Studies on intramolecular processes of excited polyatomic molecules by means of Zeeman quantum beat spectroscopy

本工作旨在利用量子拍谱技术阐明电子激发的多原子分子的回振动态的能级结构。当分子被相干地激发到一个以上的电子激发能级，并且荧光从这些能级跃迁到一个共同的电子地能级时，荧光中就会出现拍频现象。拍频的谱反映了受激分子的能级结构。由于该观测是在时域内进行的，因此其分辨率高达1mhz。因此，可以讨论由极弱扰动引起的能级分裂。上述光谱方法已应用于超声速射流中CS_2 （V^1B_2）和C_2H_2 （A^^<波>^4A_u）的能级结构的观测。当CS_2在弱磁场下被脉冲激光激发到V^1B_2态的选定振动能级时，由于相关能级的塞曼分裂而产生量子拍。这一观测结果表明，激发态CS_2具有一个磁矩，该磁矩可归因于单重态^1B_2与三重态^3A_2的相互作用。非常相似的是，大多数振动能级分裂成几个能级，每个能级都有其特定的辐射寿命，并且由于塞曼分裂和能级反交叉而表现出量子拍。基于这些量子拍谱数据，讨论了耦合机制的细节。

This work aims to elucidate the level structure of the rovibronic state of an electronically excited polyatomic molecule by means of the quantum beat spectroscopy. When molecules are excited coherently to more than one electronically excited levels and the fluorescence transitions from these levels occur to a common electronic ground level, a beat phenomenon appears in the fluorescence. The spectrum of the beat frequencies reflects the level structure of the excited molecule. Since this observation is made in the time domain, its resolution reaches as high as 1 MHz. Thus, it is possible to discuss a level splitting caused very weak perturbation.The above spectroscopic method has been applied to observe the level structures of CS_2 (V^1B_2) and C_2H_2 (A^^< wave >^4A_u) in supersonic jets. When CS_2 is excited to selected rovibronic levels of the V^1B_2 state under a weak magnetic field by pulsed laser excitation, the quantum beats appear due to Zeeman splitting of the relevant levels. This observation implies that excited CS_2 has a magnetic moment which attributable to interaction of the singlet^1B_2 state with the triplet ^3A_2.Quite similarly, most of rovibronic levels split into several levels each of which has its specific radiative lifetime and exhibits quantum beats due to Zeeman splitting as well as level anticrossing. Based on these quantum beat spectral data, the details of the coupling mechanism have been discussed.