High-resolution laser spectroscopy of solid hydrogen

High-resolution laser spectroscopy on condensed matter is usually considered to be beyond the reach because of the extreme broadening of both the homogeneous and inhomogeneous spectral linewidth. A marked exception to this general consensus is found in the rotation-vibration transitions in parahydrogen (p-H/sub 2/) crystals. The sharpest linewidth observed so far is 4 MHz (FWHM) of a transition of D/sub 2/ in the p-H/sub 2/ crystal. Because of the weak intermolecular interaction and a large intermolecular distance (3.783A), the rotational motion of molecules in the p-H/sub 2/ crystal is well quantized, and the rotational quantum number J remains as a good quantum number.