IRPD spectrum of Ne-tagged naph²⁺ (top panel, blue) compared with calculated anharmonic and harmonic band positions of singlet naph²⁺ and anharmonic frequencies of triplet naph²⁺. Both fundamental modes (blue) and combination modes (red) are shown. The calculated spectrum was convoluted with a Gaussian lineshape function, where the width is given by the FEL bandwidth, and the area corresponds to the calculated intensity in km mol⁻¹.