Fast nuclear-spin conversion of H2 trapped and polarized in a CO2 matrix

The clear evidence for the nuclear spin conversion of matrix-isolated ${\mathrm{H}}_{2}$ is presented. We trapped ${\mathrm{H}}_{2}$ in a ${\mathrm{CO}}_{2}$ film at 5.4 K and detected the induced infrared-absorption band including both the components of ortho and para ${\mathrm{H}}_{2}$. By analyzing its time evolution, the conversion rate was determined to be $9.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{4pt}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. We confirmed that this fast conversion was induced just by the ${\mathrm{CO}}_{2}$ film with the uniquely unstable structure. We also identified ${\mathrm{H}}_{2}$ at another kind of trapping sites, accompanied by higher anisotropy of the confining potential, and estimated its conversion rate to be $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{4pt}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$.