High density collection of molecular ion clusters and their photoexcitation dynamics

为了研究高凝聚态分子离子团簇的电子态和动力学，我们设计了一个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。用绝热大爆炸产生的液滴分子束直接注入10英寸扩散泵。离子提取垂直于束轴，质量选择，减速，并轻轻地嵌入在稀有气体矩阵。作为一个测试实验，苯阳离子沉积在氩矩阵在4 K。污染的背景中性苯是如此之少，10小时的沉积后，我们可以检测到的中性紫外吸收光谱。苯正离子的电子能谱表明，其周围氩原子的电子轨道被苯正离子高度扭曲和极化。高密度的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表面层频繁的爆炸烧蚀。我们还观察到苯三聚体阳离子在振动激发离子分子的C-H伸缩运动后的电荷跳跃现象。在同位素混合物（C_6H_6）^+（C_6D_6）_2的激发下，裂解生成（C_6D_6）_2^+和[（C_6D_6）（C_6H_6）]^+，二者的比例为1：2。这是一个统计极限，表明跳跃发生得比解离过程快得多。C-H伸缩谱带的谱线宽度随团簇中带电分子的位置和相邻分子的质量而变化。相当宽的线宽表明弛豫是100- 150 fs的量级。

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.