High Aggregation Number Silver Clusters by Matrix-Assisted Laser Desorption/Ionization: Role of Matrixes on the Gas-Phase Reduction of Silver Ions

摘要

Singly positively and negatively charged silver-cluster ions were effectively produced under matrix-assisted laser desorption/ionization (MALDI) conditions up to the aggregation number of n approx= 200, using reductive polar organic matrixes and silver trifluoroacetate (AgTFA). It was found that the matrix greatly influences the resulting cluster ion abundances. The effect of the matrixes with various chemical structures on the cluster formation was systematically investigated. It was also observed that the ion intensities decreased sharply at particular cluster numbers called "magic" numbers, independently of the matrix used, and this is in good agreement with the jellium model theory. The highest observed magic numbers were achieved with HABA [2-(4-hydroxyphenylazo)benzoic acid] and are 139 and 137 for positively and negatively charged cluster ions, respectively. The fragmentations of both singly positively and negatively charged clusters ions were also studied. It was found that even-number clusters lose one silver atom, while clusters with an odd-number emit a single or two silver atoms. The fragmentation behavior of clusters as a function of the cluster size is reported. On the basis of the results a possible mechanism for the cluster ion formation is also presented.