Fig. 5.

Photoelectron spectra of C${}_2^{-}$ at a range (290–355 nm) of detachment wavelengths. The vibrational origin transitions are observed for the ground $X^1{\mathrm{\Sigma }}_{\mathrm{g}}^{+}$ and first excited $a^3{\mathrm{\Pi }}_{\mathrm{u}}$ electronic states. The OPO measurements (290–325 nm) also show the (1, 0) vibrational transition for each state