Stokes Components of Raman-Induced Singlet Oxygen

We studied the Stokes signals generated by the Raman photoexcitation of dissolved oxygen in water. When a water sample is pumped with intense nanosecond radiation, Stokes signals of different origins are generated. These signals form a characteristic nonlinear diffraction pattern, exhibiting a central spot and concentric rings whose radii depend on the Stokes wavelengths. Most of the Stokes signals correspond to the stretching vibrations of water molecules. However, we also observed a small contribution from dissolved oxygen molecules. This contribution can be separated from the others using appropriate spectroscopic filters and analyzed with a spectrometer. We report on Stokes components assigned to singlet oxygen excitation detected in the central spot, as well as in the diffraction pattern’s ring structure. The signal detected from the ring exhibits a single peak, while that from the ring itself shows a two-peak structure. The two observed peaks are interpreted as Stokes signals corresponding to Raman transitions to the two lowest vibrational sublevels of the singlet-oxygen electronic state. We also report exponential growth in the Stokes signal, in agreement with the standard stimulated Raman theoretical model.