An optical spectrum of a large isolated gas-phase PAH cation: C₇₈H₂₆^.

A gas-phase optical spectrum of a large polycyclic aromatic hydrocarbon (PAH) cation - C₇₈H₂₆⁺- in the 410-610 nm range is presented. This large all-benzenoid PAH should be large enough to be stable with respect to photodissociation in the harsh conditions prevailing in the interstellar medium (ISM). The spectrum is obtained via multi-photon dissociation (MPD) spectroscopy of cationic C₇₈H₂₆ stored in the Fourier Transform Ion Cyclotron Resonance (FT-ICR) cell using the radiation from a mid-band optical parametric oscillator (OPO) laser. The experimental spectrum shows two main absorption peaks at 431 nm and 516 nm, in good agreement with a theoretical spectrum computed via time-dependent density functional theory (TD-DFT). DFT calculations indicate that the equilibrium geometry, with the absolute minimum energy, is of lowered, nonplanar C₂ symmetry instead of the more symmetric planar D_2h symmetry that is usually the minimum for similar PAHs of smaller size. This kind of slightly broken symmetry could produce some of the fine structure observed in some diffuse interstellar bands (DIBs). It can also favor the folding of C₇₈H₂₆⁺ fragments and ultimately the formation of fullerenes. This study opens up the possibility to identify the most promising candidates for DIBs amongst large cationic PAHs.