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.