Infrared spectroscopic study of rovibrational states of methane trapped in parahydrogen crystal

The ${\nu }_3$ and ${\nu }_4$ vibrational transitions of methane trapped in solid parahydrogen have been observed by using Fourier transform infrared and high resolution laser spectroscopy. The observed spectrum is interpreted in terms of rovibrational states of the spherical rotor which are subjected to the crystal field splitting. The ${\nu }_4$ band shows extremely sharp lines of a width of $\text{∼0.003\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}},$ while the ${\nu }_3$ band exhibits broader lines of a width of $\text{1\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}.$ The infrared selection rules derived from an extended group theory to take into account the field effect are consistent with the observed spectra. The intermolecular interaction and the field effect in solid parahydrogen are analyzed quantitatively.