Bonhommeau David, Halberstadt Nadine, Viel Alexandra
Laboratoire de Physique Quantique, IRSAMC, UMR 5626, CNRS et Université P. Sabatier, F-31062, Toulouse cedex 09, France.
J Chem Phys. 2006 May 14;124(18):184314. doi: 10.1063/1.2194552.
The fragmentation dynamics of argon clusters ionized by electron impact is investigated for initial cluster sizes up to n = 11 atoms. The dynamics of the argon atoms is modeled using a mixed quantum-classical method 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 with the addition of the induced dipole-induced dipole and spin-orbit interactions. The results show extensive and fast fragmentation. The dimer is the most abundant ionic fragment, with a proportion increasing from 66% for n = 2 to a maximum of 95% for n = 6 and then decreasing down to 67% for n = 11. The next abundant fragment is the monomer for n < 7 and the trimer otherwise. The parent ion dissociation lifetimes are all in the range of 1 ps. Long-lived trajectories appear for initial cluster sizes of seven and higher, and favor the formation of the larger fragments (trimers and tetramers). Our results show quantitative agreement with available experimental results concerning the extensive character of the fragmentation: Ar+ and Ar2(+) are the only ionic fragments for sizes up to five atoms; their overall proportion is in quantitative agreement for all the studied sizes; Ar2(+) is the main fragment for all sizes; stable Ar3(+) fragments only appear for n > or = 5, and their proportion increases smoothly with cluster size from there. However, the individual ionic monomer and dimer fragment proportions differ. The experimental ones exhibit oscillations with initial cluster size, with a slight tendency to decrease on average for the monomer. In contrast our results show a monotonic, systematic evolution, similar to what was found in our earlier studies on neon and krypton clusters. Several hypotheses are discussed in order to find the origin of this discrepancy. Finally, the metastable II(1/2)u and II(1/2)g states of Ar2(+) are found to decay with a lifetime of 3.5 and 0.1 ps, respectively, due to spin-orbit coupling. The difference with the commonly accepted microsecond range value for rare-gas dimer ions could originate from the role of autoionizing states in the formation of the parent ions.
研究了电子碰撞电离的氩团簇在初始团簇大小达到(n = 11)个原子时的碎片化动力学。采用混合量子 - 经典方法对氩原子的动力学进行建模，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。势能面由分子中双原子模型导出，并加入诱导偶极 - 诱导偶极和自旋 - 轨道相互作用。结果表明存在广泛且快速的碎片化。二聚体是最丰富的离子碎片，其比例从(n = 2)时的(66%)增加到(n = 6)时的最大值(95%)，然后降至(n = 11)时的(67%)。下一个丰富的碎片在(n < 7)时是单体，否则是三聚体。母离子解离寿命均在(1)皮秒范围内。对于初始团簇大小为七个及以上的情况，出现了长寿命轨迹，且有利于形成较大的碎片（三聚体和四聚体）。我们的结果与关于碎片化广泛特征的现有实验结果在定量上一致：对于大小达五个原子的团簇，(Ar^+)和(Ar_2^+)是仅有的离子碎片；对于所有研究的大小，它们的总体比例在定量上一致；(Ar_2^+)是所有大小的主要碎片；稳定的(Ar_3^+)碎片仅在(n \geq 5)时出现，并且从那时起其比例随团簇大小平滑增加。然而，单个离子单体和二聚体碎片的比例不同。实验结果显示其随初始团簇大小有振荡，单体平均略有下降趋势。相比之下，我们的结果显示出单调、系统的演化，类似于我们早期对氖和氪团簇的研究结果。讨论了几个假设以找出这种差异的根源。最后，由于自旋 - 轨道耦合，发现(Ar_2^+)的亚稳(II_{1/2u})和(II_{1/2g})态分别以(3.5)皮秒和(0.1)皮秒的寿命衰变。与稀有气体二聚体离子通常接受的微秒范围值的差异可能源于自电离态在母离子形成中的作用。
