$A$-band methyl halide dissociation via electronic curve crossing as studied by electron energy loss spectroscopy

Excitation of the $A$-band low-lying electronic states in the methyl halides, $CH3I$⁠, $CH3Br$⁠, $CH3Cl$⁠, and $CH3F$⁠, has been investigated for the $(n→σ∗)$ transitions, using electron energy loss spectroscopy (EELS) in the range of 3.5–7.5 eV. For the methyl halides, $CH3I$⁠, $CH3Br$⁠, and $CH3Cl$⁠, three components of the $Q$ complex (⁠$Q31$⁠, $Q30$⁠, and $Q11$⁠) were directly observed, with the exception of methyl fluoride, in the optically forbidden EELS experimental conditions of this investigation. The effect of electronic-state curve crossing emerged in the transition probabilities for the $Q30$ and $Q11$ states, with spin-orbit splitting observed and quantified against results from recent ab initio studies.