THE EFFECTS OF OPTICAL RESONANCES ON RAMAN ANALYSIS OF LIQUID AEROSOLS

The distillative evaporation of a single levitated microdroplet compos ed of a mixture of 1-iodododecane and 1-bromotetradecane was followed by means of Raman spectroscopy to determine the time-dependent chemica l composition and to explore the effects of morphology-dependent reson ances (MDRs) on the Raman data. Additional measurements of the single- component evaporation rates were performed to obtain the parameters ne eded to analyze the binary droplet data. The enhancement of Raman sign als by MDRs is analyzed and interpreted by means of Mie theory to dete rmine the modes and optical characteristics which produce such enhance ment. It is shown that only a limited number of all possible resonance s produce the effect and that enhancement is very sensitive to the mag nitude of the imaginary part of the complex refractive index. The narr owest resonances are damped and do not lead to Raman enhancement. Such analysis together with observation of the resonance modes which produ ce Raman enhancement make it possible to estimate the magnitude of the imaginary component of the refractive index. Multi-component evaporat ion theory is coupled with light-scattering theory to predict the size and refractive index of a distilling binary aerosol droplet. Predicte d enhancements and measured Raman signals are shown to be in good agre ement. Copyright (C) 1996 Elsevier Science Ltd.