Study of Collision Process of Ultracold Ions by Means of Quantum Jump Signals

利用激光诱导荧光（LIF）光谱和激光冷却技术研究了冷离子与原子和分子碰撞过程的特征。主要成果如下。囚禁离子的碰撞冷却在室温下，Mg ^+离子被限制在射频阱中，通过与缓冲气体的弹性碰撞冷却。我们研究了离子温度的依赖关系，这是从激光诱导荧光光谱的多普勒展宽测量，对缓冲气体的质量和压力。一个描述与缓冲气体碰撞引起的射频加热的理论模型被成功地应用于重现观测结果.囚禁离子的激光冷却观察了几十个激光冷却离子的LIF谱。由于红移区的激光冷却和蓝移区的激光加热的影响，光谱具有窄的和不对称的轮廓。基于我们模型的分析表明，在强激光辐射下，离子的冷却极限为几K，离子与中性原子的碰撞几乎不加热.离子束的观察少数离子被激光冷却到10 mK，并定域在库仑势的平衡点上，形成团簇。我们观察到的二维图像的荧光从集群组成的三个离子。荧光图像使得能够实时观察由与残余气体碰撞引起的簇的分解。

The present research has been carried out in order to clarify characteristics of collision processes of cold ions with atoms and molecules by means of the laser-induced-fluorescence (LIF) spectroscopy and the laser-cooling technique. Major outcomes are described below.1. Collision Cooling of Trapped IonsMg^+ ions confined in a rf trap were cooled via elastic collisions with buffer gas at room temperature. We investigated dependences of the ion temperature, which was measured from the Doppler broadening of the LIF spectrum, on the mass and the pressure of the buffer gas. A theoretical model describing the rf heating induced by the collision with the buffer gas was successfully applied to reproduce the observed results.2. Laser Cooling of Trapped IonsThe LIF spectrum from several tens of laser-cooled ions was observed. The spectrum has a narrow and asymmetric profile because of the effects of the laser cooling in the red-shifted region and the laser heating in the blue-shifted region. The analysis based on our model revealed that the cooling limit was several K.and that the ions were little heated by the collisions with neutral atoms under strong laser radiation.3. Observation of Ion ClusterA few ions were laser-cooled down to 10 mK, and localized on the equilibrium points of the Coulomb potential, forming a cluster. We observed two-dimensional images of the fluorescence from the clusters consisting of up to three ions. The fluorescence images enabled a real-time observation of breakdown of the cluster caused by the collision with residual gas.