Communication: photoelectron angular distributions of CH(-) reveal a temporary anion state

Photoelectron imaging has broadened the scope of traditional photoelectron spectroscopy by combining a simultaneous photoelectron angular distribution, PAD, measurement with kinetic energy analysis. A fundamental understanding of PADs has been largely limited to simple atomic systems. However, a new model has recently been developed that predicts PADs as a function of electron kinetic energy for a simple linear combination of s and p atomic orbitals. We used CH(-) to test this model by acquiring PADs in a photoelectron imaging spectrometer at wavelengths from 600 to 355 nm. The PADs for electron detachment from the HOMO (1π) of CH(-) fit model predictions. However, the PADs associated with detachment from the HOMO-1 (3σ) orbital exhibit anomalous behavior at low electron kinetic energies because of a resonant process that arises from a previously undetected excited state of CH(-).