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

本文利用激光诱导荧光（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性。主要成果如下：1。在室温下，通过与缓冲气体的弹性碰撞，对困在射频阱中的离子smg ^+进行了冷却。我们研究了离子温度与缓冲气体的质量和压力的关系，离子温度是由LIF谱的多普勒展宽测量得到的。一个描述与缓冲气体碰撞引起的射频加热的理论模型成功地复制了观察到的结果。捕获离子的激光冷却对几十个激光冷却离子的LIF谱进行了观察。由于激光在红移区冷却和在蓝移区加热的影响，光谱具有狭窄和不对称的轮廓。基于该模型的分析表明，在强激光辐射下，离子与中性原子的碰撞几乎不受热，冷却极限为几个k。将少量离子激光冷却至10 mK，定位在库仑势的平衡点上，形成一个离子簇。我们观察到由多达三个离子组成的簇的二维荧光图像。荧光图像能够实时观察到由于与残余气体碰撞而导致的星团破裂。

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.