Measuring the Efficiency of Raman Photoexcitation of Singlet Oxygen in Distilled Water

We determine the efficiency of singlet oxygen generated through a Raman excitation process in aerated distilled water. Focused nanosecond light pulses in the spectral blue region induce Raman transition toward the singlet oxygen state, generating a Stokes signal in the red spectral region. The signal is proportional to the number of photons corresponding to the amount of excited oxygen molecules. We calculate the efficiency by dividing the number of generated singlet oxygen molecules by the number of incoming pumping photons. We determine an efficiency of (8±2) 10-5 for water when pumping at 410 nm with a pulse energy of 13 mJ. We demonstrate that the Raman method exhibits no photobleaching, a phenomenon typically observed when photosensitizers are used. Thanks to this property, Raman excitation continues as long as the sample is irradiated, generating more singlet oxygen molecules over time than the photosensitization method.