Role of the charge-transfer state in the electronic absorption of protonated hydrocarbon molecules.

The vibrationally resolved electronic spectra of isolated protonated polycyclic aromatic hydrocarbons (PAHs)--naphthalene, anthracene, and tetracene--have been recorded via neutral photofragment spectroscopy. The S1←S0 transitions are all in the visible region and do not show a monotonic red shift as a function of the molecular size, as observed for the neutral analogues. Comparison with ab initio calculations indicates that this behavior is due to the nature of the excited state, which has a pronounced charge-transfer character for protonated linear PAHs with an even number of aromatic rings.