We report on the direct observation of recurrent fluorescence (RF) from initially cold ${\mathrm{C}}_4{}^{-}$ anions obtained at specific photoexcitations from the electronic ground state ($\mathrm{X}{}^2{\mathrm{\Pi }}_g$) under conditions mimicking the environment in interstellar space. Using a combination of buffer gas cooling in a (small) radio-frequency ion trap and laser excitation in an electrostatic ion storage ring, we explicitly illustrate the cooling of a molecular system due to RF and meet an experimental challenge posed almost 40 years ago. In the effort to directly observe RF, we also reexamine the spectroscopy of the $\mathrm{C}{}^2{\mathrm{\Pi }}_u\leftarrow \mathrm{X}{}^2{\mathrm{\Pi }}_g$ transition in ${\mathrm{C}}_4{}^{-}$ as probed by two-photon photodetachment spectroscopy and assign all observed transitions from cold ${\mathrm{C}}_4{}^{-}$. Our assignment and modeling of the observed photoabsorption spectrum allow us to determine spin-orbit coupling constants for both the ground ($\mathrm{X}{}^2{\mathrm{\Pi }}_g$) and third excited ($\mathrm{C}{}^2{\mathrm{\Pi }}_u$) state of ${\mathrm{C}}_4{}^{-}$ and reveal their variation with vibrational excitation energy for the ${\nu }_{2.\mathrm{C}}$ symmetric stretch mode and the ${\nu }_{3.\mathrm{C}}$ asymmetric stretch mode of the excited state.

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• Atomic & molecular structure
• Cooling & trapping
• Electronic excitation & ionization
• Electronic structure of atoms & molecules
• Electronic transitions
• Photodetachment
• Photoemission
• Rotational states
• Spin-orbit coupling
• Spontaneous emission
• Ultrafast phenomena
• Vibrational states