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

Abstract

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.