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

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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.