Rovibrational transitions and nuclear spin conversion of methane in parahydrogen crystals

Abstract

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.