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

摘要

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 (~0.05e), 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.