Accurate Determination of the Electron Affinity and the First Excited State ( a ⁴ Σ – ) Energy Level of CH

The methylidyne CH radical is one of the first molecules detected in the interstellar medium (ISM) and also one of the most common radicals in combustion. It plays an important role in both astronomical and combustion studies. However, for this simple yet important molecule, knowledge about its first excited state ( a 4 Σ – ) remains scarce. In this work, we employ slow electron velocity-map imaging (SEVI) spectroscopy to precisely determine the electron affinity (EA) of CH as 9795.8(3) cm –1 or 1.21452(4) eV. The term energy of the a 4 Σ – state is determined to be 6054.9(9) cm –1, corresponding to a wavelength of 16515(2) Å for the transition to the ground state. Additionally, the spectral profile of the forbidden transition between the CH a 4 Σ – and X 2 Π states is simulated, which can be used to determine the column density of dense interstellar clouds.