High-resolution laser spectroscopy of methane clusters trapped in solid parahydrogen

Clusters of methane are isolated in solid parahydrogen. The vibrational spectral region of the ${\nu }_4$ fundamental of methane molecule is surveyed with a Fourier transform infrared spectrometer and a high-resolution difference-frequency infrared laser system. More than 200 sharp absorption lines are discovered whose linewidth is as narrow as $\text{0.007\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ (200 MHz). The spectrum indicates that the rovibrational levels of the clusters of small sizes are well quantized in solid parahydrogen.