Stimulated Raman excitation of singlet oxygen in the water and heavy water.

Blue light (410–440 nm) generates red (600–670 nm) singlet oxygen's Stokes peak.

Oxygen quencher depletes the singlet oxygen's Stokes peak.

Irradiated uric acid UV spectra confirm the photogeneration of singlet oxygen.

Singlet oxygen is traditionally produced via photosensitizer molecules such as methylene blue, which function as catalysts. Here we investigate stimulated Raman generation of singlet oxygen from dissolved oxygen in both water (H2O) and heavy water (D2O) using nanosecond-pulsed visible blue laser light in the 400–440 nm spectral region without singlet oxygen photosensitizers. We report an oxygen-dependent Stokes peak in the red spectrum (600–670 nm) that is identical when produced in H2O and D2O. These red Stokes photons are not detected when an oxygen quencher is present. Temporal photodepletion of the uric acid absorbance peak at 294 nm is consistent with singlet oxygen generation. We postulate that a two-photon stimulated Raman process produces singlet oxygen from O2 dissolved within the solvents. We note that the energy difference between input and output photons of 0.97 eV is precisely the energy needed to excite O2 to its singlet state.

Singlet oxygen

Raman spectroscopy

Raman transitions

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