New Method to Study Ion-Molecule Reactions at Low Temperatures and Application to the H2+ + H2 → H3+ +H Reaction
New Method to Study Ion-Molecule Reactions at Low Temperatures and Application to the H2+ + H2 → H3+ +H Reaction
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DOI：
10.1002/cphc.201600828
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发表时间：
2016-11-01
期刊：
影响因子：
2.9
通讯作者：
Merkt, Frederic
中科院分区：
文献类型：
--
作者：
Allmendinger, Pitt;Deiglmayr, Johannes;Merkt, Frederic
在低温下研究离子-分子反应是困难的，因为反应体积中的杂散电场会影响带电反应伙伴的动能。本文介绍了一种研究低温下离子-分子反应的新实验方法，并以H_2 + H_2 +H反应为例，给出了在碰撞能E-col/k（B）=5- 60 K范围内，制备的单振转态离子的测量结果。为了达到如此低的碰撞能量，我们使用合并束方法并观察里德伯电子轨道内的反应，里德伯电子屏蔽了离子免受杂散场的影响。第一束是纯基态H-2分子的超声束，第二束是被激发到主量子数n在20-40范围内选择的里德堡-斯塔克态的H-2分子的超声束。最初，两个光束沿相隔10度角的轴沿着传播。为了合并这两个光束，后一个光束中的里德伯分子使用表面电极里德伯-斯塔克偏转器偏转。通过测量两个光束的速度分布来确定合并光束的碰撞能量，并且通过改变用于产生基态H-2光束的脉冲阀的温度和通过适应施加到偏转器的电极的电势函数来调整它们。碰撞能量变化到低于E-col/k（B）= 10 K，即低于E-col约为1 meV，能量分辨率为100 eV。我们表明，里德伯电子作为一个旁观者，并不影响的横截面，这被发现密切遵循经典的Langevin捕获模型的碰撞能量范围内调查。由于所有的中性原子和分子都可以被激发到里德伯态，这种研究离子-分子反应的方法也适用于其他涉及单电荷阳离子的反应。
Studies of ion-molecule reactions at low temperatures are difficult because stray electric fields in the reaction volume affect the kinetic energy of charged reaction partners. We describe a new experimental approach to study ion-molecule reactions at low temperatures and present, as example, a measurement of the H2+H2+H reaction with the ion prepared in a single rovibrational state at collision energies in the range E-col/k(B)=5-60K. To reach such low-collision energies, we use a merged-beam approach and observe the reaction within the orbit of a Rydberg electron, which shields the ions from stray fields. The first beam is a supersonic beam of pure ground-state H-2 molecules and the second is a supersonic beam of H-2 molecules excited to Rydberg-Stark states of principal quantum number n selected in the range 20-40. Initially, the two beams propagate along axes separated by an angle of 10 degrees. To merge the two beams, the Rydberg molecules in the latter beam are deflected using a surface-electrode Rydberg-Stark deflector. The collision energies of the merged beams are determined by measuring the velocity distributions of the two beams and they are adjusted by changing the temperature of the pulsed valve used to generate the ground-state H-2 beam and by adapting the electric-potential functions applied to the electrodes of the deflector. The collision energy is varied down to below E-col/k(B)=10K, that is, below E-col approximate to 1meV, with an energy resolution of 100eV. We demonstrate that the Rydberg electron acts as a spectator and does not affect the cross sections, which are found to closely follow a classical Langevin-capture model in the collision energy range investigated. Because all neutral atoms and molecules can be excited to Rydberg states, this method of studying ion-molecule reactions is applicable to other reactions involving singly charged cations.