We measured the thresholds for photodetachment from the first and second excited rotational levels of ${\mathrm{CH}}^{-}$ to the lowest vibrational, rotational, and fine-structure level of CH to be $E_1=1.213\pm 0.002\mathrm{eV}$ and $E_2=1.206\pm 0.002\mathrm{eV}$, respectively. Based on these measurements and the rigid rotor approximation, we arrive at an electron affinity of $\text{EA}=E_1+\frac{1}{2}(E_1-E_2)=1.217\pm 0.002\mathrm{eV}$. This value deviates from earlier experimental results but agrees with the calculation by Feller [J. Chem. Phys. 144, 014105 (2016)]. In the present experiment, we stored ensembles of initially hot ${\mathrm{CH}}^{-}$ in the cryogenic ion-beam storage ring Double ElectroStatic Ion-Ring ExpEriment (DESIREE) for tens of seconds such that the vast majority of the ions were in the few lowest excited rotational levels of the electronic and vibrational ground state. We identified the initial states for photodetachment channels with threshold energies $E_1$ and $E_2$ by comparing the time dependences of measured photodetachment signals with radiative rotational-cooling rates calculated using the literature values of the dipole moment of ${\mathrm{CH}}^{-}$. The conditions of a few occupied rotational levels are superior to those of previous studies of this system and an important step toward future studies with an all-rotational-ground-state ion beam.

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• Atomic & molecular structure
• Cooling & trapping
• Photodetachment
• Rotational states