Cryogenic Ion Spectroscopy of the Green Fluorescent Protein Chromophore in Vacuo.

We present the spectrum of the S ₁ ← S ₀ transition of an anionic model for the chromophore of the green fluorescent protein in vacuo at cryogenic temperatures, showing previously unresolved vibrational features, and resolving the band origin at 20 930 cm ^-1 (477.8 nm) with unprecedented accuracy. The vibrational spectrum establishes that the molecule is in the Z isomer at low temperature. At increased temperature, the S ₁ ← S ₀ band shifts to the red, which we tentatively attribute to emergent population of the E isomer.