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

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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.