“…Rg atoms are convenient ligands, as the weak intermolecular interaction usually causes only minor perturbations of the X ϩ chromophore. To reveal the spectrum of aromatic cations, often their complexes with Ar are investigated, 5-8 as the intermolecular interaction is weak enough to cause only small perturbations of the bare cations, 5,9 but strong enough to allow for sufficient production of X ϩ -Ar dimers, in most cases achieved by soft resonant multiphoton ionization of the neutral complex in molecular beams. Indeed, for aromatic cations, the vibrational IR spectra of X ϩ -Ar dimers appear to closely resemble those of the bare aromatic cations.…”