Vibronic structure in $C2H$ and $C2D$ from anion slow electron velocity-map imaging spectroscopy

The $C2H$ and $C2D$ radicals are investigated by slow electron velocity-map imaging (SEVI) of the corresponding anions. This technique offers considerably higher resolution $(<0.5meV)$ than photoelectron spectroscopy. As a result, SEVI spectra of the two isotopomers yield improved electron affinities and reveal many new structures that are particularly sensitive to vibronic coupling between the ground $Σ+2$ and low-lying excited $Π2$ states. These structures, which encompass more than $5000cm−1$ of internal excitation, are assigned with the aid of previous experimental and theoretical work. We also show that SEVI can be applied to photodetachment transitions resulting in ejection of an electron with orbital angular momentum $l=1$⁠, a $p$ wave, in contrast to anion zero-electron kinetic energy spectroscopy which is restricted to $s$-wave detachment.