School of Chemistry, University of Birmingham, UK.
Phys Chem Chem Phys. 2012 Mar 21;14(11):3935-48. doi: 10.1039/c2cp23878k. Epub 2012 Feb 9.
The dissociative photoionization mechanism of internal energy selected C(2)H(3)F(+), 1,1-C(2)H(2)F(2)(+), C(2)HF(3)(+) and C(2)F(4)(+) cations has been studied in the 13-20 eV photon energy range using imaging photoelectron photoion coincidence spectroscopy. Five predominant channels have been found; HF loss, statistical and non-statistical F loss, cleavage of the C-C bond post H or F-atom migration, and cleavage of the C=C bond. By modelling the breakdown diagrams and ion time-of-flight distributions using statistical theory, experimental 0 K appearance energies, E(0), of the daughter ions have been determined. Both C(2)H(3)F(+) and 1,1-C(2)H(2)F(2)(+) are veritable time bombs with respect to dissociation via HF loss, where slow dissociation over a reverse barrier is followed by an explosion with large kinetic energy release. The first dissociative ionization pathway for C(2)HF(3) and C(2)F(4) involves an atom migration across the C=C bond, giving CF-CHF(2)(+) and CF-CF(3)(+), respectively, which then dissociate to form CHF(2)(+), CF(+) and CF(3)(+). The nature of the F-loss pathway has been found to be bimodal for C(2)H(3)F and 1,1-C(2)H(2)F(2), switching from statistical to non-statistical behaviour as the photon energy increases. The dissociative ionization of C(2)F(4) is found to be comprised of two regimes. At low internal energies, CF(+), CF(3)(+) and CF(2)(+) are formed in statistical processes. At high internal energies, a long-lived excited electronic state is formed, which loses an F atom in a non-statistical process and undergoes statistical redistribution of energy among the nuclear degrees of freedom. This is followed by a subsequent dissociation. In other words only the ground electronic state phase space stays inaccessible. The accurate E(0) of CF(3)(+) and CF(+) formation from C(2)F(4) together with the now well established Δ(f)H(o) of C(2)F(4) yield self-consistent enthalpies of formation for the CF(3), CF, CF(3)(+) and CF(+) species.
采用成像光电子-光离子符合能谱技术，在 13-20 eV 光子能量范围内研究了内标 C(2)H(3)F(+)、1,1-C(2)H(2)F(2)(+)、C(2)HF(3)(+)和 C(2)F(4)(+)阳离子的离解光电子成像的机制。发现了五个主要的通道：HF 损失、统计和非统计 F 损失、C-C 键的断裂后 H 或 F-原子迁移，以及 C=C 键的断裂。通过使用统计理论对分解图和离子飞行时间分布进行建模，确定了母体离子的实验 0 K 出现能 E(0)。C(2)H(3)F(+)和 1,1-C(2)H(2)F(2)(+)都是通过 HF 损失进行解离的定时炸弹，其中在反向势垒上缓慢解离后会发生大动能释放的爆炸。C(2)HF(3)和 C(2)F(4)的第一个离解电离途径涉及 C=C 键上的原子迁移，分别生成 CF-CHF(2)(+)和 CF-CF(3)(+)，然后它们分别解离形成 CHF(2)(+)、CF(+)和 CF(3)(+)。发现 C(2)H(3)F 和 1,1-C(2)H(2)F(2)的 F-损失途径具有双峰性质，随着光子能量的增加，从统计行为切换到非统计行为。发现 C(2)F(4)的离解电离由两个区域组成。在低内部能量下，形成 CF(+)、CF(3)(+)和 CF(2)(+)，这是统计过程。在高内部能量下，形成一个长寿命的激发电子态，它以非统计过程失去一个 F 原子，并在核自由度之间进行统计能量再分配。随后是随后的解离。换句话说，只有基态电子态相空间无法进入。C(2)F(4)形成 CF(3)(+)和 CF(+)的准确 E(0)以及现在已经确立的 C(2)F(4)的Δ(f)H(o)，为 CF(3)、CF、CF(3)(+)和 CF(+)物种的生成热提供了一致的形成热。
