Fragmentation mass (panels a–f) and IRMPD spectra (g–o) recorded for three different anthracene isotopologs: protonated anthracene – [H − C₁₄H₁₀]⁺, m/z = 179.09 (left column); deuteronated anthracene – [D − C₁₄H₁₀]⁺, m/z = 180.09 (middle column); protonated, perdeuterated anthracene – [H − C₁₄D₁₀]⁺, m/z = 189.09 (right column). For all mass spectra, black traces represent the fragmentation mass spectra, while gray traces are reference precursor mass spectra (with laser on or off-resonance). The gray traces are shifted up 0.2 amu to enhance their visibility. The top row are mass spectra following UV irradiation, the second row following IR irradiation at the indicated IR frequency. Experimental IR spectra are shown in black, superimposed on color shaded calculated spectra for each species with the protonation (deuteronation) site indicated in the structures left of the panels. The theoretical IR spectra were calculated using B3LYP/6-311++G(2d,p) and the ground-state energies in M06-2X/6-311++G(2d,p). Spectra that are considered to “match” are colored green; the ones that are considered to not match are red. Relative energies for each structure are shown above their respective spectra. The dotted curve in panels g,h,i represents the measurement assisted by the CO₂-laser.