Structure and Photodetachment Spectrum of the Atomic Carbon Negative Ion

The continuous absorption (photodetachment) spectrum of a beam of atomic carbon negative ions from a hot cathode arc was measured in the spectral region 0.4 to 2.6 μ. Between 0.4 and 0.8 μ the ${\mathrm{C}}^{-}$ photodetachment cross section was measured relative to that for ${\mathrm{O}}^{-}$ and was found to be almost independent of wavelength, consistent with the assignment of ${}_{}{}^4{}_{}{}^{}S$ to the ${\mathrm{C}}^{-}$ ground state. In this region the ${\mathrm{C}}^{-}$ cross section is approximately twice that for ${\mathrm{O}}^{-}$. The variation of the absorption near one micron is consistent with the expected threshold behavior and yields a value for the ground-state binding energy (carbon electron affinity) of 1.25±0.03 ev. Weak absorption in the region from 1 to 2.6 μ is felt to demonstrate the stability of a metastable negative-ion state, probably ${\mathrm{C}}^{-}$ $1s^{2}2s^{2}2p^3{}_{}{}^2{}_{}{}^{}D$. No such absorption is observed at wavelengths longer than the ground-state threshold in ${\mathrm{O}}^{-}$, in which metastable states are not expected.