Detection of Water Droplet Size and Anion Species by Nonlinear Optical Scattering

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Abstract
The liquid‐air interface causes the droplet to act as an optical cavity which provides large internal fields and feedback. The stimulated Raman scattering (SRS) spectra of micrometer‐size water droplets containing different anions (e.g., NO, SO, and PO) can be generated by a single laser pulse and detected by an optical multichannel analyzer. When the spontaneous Raman intensity of the ν1 mode of the anions is larger than that of the O‐H stretching mode of water, the SRS spectra consist of the peak at ν1 and multiple peaks of nearly equal wavelength separation within the O‐H stretching mode of water. These multiple peaks are associated with the morphology‐dependent resonances of a sphere and the refractive index of water. Thus, the wavelength spacing of these SRS peaks provides information on the droplet radius. At higher anion concentrations, sequentially pumped SRS involving the ν1 anion mode can also be detected at Raman shifts equal to 2ν1 and equal to the O‐H stretching mode plus ν1. The Raman shift of the ν1 mode in wavenumbers provides molecular specific information.