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. Between 0. 4 and 0. 8 the C^- photodetachment cross section was measured relative to that for O^- and was found to be almost independent of wavelength, consistent with the assignment of ^4S to the C^- ground state. In this region the C^- cross section is approximately twice that for 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. 250. 03 ev. Weak absorption in the region from 1 to 2. 6 is felt to demonstrate the stability of a metastable negative-ion state, probably C^- 1s^22s^22p^3^2D. No such absorption is observed at wavelengths longer than the ground-state threshold in O^-, in which metastable states are not expected.