Microwave Absorption in Solid Hydrogen

We report the first observations of direct transitions between the split rotational levels of nearest-neighbor pairs of (J=1) ortho-H2 in otherwise pure para-H2. Since the observed absorption lines are very narrow (Δν∼0.1 GHz at ν≃75 GHz), and since the theory of the pair spectrum seems tractable, we can in principle derive details of the intermolecular potential and of certain solid-state effects with much higher accuracy than was previously possible.