Experimental exploration of structural transformation of isolated small many-body systems

该研究项目旨在通过对孤立气相团簇的实验探索，加深对结构相变（如熔化）的理解，这些团簇是具有有限自由度的典型系统。我们计划通过利用激光光谱学来表征与结构转变相关的振动的能级结构，并通过飞秒时间分辨光谱学来沿着结构转变的分子运动进行真实的探测。我们获得了一些重要体系的实验数据，发展了一种新的实验方法，具体如下：1）用质量选择共振增强电离方法重新研究了苯团簇的电子光谱。相比之下，六聚体和较小的簇表现出尖锐的电子振动带，较大的簇表现出广泛的吸收。这一观察结果意味着许多结构异构体的共存和/或大的结构波动较大的集群。与结构转变有关的势能表面形态的突然变化可能发生在该簇尺寸下。2）含有芳香族分子与质子溶剂的簇，即，蒽-氨和苯-水体系。利用激光双共振技术，观察到了与分子间模有关的弱电子振动带，具有良好的信噪比。这些数据对于构造高精度的分子间相互作用势是必不可少的。3）发展了一种用于实时探测结构相变运动的飞秒时间分辨光谱方法。该方法的多功能性已被各种分子的振动和转动波包的成功观测所证实。特别地，首次观察到了甲基内旋转的波包。

This research project aims at deeper understanding on structural phase transformation, such as melting, via experimental exploration on isolated gas-phase clusters, which are typical systems with limited number of degrees of freedom. We have planned to characterize their energy-level structure of vibrations pertinent to structural transformation by utilizing laser spectroscopy, and to probe in real time the motion of molecules along with the structural transformation by femto-second time-resolved spectroscopy. We have achieved experimental data on some important systems and developed a new experimental method, as follows.1) Electronic spectra of benzene clusters have been reexamined by using mass-selective resonance-enhanced ionization. In contrast that hexamer and the smaller clusters exhibit sharp vibronic bands, the larger clusters show broad absorption. This observation implies the coexistence of many structural isomers and/or large structural fluctuation for the larger clusters. Sudden change in the morphology of potential energy surfaces pertinent to structural transformation may occur at this cluster size.2) Clusters containing aromatic molecules with protic solvents, i.e., anthracene-ammonia and benzene-water, have been studied. Weak vibronic bands pertinent to intermolecular modes have been observed with good S/N by utilizing laser double resonance. The present data will be essential to construct the intermolecular potential with high accuracy.3) A time-resolved spectroscopic method in femto-second regime has been developed for real-time probing of structural transformation motions. The versatility of the method have been demonstrated by the successful observation of vibrational and rotational wavepackets of various molecules. In particular, wavepacket of the internal-rotation of a methyl group has been observed for the first time.