关键词: 拉曼光谱, 过饱和液滴, 氢键成分, 相对湿度, 光谱拟合

Abstract: High signal to noise (S/N) ratio Raman spectra of NH₄NO₃ 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 v₁-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 H₂O molecules hydrogen-bonded with the O atoms of NO₃^- with NH₄⁺ ions leaves the frequency of v₁-NO₃^- relatively unchanged, indicating that both H₂O and NH₄⁺ 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 NH₄⁺ umbrella bending, the combination band of NH₄⁺ umbrella bending and rocking vibrations, the NH₄⁺ symmetric stretching vibration, and the NH₄⁺ 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 NH₄⁺ and NO₃^- .

Key words: Raman spectrum, Supersaturated droplet, Hydrogen bonding component, Relative humidity, Spectral fitting