Temperature Dependence for the Relative Raman Cross Section of the Ammonia/Water Complex

Abstract

The Raman spectra of aqueous ammonia solutions have been obtained between −40 and 25°C. The Raman spectrum of neat water was also obtained at 25°C and is characterized by two broad peaks observed at 3200 and 3400 cm−1. The spectrum due to water is subtracted to determine the NH3 spectrum at all temperatures. In ammonia–water solutions, the spectrum shows three features at measured displacements of 3250, 3316, and 3400 cm−1. The feature at 3316 cm−1 is assigned to the Q branch of the symmetric stretch. The broad, weak features at 3250 and 3400 cm−1, previously assigned to rotational bands, are assigned to combination bands. The NH3 combination bands are assigned by comparing with sum frequency generation (SFG) experiments, monitoring changes with temperature, and analyzing the polarization data. The rotational structure of the Q band is also discussed. As the temperature is lowered from 25 to −40°C, an increase in the Raman intensity is observed for all bands. The relative Raman scattering cross section is determined from the numerically integrated area of the NH3Q branch at each temperature.