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（自由度有限的典型系统）的实验探索，更深入地了解结构相变，例如熔化。我们计划利用激光光谱来表征与结构转变相关的振动能级结构，并通过飞秒时间分辨光谱实时探测分子的运动以及结构转变。我们已经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：1）利用质量选择共振增强电离重新检查了苯簇的电子光谱。相比之下，六聚体和较小的簇表现出尖锐的振动带，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/或较大簇的大结构波动。在该簇尺寸下，可能会发生与结构转变相关的势能表面形态的突然变化。2)已经研究了含有质子溶剂（即蒽-氨和苯-水）的芳香族分子的簇。通过利用激光双共振，已经观察到与分子间模式相关的弱电子振动带，具有良好的信噪比。目前的数据对于构建高精度的分子间势至关重要。3）已经开发了一种飞秒范围内的时间分辨光谱方法，用于实时探测结构转变运动。通过成功观察各种分子的振动和旋转波包，证明了该方法的多功能性。特别是，首次观察到甲基内旋波包。

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.