When a muonium in liquid water encounters with a singlet oxygen molecule, it will go through a different reaction/spin conversion mechanism compared with the known case of the ground-state triplet oxygen molecule. The difference is manifested as a change in the relaxation rate of the triplet Mu precession, or as a repolarisation curve associated with a Mu-oxygen radical molecule. The laser setup in HiFi is used to generate the singlet oxygen with a help of photosensitizer. A successful experiment will show a unique application of photo-MuSR technique.