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

本课题旨在通过对孤立气相团簇这一典型的有限自由度体系的实验探索，加深对熔炼等结构相变的认识。我们计划利用激光光谱来表征它们与结构转变相关的振动能级结构，并利用飞秒时间分辨光谱实时探测分子运动与结构转变。我们获得了一些重要系统的实验数据，并开发了一种新的实验方法，如下所示。1)用质量选择共振增强电离法重新研究了苯簇的电子能谱。相比之下，六聚体和较小的团簇表现出尖锐的振动带，较大的团簇表现出广泛的吸收。这一观察结果表明，对于较大的团簇，存在许多结构异构体和/或较大的结构波动。与结构转变相关的势能表面形态的突然变化可能发生在这个簇大小上。2)研究了芳香分子团簇与质子溶剂（蒽-氨和苯-水）的关系。利用激光双共振，观察到与分子间模式相关的弱振动带，具有良好的信噪比。目前的数据对于高精度地构建分子间电位是必要的。3)提出了一种飞秒波段的时间分辨光谱方法，用于实时探测结构转变运动。成功地观察了各种分子的振动和旋转波包，证明了该方法的通用性。特别是首次观察到甲基内旋的波包。

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.