Structure and Photodetachment Spectrum of the Atomic Carbon Negative Ion

Abstract

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 \ensuremath{\mu}. Between 0.4 and 0.8 \ensuremath{\mu} the ${\mathrm{C}}^{\ensuremath{-}}$ photodetachment cross section was measured relative to that for ${\mathrm{O}}^{\ensuremath{-}}$ and was found to be almost independent of wavelength, consistent with the assignment of $^{4}S$ to the ${\mathrm{C}}^{\ensuremath{-}}$ ground state. In this region the ${\mathrm{C}}^{\ensuremath{-}}$ cross section is approximately twice that for ${\mathrm{O}}^{\ensuremath{-}}$. 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\ifmmode\pm\else\textpm\fi{}0.03 ev. Weak absorption in the region from 1 to 2.6 \ensuremath{\mu} is felt to demonstrate the stability of a metastable negative-ion state, probably ${\mathrm{C}}^{\ensuremath{-}}$ $1{s}^{2}2{s}^{2}2{p}^{3}^{2}D$. No such absorption is observed at wavelengths longer than the ground-state threshold in ${\mathrm{O}}^{\ensuremath{-}}$, in which metastable states are not expected.