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

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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.