VUV photochemistry and nuclear spin conversion of water and water–orthohydrogen complexes in parahydrogen crystals at 4 K

Samples of H2O{,} HDO{,} and D2O were isolated in solid parahydrogen (pH2) matrices and irradiated by vacuum ultraviolet (VUV) radiation at 147 nm. Fourier-Transform Infrared (FTIR) spectra showed a clear depletion of D2O and an enrichment of both HDO and H2O by 147 nm irradiation. These irradiation-dependent changes are attributed to the production of OH and/or OD radicals through photodissociations of H2O{,} HDO{,} and D2O. The radicals subsequently react with the hydrogen matrix{,} leading to the observed enrichment of H2O. No trace of isolated OH or OD was detected in the FTIR spectra{,} indicating that the OH/OD radicals react with the surrounding matrix hydrogen molecules via quantum tunneling within our experimental timescale. The observed temporal changes in concentrations{,} especially the increase of HDO concentration during VUV irradiation{,} can be interpreted by a model with a rapid conversion from orthohydrogen (oH2) to pH2 in water–oH2 complexes upon VUV photodissociation{,} indicating either the acceleration of the nuclear spin conversion (NSC) of H2 due to the magnetic moment of the intermediate OH/OD radical{,} or the preferential reaction of the OH/OD radical with a nearby oH2 molecule over other pH2 molecules. We have also identified and quantified an anomalously slow NSC of H2O and D2O complexed with oH2 in solid pH2.