Ultra High-Resolution Molecular Saturation Spectroscopy Using Solid Parahydrogen as the Matrix

由于凝聚态固有的均匀和不均匀谱线展宽，与气相相比，凝聚态物质的光谱分辨率受到严重限制。然而，固体对氢作为基体被发现有潜力提供非常高分辨率的光谱信息。本研究旨在将氢基质分离技术与饱和光谱技术相结合，进一步提高分辨率。作为第一步，我们试图发现氢基体中不均匀展宽的任何可能原因。为此，我们在不同的实验条件下测量了第一和第二纯振动泛音的带宽。因此，我们认为，增宽的主要原因是由于残余正氢未能转化为对氢的四极矩引起的斯塔克效应。结果表明，只要减少正氢的污染，就可以获得足够分辨率的光谱，而不需要使用预期的饱和光谱。有了这一发现，我们成功地获得了基本分子（如甲烷）的高分辨率旋转振动光谱。凭借获得的高分辨率，我们能够观察到单个旋转振动线非常细微的晶体场分裂，从中我们获得了氢和甲烷分子之间相互作用的定量信息。此外，发现固有线宽随振动状态的变化而变化，这应该为振动激发态的松弛机制提供有价值的信息。

The spectroscopic resolution of condensed matter is severely limited compared with the gas phase because of the homogeneous and inhomogeneous line broadening inherent to the condensed phase. However, solid parahydrogen used as the matrix is found to have potentiality to provide very high resolution spectroscopic information. The present research was intended to further improve the resolution by combining the hydrogen matrix-isolation with the saturation spectroscopic technique.As the first step, we attempted to discover any possible cause of the inhomogeneous broadening in the hydrogen matrix. For this purpose we measured the bandwidth of the first and the second pure vibrational overtones under various experimental conditions. As a result, we round that the main cause of the broadening was due to the Stark effect originating from the quadrupole moment of residua1 orthohydrogen which had failed to be converted to parahydrogen.It turned out that sufficiently resolved spectra were obtainable by just reducing the contamination of orthohydrogen and that the emp1oyment of the intended saturation spectroscopy was not necessary. With this finding we made success to obtain highly resolved rovibrational spectra of fundamental molecules such as methane.By virtue of the high resolution attained we were able to observe very subtle crystal field splittings of a single rovibrational line, from which we obtained quantitative information on the intermolecular interaction between hydrogen and methane molecules, for example. Furthermore, it was found that the inherent linewidth varied depending upon the vibrational state, which should provide valuable information on the relaxation mechanism of the vibrationally excited state.