Nonlinear Optical Spectroscopy of Liquid Droplets: Chemical Composition Within the Interface.

摘要

The combustion of liquid propellant spray offers new challenges for optical diagnostics. During the four years of ARO support, we have attempted to apply nonlinear optical spectroscopy as an in-situ probe to the determination of the chemical composition in the droplet and the droplet size. Droplets of low molarity ammonium nitrate in water are used as test samples. The SRS spectra of such droplets consist of two peaks at the vibrational-shifted frequencies of the nitrate ions and of the water molecules. The frequency shifts of the simulated Raman scattering (SRS) from the input laser frequency are equal to the vibrational frequencies. The dominant SRS peaks associated with the nitrate ions and water molecules are noted to consist of a series of equally spaced peaks which are attributed to morphology-dependent resonances of a droplet. The droplet radius can be accurately deduced from the wavelength spacing of the series of SRS peaks. The SRS intensity ratio of the nitrate peak and the water peak is proportional to the logarithmic ratio of the nitrate concentration and water concentration. The incident laser and SRS intensities inside the droplets are high enough to produce third order sum frequency generation and two-photon absorption.