Nuclear spin conversion of methane in solid parahydrogen

The nuclear spin conversion of C H4 and C D4 isolated in solid parahydrogen was investigated by high resolution Fourier transform infrared spectroscopy. From the analysis of the temporal changes of rovibrational absorption spectra, the nuclear spin conversion rates associated with the rotational relaxation from the J=1 state to the J=0 state for both species were determined at temperatures between 1 and 6 K. The conversion rate of C D4 was found to be 2-100 times faster than that of C H4 in this temperature range. The faster conversion in C D4 is attributed to the quadrupole interaction of D atoms in C D4, while the conversion in C H4 takes place mainly through the nuclear spin-nuclear spin interaction. The conversion rates depend on crystal temperature strongly above 3.5 K for C H4 and above 2 K for C D4, while the rates were almost constant below these temperatures. The temperature dependence indicates that the one-phonon process is dominant at low temperatures, while two-phonon processes become important at higher temperatures as a cause of the nuclear spin conversion.