Direct and Indirect Electron Emission from the Green Fluorescent Protein Chromophore

Photoelectron spectra of the deprotonated green fluorescent protein chromophore have been measured in the gas phase at several wavelengths within and beyond the S-0-S-1 photoabsorption band of the molecule. The vertical detachment energy (VDE) was determined to be 2.68 +/- 0.1 eV. The data show that the first electronically excited state is bound in the Franck-Condon region, and that electron emission proceeds through an indirect (resonant) electron-emission channel within the corresponding absorption band.