Charge transfer by electronic excitation: Direct measurement by high resolution spectroscopy in the gas phase

We report a quantitative measurement of the amount of charge that is transferred when the single ammonia complex of the photoacid β -naphthol (2HNA) is excited by light. The measurement was made by comparing the permanent electric dipole moments of cis -2HNA in its ground ( S 0 ) and excited ( S 1 ) states, determined by Stark-effect studies of its fully resolved S 1 ← S 0 electronic spectrum. While the increase in electron transfer from the donor ( NH 3 ) to the acceptor (2HN) upon excitation is small ( ∼ 0.05 e ) , it is sufficient to redshift the electronic spectrum of the complex by ∼ 600 cm − 1 ( ∼ 0.1 eV ) . Thereby explored is the incipient motion of the acid-base complex along the excited state (electron-coupled) proton transfer coordinate.