Raman observation of the interactions between NH4+, SO42- , and H2O in supersaturated (NH4)2SO4 droplets

High signal-to-noise ratio (S/N) Raman spectra of (NH4)2SO4 droplets deposited on a quartz substrate were obtained from dilute to supersaturated states upon decreasing the relative humidity (RH). When the molar water-to-solute ratio (WSR) decreases from 16.8 to 3.2, the v1-SO42- band changes very little, that is, showing a red-shift of only about 1 cm-1 (from 979.9 to 978.8 cm-1) and an increase of its full width at half-maximum (fwhm) from 8.3 to 9.8 cm-1. Other vibration modes such as v2- and v4-SO42- bands appear almost constantly at 452 and 615 cm-1. Such kind of a spectroscopic characteristic is different from previous observation on other cations, indicating that the interactions between SO42- and NH4+ in supersaturated states are similar to those between SO42- and H2O in dilute states. After fitting the Raman spectra with Gaussian functions in the spectral range of 2400−4000 cm-1, we successfully extracted six components at positions of 2878.7, 3032.1, 3115.0, 3248.9, 3468.4, and 3628.8 cm-1, respe...