Knowledge Bank

We have studied nuclear spin conversion of $CH4$ and $CD4$ trapped in solid $H2$ and solid $D2$ by Fourier-transform infrared spectroscopy. It has been shown that the rotational quantum number J of methane is still a good quantum number in solid hydrogen. The effective rotational constant has been found to be about 90% of the gas phase value. The vibration-rotation absorption of both $CH4$ and $CD4$ exhibited time-dependent intensity changes at liquid He temperatures. These changes are interpreted to be a result of the $I=1\to I=2$ nuclear spin conversion which accompanies the $J=1\to J=0$ rotational relaxation. The conversion rate is on the order of $10-3min-1$, but the rate depends on the temperature of the crystal. Mechanisms of the nuclear spin conversion in solid hydrogen will be discussed.