Knowledge Bank

The infrared fundamental band of solid normal hydrogen has been investigated over the temperature range 1.3 to $1.7\circ K$. Two features in the spectrum, the $S1(0)$ line and the $S1(1)$ line, vanish at a temperature of about $1.4\circ K$ when the crystal is cooled. The features reappear at about $1.6\circ K$ when the crystal is warmed. The temperature at which the transition takes place upon warming is close to the $\lambda$-a***omaly in the specific heat. The vanishing of the two spectral features can be explained if one assumes a phase transition to a crystal structure possessing inversion symmetry.