Collision processes of singly and multiply charged ions in cold helium gas

我们的目标是研究单电荷离子和多电荷离子在3-300K与冷氦气体碰撞时的电子能量转移。虽然氦原子与其他原子物种之间的对势在其他原子间相互作用势中是众所周知的，但关于冷氦原子与其他原子之间碰撞的实验研究却很少。此外，即使在室温下，氦原子和原子离子的碰撞截面也只测量到很少的几个。这大概是因为在冷氦气环境中很难产生原子离子。为了解决这一问题，我们将激光烧蚀技术应用于冷氦气体中的固体样品，产生原子离子，并用激光光谱方法对其进行检测。利用可调谐染料激光将产生的离子激发到电子激发态，同时观察到激发态离子的激光诱导荧光(LIF)和离子与氦原子碰撞转移到的敏化荧光(SF)。这一过程使得确定离子和冷氦原子的碰撞截面成为可能。特别是在高密度冷氦气体中，由于激发离子与两个氦原子的三体碰撞，碱土离子如Ba2+等显示出了形成激基络合物(如Ba2++-He)的迷人特性。这种激基络合物和氦原子的另一种二体碰撞过程导致了Ba^&lt；+*&He解离为Ba^+离子和氦原子。我们首次阐明了在3-30K温度下由二体和三体碰撞引起的截面与温度的特殊关系。

We are aiming at studying electronic energy transfer of singly and multiply charged ions in collision with cold helium gas at 3-300K. Although pair-potentials between helium atom and other atomic species are relatively well known among other interatomic potentials, experimental studies of collisions between cold helium atoms and other atoms are scarce. Moreover, only a few collision cross sections are measured for the combination of helium atom and atomic ions even at room temperature. This is presumably because of the difficulty of producing atomic ions in the environment of cold helium gas. In order to solve the problem, we applied laser ablation technique to a solid sample in cold helium gas to produce atomic ions and detected them with laser-spectroscopic method. The results will leads to the information on interatomic potentials between ions and atoms.Produced ions are excited to the electronically excited state with a tunable dye laser, then both laser-induced fluorescence (LIF) from ions in the excited state and sensitized fluorescence (SF) from ions in the state to which ions are transferred by collision with helium atoms are observed simultaneously. This procedure makes it possible to determine collision cross sections of ions and cold helium atoms. In particular, alkali-earth ions such as Ba^+ reveal fascinating character of forming molecules called exciplex (e.g. Ba^<+*>-He) in high density cold helium gas due to the three-body collision of excited ions and two helium atoms. Another two-body collision process of such exciplex and helium atoms results in dissociation of Ba^<+*>-He into Ba^+ ion and helium atom. We have clarified a peculiar temperature dependence of cross sections caused by two- and three-body collisions at temperature 3-30 K for the first time.