Confocal raman spectroscopy studies on the interactions between NH ₄NO ₃ and H ₂O in supersaturated nh4no3 droplets

High signal to noise (S/N) ratio Raman spectra of NH ₄NO ₃ droplets deposited on a quartz substrate were obtained from dilute to supersaturated states by reducing the relative humidity (RH) of the environment, allowing for accurate control over the concentration of solute within the droplet. When the RH was reduced from 72.1% to 37.9%, the peak position of the 1/1-NO3 ^- band of the NH4NO3 droplet did not shift from its original position at 1048 cm- ¹ and a similar full width at half-maximum (FWHM) of 10 cm- ¹ was also observed. It was concluded that the replacement of H ₂O molecules hydrogen-bonded with the O atoms of NO ^- ₃ with NH ₄ ⁺ ions leaves the frequency of 1/ ₁-NO ₃ ^- relatively unchanged, indicating that both H ₂O and NH ₄ ⁺ forming hydrogen bonds have the same strength. From component band analysis in the spectral range of 2500-4000 cm- ¹, six peaks at 2890, 3090, 3140, 3220, 3402, 3507 cm- ¹ were identified and assigned. The first four components were assigned to the second overtone of NH ₄ ⁺ umbrella bending, the combination band of NH ₄ ⁺ umbrella bending and rocking vibrations, the NH ₄ ⁺ symmetric stretching vibration, and the NH ₄ ⁺ antisymmetric stretching vibration. The latter two peaks originated from strong and weak hydrogen bonds. The signature of the strong hydrogen bonding component was observed to decrease in intensity with the decrease in RH over the full range from 72.1% to 37.9%, while the signature of the weak hydrogen bonding component was shown to increase as the RH was reduced. The observed trend in the hydrogen bonding component resulted from the interactions between NH ₄ ⁺ and NO ^- ₃.