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

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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.