Photoelectron spectra of the $C2nH−$ $(n=1–4)$ and $C2nD−$ $(n=1–3)$ anions

Anion photoelectron spectra of the carbon monohydrides, $C2nH−$ for $n=1–4$ and $C2nD−$ for $n=1–3,$ have been measured. The spectra were recorded at a wavelength of 266 nm (4.657 eV) and yield electron affinities for each species. The spectra are vibrationally resolved, and some of the vibrational modes in the neutral $C2nH(D)$ radicals are assigned. In addition, photoelectron angular distributions allow one to distinguish between photodetachment transitions to the $Σ+2$ and $Π2$ states of the neutrals. The spectra confirm previous work showing that $C2H$ and $C4H$ have $Σ+2$ ground states, while $C6H$ and $C8H$ have $Π2$ ground states. In addition, we observe the low-lying $Π2$ or $Σ+2$ excited states for all four radicals. The photoelectron angular distributions also serve as a probe of vibronic coupling between the $Σ+2$ and $Π2$ states. These effects are particularly prominent in the $C2H−$ and $C4H−$ spectra.