(H_(2)O)_(n)^(+)(n=2~5)团簇离子在308 nm处的光解动力学

Photodissociation Studies on (H_(2)O)_(n)^(+)(n=2−5) Clusters at 308 nm

本文利用一套高分辨圆柱形低温离子阱速度成像谱仪装置研究了小团簇离子(H_(2)O)_(n)^(+)(n=2-5)在308 nm处的光解动力学.通过记录光解产物碎片质谱和速度影像,发现二聚体水团簇(H_(2)O)_(2)^(+)解离生成H_(3)O^(+)和H_(2)O^(+)光碎片,表明其同时存在质子转移(H_(3)O^(+)-OH)和半束缚(H_(2)O-OH_(2))+两种构型,水团簇离子(H_(2)O)_(n)^(+)(n=3~5)普遍通过同时丢失OH和H_(2)O部分解离得到H+(H_(2)O)_(n-2,…,1)光碎片,并且(H_(2)O)_(5)^(+)团簇离子有一个仅通过丢失OH的额外通道得到H+(H_(2)O)4.前者表明(H_(2)O)_(n)^(+)(n=3~5)的构型是(H_(2)O)_(n-2)H_(3)O^(+)OH,后者表明在(H_(2)O)_(5)^(+)团簇离子中核H_(3)O^(+)和OH被H_(2)O分开.基于实验影像得到的光碎片各向异性参数为负值并且值较小,表明(H_(2)O)_(n)^(+)(n=2~5)团簇离子在光子吸收后经历垂直电子跃迁,随后缓慢解离,并导致光碎片的高内部激发.

The photodissociationdynamicsof small(H_(2)O)_(n)^(+)(n=2-5)clusters have been studied at 308 nm using a high resolution cryogenic cylindrical ion trap velocity map imaging spectrometer.Time-of-flight mass spectra and images of ionic photofragments are recorded.(H_(2)O)_(2)^(+)clusters dissociate to yield H_(3)O^(+) and H_(2)O^(+) photofragments,indicating the presence of both proton-transferred(H_(3)O^(+)-OH)and hemibonded(H_(2)O-OH_(2))^(+) structures for the dimer cluster.(H_(2)O)_(n)^(+)(n=3-5)clusters prevailingly dissociate to the H+(H_(2)O)_(n-2,...1) photofragments by losing both of OH and H_(2)O components,and the(H_(2)O)_(5)^(+) cluster shows an additional channel to produce H^(+)(H_(2)O)_(4) by only losing OH.The former suggests the(H_(2)O)_(n-2)H_(3)_(O)^(+)OH structures for the(H_(2)O)_(n)^(+)(n=3-5)clusters,while the latter suggests in(H_(2)O)_(5)^(+) that,the H_(3)O^(+) core and OH are separated by H_(2)O.The results elucidate the structure progresses of small(H_(2)O)_(n)^(+) clusters.The experimental images yield negative and small values for the anisotropy parameters of photofragments,indicating that(H_(2)O)_(n)^(+)(n=2-5)clusters undergo vertical electronic transitions upon photon absorption followed by slow dissociation,and lead to highly internally excited photofragments.