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 ν4 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 0.007 cm−1 (200 MHz). The spectrum indicates that the rovibrational levels of the clusters of small sizes are well quantized in solid parahydrogen.