We investigated two deprotonated paracoumaric acid model chromophores of the photoactive yellow protein, ${\mathrm{pCE}}^{-}$ and ${\mathrm{pCK}}^{-}$, using action-absorption spectroscopy. Employing cryogenic cooling, we determined the ${\mathrm{S}}_0\text{−}{\mathrm{S}}_1$ band origins and associated vibrational resonances, which were found to reside within the electronic continuum, rendering them susceptible to autodetachment. Resonances observed below the electron-detachment threshold were attributed to dipole-bound states of the neutral radical core, probed via resonant two-photon excitation and detachment. Above the electron-detachment limit, we identified dipole-resonance states susceptible to autodetachment. The experimental approach, combining 6 K helium-buffer gas cooling with an ion-storage ring, proved effective in resolving vibrational modes of both the neutral radical core and the valence-excited ${\mathrm{S}}_1$ state, revealing soft-mode resonances down to 50–100 ${\mathrm{cm}}^{-1}$. Time-resolved action detection indicates that internal conversion competes with autodetachment.

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• Cold and ultracold molecules
• Electronic transitions
• Molecular spectra
• Photodetachment
• Vibrational states