Rovibrational transitions and nuclear spin conversion of methane in parahydrogen crystals

Solid parahydrogen is an excellent matrix for matrix-isolation spectroscopy because of its high spectral resolution. Here we describe the rovibrational structure and nuclear spin conversion of $CH4$ embedded in parahydrogen crystals studied by infrared absorption spectroscopy. The vibration–rotation absorptions of $CH4$ exhibit time-dependent intensity changes at 4.8 K. These changes are interpreted to be a result of the $I=1→I=2$ nuclear spin conversion that accompanies the $J=1→J=0$ rotational relaxation. The half-lifetime of the upper $J=1$ rotational state is unchanged by the addition of up to 2% orthohydrogen molecules but decreases with more than 10% orthohydrogen molecules. The increase of the decay rate at higher orthohydrogen concentration indicates that the magnetic field gradient across $CH4$ due to the orthohydrogen molecules mixes the nuclear spin states, which accelerates the conversion.