Reactions in Clusters as Studied by Laser Ablation-Molecular Beam Method

在本课程组开发的激光烧蚀-分子束（LAMB）方法中，脉冲激光聚焦在金属表面。激光从金属表面烧蚀的金属离子（M^+）被允许在注入附近的分子束中与分子或团簇发生反应。产品离子由四极质谱仪检测。用LAMB方法研究了金属离子与二元团簇（如氨-甲醇混合团簇）的反应。所得到的混合配体金属配合离子M^+ (NH_3) _m (CH_3OH) _n表现出一种特殊的类幻数行为，这种行为依赖于金属。例如，当观察到M=Mg和Mn的混合配体配合物M^+(NH_3)_m(CH_3OH)_n的相对丰度与M和n对应时，这些分布中最突出的特征是具有一定数量氨成分的离子的优势。例如，在n=1的序列中，m=3的离子丰度最大，而n=2的离子丰度最大。研究发现，这种特性在氨/乙醇混合比例（从600托/10托到600托/60托）的很大范围内都能持续存在。这些特征无疑表明，在这些混合配体配合离子中，氨直接与金属离子配位。m的最大值必须与第一配位球中氨配体的数目相对应。例如，三个氨分子直接与Mg^+和Mn^+结合。甲醇和其他氨分子与这三个氨配体结合。一旦第一个配位球被三个氨分子填满，一个或两个甲醇分子与它们的结合比第四个氨分子更强。在m= Al的情况下，金属络合离子的最大值出现在m=2处。综上所述，找到了氨配体的优先配位，并确定了它们在第一配位球中的配位数。

In the laser ablation-molccular beam (LAMB) method developed by the present author's group, pulsed laser light jis focused on a metal surface. Metal ions (M^+) laser-ablated from the metal surface are allowed to react with molecules or clusters in a molecular beam injected nearby. Product ions are detected by a quadrupole mass spectrometer.The LAMB method is used to study reactions of metal ions with binary clusters like ammonia-methanol mixed clusters. Resulting mixed-ligand metal complex ions M^+ (NH_3) _m (CH_3OH) _n manifest a peculiar magic number-like behavior which is metal-dependent. For example, when observed relative abundances of mixed-ligand complexes M^+(NH_3)_m(CH_3OH)_n for M=Mg and Mn are plotted against m and n, the most prominent features in these distributions are the preponderance of ions with a certain number of ammonia constituents. For examle, ions with m=3 have largest abundances through the series of n=1, and those with n=2. Such a feature was found to persist through a broad range of ammonia/cethanol mixing ratio (from 600 Torr/10 Torr to 600 Torr/60 Torr). Such features indicate, without doubt, that ammmonia is directly coordinated to metal ions in these mixed-ligand complex ions. The values of m at the maxima must correspond to the number of ammonia ligands in the first coordination sphere. For examle, three ammonia molecules are bound directly to Mg^+ and Mn^+. Methanol and other ammonia molecules are bound to these three ammonia ligands. Once the first coordination sphere is comploted by three ammonia molecules, one or two methanol molecule (s) are bound to them more strongly than the fourth ammonia molecule. Such maxima of metal complex ions appeared at m=2 for the case M=Al.In summry, preferential coordination of ammonia ligands are found and their coordination number in the first coordination sphere is determined.