MEASUREMENT OF CHARGE TRANSFER CROSS SECTION IN VERY LOW ENERGY REGION

在射频离子阱中，在极低能区成功地观察到了激光冷却镁离子与热钡原子之间的电荷转移过程。离子阱实验的优点是产生的离子也被捕获，并获得很高的灵敏度。通过观察钡离子的激光诱导荧光证实了钡离子的产生，截面估计为10^<-4>nm^2。这与参考文献（D. Rapp和W.E.弗朗西斯，J.Chem.Phys.37，2631（1962））。电荷转移过程允许我们实现各种离子的感应冷却。由于镁离子的能级结构简单，很容易被单束激光冷却。镁离子的电荷和“冷温度”通过碰撞转移到客体离子。我们观测到同向冷却的钡离子的温度为500K，远低于氦缓冲气体冷却的钡离子的温度（4000K）。计算了共振冷却的极限温度随离子温度和离子质量的变化。我们的理论模型的实验验证是在一对镁和钡离子。该理论预测，当主体离子的温度较低时，较重的客体离子被更有效地冷却，而客体离子的温度在较高的温度区域随着质量的增加而迅速增加。通过电荷转移过程研究了共振冷却离子在射频阱中的共振振荡模式。观察到两种类型的集体振荡。用理论模型计算了振荡频率，与观测值吻合较好。通过施加与离子的适当频率共振的射频电压，可以从阱中去除不相关的离子。

The charge transfer process between a laser cooled magnesium ion and a thermal barium atom is successfully observed in a radio frequency ion trap in extremely low energy regime. The merit of ion trap experiment is that the generated ion is also trapped and very high sensitivity is achieved. The production of barium ion is confirmed by observing the laser-induced fluorescence from the ion.The cross section is estimated to be 10^<-4> nm^2. This well agrees with the value calculated from a theory in the reference (D. Rapp and W.E.Francis, J. Chem. Phys. 37, 2631(1962)).The charge transfer process allows us to achieve sympathetic cooling of various ions. The magnesium ion is easily cooled by a single laser, because its energy level structure is simple. The charge and "cold temperature" of magnesium ion are transferred to guest ions through collisions. We observed the temperature of sympathetically cooled barium ion of 500K, which is much smaller than that of the ion cooled by Helium buffer gas (4000K). We have calculated the limiting temperatures achieved by the sympathetic cooling as functions of ion temperatures and ion masses. The experimental confirmation of our theoretical model is achieved in a pair of magnesium and barium ions. The theory predicts that when the temperature of host ion is low the heavier guest ions are cooled more efficiently, while the guest ion temperature increases rapidly with increasing mass in higher temperature region. The resonant oscillation modes of sympathetically cooled ions in a radio frequency trap are also investigated through charge transfer process in the trap. Tow types of collective oscillation are observed. The frequency of oscillation can be calculated by a theoretical model, which agrees well with the observation. Irrelevant ions can be removed from the trap by applying an rf-voltage resonant to the proper frequency of the ions.