Knowledge Bank

Because of the weak intermolecular interactions, molecular rotations and vibrations are nearly free in para-$H2$ crystals, and their quantum states have long relaxation $timesh$. Vibration-rotation transitions appear extremely sharp with line widths that are smaller than those of gaseous lines by one to two orders of magnitude ($\Delta \nu /\nu \sim 10-7-10-8$). We can thus conduct high resolution spectroscopy and study solid states with unprecedented accuracy and clarity. I will discuss pure rotational (e.g. $J=6\leftarrow 0$), pure vibrational ($v=1\leftarrow 0,J=0\leftarrow 0)$, and vibration-rotation pair transitions with emphasis on qualitative and quantitative difference between solid and gaseous spectroscopies.