Study of formation and stability of helium cluster ions with a very low temperature drift tube

将长度为10 cm的漂移管放置在低温系统中，用液氦进行冷却。在正常情况下，管内氦气的温度约为4.5K，用抽气系统抽真空液氦储罐后，温度降至3.5K，选择的离子被注入管内。这些离子通过与氦原子的弹性碰撞而迅速升温，并在均匀的电场作用下漂移到管子的末端。团簇是在离子漂移过程中与He原子发生三体碰撞而形成的。从试管末端发出的离子用四重质量滤光器进行质量分析。通过改变管内电场强度来控制漂移离子的有效温度。利用这一技术，进行了以下实验1)。研究了RgHe^_(Rg=He，Ne，Ar和Kr)的稳定性。当离子的有效温度较高时，较大尺寸的团簇由于与He原子的碰撞而解离。因此，通过测量不同电场强度下团簇的尺寸分布，我们可以得到关于团簇稳定性的信息。结果表明，RgHe_n^(Rg=He，Ne，Ar和Kr^)具有稳定的二十面体结构，核心离子分别为He_2^，Nehe^，Ar^和Kr^。2)H^He_n和H_3^He_n的稳定性。3)在4.4K的气体温度下，测量了He^、Ne^、Ar^和Kr^的离子迁移率，观察到了许多有趣的特征。特别是在He^的情况下，发现折合迁移率降低到E/N=5Td以下，这可能归因于轨道共振。

A drift tube whose length is 10 cm is settled in a cryogenic system and cooled by liquid helium. In normal condition, the temperature of helium gas in the tube is about 4.5 k. When the liquid helium reservoir is evacuated with a pumping system, the temperature is lowered down to 3.5 k. Ions mass selected are injected into the tube. These ions are quickly thermalized by elastic collisions with He atoms and drift toward the end of the tube by a uniform electric field. Clusters are formed through three body collisions with He atoms during drifts of ions. The ions exited from the end of the tube are mass analyzed with a quadruple mass filter. Effective temperatures of the drifting ions are controlled by changing the electric field strength in the tube. Using this technique, following experiments have been made.1). Stability of RgHe^+_ (Rg=He, Ne, Ar and Kr) have been studied. When the effective temperature of the ions are rather high, the clusters with larger size are dissociated by collisions with He atoms. Therefore, by measuring size distributions of clusters at various electric field strength, we can obtain an information on the stability of the clusters. It is suggested that an icosahedral structure with core ions He_2^+, NeHe^+, Ar^+ and Kr^+ is stable for RgHe_n^+ (Rg=He, Ne, Ar and Kr), respectively.2) Stability of H^+He_n and H_3^+He_n. In the case of H^+He_n clusters, it is suggested that the stable structure is the icosahedron with the core ion HHe^+ like rare gas ions. On the other hand, for H_3^+He_n quite different structure with the magic number of n=10 and 11 is suggested to be stable.3) Ion mobilities of He^+, Ne^+, Ar^+ and Kr^+ have been measured at gas temperature of 4.4 k. Many interesting features have been observed. Especially in the case of He^+, it has been found that the reduced mobility decreases below E/N=5 Td which may be attributed to an orbitting resonance.