极紫外光源及高荷态离子诱导下甲烷的脱氢通道碎裂机制

Fragmentation mechanism of methane dehydrogenation channel induced by extreme ultraviolet and high charge ions

CH_(4)广泛存在于行星大气之中,研究CH_(4)的解离动力学对了解宇宙中气体演化的过程具有重要的价值.目前,CH_(4)^(2+)→CH_(3)^(+)+H^(+)碎裂通道已被大量研究,但针对该通道的解离机制的解释尚存在一定争议.本实验利用高分辨反应显微成像谱仪,开展了25—44 eV的极紫外(extreme ultraviolet,XUV)光电离实验及1 MeV Ne^(8+)与CH_(4)的碰撞实验.通过符合测量得到了CH_(3)^(+)和H^(+)两种离子的动能,重构了两体解离的动能释放(kinetic energy release,KER),并研究了CH_(4)^(2+)解离产生CH_(3)^(+)+H^(+)解离路径下的碎裂动力学过程.在光电离实验中,观测到KER谱上存在4.75 eV和6.09 eV两个峰,结合前人的工作及XUV的能量范围,对每个峰的机制归属进行讨论.特别是4.75 eV峰,分析认为可能来自于CH_(4)^(2+)直接解离机制的贡献.另外,在1 MeV Ne^(8+)离子碰撞实验中,可观测到3个KER峰,将每个峰的分支比与以往的实验结果进行对比,未发现速度效应对KER谱的显著影响.

CH_(4)is abundant in planetary atmosphere,and the study of CH_(4)dissociation dynamics is of great importance and can help to understand the atmospheric evolution process in the universe.At present,the CH_(4)^(2+)→CH_(3)^(+)+H^(+)channel has been extensively studied,but the explanation of the dissociation mechanism for this channel is controversial.In this work,the double-photoionization experiment of CH_(4)by extreme ultraviolet photon(XUV)in an energy range of 25-44 eV and the collision experiment between 1 MeV Ne^(8+)and CH_(4)are carried out by using the reaction microscope.The three-dimensional(3D)momenta of CH_(3)^(+)and H^(+)ions are measured in coincidence,the corresponding kinetic energy release(KER)is reconstructed,and fragmentation dynamics from the parent ion CH_(4)^(2+)to the CH_(3)^(+)+H^(+)ion pair are investigated.In the photoionization experiment,two peaks in the KER spectrum are observed:one is located around 4.75 eV,and the other lies at 6.09 eV.Following the conclusions of previous experiments and the theoretical calculations of Williams et al.(Williams J B,Trevisan C S,Schöffler M S,et al.2012 J.Phys.B At.Mol.Opt.Phys.45194003),we discuss the corresponding mechanism of each KER peak.For the 6.09 eV peak,we attribute it to the CH_(4)^(2+)dissociation caused by the Jahn-Teller effect,because this value is consistent with the energy difference in energy between the CH_(4)^(2+)1E initial state and the CH_(3)^(+)/H^(+)final state involving the Jahn-Teller effect.For the 4.75 eV peak,we believe that it may come from the direct dissociation of CH_(4)^(2+)without contribution from the Jahn-Teller effect.More specifically,Williams et al.presented the potential energy curve for one C—H bond stretching to 8 a.u.,while other C—H bonds are fixed at the initial geometry of the CH_(4)molecule.In the reflection approximation,we infer that the extra energy is released from the internuclear distance of 8 a.u.to infinity.It is found that the KER is 4.7 eV,which is consistent with the experimental observation,suggesting that the KER peak at 4.75 eV may arise from the direct dissociation of CH_(4)^(2+)without contribution from the Jahn-Teller effect.In addition,in the 1 MeV Ne^(8+)ion collision experiment,it is observed that the released energy values corresponding to the three KER peaks are about 4.65,5.76,and 7.94 eV.By comparing the branching ratio of each peak with the previous experimental result,it is suggested that the velocity effect is not significant in KER spectra.