Dissociative photoionization of 1,1,1,4,4,4-hexafluorobutane: A proposed nonadiabatic autoionization mechanism

Understanding the dissociative ionization of hydrofluorocarbons is essential for predicting their fragmentation pathways in plasma etching environments. Notably, photoionization over the 10–26 eV range yields no stable parent radical cation. Contrary to the conventional adiabatic bond cleavage paradigm, the dominant fragmentation of 1,1,1,4,4,4-hexafluorobutane proceeds via a rearrangement pathway yielding CF2CHCH2+ (m/z 77) through concerted loss of CF3 radical and HF. This channel opens at an appearance energy of 12.80 ± 0.1 eV and constitutes 30%–40% of the total ion yield from its threshold up to 26 eV. In contrast, the stepwise adiabatic pathway faces a calculated barrier of 14.2 eV, rendering it kinetically uncompetitive near the threshold. Time-dependent density functional theory calculations suggest a nonadiabatic autoionization hypothesis that circumvents this barrier. Thus, the intrinsic co-production of a C3-flux with the desired CF3 radical explains the empirically observed mask clogging in high-aspect-ratio etching and provides molecular-level insights for evaluating hydrofluorocarbons as alternative plasma etchants.