state frequencies. The main vibrational progressions (indicated in blue in panel a) involve, essentially, the two symmetric modes ν₁₀ at 383 cm^-1 (ring breathing) and ν₁₈ at 587 cm^-1. These vibrational modes are illustrated schematically above the experimental spectrum, with arrows representing the direction and relative magnitude of the displacement of each atom. The intensities of the out of plane modes ν₁ and ν₃ are overestimated in the simulation.