Development of high resolution photoelectron spectroscopy by using higher Rydberg state and spectroscopy of cations

利用高里德堡态的高分辨光电子能谱已经发展起来。获得了0.6 meV(5 cm-1)的分辨率。这种分辨率比传统的光电子能谱高100倍。这种新的光谱学只需将自制的脉冲发生器连接到通常用于MPI光谱学的真空室即可实现。没有使用特殊的设备，如磁屏蔽。这是因为该光谱探测到的是较高里德堡态的场致电离信号，而不是光电子。Rydberg分子对外场不敏感，因此可以极大地简化实验装置。我们将该技术应用于二氮杂环辛烷、邻氟甲苯、间氟甲苯和对氟甲苯、1，2，4，5-四氟苯和对脱氟苯。观察到了高分辨的阳离子振动结构。特别是对氟甲苯阳离子中甲基的内旋能级进行了分析，并测定了电势曲线。分析表明，电离后的内旋势垒高度有很大提高。从势垒高度的增加和内转动常数的减小之间的良好相关性可以得出结论，势垒的增加是由于电离后的几何变化所致，我们还在光谱中发现了偏振效应。当激发光的偏振从线偏振态变为圆偏振态时，光谱中的能带强度发生了很大的变化。能带强度的变化与阳离子中振动能级的对称性有关。结果表明，该光谱的极化效应有助于确定分子阳离子的振动对称性。综上所述，我们利用高里德堡态建立了新的高分辨光电子能谱，实验装置简单。它将成为研究大分子阳离子的有力工具。

High resolution photoelectron spectroscopy by using high Rydberg state has been developed. Resolution of 0.6meV(5cm-1)has been achieved. This resolution is 100 times higher than that of traditional photoelectron spectroscopy. This new spectroscopy can be achieved simply by connecting a home made pulse generator to an usual vacuum chamber for MPI spectroscopy. No special equipment, such as magnetic shield, has been used. It is because this spectroscopy detects field ionization signal of higher Rydberg state instead of photoelectron. The Rydberg molecule is insensitive to an external field therefore the experimental setup can be simplified extremely.We applied this technique to diazabicyclooctane, o-, m-, and p-fluorotoluenes, 1,2,4,5-tetrafluorobenzene, and p-defluorobenzene. Well-resolved vibrational structures of cations have been observed. In special, internal rotational levels of methyl groups in fluorotoluene cations have been analyzed and potential curves have been determined. The analysis has revealed great increase of the barrier height of the internal rotation after ionization. From the good correlation between the increase of the barrier height and the decrease of internal rotational constant, it is concluded that the increase of the barrier is due to geometrical change after ionization.We have also found polarization effect in this spectroscopy. The band intensity in the spectrum changes drastically when the polarization of the exciting light changes from linear to circular. The change in band intensity correlates with the symmetry of the vibrational level in the cation. It shows that the polarization effect of this spectroscopy is useful to assign the vibrational symmetry of the molecular cation.In conclusion, we have established the new high resolution photoelectron spectroscopy by using higher Rydberg state with a simple experimental setup. It will be a powerful tool to study a large molecular cation.