Bonhommeau David, Bouissou Thomas, Halberstadt Nadine, Viel Alexandra
Laboratoire de Physique Quantique, IRSAMC, UMR 5626, CNRS et Université Paul Sabatier, F-31062 Toulouse Cedex 09, France.
J Chem Phys. 2006 Apr 28;124(16):164308. doi: 10.1063/1.2186645.
We present the first prediction for the fragmentation dynamics following electron impact ionization of neutral krypton clusters from 2 to 11 atoms. Fragment proportions and parent ion lifetimes are deduced from a molecular dynamics with quantum transitions study in which the nuclei are treated classically and the transitions between electronic states quantum mechanically. The potential-energy surfaces are derived from a diatomics-in-molecules model to which induced dipole-induced dipole and spin-orbit interactions are added. The results show surprisingly fast and extensive fragmentation for clusters of such a heavy atom, although not as extensive as in the case of neon clusters studied previously [D. Bonhommeau et al., J. Chem. Phys. 123, 54316 (2005)]. The parent ion lifetimes range from 2.8 to 0.7 ps, and the most abundant fragment is Kr(2) (+) for all studied sizes, followed by Kr(+) for sizes smaller than 7 atoms and by Kr(3) (+) for larger sizes. Trimer and larger fragments are found to originate from the lower electronic states of parent ions. The comparison with preliminary results from experiments on size-selected neutral clusters conducted by Steinbach et al. (private communication) reveal a good agreement on the extensive character of the fragmentation. It is checked that the additional internal energy brought by the helium scattering technique used for size selection does not affect the fragment proportions. In addition, the existence of long-lived trajectories is revealed, and they are found to be more and more important for larger cluster sizes and to favor the stabilization of larger fragments. The implications of this work for microsecond-scale dynamics of ionized rare-gas clusters are discussed. In particular, given the extent of fragmentation of the parent clusters and the fast kinetics of the whole process, the small cluster ions that exhibit a monomer loss in the microsecond time window must originate from much larger neutral precursors. The decay rate of the II(12)(u) state of the ionic dimer Kr(2) (+) by spin-orbit coupling is found to be of the order of 3 ps, in contrast to the expected tens of microseconds, but only reasonably faster than the corresponding state of HeNe(+). Finally, the spin-orbit interaction strongly affects both the Kr(+)Kr(2) (+) ratio and some of the characteristic times of the dynamics, especially for smaller sizes, but not the overall dependence of the fragment proportions as a function of cluster size.
我们给出了对含2至11个原子的中性氪团簇电子碰撞电离后碎片化动力学的首次预测。碎片比例和母离子寿命是通过量子跃迁的分子动力学研究推导出来的，其中原子核采用经典处理，电子态之间的跃迁采用量子力学处理。势能面是从分子中的双原子模型推导出来的，并加入了诱导偶极 - 诱导偶极和自旋 - 轨道相互作用。结果表明，对于这样一个重原子的团簇，碎片化速度惊人地快且范围广泛，尽管不如先前研究的氖团簇那么广泛[D. 博诺莫等，《化学物理杂志》123, 54316 (2005)]。母离子寿命范围从2.8皮秒到0.7皮秒，对于所有研究的尺寸，最丰富的碎片是Kr(2) (+)，对于小于7个原子的尺寸，其次是Kr(+)，对于更大尺寸则是Kr(3) (+)。发现三聚体及更大的碎片源自母离子的较低电子态。与施泰因巴赫等人（私人通信）对尺寸选择的中性团簇实验的初步结果比较显示，在碎片化的广泛特征方面有良好的一致性。经检验，用于尺寸选择的氦散射技术带来的额外内能不会影响碎片比例。此外，揭示了长寿命轨迹的存在，并且发现它们对于更大的团簇尺寸变得越来越重要，并且有利于更大碎片的稳定。讨论了这项工作对电离稀有气体团簇微秒级动力学的影响。特别是，鉴于母团簇的碎片化程度和整个过程的快速动力学，在微秒时间窗口内表现出单体损失的小团簇离子必定源自大得多的中性前驱体。发现离子二聚体Kr(2) (+)的II(12)(u)态通过自旋 - 轨道耦合的衰变率约为3皮秒，与预期的几十微秒相反，但仅比HeNe(+)的相应态合理地快一些。最后，自旋 - 轨道相互作用强烈影响Kr(+) / Kr(2) (+)比率以及动力学的一些特征时间，特别是对于较小尺寸，但不影响碎片比例作为团簇尺寸函数的整体依赖性。
