应用反射式飞行时间质谱仪研究氨团簇离子解离动力学

用直射式和反射式飞行时间质谱研究了氨分子团簇体系在 355 nm激光下的多光子电离，得到一系列的质子化团簇离子 (NH3)nH+，同时还观察到超价氨团簇离子 (NH3)n H2＋。在反射式飞行时间质谱研究中观测到质子化氨团簇离子在自由飞行过程中的解离现象，表明在该实验条件下生成的质子化氨团簇离子是一些亚稳态团簇离子。对子离子产率的分析，得到质子化团簇离子解离速率常数，从而可以估计亚稳态团簇离子的寿命。团簇尺寸从 n＝3增大到 20，其寿命从 21 ms减小到 120 μs，大约小了两个数量级。解离速率在 n＝5到 6有一个阶跃式上升，这是由于 5个氨组成的质子化团簇离子(NH3)4NH4+ 的结构相对比较稳定。

Kinetic Study of the Metastable Ammonia Cluster Ions Using Reflection Time-of-Flight Mass Spectroscope

The multiphoton ionization of the ammonia clusters at 355nm was studied using both conventionaland reflection time-of-flight mass spectrometers. Aseries of protonated ammonia clusters(NH3)nH+were obtained,and the hypervalent ammonia cluster ions (NH3)nH2+(n>2 ) were observed. The daughter ions which were produced from dissociation of the metastable am-monia cluster ions during their free flight were also observed. The dissociation rate constants were derived from the experimental result which depends upon the number of the ammnia molecules in a cluster ion. The life times of the metastable cluster ions were estimated, which range from 21ms to 122μs for different cluster size fromn=3 to 20. The larger the cluster size, the faster the dis-sociation rate and the shorter of the life time of the cluster ions. The kinetics shows that withn=5 or smaller the cluster ionsare more stable comparing to those withn≥6, which is consistant with the observation reported in literature.