Dynamics of Clusters Initiated by Photon and Surface Impact

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• 作者：Akira Terasaki
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：August 16, 2007
• 年：2007
• 卷：111
• 期：32
• 页码：7671 - 7689
• 全文大小：670K
• 年卷期：v.111,no.32(August 16, 2007)
• ISSN：1520-5215

文摘

Clusters of atoms/molecules show dynamics characteristic of the method of excitation. Two contrasted processesare discussed: (1) electronic excitation via single-photon absorption and (2) impulsive excitation of nuclearmotions by surface impact. Process 1 is exemplified by photodissociation dynamics of size-selected metalcluster ions. The electronic energy is converted most likely to vibrational energy of internal modes; dissociationfollows via statistical mechanism to produce energetically favored fragments. Exceptionally, a silver clusterion, Ag₄⁺, is shown to undergo nonstatistical dissociation along the potential-energy surface of the excitedstate. Energy partitioning to translational and vibrational modes of fragments is analyzed as well as bonddissociation energies. Furthermore, the spectrum of the photodissociation yield provides electronic andgeometrical structures of a cluster with the aid of ab initio calculations; manganese, Mn_N⁺, and chromium,Cr_N⁺, cluster ions are discussed, where the importance of magnetic interactions is manifested. On the otherhand, momentum transfer upon surface impact plays a role in process 2. An impulsive mechanical forcetriggers extraordinary chemical processes distinct from those initiated by atomic collision as well asphotoexcitation. Experiments on aluminum, Al_N^-, silicon, Si_N^-, and solvated, I₂^-(CO₂)_N, cluster anions provideevidence for reactions proceeding under extremely high temperatures, such as pickup of surface atoms, annealingof products, and mechanical splitting of chemical bonds. In addition, a model experiment to visualize andtime-resolve the cluster impact process is performed by using a micrometer-sized liquid droplet. Multiphotonabsorption initiates superheating of the droplet surface followed by a shock wave and disintegration into anumber of small fragments (shattering). These studies further reveal how the nature of chemical bonds influencesthe dynamics of clusters.