Holm Anne I S, Donald William A, Hvelplund Preben, Larsen Mikkel K, Nielsen Steen Brøndsted, Williams Evan R
Department of Physics and Astronomy, University of Aarhus, Denmark, and Department of Chemistry, University of California, Berkeley, California 94720-1460, USA.
J Phys Chem A. 2008 Oct 30;112(43):10721-7. doi: 10.1021/jp8019655. Epub 2008 Oct 4.
Ion nanocalorimetry is used to investigate the internal energy deposited into M (2+)(H 2O) n , M = Mg ( n = 3-11) and Ca ( n = 3-33), upon 100 keV collisions with a Cs or Ne atom target gas. Dissociation occurs by loss of water molecules from the precursor (charge retention) or by capture of an electron to form a reduced precursor (charge reduction) that can dissociate either by loss of a H atom accompanied by water molecule loss or by exclusively loss of water molecules. Formation of bare CaOH (+) and Ca (+) by these two respective dissociation pathways occurs for clusters with n up to 33 and 17, respectively. From the threshold dissociation energies for the loss of water molecules from the reduced clusters, obtained from binding energies calculated using a discrete implementation of the Thomson liquid drop model and from quantum chemistry, estimates of the internal energy deposition can be obtained. These values can be used to establish a lower limit to the maximum and average energy deposition. Not taking into account effects of a kinetic shift, over 16 eV can be deposited into Ca (2+)(H 2O) 33, the minimum energy necessary to form bare CaOH (+) from the reduced precursor. The electron capture efficiency is at least a factor of 40 greater for collisions of Ca (2+)(H 2O) 9 with Cs than with Ne, reflecting the lower ionization energy of Cs (3.9 eV) compared to Ne (21.6 eV). The branching ratio of the two electron capture dissociation pathways differs significantly for these two target gases, but the distributions of water molecules lost from the reduced precursors are similar. These results suggest that the ionization energy of the target gas has a large effect on the electron capture efficiency, but relatively little effect on the internal energy deposited into the ion. However, the different branching ratios suggest that different electronic excited states may be accessed in the reduced precursor upon collisions with these two different target gases.
离子纳米量热法用于研究在与铯或氖原子靶气体发生100 keV碰撞时，沉积到M(2+)(H₂O)ₙ（M = Mg，n = 3 - 11；M = Ca，n = 3 - 33）中的内能。解离通过前驱体失去水分子（电荷保留）或捕获一个电子形成还原前驱体（电荷减少）而发生，还原前驱体可以通过伴随水分子损失失去一个氢原子或仅通过水分子损失而解离。通过这两种各自的解离途径形成裸CaOH(+)和Ca(+)分别发生在n高达33和17的团簇中。从使用汤姆逊液滴模型的离散实现和量子化学计算的结合能获得的还原团簇中水分子损失的阈值解离能，可以得到内能沉积的估计值。这些值可用于确定最大和平均能量沉积的下限。不考虑动力学位移的影响，超过16 eV的能量可以沉积到Ca(2+)(H₂O)₃₃中，这是从还原前驱体形成裸CaOH(+)所需的最小能量。Ca(2+)(H₂O)₉与铯碰撞时的电子捕获效率比与氖碰撞时至少高40倍，这反映了铯（3.9 eV）的电离能低于氖（21.6 eV）。对于这两种靶气体，两种电子捕获解离途径的分支比有显著差异，但从还原前驱体损失的水分子分布相似。这些结果表明，靶气体的电离能对电子捕获效率有很大影响，但对沉积到离子中的内能影响相对较小。然而，不同的分支比表明，与这两种不同的靶气体碰撞时，还原前驱体中可能会进入不同的电子激发态。
