High-resolution stimulated Raman gain spectroscopy of parahydrogen crystals

High-resolution stimulated Raman gain spectroscopy of the Q(1)(0) transition of para-H-2 crystals with varying ortho-H-2 concentrations is reported. A crystal containing 0.06% of ortho-H-2 showed a single sharp Raman transition with the width of less than 7 MHz. The Raman signal became broader with increasing concentration of ortho-H-2. At the concentration of 2%, the spectrum splits into a doublet. The components of the doublet have different dependence on the relative polarization of the pump and probe laser radiations. Physical origin of the splitting and the polarization dependence is yet to be understood.