High density collection of molecular ion clusters and their photoexcitation dynamics

为了研究高度凝聚的分子离子团簇的电子态和动力学，我们设计了一种带有软着陆离子沉积和红外、紫外吸收光谱仪的分子束装置。将绝热大爆炸产生的液滴形成的分子束直接注入10英寸扩散泵。离子被提取垂直于光束轴，大量选择，减速，并轻轻地嵌入在一个罕见的气体矩阵。在实验中，苯离子在4k时沉积在氩气基体中。本底中性苯的污染非常小，沉积10小时后，我们可以用紫外吸收光谱法检测到中性苯。苯阳离子的电子能谱表明，围绕苯阳离子的氩原子的电子轨道被阳离子高度扭曲并极化。高密度沉积过程伴随着很大的困难，没有任何负离子的共沉积。在沉积过程中观察到新面层频繁的爆炸烧蚀。有必要在室内设计一种自然负离子沉积机制。我们还观察到在离子分子的C-H拉伸运动的振动激发后，苯三聚体阳离子中的电荷跳变现象。同位素混合物（C_6H_6） ^+ (C_6D_6)_2的激发产生（C_6D_6）_2^+和[(C_6D_6) (C_6H_6)]^+，两者的比值为1:2。这是一个统计极限，表明跳跃发生的速度比解离过程要快得多。观察到的C-H拉伸带的线宽显示出依赖于团簇中带电分子的位置和邻近分子的质量的偏转。较宽的线宽表明弛豫的数量级为100-150fs。

In order to study electronic states and dynamics of highly condensed molecular ion clusters, we designed a molecular beam apparatus with a soft-landing ion deosition and IR and UVabsorption spectrometers. A molecular beam generated from liquid droplets by adiabatic Big-Bang explosion is directly injected against 10-inch diffusion pump. Ions are extracted perpendicular to the beam axis, mass-selected, decelerated, and gently imbedded in a rare gas matrix. As atest experiment, benzene cations are deposited in argon matrices at 4 K.Contamination of the background neutral benzene was so little that after 10 hour deposition we can detect the neutrals by UV absorption spectrometry. The electronic spectrum of benzene cation showed that the electronic orbitals of argon atoms around the cation is highly distorted and polarized by the cation. The high dnsity deposition process was accompanied with a big difficulty without any codeposition of negative ions. Frequent explosive ablation of the new surface layrs was observed during the deposition. It is necessary to design a natural negative ion deposition mechanism in a chamber.We also observed a charge hopping phenomenon in benzene trimer cations after vibrational excitation of a C-H stretching motion of an ionic molecule. The excitation of an isotopic mixture (C_6H_6) ^+ (C_6D_6)_2 resulted in the fragmentation yielding (C_6D_6)_2^+ and [ (C_6D_6) (C_6H_6) ]^+ with a ratio of 1 : 2. This is a statistical limit suggesting the hopping occurs sufficiently faster than the dissociation process. The observed line-widths of the C-H stretching bands showed sectral shifts dependent on the position of the charged molecules in the cluster and also on the masses of the neighboring molecules. Rather broad line-widths indicates that the relaxation is of the order of 100-150fs.